The Knee

FS T2W FSE sequence to assess marrow oedema and extra-articular fluid collections ●● indirect:26 involves the injection of 0.1 mmol/kg intravenous gadolinium followed by FS T1W SE imaging in

all 3 planes, with an additional FS T2W FSE sequence for assessment of extra-articular disease: ●● active or passive joint exercise for approximately 10 min may improve intra-articular contrast ●● external heat27 may also be applied to the affected knee and can result in a doubling of SI compared with

the contralateral knee ●● indications for MR arthrography include assessment of:

●● the post-operative meniscus: the reported diagnostic accuracy varies between 66% and 80% for conventional MRI and 85% and 92% for MR arthrography performed at 1.5T28,29

● using 3T imaging, conventional MRI has a sensitivity and specificity of 78% and 75% respectively, compared with 88% and 100% with MR arthrography

● occasionally, contrast material may not imbibe into a meniscal re-tear and routine MRI may reveal the tear30

● enhancement of fibrovascular scar tissue with indirect MR arthrography may result in false positive diagnosis of recurrent meniscal tear, and indirect MR arthrography may not result in a significant increase in sensitivity or specificity for recurrent tears

subchondral bone due to increased vascularity, and an enhanced fluid-filling cartilage defect ●● intra-articular bodies

●● the medial and lateral menisci: are C-shaped fibrocartilaginous structures (Fig. 5.3a), which lie between the femoral condyles and tibial plateau

●● function: they act as shock absorbers, increase stabilisation of the knee by deepening the contact surface between the femoral condyles and tibial plateau, distribute axial load, aid joint lubrication and proprioception

●● they are divided longitudinally into 3 segments: the anterior third, body (middle third) and posterior third and measure approximately 4-7 mm in peripheral height, tapering to a thin, sharp inner margin

●● meniscal blood supply: is derived from the medial and lateral genicular arteries: ●● in adults, this results in an inner avascular or white-white zone (>5 mm from the capsule), a middle

hypovascular or red-white zone (3-5 mm from the capsule), and a peripheral vascular or red-red zone (<3 mm from the capsule)

●● the menisci: demonstrate diffuse low SI on all pulse sequences due to their fibrocartilaginous nature ●● on peripheral sagittal images: they have a ‘bow-tie’ appearance representing the body (Fig. 5.3b), with

the anterior and posterior thirds appearing as triangular structures with sharply pointed inner margins (Fig. 5.3c)

●● on coronal images: the body appears as a triangle with the base against the capsule (Fig. 5.3d), while the anterior and posterior thirds appear as band-like structures (Figs 5.3e, f )

●● an MRI grading system: for intrameniscal signal intensity (SI) has historically been described, but is rarely used in clinical practice: ●● grade 1: increased intrameniscal SI that does not reach the articular surface (Fig. 5.4a) ●● grade 2: increased linear SI that reaches the capsular margin (Fig. 5.4b) ●● grade 2C: is defined as extensive wedge-shaped or triangular increased intrameniscal SI that does not

reach the articular surface on more than one image (Fig. 5.4c): ● it has a prevalence of 1.5% and in 50% of cases represents a meniscal tear, with no differentiating MRI

features between those with tears and those without35 ●● grade 3: linear or complex increased intrameniscal SI that reaches an articular surface on more than one

image (Fig. 5.4d): ● in the absence of previous surgery, this is an unequivocal sign of meniscal tear

●● increased intrameniscal SI is also seen in: ● the paediatric meniscus: due to the normal increased vascularity of the meniscus in children

(Fig. 5.5a), seen in 60% of children 8-15 years of age ● the meniscus of elderly patients: due to degeneration, with such menisci also showing contour

irregularity due to fraying (Figs 5.5b, c) ●● axial loaded MRI: allows evaluation of meniscal deformation and movement and can be performed at

differing degrees of knee flexion36

●● the medial meniscus: is more tightly adherent to the joint capsule compared to the lateral meniscus and is therefore less mobile, accounting for the increased incidence of medial meniscal tears

●● the anterior third: is approximately 1/3-1/2 the size of the posterior third (Fig. 5.3b) and has 2 segments (Fig. 5.6a): ●● the antero-inferior segment: is attached to the medial tibial spine just anterior to the ACL insertion ●● a common variant of the anterior root attachment is an insertion along the anterior margin of the tibia

(~15%), which can mimic pathological subluxation (Fig. 5.6b)38,39 ●● the posterosuperior segment: gives rise to the transverse intermeniscal ligament (Fig. 5.6c)

●● the posterior third: attaches anterior to the tibial insertion of the PCL (Fig. 5.6d) via the meniscal root, a ligamentlike structure sometimes referred to as the posterior medial meniscus root ligament (PMMRL) (Fig. 5.6e):40 ●● this prevents the meniscus from being extruded and allows the meniscus to generate hoop stress

●● the medial meniscus: is attached to the femoral condyle and tibial plateau by the coronary ligaments, which form the deep portion of the MCL and comprise the meniscofemoral and meniscotibial ligaments: ●● they are demonstrated on coronal MR images at the level of the superficial part of the MCL (Fig. 5.6f)

●● the meniscus: is firmly adherent to the capsule peripherally except for the innermost portion of the posterior third, where a variable amount of fibrofatty connective tissue is present between the meniscal margin and the posterior capsule (Fig. 5.6g): ●● occasionally a small recess may be present along the posterior horn of the medial meniscus at the

meniscocapsular junction (Fig. 5.6h): ● such recesses should not extend all the way superoinferiorly, which would suggest a complete

meniscocapsular tear41

●● the lateral meniscus: is smaller than the medial meniscus, with the anterior and posterior thirds being of equal size (Fig. 5.7a)

●● the anterior third: attaches to the tibia anterior to the tibial spine and posterolateral to the tibial attachment of the ACL (Figs 5.7b, c)

●● the posterior third: attaches to the tibia just posterior to the tibial spine (Fig. 5.7d) and anterior to the central attachment of the posterior third of the medial meniscus

●● the lateral meniscus: is loosely attached to the joint capsule, particularly in the posterolateral corner where the posterior third attaches to the capsule via the superior and inferior popliteomeniscal fascicles (Fig. 5.7e):42 ●● the fascicles: are identified on 97% of MRI studies, optimally on sagittal T2W sequences43,44 ●● the popliteus tendon: passes posterior to the lateral meniscus between the superior and inferior

popliteomeniscal fascicles (Fig. 5.7e)

●● 2 meniscofemoral ligaments (MFL): attach the posterior third of the lateral meniscus to the inner margin of the medial femoral condyle or the PCL: ●● the MFL of Humphrey: runs anterior to the PCL ●● the MFL of Wrisberg: runs posterior to the PCL

●● they function to stabilise the posterior horn of the lateral meniscus ●● either one or both are identified in 83-93% of knee MRI studies, most commonly the ligament of Wrisberg,

while both are present in approximately 37% of knees46 ●● on coronal images: the MFL appears as a hypointense oblique band in the intercondylar notch (Fig. 5.8a) ●● on sagittal images: the MFL appears as a hypointense dot running anterior (Humphrey) (Fig. 5.8b) or

posterior (Wrisberg) to the PCL (Fig. 5.8c) ●● anatomical variations: exist in proximal and distal attachments:47

●● the proximal attachment: 46% attach to the medial femoral condyle (Fig. 5.8a), 31% to the proximal half of the PCL (Fig. 5.8d) and 21% to the distal half of the PCL

●● the distal attachment: to the posterior third of lateral meniscus may be oblique in 65% of cases, vertical in 9% and indeterminate in the remainder

●● the ligaments also vary in their thickness, a thick MFL of Humphrey potentially being mistaken for a displaced meniscal fragment, mimicking the ‘double PCL sign’ of a bucket handle tear (Figs 5.8e, f )

●● the anteromedial MFL:48-50 is a rare ligament running from the anterior horn of the medial meniscus to the posterolateral wall of the intercondylar notch: ●● it appears as a thin hypointense band extending from the posterior aspect of the central portion of the

anterior medial meniscus to the posterolateral wall of the intercondylar notch, running anterior to and parallel to the ACL (Figs 5.8g, h)

●● anomalous insertion of the medial meniscus into the ACL is rare: a hypointense band is seen running from the anterior horn of the medial meniscus to the inferior or middle third of the ACL50

●● the anterolateral ligament: also referred to as the mid-third capsular ligament, represents a thickening of the lateral capsule extending from the prominence of the lateral femoral epicondyle, slightly anterior to the origin of the lateral collateral ligament, passing anteriorly and inferiorly to its attachment just posterior to Gerdy’s tubercle on the proximal tibia

●● it has femoral and tibial components and a meniscal portion, attached to the outer margin of the lateral meniscus (Figs 5.9a, b), and can be visualised in its entirety in up to ~72% of knees54

●● the transverse intermeniscal ligament: represents a fibrous band that runs between the anterior aspects of the medial and lateral menisci, most commonly between the anterior third of the medial meniscus and the anterior margin of the lateral meniscus56

●● it is present in approximately 94% of knees at arthroscopy,58 being identified in ~83% of MRI studies;57 it functions to stabilise the anterior third of the medial meniscus59 and may become entrapped during closed reduction of a tibial eminence fracture, with or without co-existing meniscal entrapment60

●● on axial and coronal MR images: it appears as a hypointense band running through the posterior aspect of Hoffa’s fat pad (Figs 5.10a, b)

●● on sagittal images: it appears as a hypointense spot within the posterior aspect of the fat pad, running between the anterosuperior aspects of the menisci (Fig. 5.10c)

●● the OMML: runs between the posterior third of one meniscus to the anterior third of the other meniscus: ●● a medial OMML: runs between the anterior third of the medial meniscus and posterior third of the lateral

meniscus ●● a lateral OMML: runs between the anterior third of the lateral meniscus and the posterior third of the

medial meniscus ●● both traverse the intercondylar notch between the ACL and PCL and are reported to be present in 1-4% of

knees ●● it has no known function, appearing on MRI as a thin, horizontal, hypointense band identified within the

intercondylar notch posterior to the ACL and anterior to the PCL (Fig. 5.11a) ●● it may simulate the double-PCL sign of a bucket-handle tear (Figs 5.11b, c)

●● the meniscofibular ligament: represents a posterolateral capsular thickening that extends between the tip of the fibular head and the posterior third of the lateral meniscus, and is thought to aid in proper positioning of the lateral meniscus within the knee joint

●● it is seen in ~42% of MRI studies, more commonly seen (~63%) in the presence of fluid in the posterolateral corner63

●● it lies anterior to the popliteus tendon, and can be visualised optimally on sagittal oblique and/or coronal FS PDW FSE/T2W images as a linear or curved hypointense band of variable thickness in the posterolateral corner (Figs 5.12a, b)

●● a discoid meniscus: represents a congenital/developmental anomaly in which the meniscus has a thickened, disc-like shape (Fig. 5.13a), being defined as a meniscus having a minimal width of >15 mm: ●● they are more common on the lateral side and have a reported overall incidence of ~1% of knees

●● the classification system: for discoid menisci is according to that of Watanabe: ●● partial or complete: depending upon the degree of tibial plateau covered (Figs 5.13b, c), with both types

having normal tibial attachments (Fig. 5.13a) and being usually asymptomatic unless torn

●● Wrisberg type: a very rare type of meniscus with nearly normal morphology but lacking posterior capsular attachments (Fig. 5.13d):

● it is consequently hypermobile and may be associated with snapping knee syndrome in childhood ● the posterior horn of a Wrisberg type discoid meniscus may rarely flip anteriorly, which can mimic a

●● they are bilateral in 20-79% of subjects, although when bilateral are of the same subtype in only 65% of cases67,70

●● discoid medial menisci are extremely rare (Figs 5.13e, f ) ●● MRI findings:71

●● continuity of the anterior and posterior thirds of the meniscus (Fig. 5.13a) on 3 or more consecutive 5 mm sagittal slices or 4 or more consecutive 4 mm sagittal slices

●● meniscal width of ≥15 mm: has a sensitivity of 80% and specificity of 98% (Figs 5.13b, c) ●● the ratio of meniscus to tibia (RMT): the minimum width of the meniscus to width of the tibia (in the

coronal plane) ≥20% has a sensitivity of 87% (Fig. 5.13c) ●● if the meniscus covers part of the tibial condyle, it is consistent with a partial discoid (Fig. 5.13c) ●● if the meniscus covers the whole of the tibial condyle, it is consistent with a complete discoid (Fig. 5.13b):

● a RMT ≥0.32 is suggestive of a complete discoid lateral meniscus (Fig. 5.13b)72

●● meniscal flounce: is the term used to describe a rippled appearance of the free, inner margin of the meniscus, and is thought to be a normal variant with a reported prevalence on sagittal MRI of 0.2-6.0%, most commonly occurring medially

●● it is not associated with a meniscal tear but may occur due to valgus deformity secondary to MCL injury, or external rotation deformity due to ACL tear

●● MRI findings: ●● an S-shaped, wavy appearance of the free inner meniscal margin on sagittal images (Figs 5.14a, b) and a

truncated appearance on coronal images (Fig. 5.14c), which should not be mistaken for a radial tear ●● meniscal malformations:38,75 a ring-shaped and flipped lateral meniscus may result in the appearance of a

meniscal fragment in the intercondylar notch, simulating a bucket-handle tear (Figs 5.14d, e): ●● both are rare congenital anomalies, with a ring-shaped meniscus accounting for 4 of 164 lateral meniscal

variants69 ●● a ring-shaped medial meniscus:76 is very rare and may simulate a displaced meniscal fragment, from which

it can be differentiated by the presence of a perfect isosceles triangle appearance of the meniscus within the central portion of the joint, and absence of a defect in the remainder of the meniscus

●● magic angle phenomenon:78,79 results in increased SI in the up sloping inner portion of the posterior third, typically of the lateral meniscus on short-TE sequences (Fig. 5.15a)

●● the anterior third of the lateral meniscus:80 speckled, striated or comb-like areas of high SI can be seen in the central aspect of the anterior third of the lateral meniscus as a normal variant (Fig. 5.15b)

●● edge artefact:81 a horizontal line of increased SI may be seen on the most peripheral sagittal image due to the concave outer margin of the meniscus (Fig. 5.15c), which may be mistaken for a horizontal tear

●● chondrocalcinosis:82 results in increased intrameniscal SI on T1W SE, PDW FSE and inversion recovery (IR) sequences (Figs 5.15d, e), thereby reducing the sensitivity, specificity and accuracy of MRI in the diagnosis of meniscal tears

●● haemosiderin-vacuum phenomenon:81 haemosiderin related to previous haemarthrosis and vacuum phenomenon may result in linear areas of signal void within the joint mimicking a displaced meniscal fragment, appearing most prominent on GRE images

Intra-Articular Ligaments and Tendons ●● meniscofemoral:83 sagittal MR images may show a pseudotear in the central aspect of the posterior third of

the lateral meniscus due to the meniscal insertion of the MFL, which appears as either an oblique (Fig. 5.16a) or vertical (Fig. 5.16b) line of increased SI: ●● however, the meniscal attachment of the MFLs should not extend >14 mm lateral to the lateral margin

of the PCL, beyond which a true tear (sometimes referred to as a ‘Wrisberg rip’) should be suspected, especially in the presence of an ACL tear77,84

●● transverse intermeniscal: on sagittal images, the attachment of the transverse ligament to the anterior third of the lateral meniscus may be mistaken for a tear (Figs 5.16c, d)

●● oblique meniscomeniscal:61 may mimic a displaced tear of the posterior third of the lateral meniscus on coronal images (Figs 5.11a-c)

●● popliteus tendon: a vertical high SI line is produced by the passage of the popliteus tendon behind the posterior third of the lateral meniscus, mimicking a vertical peripheral tear (Figs 5.16e, f )

●● meniscal subluxation: represents protrusion of the edge of the meniscus beyond the margin of the tibial plateau and is defined arbitrarily as: ●● protrusion of >25% of the width of the meniscus85 or protrusion of >3 mm of the medial meniscus or

>1 mm of the lateral meniscus87,88 ●● in normal/asymptomatic controls:85 medial meniscal subluxation is identified in 6.5% and 15% of sagittal and

coronal MR images and lateral meniscal subluxation is identified in 2% and 13% of sagittal and coronal MR images

●● abnormal meniscal subluxation: is reported in 8% of symptomatic knees,87 being associated with joint effusion and osteoarthritis (OA) (Fig. 5.17a),85,87,89 with the degree of subluxation being related to the degree of joint space narrowing:89 ●● posterior subluxation of lateral meniscus: is associated with ACL insufficiency (Fig. 5.17b)85

●● meniscal extrusion is also associated with:90 severe meniscal degeneration and meniscal tears, the latter being extensive complex or radial tears (Fig. 5.17c), or tears involving the meniscal root:88,91 ●● ~ 76% of patients with medial meniscal root tears will demonstrate pathological extrusion and conversely

39% of patients with extrusion are found to have a meniscal root tear92 ●● meniscal extrusion in young athletes:93 is reported in 48% of knees, being associated with joint effusion,

meniscal tears and ACL rupture

●● meniscal degeneration: represents mucoid change in the collagen fibres, which occurs with advancing age ●● with increasing intrasubstance degeneration, interstitial tears may develop, which are not clinically relevant ●● meniscal degeneration is identified in approximately 25% of knee MRI studies, appearing as intermediate

SI on T1W/PDW sequences and increased SI on FS PDW/T2W FSE sequences, which does not reach the articular surface (Fig. 5.4d): ●● it is typically unchanged on long-term follow-up and does not appear to represent a risk factor for

meniscal tear94,95

●● meniscal tears: may result as a consequence of progressive degeneration and extension of interstitial tears to the meniscal surface, or as an acute traumatic event

●● clinically: patients present with symptoms of locking, grinding and joint line tenderness with positive McMurray’s tests on examination

●● pathologically: tears occur most commonly in the posterior third of the medial meniscus, while tears of the anterior third are rare and over-reported at MRI, with a reported true-positive rate of 26%:97 ●● increased SI in the anterior third of the meniscus usually does not represent a clinically significant lesion

●● meniscal tears in asymptomatic knees:98 are reported in 67% of asymptomatic knees of patients with symptomatic meniscal tears on the contralateral side: ●● asymptomatic tears are usually horizontal or oblique types (degenerative), with asymptomatic radial,

complex or displaced tears being rare ●● clinical follow-up of asymptomatic meniscal tears has shown an increased incidence of knee pain, stiffness

and impaired function during daily activities after 2 years compared to individuals with no meniscal tears99

●● MRI criteria: for meniscal tears may be divided into primary and secondary signs ●● primary signs: include increased intrameniscal SI and abnormal meniscal morphology:

●● increased intrameniscal SI that reaches an articular surface:100 >90% of menisci with increased SI contacting the articular surface on more than one image are torn at arthroscopy (Fig. 5.4d):

● the signal to the surface must be in the same location on two images, although one image can be in the coronal plane and another in the sagittal plane, this observation being termed the ‘two-slice touch rule’101

●● in contrast, only 55% of medial and 30% of lateral menisci with increased SI contacting the articular surface on one slice image are torn:

● at 3T, increased SI reaching the articular surface on a single slice remains an equivocal finding, with utilisation of the two-slice touch rule resulting in an improvement in accuracy:102 – specificity and PPV for medial meniscal tears improves from 80% and 89% to 91% and 94%, respectively – specificity and PPV for lateral meniscal tears improves from 91% and 73% to 93% and 78%, respectively

● similarly, unequivocal grade 3 intrameniscal SI reveals tears at arthroscopy in 89% of cases, whereas equivocal grades 2-3 SI is associated with tears in only 10% of cases100

●● abnormal meniscal morphology: a small (Fig. 5.18a) or blunted (Fig. 5.18b) meniscus, in the absence of previous meniscectomy, indicates loss of meniscal tissue and should prompt the search for a displaced fragment

●● secondary signs: ●● a disrupted posterosuperior popliteomeniscal fascicle:103,104 is highly associated with a tear of the posterior

third of the lateral meniscus (Fig. 5.18c), with a PPV of 79-100% ●● tibial subchondral marrow oedema beneath a meniscus:105 has a 92-100% PPV for an overlying meniscal

tear (Fig. 5.18d) ●● the presence of a parameniscal cyst:106 is strongly associated with an underlying meniscal tear (90-100%):

● an exception is along the anterior third of the lateral meniscus, where an underlying tear is only found in 64% of subjects

●● classification:34,107 meniscal tears are broadly classified into 2 basic types, vertical and horizontal: ●● further subclassification by the International Society of Arthroscopy, Knee Surgery & Orthopaedic Sports

Medicine aims to unify descriptions and facilitate outcome assessment: ● vertical tears: are typically post-traumatic, being subdivided into radial and longitudinal ● horizontal (cleavage) tears: are typically degenerative ● flap tears: are divided into horizontal-flap and vertical-flap ● complex tears: are a combination of more than one morphology (Fig. 5.18e)

●● sensitivity and specificity are 93.3% and 88.4% for medial meniscal tears and 79.3% and 95.7% for lateral meniscal tears109

meniscocapsular junction), which have a high chance of spontaneous healing due to the excellent vascularity of the outer (red) zone of the meniscus

●● surgical options include: ● meniscal repair: usually undertaken for longitudinal and oblique tears ● meniscectomy (partial or complete): for radial, horizontal, complex or unstable tears:

– the goal is to preserve as much meniscal tissue as possible, since there is a direct correlation between the amount of meniscus resected and the time of onset and severity of OA

●● a radial tear: represents a vertical tear running perpendicular to the long axis of the meniscus, arising from the free edge and running into the meniscus: ●● they account for 14-15% of arthroscopically detected tears and are often associated with meniscal

extrusion (Fig. 5.17c)91 ●● they may be small and limited to the free edge of the meniscus (Fig. 5.19a), or large running through the

whole width of the meniscus (Fig. 5.19a), with full-thickness radial tears severely compromising meniscal function, leading to premature degenerative change:113 ●● therefore, the depth of a radial tear should be described as partial or complete

●● the most common locations:111 are the posterior third of the medial meniscus (53%) (Fig. 5.17c), the posterior third of the lateral meniscus (26%) (Fig. 5.19b) and the lateral meniscal body (16%) (Fig. 5.19c): ●● radial tears of the body or anterior medial meniscus are very rare

tip of the meniscal inner margin (Fig. 5.18b) ●● the cleft sign: represents a vertical high SI line passing through the meniscus (Figs 5.19b, c) ●● the marching cleft sign: is similar to the cleft sign but indicates the presence of the vertical tear on

consecutive slices, with the position moving either anteriorly or posteriorly on sagittal images, or sideways on coronal images

●● the ghost meniscus sign: represents a triangular hyperintense meniscal fragment due to the image slice passing through the tear (Fig. 5.19d), with a normal appearance to the meniscus being seen on adjacent slices

demonstrated on axial FS PDW FSE images (Fig. 5.19e)

●● longitudinal tears: run parallel to long axis of meniscus at a constant distance from its peripheral margin (Figs 5.20a-c): ●● they can involve a single or both articular surfaces, and are typically seen in younger patients after trauma,

being strongly associated with ACL tears:34,114 ● ~82% of peripheral longitudinal tears are seen in the setting of an acute ACL rupture/chronic ACL

deficiency ●● longitudinal tears also comprise approximately one-third of all meniscal tears seen in stable knees115 ●● if a tear extends significantly through the length of the meniscus, the inner fragment may become

displaced into the intercondylar notch, producing a bucket-handle tear (BHT) ●● BHTs:34,116-118 may present with locking and account for ~10% of meniscal tears, being far more common on

the medial side ●● MRI findings:

●● longitudinal tear: a vertically orientated linear increased SI focus within the substance of the meniscus, most commonly occurring in the outer one-third (Figs 5.20a-c)

●● MRI has a lower PPV for longitudinal tears than for other tear types:41 ● false positives may occur with tears located at the periphery/meniscocapsular junction and those

reaching only the superior surface, which can spontaneously heal by the time of arthroscopy ● although commonly associated with ACL tears, the sensitivity for detecting longitudinal tears is

reduced in the presence of an ACL injury119

●● BHT: a variety of signs have been described for the displaced fragment: ● the double-PCL sign: results when the meniscal fragment lies anterior and parallel to the PCL, being

seen on both sagittal and coronal images (Figs 5.21a, b): – has a reported sensitivity of ~30-53%120 and specificity of 100%118 – the use of coronal STIR116 and 3D volume sequences121 are reported to improve sensitivity

● the flipped meniscus (or double anterior horn) sign: results when a displaced posterior meniscal fragment flips anteriorly to lie adjacent to the anterior third, making the anterior third of the meniscus appear too large (Figs 5.21c, d): – has a reported sensitivity of 44% for medial BHT and 27% for lateral BHT and a specificity

of 89.7%118

● fragment in the intercondylar notch sign: results when the fragment is displaced into the intercondylar notch adjacent to the tibial spine (Figs 5.21e, f ): – notch fragments can also be identified on axial images, typically orientated in a sagittal plane (Fig. 5.21g) – has a reported sensitivity of 69% and specificity of 94%122

●● BHT: signs related to the residual meniscus include: ● the absent ‘bow-tie’ sign: a ‘bow-tie’ sign is present when the body of the meniscus appears as a bow-tie

in 2 consecutive 4 mm sagittal images, but is absent if the body appears as a ‘bow-tie’ on only 1 or no sagittal images:120 – has reported sensitivities and specificities of 71-98%117,118,121 and PPV of 76%121 – however, a false positive diagnosis may occur with small menisci or with meniscal subluxation

● the disproportionate posterior horn sign: results when the central portion of the posterior third appears larger than the peripheral portion of the posterior third of the meniscus (Figs 5.22a-c) and is indicative of a centrally displaced posterior third tear, being reported in 21-27% of cases117,123

● the truncated meniscus sign: results when the inner margin of the meniscus appears blunted on coronal images (Fig. 5.18b), being reported in 64% of cases120

●● the double ACL sign:124 has been reported as a sign of a displaced tear of the lateral meniscus, where the displaced fragment lies postero-inferior to the ACL (Fig. 5.22d):

● it may also refer to the rare displacement of a BHT of the medial meniscal fragment lying anterior to the ACL125

●● the quadruple cruciate sign:126 has been reported in the rare event of simultaneous BHTs of the medial and lateral menisci (Fig. 5.22e), resulting in 4 structures in the intercondylar notch on coronal images (Fig. 5.22f), due to the 2 displaced fragments, the ACL and the PCL

●● horizontal cleavage tears: extend through the meniscus in a plane parallel to the tibial plateau, involving either the free edge or one of the articular surfaces and propagating peripherally, thereby dividing the meniscus into superior and inferior portions (Figs 5.23a, b): ●● they are often confined to the posterior third (Figs 5.23c, d), but may also propagate to the body or

anterior third ●● horizontal tears are considered to be degenerative in aetiology, typically seen in patients over 40 years of

age, and are the commonest type of tear associated with a meniscal cyst ●● MRI findings:

●● a horizontal line of increased SI running through the meniscus, usually extending to the tibial articular surface (Figs 5.23a-d)

●● flap tears: refer to unstable injuries in which a part of a torn meniscus becomes displaced or can be displaced by a probe at arthroscopy

●● clinically: symptoms include those of mechanical obstruction in the form of locking, in addition to pain ●● preoperative identification is important as the displaced component may otherwise be overlooked at

arthroscopy

●● flap tears: can be subdivided into horizontal and vertical types: ●● horizontal flap tears: involve a short segment horizontal tear of the meniscus with displacement of a

meniscal leaf: ● displaced meniscal fragments128 more commonly affect the medial meniscus:

– in two-thirds of cases, the fragment displaces posteriorly toward the posterior intercondylar notch (Figs 5.24a, b) or posterior to the PCL

– in a third of cases, the fragment displaces towards the medial gutter

– inferior flap tears:129 are a subtype of horizontal flap tears, which involves displacement of a portion of the medial meniscal body into an adjacent synovial recess (usually the inferior) deep to the MCL (Figs 5.24c, d)

● hemi-bucket-handle tears:130 represent an uncommon type of horizontal flap tear, which have only been reported in the medial meniscus: – there is often an inferior surface horizontal flap tear (Fig 5.24e), typically with an intact superior

surface (~73%), in contrast to a classical bucket handle tear (see earlier) – the displaced meniscal tissue is directed towards the intercondylar notch (Fig 5.24e) – these should be distinguished from a typical BHT, as they are thought to have a poorer healing

capacity, which may influence surgical management ● horizontal flap tears may occur on the lateral side: the displaced fragment located in the posterior joint

or in the lateral recess (Figs 5.24f, g) with similar frequency128 ● the reported sensitivity and specificity of MRI for identifying the recess fragment is 71% and 98%

respectively122 ●● vertical flap tears: consist of both radial and longitudinal components (Figs 5.24h, i) typically resulting in

a flap of unstable meniscal tissue sometimes referred to as a ‘parrot beak’ tear: ● vertical flap tears usually commence as a radial tear and propagate as a longitudinal vertical tear, the

resulting unstable portion tending to displace centrally

●● a peripheral tear: represents a tear confined to the outer third of the meniscus (the red zone) (Fig. 5.25a), and is more likely to heal with conservative therapy due to the rich peripheral blood supply (Fig. 5.25b): ●● a false positive MRI diagnosis of peripheral tears can occur due to spontaneous healing by the time of

arthroscopy77,119

ligament-like properties of the root attachment ●● PMMRL lesions may be seen in up to ~29% of symptomatic knees40

●● lateral meniscal root tears: are typically traumatic and commonly associated with ACL rupture, in which case they have a reported incidence of 7-12%79

●● degeneration/partial tear: the presence of increased SI within a medial meniscal root, and/or focal linear subcortical marrow oedema deep to the root attachment (Fig. 5.26a) are thought to represent abnormal stress through a poorly functioning root attachment: ●● this is believed to be highly predictive of subsequent root tearing when re-imaged after a mean duration of

13.5 months133 ●● complete tear: results in loss of hoop stress, being almost functionally identical to total meniscectomy134 and is

reported to be a critical factor in the development of early OA86,135

●● the site of root tearing: may be at the junction of the posterior third with the root, within the mid-substance of the root, or at the enthesis attachment

●● degeneration: characterised by thickening of the root and intrasubstance increased SI that does not reach the articular surface (found in up to ~14% of symptomatic knees)

●● partial tear: characterised by abnormal SI which extends to the articular surface or abnormal morphology with partial discontinuity (up to ~12% of symptomatic knees) (Figs 5.26b, c)

●● complete tear: appears as a complete discontinuity of the affected root (<3% of symptomatic knees) (Figs 5.26d, e):

● coronal images are most reliable and accurate with the tear typically appearing as a vertical linear defect:112 – sensitivity, specificity and accuracy are reported to be 86-90%, 94-95% and 94%, respectively

● sagittal images may demonstrate a ghost meniscus sign (Fig. 5.19d) reflecting the absence of meniscus in the image plane with a normal meniscal appearance on the immediately adjacent images

●● extrusion of the meniscus is observed in ~76% of PMMRL tears

●● meniscocapsular separation: refers to an injury (typically a longitudinal-vertical tear) in which the meniscus separates from the capsule and may result in meniscal instability: ●● it is more common on the medial side but may also occur along the posterolateral corner ●● since it occurs in the red zone of the meniscus, spontaneous healing may occur, being reported in up to

94% of cases at the time of arthroscopy140

●● on the medial side: tears may occur at the meniscocapsular junction, at the meniscofemoral and meniscotibial (coronary) ligaments, which form part of the deep layer of the MCL, or within the peripheral portion of the meniscus (peripheral tear):

● symptomatic separation may involve a segment of less than 5 mm in length, which could be overlooked at MRI141

● posteromedial meniscocapsular separation occurs with ACL rupture, having an incidence of ~9% and is often unrecognised at MRI142

●● on the lateral side: tears may involve the popliteomeniscal fascicles,103,143 resulting in symptomatic lateral compartment knee pain associated with a hypermobile meniscus

●● the ‘floating meniscus’:144 describes a post-traumatic situation in which the meniscus is surrounded by joint fluid, indicative of tearing of the coronary ligaments due to meniscal avulsion from the tibia: ●● clinically: the condition is seen in cases of severe acute knee trauma such as dislocation, and more

commonly involves the lateral meniscus ●● meniscal avulsion: may be mimicked by the Wrisberg type of discoid meniscus, since this has no

attachment to the tibial plateau posteriorly ●● MRI findings:

●● meniscocapsular separation: meniscal displacement relative to the tibia with fluid between the meniscus and capsule (Fig. 5.27a):

● a meniscal corner tear: occurring at the outer edge of the meniscus but also involving the superior or inferior meniscal corner (Fig. 5.27b)

● an irregular meniscal margin and tears of the coronary ligaments (Fig. 5.27c)

● laterally: disruption of popliteomeniscal fascicles (Fig. 5.18c), with lateral meniscal displacement due to meniscal hypermobility

● all of the described features have a poor PPV for the diagnosis of meniscocapsular separation138 since meniscal displacement relative to the tibia may normally be as much as 10 mm medially or 13 mm laterally, and perimeniscal fluid may also be seen medially within synovial recesses, within the MCL bursa (which lies between the deep and superficial portions of the MCL), in association with MCL tears and within small meniscal cysts

● in the presence of a tibial plateau fracture: abnormal increased SI alongside the lateral meniscus (>7 mm thickness) and increased distance between the lateral collateral ligament and lateral meniscus (>7 mm) show high accuracy (96% and 85% respectively) for lateral meniscocapsular separation145

●● ‘floating meniscus’:144 the diagnosis is made in the presence of fluid SI of >3 mm thickness in long axis dimension on either sagittal or coronal images, surrounding either the anterior or posterior third of the meniscus, or fluid between the meniscus and the tibial plateau (Figs 5.27d, e):

● the meniscotibial ligaments are torn but the meniscofemoral ligaments are typically intact and the meniscus itself is usually normal apart from the avulsion injury

● associated cruciate and collateral ligament injuries are common, as are bone contusions (Figs 5.27d, e)

●● discoid lateral menisci: are more prone to tears than normal lateral menisci and may also show intrasubstance degeneration147 (Figs 5.28a, b)

●● 71-92% of discoid lateral menisci show tears,146,147 which are commonly multiple, with peripheral and horizontal types being the most common

●● tear type is related to type of discoid meniscus:148 complete discoid menisci typically show simple horizontal tears (Figs 5.28c, d), while partial discoid menisci show radial, degenerative and complex tear types (Figs 5.28e-g)

●● in a study of 164 lateral meniscal variants (158 discoid):69 6 tear patterns were observed, including 33 simple horizontal, 21 combined horizontal, 37 longitudinal, 27 central, 14 complex and 12 radial: ●● ~27% of discoid meniscus tears are associated with articular cartilage lesions, which are most commonly

located along the lateral tibial plateau149 ●● tears of a Wrisberg-type discoid meniscus: are often complex, degenerative tears (Figs 5.28c, d), possibly

●● discoid medial menisci: are rare (incidence of <0.3%) but are more prone to tears than normal medial menisci ●● horizontal cleavage tears are the single most common tear configuration (36%) with longitudinal vertical and

bucket-handle tears encountered less often ●● anomalous insertion of the discoid medial meniscus into the ACL is sometimes present, which may influence

operative management

●● meniscal tears are identified in 91% of patients with symptomatic OA and 76% of age-matched asymptomatic controls

●● meniscal tears in association with OA have no effect on the degree of pain or functional status ●● prediction of surgical reparability:154 a longitudinal or oblique high SI line within 3 mm of the

meniscocapsular junction ●● prediction of surgical irreparability: a high SI line greater than 5 mm from the meniscocapsular junction and/

or an abnormal high SI area within the meniscal body (complex tear configuration)

●● MRI has a high accuracy and low sensitivity for identifying reparable lesions, but is both accurate and sensitive at identifying irreparable lesions

most likely to heal ●● MRI features predictive of healing also include:

● the presence of thin horizontal strands of low SI on T2W images traversing the tear ● a tear width of <2 mm ● lack of tear visualisation on fluid-sensitive images: most longitudinal tears without signal reaching the

articular surface on T2W are likely to be partially or completely healed at subsequent arthroscopy

●● identification of unstable tears:155 an unstable tear is defined as one in which the meniscus or a meniscal fragment can be inappropriately displaced (more than 3 mm) by a probe at arthroscopy: ●● MRI features indicative of an unstable lesion include:

● identification of a displaced fragment: sensitivity 36% and specificity 94% ● visibility of the tear on more than three 3 mm thick coronal and two 4 mm thick sagittal images:

sensitivity 54% and specificity 94% ● having more than one orientation plane (complex tear) or more than one pattern (contour irregularity,

peripheral separation, tear): sensitivity 45% and specificity 94% ● having intrameniscal high SI on a T2W SE image (indicating joint fluid within the meniscus):

sensitivity 18% and specificity 100% ●● the sensitivity and specificity for the presence of at least one MRI criterion is 82% ●● with regard to the lateral meniscus, abnormality of the popliteomeniscal fascicles may indicate an unstable

lesion87

●● a meniscal cyst: represents a cyst arising within (meniscal) or more commonly adjacent to (parameniscal) the meniscus in association with a meniscal tear

●● pathologically: they are thought to result from an intrameniscal or parameniscal collection of joint fluid, which passes from the joint through a full-thickness meniscal tear, having a reported incidence of 4-8% on knee MRI studies, and beingassociated with 7.8% of meniscal tears44,158

associated with an underlying tear:106 ● up to 50% of AHLM cysts that are not associated with an underlying meniscal tear appear to be

associated with ACL cysts, the AHLM cyst possibly arising from adjacent ACL cysts dissecting along the common insertion with the root attachment of the AHLM (Figs 5.29a, b)159

●● clinically: they most commonly present in men aged 20-40 years, resulting in mechanical symptoms, pain or a focal swelling adjacent to the joint, although they are often asymptomatic: ●● they are clinically and arthroscopically more commonly seen from the lateral meniscus ●● however, MRI studies indicate an occurrence approximately twice as commonly from the medial

compartment compared to the lateral compartment156 ●● MRI findings:

●● a well-defined, lobular lesion either within (intrameniscal) (Figs 5.29c, d) and/or adjacent to (parameniscal) (Figs 5.29e, f ) the meniscus, having fluid SI on all pulse sequences (Figs 5.29c-f) and commonly with internal septation (Fig. 5.29g)

●● following contrast, there is either no or peripheral/septal enhancement (Fig. 5.29h) ●● direct contact with the adjacent meniscal tear is seen in 98% of cases,156 this being the most important

diagnostic criterion with 90% of tears showing a horizontal component(Fig. 5.29e) ●● rarely, meniscal cysts may result in erosion of the adjacent bone (Figs 5.29i, j)160 ●● patterns of cyst extension:161

● medial cysts: are most commonly located posteromedially (Fig. 5.30a), penetrating the capsule between Layer 1 and the fused Layers 2 and 3 (see later): – they may then extend anteriorly to lie superficial to the MCL (Figs 5.30a, b)

● lateral cysts: may extend anteriorly to lie deep to the iliotibial band (Figs 5.29e, f ) or posterolaterally to lie deep to the LCL (Figs 5.29i, j)

● cyst extension is also described: – anteriorly into Hoffa’s fat pad106,162 (Figs 5.30i, j) – posterior to the PCL from a posterior third medial meniscal tear (a pericruciate meniscal

cyst)163 (Figs 5.30e-g), which may simulate a PCL ganglion and is differentiated by identifying communication with a posterior third meniscal tear

– laterally from a lateral meniscal tear, which may rarely compress the common peroneal nerve164,165

Miscellaneous Meniscal Pathology ●● meniscal contusion:166,167 is seen in the setting of acute knee trauma and is associated with ACL injury and

tibial plateau fracture ●● MRI findings:

●● contusion appears as a region of poorly-defined intrameniscal hyperintensity, which reaches the articular surface, with adjacent bone bruising in the posterior aspect of the tibial plateau, and may resolve with time

●● meniscal haematoma:168,169 is a very rare phenomenon, thought to result from trauma induced bleeding from the perimeniscal capillary plexus

●● MRI findings: ●● reported cases show identical appearances to a pericruciate meniscal cyst of the posterior third of the

medial meniscus, or rarely the lateral meniscus ●● meniscal ossicle:170,171 is a rare intrameniscal bone fragment (ossicle), which is most commonly seen in the

posterior horn and root of the medial meniscus and radiographically simulates a loose body: ●● it is thought to be a post-traumatic lesion, either from heterotopic ossification within the meniscus, or less

commonly an avulsion injury of the root attachment from the tibial spine172,173

●● a corticated, marrow-containing structure within the posterior third of the medial meniscus, being hyperintense on T1W (Fig. 5.31)

●● an associated meniscal tear is seen in the majority of cases (~98%), severe cartilage loss in 51%, and prior ACL reconstruction or ACL injury is present in 58%

●● the ACL: is an intracapsular/extrasynovial structure that originates from the posteromedial aspect of the lateral femoral condyle, posterior to the intercondylar notch, and inserts into the tibia anterolateral to the anterior tibial spine, between the attachments of the anterior thirds of the menisci

●● it runs inferomedially and parallel to the roof of the intercondylar notch, parallel to Blumensaat’s line, ~55° to the plane of the tibial plateau

●● the ACL: is approximately 38 mm long, 11 mm wide and has 2 functional units: ●● the anteromedial bundle: which is tight in flexion ●● the posterolateral bundle: which is typically smaller than the anteromedial bundle178 and is tight in

extension ●● the double bundle anatomy of the ACL can be visualised in 94% of patients at 3T179

●● functionally: it is the major restraint to anterior tibial translation and also provides restraint to rotatory load

●● the pericruciate fat pad: is an intracapsular/extrasynovial structure filling the gap between the ACL and the PCL

●● MRI findings: ●● the sagittal oblique plane (Fig. 5.32a): optimally demonstrates the length of the ligament, allows

differentiation of the 2 bundles and shows the tibial attachment: ●● with the knee extended, the ligament appears straight ●● with the knee flexed, the ligament may appear lax and slightly curved ●● the coronal plane (Figs 5.32b, c): shows the ligament within the intercondylar notch, where it

runs from superolateral to inferomedial, and allows assessment of both the femoral origin and tibial insertion

●● the axial plane (Fig. 5.32d): most clearly demonstrates the femoral origin of the ACL ●● the anteromedial bundle is hypointense (Figs 5.32a, b), while the posterolateral bundle is of intermediate

SI on T1W and mildly hyperintense on FS PDW/T2W FSE/STIR images (Fig. 5.32e), having a fan-like structure with a striated appearance due to regions of fat near its tibial attachment

●● an ACL comprising three bundles: is a normal variant with a separate third bundle, the intermediate bundle, thought to be a subsidiary of the anteromedial bundle ●● it may be quite common but under-recognised, with a reported prevalence of ~20%182 ●● it differs from the anteromedial meniscofemoral ligament (see earlier) as it does not insert into the anterior

horn of the medial meniscus180

●● hypoplastic or aplastic ACL: is rare and may be associated with a hypoplastic PCL or other congenital anomalies, including meniscal anomalies, fibular hemimelia and hypoplasia of the tibial spines, intercondylar notch or patella

●● MRI findings: ●● the ACL may be hypoplastic, replaced by a low SI band or completely absent (Figs 5.33a, b)

●● acute ACL rupture: typically follows a valgus force on the knee in varying degrees of flexion, with associated external rotation of the tibia or internal rotation of the femur, the so-called ‘pivot shift’ injury: ●● it is a relatively common sports injury in skiers and American footballers, typically related to deceleration

and pivoting, with <20% occurring in the context of a contact mechanism of injury186,187 ●● clinically: examination findings include knee swelling and an anterior drawer or positive Lachman’s test ●● ACL tears: may occur in the mid-portion (commonly from contact sports) (Figs 5.34a, b), at the femoral

origin (commonly from skiing) (Figs 5.34c, d), or rarely the ligament is avulsed from its tibial insertion without (Fig. 5.34e) or with a fragment of bone (Fig. 5.34f): ●● tibial avulsion: accounts for ~5% of cases, usually in the skeletally immature

●● MRI: has a reported sensitivity of 92-94% and specificity of 95-100% for the diagnosis of complete ACL rupture

●● MRI findings: ●● primary signs: consist of morphological and SI abnormalities, with acute tears showing a poorly-defined,

oedematous ligament on T1W, PDW FSE and T2W FSE images (Figs 5.34a-d), often with focal discontinuity (Fig. 5.35a):

● tears of the femoral insertion are well-demonstrated on axial images (Fig. 5.35b) ● mid-substance tears are usually best demonstrated on sagittal images (Fig. 5.34a) ● subacute tears (2-4 weeks): show a wavy, horizontal (Fig. 5.35c) or retracted appearance:

– an abnormal course of the ACL: can be identified by Blumensaat’s angle, representing the angle between the roof of the intercondylar notch and the line of the ACL, the apex of which normally points posteriorly (Fig. 5.35d), but with ACL rupture, the apex points anteriorly (Fig. 5.35e)

– vertical orientation of the proximal ACL, horizontal orientation of the distal ACL and ligament discontinuity each have a PPV of 100% for an ACL tear189

● chronic tears (6-8 weeks): have a variable appearance, being attenuated (Fig. 5.36a), incompletely visualised or absent (Figs 5.36b, c), or the ligament may become fibrosed

● various patterns of scarring have been reported, which could result in underestimating the severity of the injury, including:190-192 – end-to-end scarring of the torn ACL – scarring of the ACL to the intercondylar roof – scarring of the distal remnant to the anatomical origin of the ACL on the posteromedial wall of the

lateral femoral condyle (least common) (Figs 5.36d-f) – scarring and attachment onto the PCL (Fig. 5.36g)

●● secondary signs: are related to bone injury in the acute stage, and to anterior tibial shift in the chronic stage ●● MRI findings:

●● bone injury: in the form of bone bruising, classically in the central portion of the lateral femoral condyle (Figs 5.37a, b) and the posterolateral aspect of the tibial plateau (Figs 5.37a, b), having a reported sensitivity of 50% and specificity of 97% for acute ACL rupture:

● tibial bone marrow changes resolve within a median time of 6 months and femoral marrow changes within 3 months following ACL injury193

● a similar pattern of bone bruising can be seen in young children with an intact ligament due to relative ligament laxity

● additional sites of bone injury include: the medial margin of the MFC (Fig. 5.37b), the posteromedial tibial plateau (Fig. 5.37c), deepening (>1.5 mm) of the condylopatellar sulcus and/or a double sulcus of the lateral femoral condyle, termed the deep lateral notch and double notch sign respectively194 (Fig. 5.37d), and an osteochondral fracture of the lateral femoral condyle (Fig. 5.37e)

● cortical fractures are common: present in 60-72% of cases:195 – cortical depression fractures with ACL tear are associated with poorer clinical outcome scores at

1 year after ACL reconstruction ● the Segond fracture:196 an avulsion injury of the lateral rim of the tibia at the attachment site of the

anterolateral ligament (Figs 5.37f, g)

●● anterior tibial shift in the form of: ● the anterior drawer sign: diagnosed when there is 5-7 mm of posterior translation of the lateral femoral

condyle with respect to the lateral tibial plateau (Fig. 5.38a) ● a buckled PCL: concavity of the posterior (tibial) portion of the PCL with acute angulation at the apex

(Fig. 5.38b) ● an abnormal PCL line: the PCL line is defined on sagittal images as a line drawn along the posterior

aspect of the PCL, which should normally intersect the posterior femoral cortex within 5 cm of the distal femur (Fig. 5.38c)

● posterior displacement of the posterior third of the lateral meniscus relative to the posterior margin of the tibial plateau, which has a sensitivity of 57% and specificity of 97-100% (Fig. 5.38d)

● the vertical fibular collateral ligament sign:197 normally, the fibular collateral ligament (FCL) is not seen in its full course on a coronal image, but with anterior tibial shift, the FCL assumes a coronal orientation, and may be seen in its entire length on a single coronal slice (Fig. 5.38e)

● undulation of the patellar tendon ●● associated injuries include:175 meniscal tears, occurring in 41-68% of acute ACL injuries, being typically

posterior third peripheral, longitudinal-vertical tears: ● in the presence of a cortical depression fracture, a meniscal tear is often present in the same compartment195

– a delay in diagnosis of an ACL tear >6 months after initial injury increases the risk of medial meniscal tearing198

● articular cartilage injuries: reported in 23% of acute injuries and 54% of chronic injuries

● posterolateral corner injury: seen particularly with a hyperextension mechanism ● tibial collateral ligament injury ● a shearing injury of the infrapatellar fat pad: must be differentiated from a normal horizontal cleft

(see later): – manifests as abnormal fat pad SI together with other signs of acute ACL rupture

●● stump entrapment: may result in locking or a block to terminal extension of the knee following ACL rupture: ●● stump entrapment has also been reported following partial ACL tears which selectively involve either the

anteromedial or the posterolateral bundle201 ●● MRI findings:

●● 2 types of abnormality are described: ● type 1: a nodular mass located at the anterior aspect of the intercondylar notch (Figs 5.39a-c),

between the lateral femoral condyle and tibia ● type 2: a tongue-like free end with angulation of the stump (Figs 5.39d, e)

●● partial ACL tears: are more difficult to diagnose than complete ACL rupture and may be stable or unstable, accounting for 10-35% of ACL injuries diagnosed at MRI

●● the addition of coronal oblique17 sequences can improve the diagnostic accuracy of MRI, with a statistically significant improvement in specificity at 3T205

●● the addition of an axial oblique intermediate-weighted sequence improves sensitivity and diagnostic accuracy for partial-thickness tears19

●● MRI findings: ●● a discrete focus of increased SI and deformity (bowing) (Figs 5.40a, b) within a ligament that has largely

intact fibres ●● ACL distortion without obvious discontinuity (Figs 5.40c, d) ●● bone bruising is less common than with complete tears ●● MRI has a reported sensitivity of 40-75% and specificity of 62-89% for the diagnosis of partial ACL rupture:202

● 3T imaging offers greater sensitivity, specificity and accuracy of 77%, 97% and 95%, respectively for partial tears206

● the use of 3D isotropic sequences at 3T does not appear to increase diagnostic accuracy for partial ACL tears207

●● axial images:9 cannot differentiate stable partial tears from intact ligaments, or unstable partial tears from completely ruptured ligaments, but can sometimes be helpful in differentiating between stable and unstable partial tears:

● stable partial tears may appear as an attenuated ligament, while unstable partial tears can appear as an isolated ACL bundle, a non-visualised ACL or a cloud-like mass

●● secondary signs of ACL insufficiency are more common with unstable tears:208 ● anterior tibial translation, uncovering of the posterior horn of the lateral meniscus and buckling of the

PCL, although of low sensitivity (23%), are highly specific for an unstable injury

●● mucoid degeneration of the ACL: is an ageing phenomenon, the ligament being functionally intact ●● it may be associated with a ganglion cyst of the ACL, or may actually represent an intraligamentous ganglion

cyst210,211 ●● it is usually an incidental finding on MRI, being reported in ~1% of knee studies,210 although it may be

associated with knee pain, which can clinically simulate a meniscal tear,213,214 and can present with a limitation of knee extension in up to 78%215

●● the ligament appears ill-defined and expanded, but with a normal orientation, and fibres typically more conspicuous on T2W images than on T1W sequences

● intermediate SI on T1W and increased SI on PDW and T2W sequences (Figs 5.41a-c), although this will not equal fluid SI, as seen with ganglion cyst formation (see below)

●● both the anteromedial and posterolateral bundles are intact throughout their entire length, in contrast to a partial tear of the ACL

●● cystic (Fig. 5.41d), oedema-like (Fig. 5.41e) or erosive (Fig. 5.41f) marrow changes may be seen at the ligament attachments

●● the PCL: is an intracapsular/extrasynovial structure, which shares the same envelope as the ACL ●● its function is to resist posterior translation, and it is twice as strong as the ACL ●● the PCL: consists of an anterolateral and posteromedial bundle, which originate from the posterolateral aspect

of the MFC and attach into the posterior intercondylar fossa of the tibia, approximately 1 cm below the tibial articular surface: ●● the anterolateral bundle is stronger and is tightest in flexion, while the posteromedial bundle is tightest in

extension

●● MRI findings: ●● on sagittal MR images, it is usually a smooth posteriorly convex structure (Fig. 5.42a), although with the

knee in extension it may be slightly buckled (Fig. 5.42b), the apex being termed the genu ●● the femoral and tibial attachments are well demonstrated on coronal (Figs 5.42c, d) and axial

(Figs 5.42e, f ) images

●● occasionally, the double bundle structure of the ligament is evident at the femoral (Fig. 5.42g) or tibial attachments (Fig. 5.42h)184

●● the ligament is typically uniformly hypointense, although slight increased SI may be seen on short TE sequences due to magic angle effect (Fig. 5.42i)

●● PCL rupture: is less common than ACL rupture, representing 2-23% of all knee injuries ●● injury mechanisms include: posterior displacement of the tibia with a flexed knee (dashboard injury),

hyperextension and rotation combined with either valgus or varus force ●● PCL injury: is commonly associated with ACL tear and posterolateral corner injury, being isolated in 30% of

cases, most commonly when due to a ‘dashboard’ mechanism of injury ●● tear patterns include: partial tear (47%), complete tear (45%) and bony avulsion from the tibial attachment

(9%) ●● acute tears occur most commonly in the mid-substance of the ligament ●● healing of the PCL:219,220 may be demonstrated with MRI as regained continuity of the ligament, which may

simulate a normal ligament particularly >6 months after injury: ●● this is seen with isolated ligament tears, or those combined with MCL injury, whereas an associated

posterolateral corner injury predisposes to failure of PCL healing ●● MRI findings:221

●● acute tears (partial or complete): show diffuse ligament thickening with indistinct margins: ● an AP dimension of the ligament distal to the genu of >7 mm has a sensitivity of 94% and specificity of

92% for PCL tears, although this can also be seen in mucoid degeneration of the PCL (see below) ● increased SI on PDW images is more commonly seen than on T2W sequences (Figs 5.43a-c) ● non-visualisation of the ligament (Figs 5.43d, e)

●● bony avulsion from tibial plateau: the ligament may appear intact but an avulsed fragment and bone oedema are seen at the posterior tibial plateau (Figs 5.44a, b)

●● complete disruption characterises complete tears and is seen in 66% of cases ●● posterior displacement of the tibia in relation to the femur may be seen (Fig. 5.44c) ●● associated injuries:216 PCL tears are associated with other knee injuries in 66-72% of cases, which

include: ● bone bruising (34%): in the anterior tibia (Figs 5.44d, e) and anterolateral femoral condyle ● ligament injury (42%): ACL, MCL and LCL, most commonly the MCL ● meniscal tears: 31-52% ● an avulsion fracture of the tibial portion of the deep MCL, termed the ‘reverse’ Segond fracture222

●● chronic tears: manifest as a thickened, attenuated (Fig. 5.44f) or buckled ligament with low SI, and chronic PCL deficiency may result in premature patellofemoral OA

●● multi-ligament injury of the knee: is a severe injury often including joint dislocation and may involve rupture of one or both of the cruciate ligaments in addition to injuries of the MCL and/or LCL

●● often due to high velocity trauma, which results in a much higher incidence of associated neurovascular compromise

●● mucoid degeneration of the PCL: is much less common than of the ACL (incidence of ~0.1%) and the pathogenesis remains uncertain

●● clinically: the ligament is typically functionally intact and asymptomatic with no instability on posterior drawer examination, but rarely it may cause pain227

●● MRI findings: ●● a striated ‘tram-track’ appearances with thickening (mean 8.5 mm AP diameter), which may simulate a

tear (Figs 5.45a, b): ● homogeneous intermediate-to-increased SI on PDW and T2W sequences extending longitudinally

throughout a diffusely thickened PCL ● the fibres along the margins of the PCL remain intact with preserved low SI, which can be helpful in

discriminating mucoid degeneration from a tear ●● a coincidental ganglion cyst of the PCL is common (~80%) ●● incidental mucoid degeneration of the ACL is observed in ~45% of cases

●● cruciate ganglion cysts: are true intra-articular ganglion cysts associated with the cruciate ligaments, which account for 75% of reported intra-articular ganglia of the knee229

pain but are not associated with knee instability ●● pathologically: they may arise within (intraligamentous) (Figs 5.46a, b) or on the surface of the ligament

(extraligamentous) (Figs 5.47a, b), and may involve the proximal ligament, distal ligament or whole length of the ligament, having a mean diameter 31 mm (range 20-73 mm)210

●● PCL ganglia:232,234 are less common than ACL ganglia, typically occurring in relation to the posterior aspect of the ligament and are therefore located in the intercondylar notch: ●● they are usually well-defined and often multilocular in appearance (Figs 5.47c-e)

●● MRI findings: ●● intraligamentous ganglia: may resemble mucoid degeneration (Figs 5.41a-c) or have a more focal cystic

appearance (Figs 5.46a, b), and are the most common type to involve the ACL ●● extraligamentous ganglia: are lobulated or fusiform in shape, simple or complex in architecture, being

hypointense on T1W and hyperintense on PDW FSE and T2W images (Figs 5.47a-e) ●● they may show rim enhancement on post-contrast FS T1W sequences, but MR arthrography is of no

additional diagnostic value229 ●● the differential diagnosis: of a cyst in the posterior capsular region includes:

● a pericruciate cyst arising from a posterior medial meniscal tear (Figs 5.30e, f ) ● a ganglion cyst arising from the ligament of Humphrey235 ● a loculated posterior capsular effusion

●● cruciate intraosseous cysts: are also referred to as insertional cysts, and are reported in 1-1.5% of knee MRI studies in association with the proximal or distal insertions of the ACL and PCL210

●● they are typically incidental findings, most commonly arising at the femoral insertion of the ACL (Figs 5.48a, b) and tibial insertion of the PCL (Figs 5.48c, d) ●● they may be associated with mucoid degeneration of the cruciate ligaments (see above) (Fig. 5.41d),210,213

or may be due to chronic avulsive stress

●● MRI findings: ●● subcortical cysts at the ligament insertions, appearing hypointense on T1W, hyperintense on PDW

(Figs 5.48a, c) and FS PDW/FS T2W/STIR images (Figs 5.48b, d) ●● they are rarely associated with marrow oedema236

●● pericruciate fat pad impingement: has been described as an uncommon cause of debilitating posterior knee pain in athletic individuals participating in intensive activity, e.g. football players

●● MRI findings: ●● increased SI on FS PDW FSE/FS T2W FSE sequences within the pericruciate fat pad between the ACL

and the PCL

●● the medial capsular structures are divided into three anatomical layers ●● layer 1: the superficial layer consists of the deep crural fascia:

●● anterosuperiorly: it is continuous with the fascia of the vastus medialis muscle ●● anteriorly: layers 1 and 2 fuse to form the medial patellar retinaculum (Fig. 5.49a) ●● centrally: it is separated from the superficial part of the MCL by a variable amount of fatty tissue

●● posteriorly: it is continuous with the sartorius muscle and superficial to the tendons of gracilis, semimembranosus and semitendinosus (Fig. 5.49a)

●● the tendons of sartorius, gracilis and semitendinosus join to form the pes anserinus (Figs 5.49b-d), which inserts into the anteromedial aspect of the proximal tibial metaphysis:

● not uncommonly, the sartorius is predominantly muscular to its distal insertion (Fig. 5.49e)184 ●● layer 2: the intermediate layer comprising mainly the superficial part of the MCL, also termed the tibial

collateral ligament (TCL): ●● the MCL runs vertically along the middle third of the medial part of the knee and functions as a restraint

to valgus stress and external rotation ●● it arises from the MFC and inserts into the medial tibia, 6-7 cm distal to the joint line (Figs 5.50a, b) ●● posterior to the MCL is the posterior oblique part of the MCL, which is considered to represent an

individual structure termed the posterior oblique ligament (POL – see later) ●● layer 3: the deep capsular layer:

●● anteriorly: it is continuous with the capsule of the suprapatellar recess ●● centrally: it lies deep to the vertical part of the superficial MCL, forming the deep MCL (also termed

the capsular ligament), a capsular thickening attached to the medial meniscal margin with superior and inferior extensions to form the (coronary) meniscofemoral and meniscotibial ligaments (Fig. 5.6f)

●● posteriorly: it fuses with layer 2 ●● the MCL bursa: lies between the superficial and deep parts of the ligament, but is not identified unless

distended with fluid (Fig. 5.50c)

●● the deep component of the MCL: is usually injured before the superficial component as it is shorter and weaker ●● isolated deep MCL injuries: typically represent meniscocapsular separations (see earlier) and do not result in

clinical valgus laxity in the presence of an intact overlying MCL241

●● MCL injuries: commonly occur in association with other ligament injuries, particularly ACL rupture and result from valgus stress, such as a clipping injury that may occur in rugby, American football or hockey245

●● isolated MCL injuries: usually heal spontaneously resulting in a thickened ligament ●● the overall accuracy of MRI for MCL injury is reported as 87%,242 although correlation between clinical

grading and MRI grading may not be very good (~65%) ●● MRI findings:

● grade 1 injury: subcutaneous oedema over the intact MCL, usually the proximal component (Fig. 5.51a) and less commonly the distal component (Fig. 5.51b)

● grade 2 injury: increased SI within the MCL on FS PDW/T2W images (Figs 5.51c, d), thickening (Fig. 5.51e) or attenuation of the ligament, and longitudinal striations within the MCL representing moderate to high-grade interstitial partial tearing

● grade 3: focal disruption of the MCL (Figs 5.51f, g)

● the ligament may be ruptured at its femoral origin, in its mid-substance (Fig. 5.51h) or least commonly at its tibial insertion (Fig. 5.51i)

● rarely, an avulsion fracture of the MFC may occur (Fig. 5.51j) ● indirect signs/associated injuries include:

– ACL rupture: is always associated with a higher grade of MCL injury than isolated MCL injury242 – medial meniscal tears – O’Donoghue’s unhappy triad:246 refers to a specific injury pattern encompassing grade 3 MCL

injury along with tears of the ACL and medial meniscus – bone bruising:247 reported in 45% of cases of isolated MCL injury, occurring either medially due

to micro-avulsions from the ligament attachment (Fig. 5.52a) or laterally from the initial valgus impaction (Fig. 5.52b)

– transient lateral dislocation of the patella: is associated with MCL injury in up to 50% of cases and thought to be due to a shared mechanism of valgus injury248

●● the Pelligrini-Stieda lesion: represents post-traumatic ossification of the femoral insertion of the MCL and/or the adductor magnus insertion: ●● calcification/ossification extending proximal to the femoral attachment of the MCL can also be seen as a

result of stripping of the femoral periosteum with an intact adductor magnus insertion ●● Pelligrini-Stieda lesions caused by periosteal stripping are associated with PCL injuries252

●● MRI findings: ●● depend upon the maturity of ligament ossification ●● 4 types of tendon ossification have been described:

● beak-like with inferior orientation and femoral attachment (commonest) ● a drop-like pattern parallel to the femoral condyle ● an elongated appearance with superior orientation parallel to the femur ● beak-like with inferior and superior orientation and femoral attachment

●● the proximal MCL shows chronic thickening ●● bone fragments may appear as areas of signal void if mainly cortical bone (Figs 5.53a, b), or may have

internal fat SI if composed of medullary bone (Fig. 5.53c)

●● grade 1 and grade 2 MCL oedema: is present in approximately 90% of cases of medial compartment OA253 (Fig. 5.54a)

●● MCL oedema:254 has also been noted in 60% of knee MRI studies in the absence of trauma, being significantly associated with the presence of medial meniscal tears (Fig. 5.54b), lateral meniscal tears, meniscal extrusion and femoral chondromalacia: ●● patients with MCL oedema were also significantly older than patients without this finding

●● calcification of the MCL: is a rare condition that can present with medial knee pain, typically in women in the 5th-6th decades and may mimic a medial meniscal tear

●● radiographs demonstrate lobular calcification, usually in the femoral component of the ligament ●● the condition responds well to open debridement of the calcific deposits ●● MRI findings:

●● a swollen ligament containing areas of lobular low SI calcification, with soft tissue and marrow oedemalike SI (Figs 5.55a-c)

●● bone erosion may also be seen

●● medial knee friction syndromes: have been described at several sites, including along the posteromedial knee and the medial tibial crest

●● posteromedial knee friction syndrome:260 is thought to represent an uncommon cause of medial knee pain and/or a medial knee snapping sensation:

●● MRI findings: ●● increased SI between the posteromedial femoral condyle (PMFC) and the deep surface of sartorius and/or

gracilis tendon (Figs 5.56a, b) ●● medial tibial crest friction:258 is a rare cause of medial knee pain ~2.5 cm inferior to the joint line, which has

been described in young, athletic individuals in the absence of a history of trauma ●● MRI findings:

●● increased SI deep to the distal fibres of the MCL with secondary marrow oedema along the tibial crest ●● it is predisposed to by a prominent tibial crest, manifest by a reduction of the tibial crest angle (Fig. 5.56c)

●● the posteromedial corner: extends from the posterior margin of the MCL to the medial margin of the PCL ●● the structures of the posteromedial corner include:

●● the posterior third of the medial meniscus and meniscotibial (coronary) ligaments ●● the posterior oblique ligament (POL): which arises from the adductor tubercle of the MFC and distally,

has 3 separate arms: ● the tibial arm: which passes inferiorly inserting onto the posteromedial aspect of the tibia and to the

posterior third of the medial meniscus ● the capsular arm: is continuous with the posterior capsule and blends with the oblique popliteal

ligament (OPL) ● the superficial arm: passes distally to insert onto the tibia together with the fascial tissues of the pes anserinus ● the POL: is optimally assessed on coronal and axial MR images as a thin hypointense structure

posterior to the MCL (Figs 5.57a, b) ●● the distal semimembranosus expansions: the semimembranosus muscle arises from the ischial tuberosity

and its distal tendon, which may normally contain a small amount of fat (Fig. 5.57c), splits at approximately the level of the knee joint line into 6 distinct expansions:265

● 1 – the anterior (tibial) arm: also termed the pars reflexa, passes anteriorly deep to the POL and inserts into the tibia deep to the MCL, being identified on sagittal and coronal MR images (Figs 5.57d, e)

● 2 – the direct arm: is attached to the tibia deep to the pars reflexa and is not seen at MR imaging ● 3 – the inferior (popliteal) arm: extends more distally than the direct and anterior arms to insert into the

tibia just above the tibial attachment of the MCL, being visualised on sagittal images (Fig. 5.57f) ● 4 – the capsular arm: is contiguous with the POL and best seen on axial images (Fig. 5.57g), and also

on sagittal oblique images when it is particularly thick (Fig. 5.57h)

● 5 – the OPL insertion: an extension of the semimembranosus tendon that blends with the OPL and posterior capsule

● 6 – the meniscal arm: usually arises from the anterior arm and is a short band-like connection to the meniscotibial band (coronary ligament) of the posterior horn of the medial meniscus (Fig. 5.57e)

●● the OPL: extends laterally to fuse with the medial limb of the arcuate ligament and is seen on axial and sagittal images, being indistinguishable from the posterior capsule

●● posteromedial corner injury: may result in the condition known as anteromedial rotatory instability (AMRI): ●● excessive opening of the medial joint space in abduction at 30° knee flexion ●● simultaneous anteromedial rotatory subluxation of the medial tibial condyle on the central axis of the

intact PCL ●● clinically: patients present with posteromedial pain and tenderness in the acute stage ●● pathologically: findings include a spectrum of lesions of the distal semimembranosus complex259,266

●● semimembranosus tendon injuries include: complete tears, myotendinous junction injuries, avulsion injuries, partial tears, tendinopathy and insertional tendinosis

●● complete tendon tears: are very rare ●● MRI findings:

●● tendon disruption and retraction with peritendinous haematoma (Figs 5.58a, b) ●● myotendinous junction injuries: typically occur in athletes ●● MRI findings:

●● oedema/haemorrhage at the myotendinous junction ●● avulsion injuries: of the posteromedial corner of the tibial plateau with associated MCL, ACL, medial

meniscal and rarely PCL tear ●● MRI findings:

●● a fracture line in the posteromedial tibial plateau, with bone and soft-tissue oedema, and tendon oedema and thickening (Figs 5.58c, d)

●● partial tears of the tendon and POL ●● MRI findings:

●● abnormal tendon or ligament SI, with fluid in the posteromedial corner of the knee and lack of visualisation of the anterior limb of the tendon insertion (see below)

●● tendinopathy: acute or chronic ●● MRI findings:

●● tendon swelling and increased SI with surrounding soft-tissue oedema and bursitis when acute (Figs 5.58e, f ), or tendon enlargement with normal SI when chronic (Fig. 5.58g)

●● insertional tendinosis: is associated with chronic repetitive injury and stress of the tibial insertion of semimembranosus

●● MRI findings: ●● swelling and oedema of the tendon insertion (Fig. 5.58h), with/without irregularity and cystic change

within the posterior medial tibia (Fig. 5.58i)

meniscofemoral ligaments (83%)

●● MRI findings: ●● periligamentous oedema (Figs 5.59a, b), ligament thickening and hyperintensity (Fig. 5.59c) on FS

PDW/T2W/STIR images and partial or complete ligament rupture (Fig. 5.59d) ●● injury to various components of the distal semimembranosus expansion (Figs 5.59e, f ) ●● associated injuries include: medial meniscal tears (43%), MCL tears (33%), ACL rupture (75%), which

may be associated with avulsion fracture of the posteromedial tibial plateau at the meniscotibial ligament insertion (the so called ‘reverse Segond’ fracture) or bone bruising at the insertion of semimembranosus219,268

● PCL rupture is rare

collateral ligament (FCL) posteriorly

●● layer 3: the capsular layer, comprising a superficial part containing the fabellofibular ligament, a deep part containing the arcuate ligament complex, and the popliteus tendon, which lies deep to the arcuate ligament

●● the functional anatomy: can be divided into anterolateral and posterolateral stabilisers

●● the anterolateral stabilisers: consist of the capsule and iliotibial tract ●● the lateral capsule: provides anterior and posterolateral stability and is reinforced by the superior and inferior

retinaculum and the vastus lateralis muscle: ●● the anterolateral ligament:51 represents a thickening of the joint capsule with attachments to the femoral

condyle, anterolateral tibial plateau ( just posterior to Gerdy’s tubercle) and lateral margin of the lateral meniscus (Fig. 5.9)

●● the iliotibial band: is an extension of the fascia lata and consists of deep and superficial layers (major tendinous component): ●● the superficial layer: inserts onto Gerdy’s tubercle on the anterolateral aspect of the proximal tibia

(Figs 5.60a, b) ●● the deep layer: inserts on the intermuscular septum of the distal femur

●● injury to the anterolateral ligament (ALL): is identified in almost 80% of cases of acute ACL rupture, the injury typically involving the distal part of the ALL

●● MRI findings: ●● discontinuity, irregular contour and oedema/thickening of the ALL, usually involving the tibial

component, with periligamentous oedema (Fig. 5.61)

●● acute ITB injury:275 is reported in ~60% of patients imaged for acute knee trauma within 4 weeks of injury, typically young males: ●● injury patterns include minor sprain (grade 1) in 78.3%, severe sprain (grade 2) in 17.4% and tear (grade 3)

in 4.3% ●● associated injuries are present in the vast majority of cases, most commonly ACL tears, acute patellar

dislocation and posterolateral corner injury

●● ITB avulsion: is associated with acute ACL rupture: ●● associated injuries commonly include: MCL tear, meniscocapsular tear of the posterior horn of the medial

meniscus, and posterolateral corner injury196 ●● the fracture fragment is typically small (mean size measures 8 × 10 × 2 mm) and the resulting marrow

oedema is not significant ●● MRI findings:

●● grade 1 sprain manifests as oedema around an otherwise normal ITB (Figs 5.62a, b), grade 2 sprain as oedema around a thickened ITB and grade 3 sprain as disruption of the ITB or avulsion from Gerdy’s tubercle (Fig. 5.62c)

●● the ITB friction syndrome: is a cause of lateral knee pain related to repetitive motion of the knee, typically occurring in long-distance runners and cyclists: ●● it is fairly common, accounting for ~15% of all overuse injuries affecting the knee

●● clinically: it presents with tenderness over the lateral femoral condyle a few centimetres proximal to the joint line, with reproduction of pain during flexion/extension of the knee while pressure is exerted over the condyle

●● MRI findings: ●● most common finding (75%): poorly-defined fluid SI between the ITB, the distal vastus lateralis and the

lateral femoral condyle, best demonstrated on coronal and axial FS PDW/T2W/STIR images (Figs 5.63a, b)

●● abnormal SI may occasionally extend posterolaterally between biceps femoris and the femoral shaft (Fig. 5.63c), distal to the lateral femoral condyle, between the ITB and LCL, and rarely superficial to the ITB (Fig. 5.63c)

●● typically, in acute/subacute cases the ITB has normal thickness and SI (Figs 5.63a, b), but it may be thickened in chronic cases (Fig. 5.63d)

●● 30% of cases show a well-circumscribed fluid collection between the ITB and lateral femoral condyle, thought to represent adventitial bursa formation (Figs 5.63e, f )

●● the posterolateral stabilisers: or posterolateral corner (PLC) is also termed the arcuate ligament complex and functions to resist varus and external rotation forces

●● the PLC consists of: ●● the FCL, which originates from the external tuberosity of the lateral femoral condyle, directly anterior to

the insertion of the lateral head of gastrocnemius, and inserts as the conjoined tendon together with biceps femoris onto the fibular head:

● it is demonstrated on coronal, sagittal and axial images, appearing as a 3-4 mm thick hypointense band (Figs 5.64a-c)

●● the biceps femoris (BF) muscle and tendon: the BF muscle has long and short heads, the long head arising from the ischial tuberosity and the short head arising from the lateral lip of the linea aspera of the distal femur:

● the BFT descends posterior to the iliotibial tract and inserts as the conjoined tendon with the FCL onto the fibular head (Fig. 5.64d)

● it is demonstrated on coronal (Fig. 5.64e), sagittal (Fig. 5.64d) and axial (Fig. 5.64c) images as a hypointense structure

● variations exist in the distal tendon, a prominent anterior slip inserting into the lateral tibia being frequently identified (Fig. 5.64f)184

●● the anterolateral ligament (also termed the mid-third capsular ligament – see earlier) (Fig. 5.9) ●● the popliteus muscle and tendon: the popliteus tendon arises below the FCL in a sulcus on the lateral

femoral condyle (Fig. 5.64c) and passes inferiorly and posteriorly deep to the FCL, descends through the popliteus hiatus deep to the arcuate ligament and joins its muscle, which inserts onto the posteromedial aspect of the proximal tibia:

● it is best assessed on a combination of coronal, sagittal and axial images (Figs 5.65a-c), and may demonstrate increased SI on low-TE images due to magic angle effect (Fig. 5.65d)

● bifurcation of the popliteus tendon is a recognised normal variant in ~0.4% of knees undergoing arthroscopy (Fig. 5.65e)286

● the cyamella: is a rarely present sesamoid bone, which is typically located in the vicinity of the popliteus myotendinous junction

●● the popliteomeniscal fascicles: are extensions from the popliteus tendon to the lateral meniscus, and comprise superior and inferior struts, which are best assessed on sagittal oblique PDW/T2W images (Fig. 5.7e):

● the popliteus also has a small muscular attachment to the posterior third of the lateral meniscus (Fig. 5.65f)

●● the popliteofibular ligament: is considered to be one of the most important stabilisers of the PLC, extending from the popliteus tendon ( just proximal to the musculotendinous junction) to the posteromedial aspect of the fibular styloid (Fig. 5.66a):

● it is identified on 38% of MR studies,270 optimally on coronal oblique images, but the ligament may also be seen on standard sagittal oblique images (Fig. 5.66b)

● the lateral inferior geniculate artery (LIGA) is an anatomical marker of its location (Figs 5.66c, d) ●● the arcuate ligament: is a Y-shaped thickening of the capsule, reported to be present in 24-87% of

knees:270 ● the medial limb curves over the popliteus tendon and joins the oblique popliteal ligament, being seen

on 25% of MR studies (Fig. 5.66c)270 ● the lateral limb extends to the fibular styloid, being identified in 23% of MR studies270

●● the OPL: a capsular ligament with lateral attachment to the posterior aspect of the lateral femoral condyle, where it merges with the medial limb of the arcuate ligament, and a medial attachment to the posterior surface of the MFC, where it merges with the tendon of semimembranosus:

● it is indistinguishable from the posterior capsule ●● the fabellofibular ligament: a capsular thickening that extends from the fabella to the styloid process of the

fibula: ● in the absence of the fabella, the ligament extends to the lateral femoral condyle and is

reported to be present in 24-80% of knees,270 although identified in only 23% of MRI studies (Fig. 5.66e)270

●● the lateral gastrocnemius tendon: inserts on the supracondylar process of the femur (Fig. 5.67a), just posterior to the FCL and contains the fabella:287

● the fabella: is a sesamoid bone located in the anterior surface of the lateral head of gastrocnemius tendon, occurring with a reported incidence of 11-13%

● its anterior surface is covered with smooth hyaline articular cartilage and it is best assessed on sagittal and axial images (Figs 5.67a, b)

●● the (sub)popliteus bursa: is an extra-articular extension of the synovial membrane of the knee joint, which extends from the popliteal hiatus along the proximal part of the popliteus tendon:

● the distended bursa appears as fluid around the popliteus tendon and muscle (Figs 5.67c, d) and may be confused with a tear of the musculotendinous junction

●● posterolateral corner (PLC) injury: is caused by combined varus force and hyperextension of the knee, and may be isolated but is usually associated with PCL and/or ACL injury

●● clinically: it presents with posterolateral rotatory instability, which manifests as posterior subluxation and external rotation of the lateral tibial plateau relative to the femur: ●● however, it may go clinically undetected due to the associated cruciate ligament injury (ACL in ~60% and

PCL in ~20%), and missed PLC injury is a recognised cause of failed cruciate ligament repair ●● MRI has high accuracy in identifying acute high-grade PLC injuries, but sensitivity is considerably reduced

●● FCL tears: which are either isolated or combined with other PLC injuries, may be proximal (Figs 5.68a, b), mid-substance (Fig. 5.68c) or distal (Figs 5.68d, e) and are graded 1 (oedema) to 3 (complete disruption)

● an irregular contour or focal discontinuity of the ligament, with hyperintensity and surrounding oedema (Figs 5.68a-c)

● avulsion of the conjoined tendon from the fibular head, which is the most common pattern (Figs 5.68d, e) ● in chronic cases, the tendon may appear diffusely thickened (Fig. 5.68f)

●● BFT injury: has been reported in 79% of cases,291 and can take the form of distal avulsion from the fibular head (most common) (Figs 5.68d, e) or avulsion fracture of the fibular styloid process:

● various degrees of tendinopathy (Fig. 5.68g) and tenosynovitis (Fig. 5.68h) may also be seen ●● arcuate fracture:292-294 an avulsion of the fibular head and styloid at the insertion points of the arcuate

complex (arcuate, popliteofibular and fabellofibular ligaments) ●● MRI findings:

● an avulsed fragment from the posterosuperior apex of the fibular styloid with marrow oedema in the fibular head (Figs 5.69a, b)

● hyperintensity and swelling of the popliteofibular ligament and soft-tissue oedema around the medial limb of the arcuate ligament (Figs 5.69c, d)

● associated injuries to the cruciate and collateral ligaments, medial and lateral menisci and bone bruising in a variety of locations, typically in the anteromedial femoral and tibial condyles

●● popliteofibular ligament injury:290 can occur in the context of arcuate fracture, or may consist of ligament disruption, partial tear or intrasubstance degeneration

●● MRI findings: ● increased SI within the ligament, focal discontinuity, or avulsion, best seen on coronal or sagittal

oblique images (Figs 5.69e, f ) ●● popliteus tendon tears:295 have been reported in 36% of patients,291 involving the musculotendinous

junction in 96% of cases and typically being associated with injuries to the cruciate/collateral ligaments and menisci, with ~8% being isolated

●● MRI findings: ● tears at the musculotendinous junction appear as muscle oedema/haemorrhage extending proximally

around the tendon, which itself may appear normal (grade 1) (Figs 5.70a-c), partially disrupted (grade 2) (Fig. 5.70d) or completely torn (grade 3) (Fig. 5.70e)

● insertional tendinosis (Fig. 5.70f) and avulsion of the tendon insertion from the lateral femoral condyle (Fig. 5.70g) are rare occurrences

●● calcification of the FCL: is a rare entity, in which calcification occurs in the proximal portion of the intact ligament

●● clinically: patients present with acute onset, severe, non-traumatic lateral knee pain ●● pathologically: the condition is likely due to hydroxyapatite deposition disease (HADD) ●● MRI findings:

●● thickening of the ligament with focal low SI areas due to calcific deposits ●● surrounding inflammatory reaction on T2W or STIR sequences and enhancement following

gadolinium ●● confirmation of crystal deposition within the tendon can be made by high resolution CT297

●● the fabella syndrome: describes pain arising in the posterolateral corner, which is exacerbated by pressure over the fabella

●● it is thought to occur secondary to chondromalacia in the younger age group and OA in older age group ●● MRI findings:

●● are not described although degenerative changes at the fabello-femoral joint are potentially symptomatic

●● pathology: of the cyamella is rare, sesamoiditis having been described in a single case report, with mechanical symptoms of pseudolocking and clicking

●● MRI findings: ●● marrow oedema within the cyamella with increased SI of the surrounding soft tissues

●● tendinopathy of popliteus: due to a cyamella can clinically mimic a lateral meniscal tear

●● the posterior capsule: extends transversely between the medial and lateral femoral condyles, bridging the posterior aspect of the intercondylar fossa

●● it is contributed to: on the medial side by the OPL and on the lateral side by the medial limb of the arcuate ligament

●● on its medial aspect: the capsule appears as a thick band-like structure adjacent to the posterior third of the medial meniscus (Fig. 5.71a): ●● the superior medial attachment is to the posterior femoral cortex, a few centimetres above the condylar

articular cartilage ●● the inferior medial attachment is to the posterior tibial cortex, a few centimetres below the joint line ●● a bursa is present between the capsule and semimembranosus tendon ●● more laterally, the medial head of gastrocnemius tendon lies posterior to the capsule (Fig. 5.71b) ●● a joint recess is present between the superior attachment of the medial gastrocnemius tendon and the

posterior femoral cortex (Fig. 5.71c) ●● in its central aspect: in the intercondylar area, the capsule consists of incomplete fibres with perforations

allowing communication between the posterior joint space and the popliteal fossa: ●● these are penetrated by vessels and nerves (Fig. 5.71d)

●● a joint (posterior cruciate ligament; PCL) recess, which may contain fluid, is located between the PCL and posterior capsule (see later)

●● the popliteal vessels are located posterior to the capsule ●● in its lateral aspect: the popliteus hiatus lies between the posterior third of the lateral meniscus and the

posterolateral capsule, allowing passage of the popliteus tendon: ●● the subpopliteus recess: may extend deep to the capsule and posterior to the proximal tibia ●● the posterolateral knee joint may communicate with the proximal tibiofibular joint in this region

(Figs 5.71e, f ) ●● medially, the posterolateral capsule is reinforced by the lateral head of gastrocnemius tendon, a joint recess

also being present deep to the lateral head tendon (Fig. 5.71g)

●● the posterior knee muscles: include the gastrocnemius, plantaris and popliteus ●● the gastrocnemius muscle: is the most superficial muscle of the calf, arising via medial and lateral heads

from the posterior surface of the femur, just proximal to the femoral condyles (Figs 5.71.b, g and 5.72a, b): ●● the 2 heads unite to form the main bulk of the muscle, which extends through the calf to terminate as the

Achilles tendon ●● function: plantar-flexion of the foot and knee flexion in the non-weight bearing state

●● several anatomical variants of the gastrocnemius are evident on knee MRI, including the accessory gastrocnemius, which arises from the central aspect of the distal posterior femur, and runs lateral to the popliteal vessels to fuse with the lateral head in the proximal calf (Figs 5.72c-e), and a small slip which extends laterally from the lateral head of gastrocnemius to fuse with the deep surface of the ITB (Figs 5.72f, g)184

●● the plantaris muscle: is a small strap-like muscle, which is absent in 7-10% of the population: ●● it arises from the lateral supracondylar line just above the attachment of the lateral head of gastrocnemius

tendon (Fig. 5.73a), and its muscle belly lies deep to the lateral head of gastrocnemius (Fig. 5.72b) ●● the plantaris tendon inserts into the calcaneus or the medial aspect of the Achilles tendon

●● the popliteus muscle:285 arises from the posteromedial aspect of the proximal tibial metaphysis and forms part of the floor of the popliteal fossa (Figs 5.73b, c): ●● its tendon inserts via the popliteus hiatus into the lateral femoral condyle ●● it also has attachments to:306 the fibular head via the popliteofibular ligament (seen in 98% of cases) and at

least one attachment to the posterior aspect of the lateral meniscus (seen in 95% of cases) ●● function: internal rotation of the tibia on the femur in the non-weight-bearing state, and external rotation

of the femur on the leg in the weight-bearing state ●● the accessory popliteus muscle:307 an accessory muscle in the popliteal fossa, which has a common origin with

the lateral head of gastrocnemius from the posterior femoral condyle: ●● its belly lies between the posterior capsule and the popliteal artery (Fig. 5.73d), and it inserts into the

posteromedial aspect of the posterior capsule (Fig. 5.73e)

●● the tensor fascia suralis:308 is a very rare superficial accessory muscle which usually arises from the distal semimembranosus (Fig. 5.73f): ●● it crosses the knee (Fig. 5.73g) to insert into the posterior fascia of the calf, the medial gastrocnemius

(Fig. 5.73h) or via a long thin tendon into the Achilles tendon ●● the popliteal artery: is the continuation of the femoral artery at the adductor hiatus, running through the

popliteal fossa behind the knee joint and popliteus muscle and deep to the popliteal vein and tibial nerve (Figs 5.74a, b): ●● the artery has muscular branches and articular branches to the knee and terminates at the lower border of

the popliteus muscle by dividing into the anterior and posterior tibial arteries ●● the tibial nerve: is the major branch of the sciatic nerve and runs within the popliteal fossa initially lateral to,

then superficial to and eventually medial to the popliteal artery (Figs 5.74b-e) ●● the common peroneal nerve: is the lesser branch of the sciatic nerve and runs in the lateral aspect of

the popliteal fossa to lie posteromedial to the BF muscle and lateral/superficial to the lateral head of gastrocnemius to run posterior to the fibular head (Figs 5.74b-e)

●● posterior capsular rupture: may occur following a hyperextension injury and is usually associated with tears of the menisci or cruciate ligaments

●● MRI findings: ●● capsular disruption with high SI oedema in the posterior capsular region on FS PDW/T2W images

(Figs 5.75a, b) ●● chronic stage: the capsule may appear irregular and thickened

●● the gastrocnemius muscle: medial head gastrocnemius strains may occur at the level of the knee joint and may be associated with tears of the semimembranosus tendon: ●● tears of the medial head of gastrocnemius: may involve the proximal muscle (‘tennis leg’), which is

relatively common (Figs 5.76a, b), while injury of the tendon at its myotendinous junction or proximal attachment is uncommon (Figs 5.76c, d)

●● tears of the lateral head of gastrocnemius occur in association with posterolateral corner injury (Fig. 5.76e) ●● MRI findings:

●● oedema at the musculotendinous junction, tendon thickening and disruption (Figs 5.76a-e) ●● the plantaris muscle: ruptures may occur in association with ACL tears or posterolateral corner injuries:

●● tears of plantaris or the medial head of gastrocnemius may result in posterior compartment syndrome ●● MRI findings:

●● increased SI on FS PDW/T2W images in the plantaris muscle or at the musculotendinous junction: ● myotendinous rupture: resulting in a mass-like appearance between the popliteus tendon and the

lateral head of gastrocnemius ● fluid collections between the medial head of gastrocnemius and soleus muscles ● associated injuries, as described above

●● the popliteus muscle: (see posterolateral corner injuries)

●● the PCL recess: represents a synovial recess that lies between the ACL and PCL and may extend to the medial wall of the posterior intercondylar fossa

●● effusion may normally collect within the PCL recess and needs to be differentiated from cystic structures in this region, including PCL ganglia (Figs 5.47c-e) and pericruciate meniscal cysts (Figs 5.30e, f )

●● MRI findings: ●● the recess has some characteristic features, which allow its differentiation from other posterior fluid

collections ●● it is a round or oval structure lying posterior to the PCL (Fig. 5.77a) and adjacent to the lateral aspect of

the MFC (Fig. 5.77b) ●● it does not contact the proximal one-third of the PCL (Fig. 5.77a) and lacks a surrounding capsule ●● the recess may be traversed by the ligament of Wrisberg ●● sagittal and coronal plane dimensions are typically ~2 × 1 cm and axial plane dimensions are ~1 × 1 cm ●● it communicates with the medial femorotibial compartment (Fig. 5.77c) ●● various pathological processes may be encountered within the recess, including loose bodies

●● various bursae are identified in the posteromedial capsular region ●● the semimembranosus bursa: is located below the level of the joint space and effusion in the bursa may lie

deep and superficial to the tendon insertion (Figs 5.78a, b) ●● the subgastrocnemius bursa: is located between the capsule and gastrocnemius tendon (Fig. 5.78c):

●● effusion in the bursa may extend between the proximal semimembranosus tendon and the MFC (Fig. 5.78d), and communicates with the semimembranosus-gastrocnemius bursa (Fig. 5.78e)

●● the semimembranosus-gastrocnemius bursa: extends between the tendons of the medial head of gastrocnemius and semimembranosus, and when pathologically distended represents a popliteal (Baker’s) cyst (see later): ●● a small effusion in this bursa is a common finding on knee MRI (Fig. 5.78f)

●● in the posterolateral capsular area: fluid may be seen in the subpopliteus recess (Figs 5.67c, d)

Introduction ●● the extensor mechanism: comprises the quadriceps muscles and tendon, the patellofemoral joint, the patellar

tendon and the associated fat pads, bursae and plicae

●● the quadriceps muscles: include rectus femoris, the vastus muscles (medialis, lateralis and intermedius) and the articular muscle

●● rectus femoris: arises from the anterior inferior iliac spine (AIIS) and travels in the anterior aspect of the thigh

●● vastus medialis: arises from the proximal femoral shaft and is located in the anteromedial thigh (Fig. 5.79a): ●● it has a longitudinal component (vastus medialis longus) and an oblique component (vastus medialis

obliquus; VMO), which arises from the AdM tendon (Fig. 5.79b), inserts into the superomedial aspect of the patella (Fig. 5.79c) and provides a major mediolateral stabilising force on the patella

●● vastus intermedius: arises from the proximal femoral shaft and is located in the anteromedial thigh adjacent to the femur

●● vastus lateralis: arises from the proximal femoral shaft and is located in the anterolateral aspect of the thigh (Fig. 5.79a)

●● the articular muscle:314 originates from the distal 1/5 of the femur and inserts into the suprapatellar bursa (Figs 5.79d, e): ●● it functions to protect the suprapatellar bursa from entrapment between the patella and femur during knee

movement ●● the quadriceps tendon: is formed by a common insertion of the quadriceps muscles into the superior pole of

the patella, and does not have a paratenon: ●● the average dimensions of the tendon are: thickness 6-10 mm and width 28-42 mm ●● it has a laminated appearance on sagittal MR images, with three layers in 56% (Fig. 5.79f) and two layers

in 30% of cases (Fig. 5.79g): ● when tri-laminar: the anterior represents rectus femoris, the middle represents vastus medialis and

lateralis, and the posterior represents vastus intermedius ●● superficial fibres of the rectus femoris tendon may extend over the anterior surface of the patella and

contribute to the patellar tendon (prepatellar quadriceps continuation) (Fig. 5.79f)315 ●● increased SI may be seen in the distal tendon on short-TE images due to magic angle artefact (Fig. 5.79h)

●● injury to quadriceps tendon: can be from a direct blow or an indirect injury (more common), typically occurring in athletes due to a rapid deceleration injury, or in the elderly (typically 6th-7th decades) due to falling on a flexed knee, particularly from the stairs

●● predisposing conditions: that may result in atraumatic bilateral tendon rupture include gout, DM, obesity, hyperparathyroidism, RA, SLE, renal failure and steroid use

●● tendon rupture: most commonly occurs within 2 cm of the upper pole of the patella due to the quadriceps tendon being relatively avascular near its insertion, and may be partial or complete: ●● rarer sites include the musculotendinous junction, muscle belly and quadriceps tendon (mid-substance)

●● partial ruptures may result in no loss of extensor function and are treated conservatively, while complete tears, which are much less common, result in loss of extensor function and are usually treated with early surgery

●● MRI findings: ●● partial tears: most commonly affect the rectus femoris tendon fibres and appear as focal high SI within

the tendon with some intact fibres resulting in disruption of the typical tri-laminar tendon anatomy (Figs 5.80a, b)311,317

● focal disruption of the prepatellar quadriceps continuation may occur with an otherwise intact quadriceps tendon insertion315

●● complete tears: show no intact fibres (Fig. 5.80c) and the quadriceps tendon may be retracted due to muscular contraction:

● the patella may be tilted anteriorly and displaced inferiorly (Fig. 5.80c), and avulsion fragments from the superior pole of the patella may be demonstrated at the retracted tendon end

●● chronic injury (beyond 2-4 weeks): may be associated with atrophy of the tendon and quadriceps musculature with associated patella baja

Quadriceps Tendinopathy ●● quadriceps tendinopathy: is predisposed to by conditions including renal failure, DM and gout, as well as by

steroid and other drug use: ●● it may also be seen in sports involving repetitive jumping, but is less common than patellar tendinopathy

●● MRI findings: ●● thickening of the tendon with intermediate/increased SI and blurring of the normal fat planes at the

quadriceps insertion (Figs 5.81a-c)

●● the quadriceps fat pad (anterior suprapatellar fat pad): is a triangular fat pad lying deep to the quadriceps tendon and anterior to the suprapatellar bursa, with its base on the superior margin of the patella

●● it is a consistent finding on knee MRI (Figs 5.82a, b) ●● the prefemoral fat pad (posterior suprapatellar fat pad): lies anterior to the femur and is separated from the

anterior suprapatellar fat pad by the suprapatellar bursa (Figs 5.82a, b)

●● quadriceps fat pad oedema: swelling with mass effect on the suprapatellar bursa/quadriceps tendon is reported in 4.2-14% of knee MR examinations

●● clinically: it is very uncommonly associated with anterior knee pain (5.4% of cases) ●● pathologically: biopsy has shown vasculitis, and removal of the fat pad has been associated with resolution of

symptoms ●● MRI findings:

●● enlargement of the fat pad with mass effect on the suprapatellar bursa, the fat pad showing intermediate T1W/PDW (Fig. 5.83a) and increased FS T2W/STIR SI indicating oedema (Figs 5.83b, c), with enhancement following contrast

● occasionally, the intratendinous fat within the distal quadriceps tendon may also be involved (Figs 5.83d, e), which is potentially more symptomatic

●● prefemoral fat pad impingement: is thought to relate to microtrauma secondary to osteophytes arising from the superior pole of the patella or patellofemoral maltracking

●● clinically: it may give rise to chronic anterior knee pain situated more proximally than the superior patellar pole, and a mechanical sensation of catching

●● MRI findings: ●● enlargement of the prefemoral fat pad, which appears oedematous and more commonly involves the lateral

aspect of the prefemoral fat pad (Figs 5.84a, b) ●● it may be seen in up to 68% of patients with patellar tendon-lateral femoral condyle friction syndrome

(see later) (Fig. 5.84c)323

●● the patellofemoral joint: consists of the articulation between the patella and the trochlear groove/adjacent femoral condyles, together with the associated soft-tissue restraints, the medial and lateral patellar retinaculum

●● the patella: is a sesamoid bone within the extensor tendon (Fig. 5.85a) ●● its articular surface: is divided by a vertical median ridge into medial and lateral facets, with a small odd facet

medially (Fig. 5.84b), and the lowest 25% being non-articular

●● the remainder of the patella is covered by hyaline cartilage, which is typically 5-6 mm in thickness (Figs 5.85a, b)

●● the patellar calcar:326 refers to a linear low SI structure within the lateral patella that runs parallel to the lateral facet and has an anteriorly convex contour: ●● it is seen in the majority of individuals and is typically ≤1 mm in thickness (Figs 5.85c, d)

●● the relationship between the patellar articular surface and the trochlear groove varies with knee position: ●● in full extension: the patella lies superior to the trochlear groove (Fig. 5.85e) ●● in 30° flexion: the patella begins to engage with the trochlea (Fig. 5.85f) ●● between 30-90° flexion: initially the inferior, then the superior patellar cartilage engages with the

trochlea ●● beyond 120° flexion: only the odd facet articulates with the femur

●● patellofemoral joint stability: relies upon passive and active stabilisers, the passive stabilisers being the patellar retinaculum and the active stabilisers, the quadriceps muscles

●● patellar type (Wiberg): 3 types are described, but their clinical relevance is unclear: ●● type 1: the medial and lateral facets are of approximately equal size (Fig. 5.85g) ●● type 2: has a smaller medial and dominant lateral facet (Fig. 5.85b) ●● type 3: has a dominant lateral facet with a very small medial facet (Fig. 5.85h)

●● bipartite patella: is a congenital variant, which occurs in approximately 2% of individuals and is usually bilateral

●● it affects the superolateral aspect of the patella and is usually asymptomatic, but may be associated with anterior knee pain

●● MRI findings: ●● the anomaly is best seen on coronal and axial images and the hyaline cartilage overlying the defect is intact

(Figs 5.86a-c): ● a symptomatic bipartite patella:328 may show evidence of oedema adjacent to the junction of the

2 bones (Fig. 5.86d) and cystic change (Fig. 5.86e)

● traumatic separation along the synchondrosis is a rare complication (Fig. 5.86f) ● the patella may rarely be tri-partite (Fig. 5.86g)

●● dorsal defect of the patella (DDP): represents a well-defined focal defect of the subchondral bone located in the superolateral aspect of the patella: ●● it is usually an incidental finding and occurs in <1% of individuals, but may be associated with anterior

knee pain that can respond to surgical treatment

●● MRI findings: ●● a hemispherical defect in the deep cortical surface of the patella with intact overlying cartilage, which

thickens to fill the defect (Figs 5.87a-c)

●● the medial and lateral patellar retinaculum: are formed from tendinous fibres of the vastus medialis and vastus lateralis muscles, respectively: ●● they extend anteriorly from the medial and lateral collateral ligaments to insert into the medial and lateral

aspects of the patella, acting as static stabilisers ●● MRI findings:

●● they appear as thin hypointense bands surrounded by fat (Fig. 5.88a) ●● the patellofemoral, patellomeniscal and patellotibial ligaments are focal condensations of the retinaculum ●● the medial patellofemoral ligament (MPFL):329 is a major medial stabiliser of the knee, arising

from the adductor tubercle/MCL and inserting into the superomedial margin of  the patella

●● MRI findings: ●● it appears as a thickening of the medial patellar retinaculum at the inferior margin of the VMO muscle,

best visualised in the axial plane (Fig. 5.88b) ●● the patellotibial ligaments extend from the patellar tendon to the medial and lateral aspects of the proximal

tibia (Fig. 5.88c)

●● patella alta: represents an abnormally high riding patella, which may predispose to patellar maltracking and lateral dislocation

●● patella baja: represents an abnormally low lying patella and occurs with: ●● neurological conditions, following trauma (quadriceps tendon rupture) and post-harvesting for ACL

reconstruction ●● patellar position: is most commonly assessed on lateral radiographs using the method of Insall and Salvati,

based on the ratio of patellar tendon length (TL) to patellar length (PL): ●● patella alta is diagnosed if the ratio is >1.2, and patella baja if the ratio is <0.8

●● a similar ratio can be measured on sagittal MR images (Fig. 5.89a): ●● PL is the diagonal length of the patella on a mid-sagittal image through the patella while TL is the shortest

length of the patellar tendon on the same image ●● patella alta: is defined as a TL/PL ratio of >1.52 for females and >1.32 for males (Fig. 5.89b) ●● patella baja: is defined as a TL/PL ratio of <0.79 for females and <0.74 for males (Fig. 5.89c)

●● chondromalacia patellae (CMP): represents anterior knee pain in young patients with imaging evidence of cartilage softening, swelling and oedema and may progress to patellofemoral OA: ●● it is associated with a reduced lateral patellar tilt angle, a reduced trochlear depth, and an increased sulcus

angle336 ●● classification:

●● grade 1: cartilage softening and swelling (Fig. 5.90a) ●● grade 2: cartilage blistering/fissuring/fraying reaching the surface with <50% cartilage thickness loss

(Figs 5.90b, c) ●● grade 3: focal ulceration with >50% cartilage thickness loss (Fig. 5.90d) ●● grade 4: full thickness cartilage loss with involvement of subchondral bone (Figs 5.90e, f )

●● MRI findings: ●● MRI is relatively insensitive to grade 1 chondromalacia, which may manifest as cartilage oedema, with

increased SI on PDW/T2W sequences without contour irregularity (Fig. 5.90a) ●● higher grades of chondromalacia: manifest as fluid-filled defects or fissures in the cartilage of varying

depth (Figs 5.90b-d) ●● subchondral oedema and cysts (Figs 5.90e, f ) may also be seen, indicating grade 4 CMP ●● MRI has a reported sensitivity for grades 3 and 4 lesions of up to 100%337

●● patellar fractures: in adults are usually due to indirect (tractional) forces and less commonly due to a direct blow: ●● indirect: due to contraction of the quadriceps muscle following a fall, with 50-80% of fractures having a

transverse orientation and the degree of fracture separation depending upon integrity of the retinaculum ●● direct: fractures are comminuted or stellate

●● stress fractures: of the patella are rare sports injuries, typically having a transverse orientation ●● patellar sleeve fracture:276,328 represents an osteocartilaginous avulsion injury of the lower pole of the patella,

usually in children: ●● rarely, sleeve fractures involve the superior pole of the patella338

●● increased subcortical patellar SI:339 transient curvilinear areas of high T2W SI have been demonstrated in 27% of patients being investigated for potential acute ACL rupture, or following arthroscopic knee surgery

●● MR findings: ●● transverse fracture line with variable separation and adjacent bone oedema ●● occult fractures: linear areas of low SI with surrounding marrow oedema (Figs 5.91a, b) ●● isolated bone bruising may occur in less severe trauma (Fig. 5.91c) ●● may be associated acute traumatic prepatellar bursitis (Fig. 5.91d) or haemarthrosis (Fig. 5.91e) ●● patellar sleeve fracture: avulsion injury typically of the inferior patellar pole (Fig. 5.91f):

● MRI can also assess the extent of patellar cartilage involvement in patellar sleeve fractures

●● OCD of the patella: is a rare post-traumatic lesion, which is usually unilateral, affecting males between 15-20 years of age

●● a full-thickness cartilage and subchondral bone defect, with subchondral cyst formation and oedema (Figs 5.92a, b)

●● the stability of the lesion is best assessed on T2W images, with an unstable lesion having a high SI line between the fragment and the host bone (Fig. 5.92c)

●● osteonecrosis (ON): rarely affects the patella, but always occurs in association with ON of the distal femur ●● clinically: patients may be asymptomatic or present with anterior knee pain

●● predisposing causes: include patellar fracture, total knee replacement and corticosteroid use, while some cases are idiopathic

●● MRI findings: ●● serpiginous regions of low SI on T1W and hyperintensity on T2W, always involving the superior half of

the patella (Figs 5.93a, b) ●● it is always associated with ON of the distal femur and/or proximal tibia

The Trochlea Normal Anatomy ●● the trochlea: represents that part of the anterior femur lying between the femoral condyles, being covered by

hyaline cartilage that articulates with the patella to form the patellofemoral joint ●● on sagittal MR images: the normal transition between the distal femur and the trochlea is smooth

(Fig. 5.94a) ●● in the axial plane: the trochlear depth is normally >3 mm (Fig. 5.94b) and the ratio between the medial and

lateral facets is normally 1:1-3 (Fig. 5.94c)

●● trochlear dysplasia: refers to morphological abnormality of the depth of the trochlear groove mainly affecting its cranial part, and dysplasia of the lateral femoral condyle

radiographs, even with axial views348 ●● MRI findings:

●● ventral trochlear prominence (VTP): assessed on a mid-sagittal image through the deepest aspect of the trochlea and determined by measuring the distance between a line parallel to the distal femoral cortex and the most ventral aspect of the trochlear cartilage (Fig. 5.95a)

●● VTP >8 mm has a reported accuracy of 79% for trochlear dysplasia ●● trochlear depth (TD): is determined by measuring the maximum depths of the medial (a) and lateral (b)

femoral condyles, and a line (c) between the deepest point of the trochlea and a line joining the posterior aspect of the condyles (Fig. 5.95b), with TD = ([a+b]/2)-c:

● TD <3 mm has a reported accuracy of 97% for trochlear dysplasia (Fig. 5.95c) ●● trochlear articular facet asymmetry (TAFA): is determined as a percentage of medial (a) to lateral (b) facet

length (Fig. 5.95d), with TAFA = [a/b] × 100%: ● TAFA <40% has a reported accuracy of 97% for trochlear dysplasia

●● patellar lateralisation: is determined by measuring the distance between the most lateral point of the patella and a line parallel to the lateral femoral condylar cortex (Fig. 5.95e):

● a measurement >6 mm has a reported accuracy of 79% for trochlear dysplasia ●● lateral trochlear inclination (LTI):349 is assessed on the most proximal axial image that contains trochlear

articular cartilage and is determined by measuring the angle between a line joining the posterior margins of the medial and lateral femoral condyles and a line parallel to the subchondral bone of the lateral trochlear facet (Fig. 5.95f):

● LTI of 11°or greater differentiates normal from dysplasia with an accuracy of 90%

●● trochlea-tubercle distance (TTD):342,350 is calculated as the distance between sagittal lines drawn through the deepest point of the trochlea and the tip of the tibial tubercle (Fig. 5.95g), and can be calculated by MRI with the same accuracy as with CT:351

● a TTD >2 cm (implying lateralisation of the tibial tubercle) is specific but relatively insensitive for the presence of patellar maltracking

● a nipple-like anterior prominence at the superior border of trochlea, with reported accuracy of 82% (Fig. 5.96a)

● a sharp, step-like transition zone, with reported accuracy of 87% (Fig. 5.96b) ●● cartilage-bone mismatch:352 in trochlear dysplasia, the cartilage and bony contour do not match in

the axial plane, the central hyaline cartilage being thicker and resulting in a greater sulcus angle (mean 186.5°), than the bony sulcus angle (mean 167.9°) (Fig. 5.96c)

●● patellar morphology:353 is also abnormal in patients with trochlear dysplasia compared to normal controls, the patella in trochlear dysplasia showing:

● a smaller transverse diameter (mean ~38 mm): representing the maximal transverse patellar diameter on axial images (Fig. 5.96d)

● a smaller mean medial patellar facet length (~19 mm): representing the medial facet cartilage length on an axial image (Fig. 5.96e)

● a mean cartilaginous Wiberg angle of 130°: representing the angle between lines drawn parallel to the lateral and medial facet cartilage on an axial image (Fig. 5.96f)

● a mean subchondral Wiberg angle of 126°: representing the angle between lines drawn parallel to the lateral and medial facet cortex on an axial image (Fig. 5.96g)

● a type 2 patellar morphology (Wiberg) is also more common in trochlear dysplasia ●● classification: 4 types of trochlear dysplasia are described, which can be accurately demonstrated on axial

and sagittal MR images:354 ● type A: a normal shape of the trochlea but a shallow trochlear groove (Fig. 5.95c) ● type B: a mark edly flattened (Fig. 5.95f) or even convex trochlea (Fig. 5.95h) ● type C: asymmetric trochlear facets, with the lateral facet being too high and the medial facet being

hypoplastic, resulting in a flattened, oblique joint surface (Fig. 5.95d) ● type D: type C with a vertical link between medial and lateral facets (cliff pattern on parasagittal

images) (Fig. 5.96b)

medial or central aspect of the trochlea ●● associated injuries include: meniscal tears (usually medial) and bone bruising, typically affecting the weight-

bearing area of the femoral condyle, and the femoral trochlea underlying the chondral injury ●● trochlear chondral injuries: are also significantly associated with chronic patellar instability (see below), and

higher grades of injury are reported with increased chronicity356 ●● MRI findings:

●● altered chondral SI, with hyperintensity on FS PDW FSE/T2W FSE images and focal chondral defects of varying depth (Figs 5.97a-c)

●● subchondral oedema (Figs 5.97b, c) and cystic change (Figs 5.97b, c)

●● the trochlea: is the least common site of OCD in the knee, being typically seen in male adolescents (mean age 14 years) presenting with anterior knee pain

●● co-existing femoral condylar OCD may be seen in 25% of patients

●● MRI findings: ●● as for OCD elsewhere, with cartilage and/or bone defects (Figs 5.98a, b) and subchondral oedema in the

acute setting ●● necrotic fragments appear hypointense on all pulse sequences (Figs 5.98c, d) ●● intra-articular bodies

●● patellofemoral alignment: refers to the static relationship between the patella and trochlea at any given degree of knee flexion

●● patellofemoral tracking: refers to the dynamic relationship between the patella and trochlea during knee motion

●● malalignment and maltracking: occur as a result of variation in the bony geometry of the patellofemoral joint and/or variation in function of the passive and active stabilisers

●● abnormal patellar alignment typically occurs between the first 30-45° of knee flexion ●● malalignment and maltracking: may result in chondromalacia, OA, patellar dislocation and Hoffa’s fat pad

impingement360,361

●● anatomical features of the patellofemoral joint which are associated with patellar instability/maltracking include:

● a shallow trochlear groove (<5 mm), which has a sensitivity of ~86% ● a larger Insall-Salvati index (>1.2), indicative of patella alta, which has a sensitivity of ~78% ● a shorter patellar nose (<9 mm), which has a sensitivity of ~95% and is defined as that portion of the PL

that is not covered by hyaline cartilage (Fig. 5.99a) ● a small morphology ratio (<1.2), which has a specificity of ~87% and is defined as the ratio between PL

and patellar articular surface length (Fig. 5.99b) ● lateral patellar tilt (>11°), which has a sensitivity of ~93% and is defined as the angle between a line

through the maximal transverse PL and a line joining the posterior aspect of the femoral condyles (Fig. 5.99c)

●● kinematic MRI: may be used for dynamic assessment of patellar maltracking: ●● images through the patellofemoral joint are obtained during active extension against a loaded quadriceps

mechanism, typically from 40° flexion to full extension with unconstrained patellar motion ●● a variety of techniques are available for loading the quadriceps ●● rapid imaging using gradient echo sequences during active extension allows multiple images through the

patella to be obtained, which are viewed using a cine-loop facility to assess patellar tracking ●● assessment of patellar tracking can be:

●● quantitative: using the measurements described previously ●● qualitative: using cine-loop assessment:

● normally, the patella remains within the centre of the femoral groove during active extension ● maltracking is indicated by demonstration of lateral subluxation and/or tilt which may be of varying

degree (Fig. 5.100)

●● lateral patellar dislocation: is often clinically occult since it may be transient with spontaneous relocation, but shows a typical combination of MRI features allowing diagnosis

●● injury mechanisms include: rarely, a direct blow to the medial patella, but more commonly forced internal femoral rotation on a fixed foot

●● predisposing conditions include:367 trochlear dysplasia, typically in conjunction with patella alta or increased TTD, which confer a 41-and 37-fold increased risk of dislocation respectively, compared with control subjects

●● lateral patellar dislocation: results in combined soft-tissue and osteochondral injury: ●● soft tissue: rupture of the medial patellar retinaculum and capsule, disruption of the medial patellofemoral

(MPFL) and patellotibial ligaments, rupture of the VMO-MPFL complex from the adductor tubercle and tendon of adductor magnus, and injury to the infrapatellar fat pad368

●● osteochondral: compression injury to the anterolateral margin of the lateral femoral condyle, osteochondral injury to the inferomedial aspect of the patella and chondral/osteochondral loose bodies from the patellar surface

●● soft tissue: ● injury or disruption (thickening/attenuation) of the medial patellar retinaculum, which may occur at

the femoral attachment (Fig. 5.101a), in the mid-substance or at the patellar insertion (Fig. 5.101b) ● injury of the MPFL369 is described in up to 96% of cases, with involvement of the femoral attachment

(Fig. 5.101c) being more common than the patellar insertion (Fig. 5.101d) or mid-substance injury, and conferring the highest risk of subsequent instability:370 – partial tears appear to be more common than complete tears, and MRI has a sensitivity, specificity

and accuracy of ~82%, 96% and 91% respectively371 ● rupture of the patellotibial ligament (Fig. 5.101e) ● oedema or haemorrhage in the distal VMO muscle (Fig. 5.101f) (45%), which may also show anterior

and superior displacement ● joint effusion/haemarthrosis (Fig. 5.101g) (55%) and infrapatellar fat pad injury (Fig. 5.101h),372

which may be a shear injury from the inferior pole of the patella, intrasubstance disruption with fluidfilled clefts or diffuse oedema: – fat pad damage can mimic a loose body

● associated MCL (11%) and medial meniscal tears (11%) may be seen ●● osteochondral:

● bone bruising involving the inferomedial pole of the patella (61%) (Figs 5.102a, b) and the anterolateral aspect of the lateral femoral condyle (80%), which is of varying degree (Figs 5.102b-e)

● a chondral or osteochondral defect from the inferomedial aspect of the patella in 70% (Fig. 5.102f), which may be located within the infrapatellar fat pad or manifest as intra-articular loose fragments (15%) (Fig. 5.102g)

● a concave impaction deformity of the patella (44%) (Figs 5.102a, f, h) ● abnormal patellofemoral alignment: lateral patellar tilt (43%), patella alta (21%) and femoral trochlear

dysplasia ● in chronic recurrent cases, ossification may be seen at the medial patellar margin (Figs 5.103a, b)

●● lateral patellar compression syndrome: is due to excessive lateral patellar tilt without lateral subluxation, and is a condition that occurs in adolescents and adults resulting in patellofemoral pain

●● a lateral position of the tibial tubercle may contribute ●● MRI findings:

●● lateral patellar tilt, with associated oedema of the lateral patellar facet and trochlear cartilage (Fig. 5.104a) ●● lateral facet and trochlear subchondral oedema (Fig. 5.104b) and cysts, and increased TTD may be present

●● medial patellar dislocation: is uncommon and is usually seen as a complication following surgical release of the lateral patellar retinaculum

●● rarely, it may occur spontaneously, usually in the context of trochlea dysplasia ●● MRI findings:

●● lateral patellar retinaculum tearing and bone bruising, which involves the anteromedial aspect of the MFC

●● the patellar tendon: is primarily composed of fibres of the rectus femoris tendon and extends from the inferior pole of the patella to the tibial tuberosity

●● its normal length is ~5 cm (similar to the height of the patella), and it has a width of ~3 cm superiorly and 2 cm inferiorly

●● AP thickness is normally 5-6 mm, being greater at the proximal and distal ends than at the mid-point ●● on sagittal MR images, it appears as a hypointense band extending from the inferior pole of the patella to the

tibial tuberosity (Fig. 5.105a), and is mildly convex anteriorly in the axial plane (Fig. 5.105b) ●● it may normally show triangular areas of intermediate SI on low TE sagittal images at its deep proximal (75%)

(Fig. 5.105c) and distal ends (43%) (Fig. 5.105d) due to magic angle effect ●● buckling of the tendon is also a common normal variant (Fig. 5.105e), but is more frequent in the presence

of joint effusion and ACL rupture

●● patellar tendinosis (tendinopathy): is an overuse injury, typically seen in athletes engaged in activities requiring repeated quadriceps muscle contraction, commonly basketball and volleyball ( jumper’s knee):378 ●● it may also rarely occur following direct blunt trauma to the anterior knee379

●● pathologically: it results from repeated microtrauma and incomplete healing with subsequent mucoid/cystic degeneration of the tendon

●● it typically occurs proximally, possibly due to relative avascularity of the osseo-tendinous junction, but diffuse tendinosis and distal disease may also occur

●● MRI findings: ●● thickening of the tendon (mean width 12 mm) involving the proximal third to a variable degree

(Figs 5.106a, b), occasionally extending into the mid-third (Fig. 5.106c), but rarely affecting the distal tendon (Fig. 5.106d), or the whole tendon (Fig. 5.106e)

●● increased intrasubstance SI on all pulse sequences affecting the posterior surface of the tendon in the central and/or medial portion best seen on sagittal and axial images (Figs 5.106a-e)

●● severe tendinosis may not be distinguishable from a partial tear, in which there is some fibre disruption (Figs 5.106f, g)

●● associated findings: can be divided into entheseal and peritendinous: ● entheseal conditions: patellar apical chondral-bone avulsion and chronic enthesopathic patellar apical

changes including remodelling, cortical defects and subcortical cystic changes (Fig. 5.107a) ● peritendinous conditions: irregularity of the anterior tendon surface and peritendinous oedema

(prepatellar and within Hoffa’s fat pad) (Figs 5.106a-c and 107a) ● miscellaneous: medial patellar avulsion fractures, patellar tendon calcification/ossification

(Figs 5.107b, c) and chronic medial retinacular tear ●● chronic tendinosis: appears as diffuse thickening of the tendon with variable SI abnormality

(Figs 5.107d, e)

●● patellar tendon rupture: is less common than tendinosis, but complicates chronic tendinosis or iatrogenic intervention (e.g. tendon harvest for ACL grafting), and also occurs in systemic diseases including DM, chronic renal failure and RA

●● acute rupture: occurs as a sports injury and usually involves the inferior pole of the patella, being either partial or complete

●● mid-substance rupture: is rare and may occur with severe trauma (forced knee flexion against a contracted quadriceps muscle)

●● partial (Figs 5.108a, b) or complete tendon discontinuity, with oedema and haemorrhage at the site of rupture

●● superior retraction of the patella occurs with complete rupture (Fig. 5.108c) and avulsion fragments from the inferior patellar pole may be seen

●● paediatric: avulsion fractures of the tibial tuberosity (Fig. 5.108d)

●● Sinding-Larsen-Johansson disease: is an overuse injury of the proximal patellar tendon and typically affects athletic adolescents

●● pathophysiology:381 it may represent a chronic traction injury of the proximal tendon with associated heterotopic ossification, or a chronic avulsion injury of the inferior patellar pole-patellar tendon junction: ●● rarely, a similar condition (Menelaus-Batten syndrome) can affect the superior pole of the patella, being

typically unilateral and more common in boys, but occasionally bilateral382 ●● MRI findings:

●● thickening and oedema of the proximal tendon with fragmentation of the inferior pole of the patella, best seen on sagittal images (Figs 5.109a, b)

●● involvement of the patellar cartilage is not a feature of Sinding-Larsen-Johansson disease, in contrast to acute patellar sleeve avulsion fracture (see earlier)

●● rarely, similar features at the superior pole of the patella (Fig. 5.109c)

●● Osgood-Schlatter disease (OSD): represents a chronic avulsion injury of the patellar tendon-tibial tuberosity junction, typically seen in athletic adolescents, and is bilateral in up to 50% of cases276

●● MRI findings: ●● thickening and oedema of the distal tendon with fragmentation of the tibial tuberosity, and bone marrow

oedema-like SI (Figs 5.110a-c)

●● fluid in the deep infrapatellar bursa in active OSD (Fig. 5.110a) ●● chronic changes include hypertrophic bone formation at the tibial tuberosity and/or in the distal tendon

(Figs 5.110d, e)

●● synovial plicae: represent remnants of embryological divisions of the joint and are formed from inward folds of synovial tissue

●● all plicae are present in ~1% of knees, whereas 10% of knees have no plica ●● the commonest are the suprapatellar and infrapatellar plicae, while the medial parapatellar plica is the most

likely to become symptomatic ●● the lateral parapatellar plica is very rare: reported in <1% of individuals

●● clinically: plica syndrome is diagnosed in the presence of painful impairment of knee function, in which a thickened (usually >2 mm), fibrotic plica is the only imaging cause found to explain the symptoms: ●● symptoms may be related to direct injury, twisting injury or repetitive overuse flexion-extension injury ●● a thickened plica becomes symptomatic when it snaps over the femoral condyle in extension and the

patella in flexion, resulting in secondary mechanical synovitis and erosions about the margin of the condyle and patella

●● the suprapatellar plica: is located between the suprapatellar bursa and joint cavity and originates from the synovium anterior to the distal femoral metaphysis

●● it runs antero-inferiorly to insert just deep to the quadriceps tendon above the patella, and is best seen on sagittal images as a band-like structure deep to the quadriceps tendon and patella (Fig. 5.111a)

●● it may impinge on the articular cartilage of the superomedial angle of the trochlea in knee flexion, while failure of resorption of the plica will result in a completely separate suprapatellar bursa (Figs 5.111b, c)

●● superior plica syndrome: is a controversial entity presenting with dull superior knee pain, which is worse with prolonged sitting and climbing stairs

●● the medial parapatellar plica: arises from the medial wall of the knee joint and extends obliquely downwards to insert into the synovium covering the infrapatellar fat pad (Figs 5.112a, b)

●● it may be connected to the suprapatellar plica (Fig. 5.112c), and if large its medial border can extend over the medial surface of the trochlea or the medial facet of the patella (Fig. 5.112d)

●● the plica can become trapped between the patella and MFC, becoming thickened and fibrotic, possibly resulting in chondromalacia

●● it is best seen on sagittal (Fig. 5.112a) and axial (Figs 5.112b, d) PDW/T2W images as a thin band between the medial facet of the patella and MFC

●● medial plica syndrome: is most common in teenagers, with onset of symptoms typically following blunt trauma, resulting in medial patellar pain at rest, or during flexion/extension activity: ●● resection of the medial plica in symptomatic individuals appears to result in a good-to-excellent long term

functional outcome in the majority of patients388 ●● MRI findings:

●● a thickened plica with associated joint effusion (Figs 5.112e, f )

●● the infrapatellar plica: is also termed the ligamentum mucosum and originates from the roof of the intercondylar notch just anterior to ACL (Fig. 5.113a), then extends anteriorly and downwards through Hoffa’s fat pad to insert into the inferior pole of the patella (Fig. 5.113b)

●● it is the most common plica and may be seen in up to ~78% of individuals ●● it may occasionally be as thick as the ACL and is best seen on sagittal MR images as a curved hypointense band-like

structure anterior to the ACL and extending for a variable distance into the infrapatellar fat pad (Figs 5.113a, b) ●● inferior plica syndrome: injury to the plica may be a cause of anterior knee pain391 ●● MRI findings:

●● thickening (usually >2 mm) and increased T2W SI associated with the plica, the increased SI typically being curvilinear (Fig. 5.113c) or occasionally globular

●● chronic injury results in fibrosis of the plica (Fig. 5.113d)

●● the lateral patellar plica: is the least common plica in the knee, appearing as a thin, longitudinal band located 1-2 cm lateral to the patella

●● it originates from the lateral wall of the knee joint above the popliteus hiatus and inserts into the infrapatellar fat pad (Figs 5.114a, b)

●● lateral plica syndrome is considered to be very rare: ●● 14 thickened fibrotic plicae were seen in 3000 arthroscopies, of which 13 resected plicae resulted in

symptomatic relief

●● the infrapatellar (Hoffa’s) fat pad: is an intracapsular, extrasynovial structure, which is limited anteriorly by the patellar tendon, posteriorly by the synovial lining of the knee joint and the articular cartilage overlying the anterior aspects of the femoral condyles: ●● superiorly it inserts into inferior pole of the patella and, inferiorly, it inserts into the periosteum of the tibia

and anterior thirds of the menisci ●● it contains: the infrapatellar plica, the transverse intermeniscal ligament, the deep infrapatellar bursa and

vertical and horizontal clefts of residual synovial tissue ●● on MRI, it appears as a uniform fat SI structure on all pulse sequences, containing multiple linear

hypointense fibrous septa (Figs 5.115a, b)

●● Hoffa’s disease: is caused by acute trauma or repetitive microtrauma to the fat pad resulting in haemorrhage and inflammation: ●● diffuse, homogeneous inflammation of Hoffa’s fat pad has also been described in HIV positive patients on

anti-retroviral therapy, the cause of which is not known394

●● fat pad hypertrophy: occurs with impingement of the apex of the fat pad between the femur and the tibia, and in the chronic phase may result in a fibrocartilaginous mass, which may occasionally ossify

●● clinically: it results in anterior knee pain, often with loss of normal hyperextension ●● MRI findings:

●● acutely: oedema in the fat pad, best assessed on sagittal and axial images (Figs 5.116a, b) ●● chronic phase: a fibrotic/ossified mass in the fat pad with heterogeneous SI (Figs 5.116c, d)

●● patellar tendon-lateral femoral condyle friction syndrome: is a manifestation of abnormal patellar tracking, in which the lateralised patellar tendon rubs against a prominent lateral femoral condyle during flexion and extension, resulting in impingement of the intervening fat pad323,361

●● clinically: it presents with chronic anterior or lateral knee pain, typically seen in young, athletic individuals ●● MRI findings:

●● oedema in the superolateral part of Hoffa’s fat pad (Figs 5.117a-c) ●● cystic change between the lateral femoral condyle and lateral retinaculum ●● abnormal patellar alignment, including patella alta, increased TTD and lateral patellar subluxation

(Fig. 5.117c)396

●● a posterior recess (Hoffa’s recess) in the fat pad: is a normal synovial lined structure, which arises anterior to the distal insertion of the ACL and communicates with the knee joint: ●● its roof is formed by the infrapatellar plica and it is reported to be present in 15-45% of knee MR studies ●● it is also termed the infrahoffatic recess399 and is of variable shape, appearing linear or ovoid in ~65%

of cases (Fig. 5.118a), globular in ~25% of cases (Fig. 5.118b) and pipe-shaped in ~9% of cases (Fig. 5.118c)

●● a further recess arising adjacent to the inferior pole of the patella is termed the suprahoffatic recess and is identified in 71% of knee MRI studies (Fig. 5.118b)

●● the recesses may represent the site of occurrence of various pathological processes including: ●● loose bodies (Fig. 5.118d), ganglion cysts, nodular synovitis and amyloidosis

●● Hoffa’s ganglion: is typically located adjacent to the anterior margin of the lateral meniscus and accounted for 3 of 23 intra-articular ganglia in a review of 1767 knee MR studies

●● MRI findings: ●● lobular, septated fluid SI mass (Figs 5.119a, b), which is differentiated from an anterior lateral meniscal

cyst by the absence of an adjacent meniscal tear ●● solid lesions include: nodular PVNS, synovial chondromatosis, synovial chondroma, synovial

haemangioma and gouty tophus (see Chapter 7)

●● hyaline cartilage injury: may be either acute as occurs with acute knee trauma, or more chronic as in the setting of OA: ●● acute cartilage injury: is relatively common, being identified in >60% of younger patients undergoing

arthroscopy, and can occur secondary to meniscal tears and ACL rupture ●● at arthroscopy: damaged cartilage may manifest by softening with/without fissuring/fibrillation, a

chondral flap or overt chondral fracture, possibly with a displaced chondral fragment ●● delamination injuries:405 represent a separation of the articular cartilage from the underlying subchondral

bone, occurring as a result of shearing stress concentrated at the junction of the non-calcified and calcified cartilage, resulting in an injury that runs parallel to the articular surface

●● the role of MRI in treatment planning:403 management options for chondral injury depend upon the location, depth and size of the defect: ●● full-thickness defects are most commonly symptomatic and are the most likely to be treated ●● the larger the lesion, the less likelihood of a good outcome ●● femoral condyle lesions are most easily treated, while patellar lesions have the poorest prognosis and tibial

plateau lesions are also difficult to treat ●● cartilage injuries associated with OA also have a poorer outcome, OA manifesting as marginal

osteophytes, subchondral cysts and sclerosis ●● MRI findings:

●● the accuracy of MRI in identifying cartilage defects is reported as ~90%, although MRI has only a moderate accuracy for grading the depth of cartilage injury

●● on FS T1W 3D SPGR images: cartilage defects are seen as focal areas of reduced SI, with reported sensitivity, specificity and accuracy of 81-93%, 94-97% and 91-97%, respectively, for the detection of cartilage abnormalities

●● on PDW FSE/FS PDW/T2W FSE images: cartilage defects are seen as linear (Figs 5.120a-c) or irregular focal areas of increased SI of variable size (Figs 5.120d, e), with 86-94% sensitivity, 94-99% specificity and 81-98% accuracy for detection of cartilage abnormalities

●● at 3T, the reported specificity and accuracy for detecting cartilage lesions of the knee are significantly improved compared to 1.5T, but sensitivity is not appreciably altered406

●● secondary signs of articular cartilage injury include: ● focal subchondral oedema (Figs 5.120b and 5.121a, b) and cysts (Figs 5.121c, d), associated with

full-thickness cartilage defects, reported in association with 83% of cartilage defects requiring surgical treatment407

● subarticular osteophytes: reported in 15% of MR examinations of the knee ●● poorly identified types of cartilage injury include:

● flap tears, fissures, fibrillation and delamination injuries405, which appear as linear areas of fluid SI on PDW/FS T2W FSE images at the junction of the cartilage and subchondral bone (Fig. 5.121e)

●● osteochondral injuries: include bone bruising, osteochondral fracture and OCD

●● bone bruises (BBs): are thought to represent areas of post-traumatic haemorrhage, oedema and hyperaemia from trabecular microfracture, which may result from a direct blow, compressive injury or traction forces at sites of soft-tissue avulsion: ●● direct impaction or compression injuries are typically associated with more pronounced marrow oedema

than avulsion injuries413

●● clinical features: BBs may be the only finding following knee trauma (10% of cases) (Figs 5.122a, b) and may then account for knee pain: ●● patients with BBs and an associated ligament injury show significantly greater functional disability

compared to those without BB in the acute setting, but there is no such difference demonstrated at 6 month follow-up415

●● they are typically painful for ~6 weeks following injury, while some patterns of BB are associated with hyaline cartilage damage/loss and predispose to premature OA416

the latter always being associated with an osteochondral injury

●● MRI findings: ●● areas of marrow oedema-like SI, with hypointensity on T1W and hyperintensity on T2W, STIR and FS

PDW/T2W FSE images, but poorly demonstrated on GRE imaging ●● bone bruises are typically hemispheric or wedge-shaped, with the base against the subchondral plate ●● they usually resolve within 6 weeks post-injury on T1W and T2W FSE, but may persist for up to

6 months on FS T2W or STIR images ●● BBs: can be classified into 2 basic types on T1W images:

● reticular: ill-defined areas of low SI with intervening normal marrow signal adjacent to the subchondral bone plate

● geographic: diffuse areas of low SI marrow with well-defined margins extending to the subchondral bone plate

● clinical relevance: reticular lesions heal with no sequelae, whereas geographic lesions are associated with cartilage thinning, chondral defect or cortical impaction in 50% of cases at follow up MRI

●● bone marrow oedema (BMO): has various aetiologies, including ischaemic, mechanical and reactive ●● pain associated with BMO: is from raised intraosseous pressure due to the abnormally high fluid content of

the marrow

●● causes of ischaemic BMO include: ON, bone marrow oedema syndrome (BMOS), OCD and complex regional pain syndrome (CRPS)

●● ON: is more common in the lateral femoral condyle than in the lateral tibial plateau or MFC: ●● risk factors: include steroid use, alcohol abuse, haemoglobinopathies and connective tissue diseases, and

staging is as for ON of the hip: ● stage 1: focal subchondral BMO ● stage 2: subchondral area of ON surrounded by a reactive interface (plain film is still normal) ● stage 3: osteochondral fracture (Fig. 5.123a) ● stage 4: development of secondary OA

●● spontaneous osteonecrosis of the knee (SONK): is now regarded as a subchondral insufficiency fracture (SIF – see mechanical BMO, below) rather than true primary ON421,424,425

●● BMOS: is a condition of unknown aetiology and may represent early ON or may be the same as transient migratory osteoporosis: ●● imaging demonstrates normal radiographs, the diagnosis being made by MRI ●● spontaneous healing occurs in 3-12 months and the condition may heal in one location of the knee and

develop in another location in the same knee, or rarely in another joint423,426 ●● MRI findings:

● extensive, diffuse BMO involving an entire quadrant of the knee joint (Fig. 5.123b), the absence of a subchondral line differentiating from SIF

● a soft-tissue reaction may also be present (Fig. 5.123b) ●● OCD: is a manifestation of ON in childhood, at a time when the growth plate is still open:

●● BMO can be seen in all stages of the disorder (see later) ●● CRPS: is also known as algodystrophy, reflex sympathetic dystrophy syndrome (RSD) and Sudek’s

atrophy: ●● it is usually secondary to trauma/injury of unknown origin and typical symptoms include burning,

trophic disturbances and sensorimotor alterations ●● 3 stages are described: acute, dystrophic and atrophic, and the diagnosis is made typically with a

combination of clinical, plain film and scintigraphic findings

●● MRI findings: ● acute stage: diffuse BMO on both sides of the joint and periarticular soft-tissue oedema ● joint effusion is common

●● disuse osteopenia:427 may be seen in cases of chronic knee pain of any aetiology ●● MRI findings:

● spotty areas of marrow oedema-like SI located predominantly in a subcortical location (Figs 5.123c, d)

●● causes of mechanical BMO include: post-traumatic bone bruising (see above), microfracture, stress related BMO and stress fracture

●● microfracture: represents a post-traumatic injury, which commonly extends to the cortical surface ●● MRI findings:

● a linear area of low SI surrounded by BMO, with oedema possibly hiding the fracture line ●● stress related BMO: subchondral oedema secondary to mechanical overload:

●● oedema may correlate with pain in OA and is predictive of progressive disease ●● MRI findings:

● a wedge-shaped area of subchondral BMO, with the base of the wedge against the articular surface and changes of associated OA

●● stress fracture: either insufficiency or fatigue fracture, the imaging features being as for a microfracture but differentiated by the absence of a history of acute trauma

●● insufficiency fracture: include what was previously called spontaneous osteonecrosis of the knee (SONK),421,428 but is now considered to represent a subchondral insufficiency fracture (SIF) rather than primary ON: ●● it typically occurs in patients >55 years of age, being most common in women and usually affecting the

MFC (Figs 5.124a-c), although it has also been described in the lateral femoral condyle (Figs 5.124d, e) and tibial plateau (Fig. 5.124f)

●● predisposing factors include altered biomechanics of the joint, as seen with meniscal extrusion or prior debridement,429 and systemic osteoporosis is uncommon, being reported in only ~16% of cases422

●● MRI findings: ●● subchondral BMO in the weight-bearing surface of the MFC, with a low SI subchondral line indicating

subchondral fracture (Figs 5.124a, b): ● the extent of marrow oedema is greater than that typically seen with chondral loss alone, and an

associated medial meniscal tear is often present (Fig. 5.124f) ● adjacent soft-tissue oedema and fluid may be seen between the fracture and subchondral bone

(Fig. 5.124g) ● in the later stages: collapse (Fig. 5.124h) and secondary OA may occur

●● insufficiency fractures: may also occur at sites distant from the subchondral bone and such fractures are not considered to represent SONK

●● osteochondral fractures: are defined as post-traumatic injuries to the articular surface, which result in a local cartilage defect or fracture (fracture or impaction of the subchondral bone-plate)

●● osteochondral fractures: of the distal femur typically result from either impaction injuries or shearing injuries: ●● impaction injuries: are classically associated with ACL rupture and typically occur at the lateral femoral

notch (condylopatellar sulcus) (Figs 5.37d, e) ●● shearing injuries: in adults, usually result in a cartilage flap tear or defect, while in children, usually result

in an osteochondral fracture, which may progress to OCD ●● osteochondral fractures: of the proximal tibia typically occur from impaction injuries, the common locations

being: ●● the posterior lateral tibial plateau after ACL rupture (Figs 5.125a, b) or the weight-bearing surface after

axial loading injury, usually affecting the lateral tibial plateau: ● fractures with >5 mm depression may result in premature OA and are usually treated surgically

●● associated soft-tissue injuries with tibial plateau fractures include:430 MCL rupture (55%), lateral meniscal tears (45%), FCL rupture (34%), medial meniscal tears (21%), ACL (41%) and PCL (28%) rupture

●● MRI findings: ●● joint effusion, haemarthrosis (Figs 5.125c, d) or lipohaemarthrosis (Figs 5.125e, f ) ●● cartilage injury: manifests as high SI fluid within a cartilage fissure or defect on PDW or T2W images

(Figs 5.120 and 5.121) ●● bone injury: a discrete line of subchondral low SI on T1W/PDW images representing the fracture line

with/without surrounding bone oedema (Figs 5.125g, h) ●● the fracture line may appear hypointense if due to impaction injury (Fig. 5.125h) or hyperintense on

T2W if it contains fluid (Fig. 5.125d) ●● intramedullary fat globules may occasionally be seen (in ≤2% of cases)431

●● OCD (also termed osteochondral defect): is a chronic, post-traumatic lesion affecting convex articular surfaces and resulting in partial or complete separation of a fragment of articular cartilage and subchondral bone: ●● pathophysiology:436 is likely primarily related to repetitive trauma, but ischaemia, genetic causes and

abnormal ossification may be contributing factors437 ●● around the knee, it can affect the femoral condyle (accounting for ~75% of OCD of all joints), the femoral

trochlea438, or the patella (~5% of cases) ●● the incidence of OCD is estimated at 0.02-0.03% (based on knee radiographs) and 1.2% (based on knee

arthroscopy) ●● clinically: it classically presents in adolescence (age 10-15 years), twice as commonly in boys, usually athletic

individuals with vague joint pain and a history of trauma in ~40% of cases ●● bilateral lesions are reported in 15-30% of cases, commonly at different stages of development

●● when involving the femoral condyle, ~70% occur on the medial side, typically involving the lateral margin: ●● lateral lesions: are usually centred on the weight-bearing surface, typically posteriorly and are associated

with a discoid lateral meniscus439 ●● posterolateral lateral femoral condylar OCD involving the popliteus tendon insertion has been described

in young adults440 ●● rarely, lesions occur anteriorly

●● prognosis is dependent upon age at diagnosis: ●● juvenile OCD, prior to fusion of the distal femoral physis more commonly heals spontaneously ●● adult OCD, after closure of the distal femoral physis rarely heals without surgical intervention ●● size of lesion: lesions <0.2 cm2 are more likely to heal

(Figs 5.126a, b) ●● stage 2: osteochondral fragment with clear margins but no fluid between the fragment and the underlying

bone (Figs 5.126c, d) ●● stage 3: fluid is partially visible between the fragment and underlying bone (Figs 5.126e, f ) ●● stage 4: fluid is visible completely between the fragment and the underlying bone, but the fragment

remains in situ (Fig. 5.126g) ●● stage 5: the fragment is completely detached and is lying free within the joint (Fig. 5.126h)

●● an important criterion is stability of the lesion, with unstable lesions more likely being symptomatic

●● 4 criteria for an unstable lesion are described on T2W images: ● a high SI line at least 5 mm in length at the junction of the osteochondral fragment and underlying

bone (Fig. 5.126g) ● a cyst, measuring at least 5 mm in diameter deep to the lesion (Fig. 5.127a) ● a focal defect in the articular cartilage measuring at least 5 mm (Fig. 5.127b) ● a high SI line traversing the cartilage and subchondral bone plate (Fig. 5.127b), which may represent

either fluid or granulation tissue and is the most common sign of instability, reported in 72% of cases ●● the presence of one criteria is associated with a high sensitivity (~100%) but poor specificity (~15%)

for instability, and over diagnosis of instability can occur when the only criteria is high SI at the bonefragment interface443

●● in the presence of two or more criteria, the specificity improves to ~92% but sensitivity decreases ●● absence of these signs on T2W images indicates a stable lesion (Figs 5.126a-d) ●● with healing, the subchondral bone can return to near normal appearances (Figs 5.127c, d)

●● MR arthrographic findings: ●● indirect: an intravenous injection of gadolinium shows enhancement of the tissue between the fragment

and host bone in unstable lesions ●● direct: extension of injected contrast medium between the fragment and host bone indicates an unstable

lesion

●● soft-tissue twisting injuries: occur in the soft tissues surrounding the knee joint and are rotational injuries located at the myofascial junction

●● clinically: they can present with medial (more common) or lateral joint pain that can mimic a meniscal tear ●● MRI findings:

●● linear areas of fluid SI within the subcutaneous tissues at the myofascial junction, indicative of subcutaneous haemorrhage (Figs 5.128a, b)

●● a popliteal (Baker’s) cyst: represents the most common cystic mass around the knee, being identified in ~5-32% of knee MRI studies for internal derangement448 and ~20% of asymptomatic knees449

●● it is located in the popliteal fossa between the tendons of the medial head of gastrocnemius and semimembranosus, and is caused by extravasation of joint fluid through a weakened posteromedial joint capsule into the synovial lined semimembranosus-gastrocnemius bursa, resulting from increased intraarticular pressure associated with joint effusion

●● aetiology: in adults, popliteal cysts are associated with identifiable intra-articular pathology in 87-98% of cases, including: ●● internal derangement: meniscal tear, especially posterior third medial meniscus, cruciate and collateral

ligament injury and chondral lesions ●● chronic arthropathy: OA, RA and infection ●● in children, popliteal cysts are uncommon but may be seen with underlying arthritis or due to primary

bursitis:232,450 ● popliteal cysts in children typically reduce in size spontaneously over time451

●● clinically: patients present with a painless mass in the medial aspect of the popliteal fossa, while cyst rupture may mimic a deep vein thrombosis (DVT) or superficial thrombophlebitis

●● MRI findings: ●● a cystic mass located in medial aspect of popliteal fossa, showing fluid SI on T1W/PDW (Fig. 5.129a) and

T2W/STIR images (Figs 5.129b, c) and peripheral ‘rim’ enhancement following contrast

●● communication with the knee joint via a small tail between the medial head of gastrocnemius and semimembranosus tendons is diagnostic (Fig. 5.129c)

●● the cyst may be septated/multi-loculated (Fig. 5.129d) and show heterogeneous internal SI due to debris or chondral loose bodies (Fig. 5.129e)

●● the semimembranosus-gastrocnemius bursa may be divided by a septum, resulting in preferential distension of one component of the cyst

●● fluid is also commonly seen in the subgastrocnemius bursa (Fig. 5.129d), located between the MFC and the medial head of gastrocnemius

●● popliteal cysts may enlarge and extend in varying directions, including inferomedial (Figs 5.129a, b) and medial, while lateral or proximal extension is less common (Fig. 5.129f)

●● complications: ● cyst rupture: leads to leakage of synovial fluid into the soft tissues, manifest as oedema in the adjacent

soft tissues and fascial planes on T2W images (Figs 5.130a, b) ● infection or haemorrhage (Figs 5.130c, d) ● synovial pathologies, due to the presence of a synovial lining, including:

– synovitis, synovial chondromatosis and PVNS (see Chapter 7) ● compression of: the popliteal vessels, resulting in ischaemia or venous thrombosis or the adjacent

nerves, resulting in entrapment syndromes452

●● the SM-TCL bursa: is consistently present on cadaveric studies and is a non-communicating bursa located between the semimembranosus (SM) tendon and the TCL, with a deeper part extending between the SM tendon and the medial tibial condyle

●● SM-TCL bursitis: is caused by chronic trauma resulting from repetitive knee extension, external rotation and valgus stress

●● clinically: it presents as focal posteromedial joint pain mimicking a medial meniscal tear ●● MRI findings:

●● sagittal PDW/T2W images show a longitudinal fluid collection along the SM tendon (Fig. 5.131a), while on axial images it appears as an inverted U-shape (Fig. 5.131b)

●● the deep pocket: is located proximally between the SM tendon and the medial tibial condyle, adjacent to the posterior third of the medial meniscus (Fig. 5.131b)

●● the superficial pocket: is located between the SM tendon and TCL (Fig. 5.131b) ●● the 2 pockets are joined along the anterosuperior margin of the SM tendon (Fig. 5.131c)

●● the TCL bursa: is located between the deep and superficial portions of the TCL and is reported to be present in 93% of knees in cadaver studies

●● TCL bursitis: occurs in association with medial compartment OA, due to osteophyte formation resulting in bursal inflammation, and chronic friction on the medial side of the knee associated with horse riding or motorcycling

●● clinically: it presents with medial joint line pain

●● MRI findings: ●● the bursa is best appreciated on coronal T2W images as a well-defined, longitudinal fluid collection

between the superficial and deep layers of the TCL (Fig. 5.132a) ●● separate femoral and tibial components may be identified ●● on axial images (Fig. 5.132b), the bursa is limited anteriorly by the anterior margin of the superficial TCL

and posteriorly by the junction between the superficial and deep components of the TCL ●● fluid in the TCL bursa: may also be seen in association with meniscocapsular separation and TCL tears

●● the pes anserinus: refers to the conjoined tendinous insertion of the sartorius, gracilis and semitendinosus muscles into the medial aspect of the proximal tibial metaphysis, 5-6 cm below the level of the knee joint

●● the pes anserinus bursa: is located deep to the pes anserinus tendon, superficial to the tibial attachment of the TCL and just distal to the insertion of the semimembranosus tendon

●● clinically: chronic bursitis typically occurs in overweight, middle aged or elderly women with OA or inflammatory arthritis,455 while acute bursitis is typically an overuse injury in runners: ●● presentation is with medial knee tenderness 5-6 cm below the joint line, and exacerbation of symptoms

on climbing or descending stairs

●● MRI findings: ●● a lobular cystic lesion lying below the level of the knee joint and superficial to the TCL (Figs 5.133a, b) ●● synovial thickening may be seen in chronic cases ●● the differential diagnosis includes:

● meniscal cyst: differentiated by the absence of communication with the joint ● PVNS of the anserine bursa (Figs 5.133c, d)

●● the prepatellar bursa: is located anterior to the patella and proximal patellar tendon ●● chronic bursitis: occurs secondary to repeated trauma from kneeling (housemaid’s or carpet layer’s knee) and

may be a manifestation of gout456 ●● acute haemorrhagic bursitis: may occur due to direct trauma (Fig. 5.134a) ●● infectious bursitis: may occur secondary to penetrating trauma and is usually due to Staphylococcus aureus ●● clinically: bursitis results in pain and focal swelling over the patella ●● MRI findings:

●● the bursa is optimally visualised on sagittal and axial images as a fluid collection overlying the patella, or anterior to the superior part of the patellar tendon (Figs 5.134b, c)

●● associated peribursal oedema results in poor definition of the bursa (Fig. 5.134d)

●● the superficial infrapatellar bursa: is located between the tibial tuberosity and the overlying skin ●● bursitis: may be caused by chronic trauma due to occupational kneeling (clergyman’s knee) ●● clinically: bursitis presents with inflammation and pain anterior to the tibial tubercle ●● MRI findings:

●● bursitis appears as fluid distension anterior to the tibial tuberosity, with inflammatory changes in the adjacent subcutaneous fat (Figs 5.135a, b)

●● the deep infrapatellar bursa: is located between the posterior margin of the distal patellar tendon and the anterior tibial cortex, inferior to Hoffa’s fat pad

●● a small amount of fluid is seen in the bursa on sagittal T2W images in 41-68% of asymptomatic knees, typically measuring 9 × 2 mm (Figs 5.136a, b)

●● chronic bursitis: results as an overuse injury, typically in runners and jumpers but is also associated with Osgood-Schlatter disease

●● clinically: bursitis presents with anterior knee pain aggravated by palpation of the distal patellar tendon

●● MRI findings: ●● the bursa is optimally demonstrated on sagittal and axial T2W images as fluid between the distal patellar

tendon and the tibia (Figs 5.136a, b) ●● bursitis appears as a poorly-defined, fluid distended bursa (Figs 5.136c, d)

●● the iliotibial bursa: is located between the distal part of the ITB and the adjacent tibial surface

●● iliotibial bursitis: is an overuse injury from varus stress on the knee resulting in anterolateral knee pain, typically in runners: ●● it may mimic ITB friction syndrome, which represents inflammation of an adventitial, rather than

primary bursa (see earlier)460 ●● MRI findings:

●● a well-defined fluid collection between the insertion of the ITB into Gerdy’s tubercle and the adjacent anterolateral tibial surface

Fibular Collateral Ligament-Biceps Femoris (FCL-BF) Bursa ●● the FCL-BF bursa: is located superficial to the distal FCL and deep to the BFT ●● clinically: bursitis presents with lateral knee pain:

●● appears on axial T2W images as an inverted J-shape, with the long arm of the J extending lateral to the FCL

●● the distal aspect of the bursa is located just proximal to the fibular insertion of the BFT (Fig. 5.137)

●● ganglion cysts: are considered to result from mucoid/cystic degeneration in a collagenous structure, usually adjacent to the joint capsule or a tendon

●● they have no synovial lining and may be intra-articular, extra-articular, intraosseous or periosteal

●● extra-articular ganglia: may arise from bursae, ligaments, tendons, muscles or nerves and rarely communicate with the joint cavity

●● clinically: they may be asymptomatic, incidental findings (particularly from the gastrocnemius origins posterior to the femur), may result in a soft-tissue mass or may cause symptoms due to pressure on adjacent structures e.g. the peroneal nerve

●● MRI findings: ●● ganglia appear as well-defined, lobular structures, which show fluid SI on all pulse sequences

(Figs 5.138a-d) ●● they commonly contain thin, hypointense septa, which enhance after contrast

●● intraosseous ganglia: are located in the subchondral region and may be related to the insertions of the cruciate ligaments (Fig. 5.48), or may be associated with chronic arthropathy, particularly OA

●● they are occasionally large, and may mimic primary bone tumours being most commonly seen in the proximal tibia

●● MRI findings: ●● they are multi-locular in 66% of cases, being hypointense on T1W (90%) (Fig. 5.139a) and hyperintense

on T2W/STIR images (Fig. 5.139b) ●● they have a mildly hyperintense rim (41%) with rim enhancement following contrast (Fig. 5.139c) and

may be associated with surrounding marrow oedema (55%) (Fig. 5.139c) ●● communication with the joint through a cortical defect is a useful diagnostic feature (Fig. 5.139a) ●● fluid levels are rare, but vertical internal trabeculae are a characteristic feature (Figs 5.139d, e), being

most prominent on T2W GRE images and extraosseous extension is also reported (38%) (Fig. 5.139e)

●● periosteal ganglia: typically occur in relation to the proximal medial tibia near the pes anserinus insertion, and are rare lesions

●● MRI finding: ●● they are hypointense to muscle on T1W (90%) (Fig. 5.140a) and hyperintense on T2W/STIR images

(Fig. 5.140b), being located adjacent to the cortex with associated cortical scalloping

●● periosteal new bone formation may occur, perpendicular to the scalloped cortex466 and hypointense internal bony septa may be seen

●● they may show thin rim enhancement following contrast (Fig. 5.140c)

●● adhesive capsulitis: of the knee is rare and its aetiology is poorly understood ●● it is believed to be similar to adhesive capsulitis in the shoulder and hip, with inflammation of the synovium

and fibrosis of the joint capsule implicated in its pathogenesis ●● clinically: pain is described initially, followed by progressive stiffness that resolves spontaneously within

2 years ●● MRI findings:467

●● increased SI and enhancement within the suprapatellar fat, ACL, PCL, popliteus insertion, pericruciate zone and along the posteromedial capsule (Figs 5.141a, b)

●● metaphyseal stripes:469 refer to the presence of thin hyperintense stripes on T2W images adjacent to the posterior distal femoral and tibial metaphyses

●● they are always seen prior to fusion of the physis, but may also be seen after physeal closure and can mimic a subperiosteal fluid collection

●● histological examination reveals loose fibrovascular connective tissue and they are considered to represent a normal anatomical finding in the paediatric age group: ●● MRI findings:

● hyperintense stripes present along the posterior aspects of the distal femoral and proximal tibial metaphyses on FS PDW/T2W images (Figs 5.142a, b)

●● anomalous ossification of the lateral femoral condyle:470,471 may mimic OCD on radiography and typically involves the posterolateral femoral condyle: ●● MRI findings:

● areas of anomalous ossification appear as islands of tissue with the same SI characteristics as the adjacent epiphysis and are surrounded by intact hyaline cartilage (Figs 5.142c, d)

● features suggestive of normal variants of ossification as opposed to stage 1 OCD include location in the posterior femoral condyle, accessory ossification centres, spiculation and lack of bone marrow oedema471

●● focal periphyseal edema (FOPE):472 refers to a localised marrow oedema pattern in the distal femur, proximal tibia or proximal fibula adjacent to a patent but narrowed physis:

●● it is of uncertain aetiology possibly an incidental finding representing early physeal fusion ●● although possibly asymptomatic, it is also reported in children with knee pain and in such cases, localised

microtrauma may be the cause of pain due a relatively less flexible, fusing physis ●● MRI findings:

● focal marrow oedema centred along both sides of a narrowed but unfused physis (Figs 5.143a-d)

●● paediatric ACL rupture:473,474 appears to be more common in girls475, and the same criteria can be used for its diagnosis as for the adult knee, including both primary and secondary signs: ●● however, tibial spine avulsion is significantly more common in the immature knee,474 the extent of

fracture displacement being an important factor influencing surgical management276 ●● osteochondral injuries:433,476 are the most common acute knee injuries in children, with a reported prevalence

of 34% of acute paediatric knee trauma: ●● meniscal tears (23%) and ACL injuries (24%) are less common ●● MRI findings:

● are as previously described for acute chondral/osteochondral injury ●● cortical desmoid:66,477 represents an avulsive cortical irregularity of the distal femur (Bufkin lesion), which is

described in children and adolescents: ●● pathogenesis: possibly a chronic traction injury at the insertion of the AdM aponeurosis or origin of the

medial head of gastrocnemius tendon, being bilateral in up to one-third of cases ●● although usually an incidental finding, it may be symptomatic

●● MRI findings: ● cortical irregularity at the insertion of the medial head of gastrocnemius, which appears hypointense

on T1W/PDW (Figs 5.144a, b) and has variable SI on T2W/STIR images, appearing either hypointense (Fig. 5.144c) or hyperintense (Fig. 5.144d): – the defect may appear convex, concave or divergent (biconvex) (Figs 5.144b, c) in shape, having a

thin hypointense margin with the host bone, and may demonstrate marrow oedema, adjacent softtissue oedema and also periostitis in the context of acute trauma478

●● physeal fractures:479 MRI is more sensitive than radiography in identifying physeal fractures and provides a better delineation of fracture extent ●● MRI findings:

● widening of a portion of the physis, with an associated metaphyseal or epiphyseal fracture line (Fig. 5.145): – rarely, avulsed periosteum may become displaced and entrapped occur along the fracture480

●● the knee: is the most common site of premature growth arrest, which may result in complex distal femoral or proximal tibial deformity

the physis is involved ●● MRI findings:

●● a fibrous bar: appears as region of low SI fibrous tissue compared to the high SI of the normal physis on T2W images (Figs 5.146a, b)

●● an osseous bridge: appears as a region of continuous trabecular bone across the physis (Figs 5.146c, d)

●● the proximal tibiofibular joint (PTFJ): is a synovial joint located between the lateral tibial condyle and fibular head (Figs 5.147a-c) and communicates with the knee joint in 10% of adults and up to 27% of cadaveric specimens via the subpopliteal recess485

●● it is surrounded by a fibrous capsule and stabilised by 2 ligaments: ●● the anterosuperior tibiofibular ligament (ASTFL) (Figs 5.147d, f ) and the more slender, and sometimes

deficient posterosuperior tibiofibular ligament (PSTFL) (Figs 5.147e, g) ●● it is also stabilised by the ligaments and tendons attached to the fibular head, including the conjoined

tendon and arcuate ligament complex ●● the PTFJ: varies in size and shape, being classified into:

●● horizontal (~25%): which has greater rotatory mobility and joint surface area ●● oblique (~75%): less rotatory mobility and surface area (Figs 5.147h, i)

●● in the clinical setting of lateral knee pain with tenderness over the PTFJ, reported MRI findings include: ●● effusion in the PTFJ ●● partial ruptures of the anterior or posterior tibiofibular ligaments, FCL or BFT

●● PTFJ OA: is reported in 7.5% of patients with knee OA, and symptomatic OA manifests as lateral knee pain at rest, on walking or on climbing stairs ●● MRI findings:

● marginal osteophytes, subchondral cysts (Figs 5.148a, b) and joint effusion (Fig. 5.148c) ● massive intraosseous ganglion formation may occur across the joint (Figs 5.148d, e) simulating a

neoplastic process ●● PTFJ involvement in ankylosing spondylitis (AS):489 is seen in up to 18% of subjects clinically diagnosed

with AS ●● MRI findings:

● include subchondral sclerosis, oedema, cartilage thinning and erosions

●● PTFJ ganglion cyst: has a reported prevalence of 0.76% in patients undergoing MRI for knee pain ●● symptoms are usually not related to the presence of a ganglion, but the ganglion may be associated with

compression neuropathy of the common peroneal or tibial nerves:492 ●● intraneural ganglia493 (see Chapter 8) may arise when fluid from a posterior PTFJ cyst extends into the

epineurium of the tibial nerve via a capsular defect, while anterior PTFJ cysts can similarly give rise to peroneal intraneural ganglia

●● ganglia typically measure 1-2.8 cm in maximum dimensions

●● MRI findings: ●● a cystic structure in the anterolateral aspect of the proximal calf (Figs 5.149a-c), with internal septa

(Figs 5.149b, c) and rim enhancement following contrast ●● communication via a ‘tail’ may be seen with PTFJ (Fig. 5.149c) ●● common peroneal nerve compression: manifests as denervation oedema/atrophy of the anterolateral calf

muscles, showing muscle oedema in the acute phase and atrophy/fatty infiltration in the chronic phase

●● acute dislocation: of the PTFJ is unusual in isolation but has been reported in sports requiring twisting movement on a flexed knee including wrestling, gymnastics and skiing: ●● typically, isolated dislocation is anterolateral, with posteromedial dislocation being rare while superior

dislocation is generally associated with high energy ankle injuries ●● subluxation: is uncommon and causes include generalised hypermobility and muscular dystrophy ●● MRI findings:

●● sprain of the PTFJ ligaments may be seen (Figs 5.150a, b)

●● operative procedures: for the management of meniscal injury include partial meniscectomy, meniscal repair and meniscal replacement

●● partial meniscectomy: is typically indicated for tears not amenable to repair, including non-peripheral tears with a horizontal, vertical flap or complex configuration especially in middle aged or older patients

●● partial meniscectomy: may be either circumferential or segmental: ●● circumferential: results in uniform resection of the inner aspect of the meniscus, resulting in preservation

of a variable amount of the outer third of the meniscus (Figs 5.151a, b) ●● segmental: where there is focal resection of almost the complete width of part of the meniscus

(Fig. 5.151c) ●● pure horizontal cleavage tears can be treated with partial resection of only the unstable portion of the

meniscus, thus commonly leaving a residual horizontal cleft within the meniscus, which is evident at MRI (Figs 5.151d, e)

●● following partial meniscectomy the meniscus appears truncated with absence of meniscal tissue being identified if >25% of the meniscus has been resected

●● taking into account the above criteria, the reported accuracy of conventional MRI for recurrent tear is 66-80%

●● meniscal repair: the criteria for a reparable meniscal tear include a tear within the vascularised peripheral third, a longitudinal morphology and a length >10 mm or instability, ideally performed in younger patients (<40 years): ●● primary repair: may be optimally used for traumatic vertical tears >7-10 mm in length in the outer

one-third of the meniscus, although repair of unstable tears in the avascular inner two-thirds may also be successful

●● the meniscal fragments can be fixed using a variety of sutures, bio-absorbable arrows, tacks or darts that span the defect in the meniscal substance

●● healing takes ~4 months and healed or partially healed menisci are usually asymptomatic, whereas failed repairs are usually symptomatic

●● following successful meniscal repair, the meniscus may appear almost normal (Figs 5.151f, g) ●● meniscal transplantation: with either allografts or synthetic meniscal replacement is usually reserved for

symptomatic younger patients who have had a previous subtotal meniscectomy or who have irreparable tears, with the aim of preventing early onset of OA: ●● synthetic partial meniscal implants: are used to fill defects measuring <5 cm, where the anterior and

posterior root attachments remain intact: ● implant scaffolds include polyurethane-based and collagen-based compositions

●● meniscal allograft transplantation requires the sacrifice of any residual normal meniscal tissue and can be performed arthroscopically via a mini-arthrotomy, with the anterior and posterior meniscal anchors fixed using bone plugs and traction sutures, while the margin of the meniscus is sutured to the joint capsule

●● the role of MRI: in the assessment of the post-operative meniscus includes: ●● assessment of stability or recurrent tear of the meniscal remnant ●● identification of a tear in another area of the meniscus ●● identification of non-meniscal causes of recurrent pain, including:

● cartilage damage, ligament pathology, intra-articular loose fragments, synovitis and ON

●● features of recurrent tear on conventional MRI include: ●● high SI joint fluid extending into a cleft within the meniscal fragment on T2W images (Figs 5.152a-d)

with accuracy reported as 80%, and the presence of a displaced fragment ●● however, if the initial meniscal resection involved removal of <25% of meniscal tissue, then MRI has a

reported accuracy of ~90% ●● at 3T, conventional MRI has a reported sensitivity of 78% and specificity of 75% for recurrent tears30

●● for meniscal repair: a high SI line extending to the meniscal surface can persist for 1 year following repair, due to the presence of granulation tissue: ●● separation of the previously repaired meniscal fragments by >1 mm may be a useful sign of re-tear ●● in a healed meniscus, no fluid signal is evident at the repair ●● fluid SI involving <50% of the repair site is consistent with a partially healed meniscus, while fluid SI

occupying >50% implies an unhealed tear ●● diagnostic accuracy: is increased by using direct gadolinium MR arthrography, the indications for which

include:498 ●● the assessment of meniscal repair, or if there has been >25% meniscal resection (in the absence of OA,

chondral injury and ON) ●● the diagnostic criteria for a repeat tear include the presence of increased intrameniscal SI (equal to joint

fluid) on a FS PDW/T2W FSE sequence, which has a reported accuracy of 88-92% ● at 3T, MR arthrography has a reported sensitivity of 88% and specificity of 100% for re-tears30 ● MR arthrography may yield false negative results when recurrent tears do not fill with injected

contrast due to granulation tissue at the meniscal surface impeding contrast extension, or possibly the viscosity of the contrast medium preventing opacification of some tears30,500

●● the diagnostic criteria for meniscal repair include full-thickness extension of contrast medium across the repair indicating a re-tear or failure of healing, whereas partial extension of contrast across the repair indicates partial healing

●● MRI following meniscal allografts:501 can provide a variety of information, including the position of the meniscus, status of the capsular attachment, allograft shrinkage and the presence of meniscal degeneration, fragmentation and tears: ●● increasing intrameniscal SI as well as a decrease in width or increase in thickness of the graft are commonly

described in the first post-operative year but do not appear to correlate with clinical outcome502 ●● MRI following synthetic partial meniscal implants:32,499 can provide useful information about the size and

position of the implant, the adjacent meniscus and the presence of synovitis: ●● polyurethane-based implants: demonstrate high SI within the implant, which is thought to relate to its

highly porous structure (Figs 5.152e, f ): ● such high SI diminishes over a number of years but never reaches the low SI of normal meniscal tissue ● mild extrusion of the medial meniscus is common, with radial displacement averaging 2 mm being

reported, but does not appear to correlate with clinical outcome503 ●● collagen-based implants:504 normally demonstrate a reduction in size over time:

● implants typically appear partially resorbed (92%), demonstrate mild increased SI (90%) and extrusion of ~3 mm (72%) with limited correlation with clinical outcome505

●● techniques for ACL reconstruction: include the use of autografts and allografts: ●● autografts: include bone-patellar tendon-bone (BPTB; most commonly used), hamstring (semitendinosus

or gracilis) tendons and bone-quadriceps tendon grafts ●● allografts: include Achilles tendon, fascia lata, BPTB and hamstring tendons ●● single-bundle ACL reconstruction: is most frequently performed and aims to reconstitute the anatomy of

the anteromedial bundle of the ligament ●● double-bundle ACL reconstruction:507 aims to separately reconstruct the anteromedial and posterolateral

bundles, and is reported to result in less rotational instability than single-bundle surgery ●● primary repair: can be used for avulsion injuries at either the femoral or tibial insertion, which are usually

seen in children ●● optimal positioning of the graft tunnels: is guided by the principles of isometry and avoidance of graft

impingement: ●● isometry: allows a constant length and tension of the graft through flexion and extension, and is dependent

upon the position of the femoral tunnel, with incorrect tunnel position resulting in graft elongation and instability

●● avoidance of impingement: depends upon position of the tibial tunnel and size of the intercondylar notch: ● impingement: most commonly occurs against the roof of the intercondylar notch in terminal

extension, against the sidewall of the notch or at the tunnel margins

●● MRI technique: the use of T2W GRE and FS PDW/T2W images should be avoided due to metal artefact and T2W FSE or STIR sequences should be considered

●● the MRI appearance: is dependent upon the type of graft, fixation technique and time since repair ●● tunnel position:

●● a correctly positioned femoral tunnel is located at the intersection of the posterior femoral cortex and the intercondylar roof (Figs 5.153a, b), and on coronal images at the posterosuperior corner of the intercondylar notch, at 10-11 o’clock for the right knee (Fig. 5.153c) and 1-2 o’clock for the left knee

●● a correctly positioned tibial tunnel is seen on sagittal images posterior to a line drawn along the roof of the intercondylar notch (Blumensaat’s line) (Fig. 5.153d), the centre of the tunnel being 1/4 to 1/2 the distance from the anterior to the posterior tibial cortex, and on coronal images centred on the intercondylar eminence (Fig. 5.153e)

●● fluid collections may be normal within the femoral or tibial tunnels for 1 year following hamstring grafts (Figs 5.153f, g)

●● double-bundle ACL graft reconstruction:511 ● the correctly positioned femoral tunnel for the anteromedial bundle is the same as for a single-bundle

ACL reconstruction, while the femoral tunnel for the posterolateral bundle should be at 9-10 o’clock for the right knee and 2-3 o’clock for the left knee on coronal images

● the correctly positioned tibial tunnel for the anteromedial bundle is the junction of the anterior and middle thirds of the tibia, while the tibial tunnel for the posterolateral bundle should be positioned just posterior to the mid-point of the tibia

●● graft fixation:176 may be achieved by interference screws, cross pins, staples or endobuttons, which can be metallic or bio-absorbable (Figs 5.154a, b): ●● if metal screws are used, they will result in a variable amount of artefact, and marrow oedema may persist

around the fixation points for up to 12 months post-reconstruction

●● graft type and appearance: ●● BPTB graft: is harvested from the central one-third of the patellar tendon, together with small fragments

of bone from the inferior pole of the patella and tibial tuberosity: ● the graft is typically 10 mm wide and 3-4 mm thick, and a normally positioned graft runs just posterior

to and parallel to the roof of the intercondylar notch (Fig. 5.153d) ● the SI of the graft depends upon graft age, the graft being avascular and therefore appearing

hypointense on T1W and T2W sequences in the first 3-4 months ● at 4-8 months: the graft undergoes revascularisation and re-synovialisation appearing as homogeneous

low SI or segmental/diffuse intermediate SI on T2W sequences, but never as bright as fluid (Figs 5.154c, d)

● by 12-17 months: it typically appears similar to native ACL, showing uniform low SI on all pulse sequences (Figs 5.154e, f ): – however, focal increased intrasubstance SI (<25% cross-sectional diameter) often persists on PDW/

T2W sequences (~64-70% of cases) for many years following surgery, with increased SI more frequently involving the posterolateral bundle, and does not necessarily indicate a poor functional outcome512,513

●● hamstring graft: is prepared from harvested semitendinosus and gracilis tendons, having an average diameter of 8-9 mm:

● the MR appearance is similar to BPTB graft, except that it may show linear areas of increased SI between individual bundles of the graft (Fig. 5.154g)

●● complications: following ACL reconstruction may be related to continued or recurrent instability and/or decreased range of motion (loss of extension)

●● recurrent instability: may be due to graft disruption or graft stretching: ●● graft disruption: the graft is most at risk during the first few post-operative months, while

revascularisation and re-synovialisation are occurring: ● the most specific sign of graft disruption is the complete discontinuity of graft fibres (Figs 5.155a, b) ● during the first 4-8 months the most specific sign of rupture is fluid SI completely traversing the graft

on T2W image ● conventional MRI has a reported sensitivity of 50% and specificity of 100% for full-thickness graft

disruption517 ● MR arthrography is reported to have 100% sensitivity and 89-100% specificity for graft rupture if

injected contrast is shown to extend through the defect in the graft fibres518 ● fibre continuity in the coronal plane is the most reliable sign of an intact graft ● secondary signs of graft failure include: anterior tibial shift greater than 7 mm (Fig. 5.155c), buckling

of the PCL, uncovering of the posterior horn of the lateral meniscus and a recurrent ‘pivot shift’ bone contusion pattern (Fig. 5.155d)

● the disrupted graft may flip into the intercondylar notch, resulting in a ‘pseudocyclops’ appearance that mimics intercondylar arthrofibrosis519

●● graft stretching: is diagnosed in the presence of clinical instability but the identification of intact graft fibres:

● it is more common with hamstring grafts and manifests as an intact but bowed (Fig. 5.155e) or buckled graft on sagittal images

● instability may be seen in the presence of an anteriorly placed femoral tunnel (Fig. 5.155f), or a posteriorly placed tibial tunnel (Fig. 5.155g)

●● decreased range of movement (extension) may be due to graft impingement or arthrofibrosis ●● graft impingement: the most common cause is anterior placement of the tibial tunnel resulting in contact

between the graft and roof of the intercondylar fossa in full extension, but less commonly notch osteophytes may result in focal impingement: ●● mechanical impingement results in fraying, fibrosis and eventual graft disruption

●● MRI findings: ●● increased SI within the graft at the site of impingement (Fig. 5.156a), kinking of the graft at the anterior

margin of the intercondylar fossa (Fig. 5.156b) and an anteriorly placed tibial tunnel (Fig. 5.156c) ●● osteophytes from the wall of the intercondylar fossa impinging the graft (Fig. 5.156d) ●● these signs have relatively poor sensitivity and specificity ●● impingement may be treated by notchplasty, which consists of resection of bone from the roof and lateral

side of the intercondylar notch, although recurrent impingement following notchplasty may occur due to fibrocartilaginous overgrowth

●● SI changes in the graft usually resolve within weeks of successful notchplasty

●● arthrofibrosis: is an uncommon complication, which typically occurs in the anterior compartment of the knee:

● it may be focal or diffuse and results in anterior knee pain and loss of full extension ● the focal form is termed the ‘cyclops’ lesion, a focal nodule of hyperplastic synovial tissue located

between Hoffa’s fat pad and the distal aspect of the ACL graft (Figs 5.157a, b) ● conventional MRI has a reported 85% accuracy for the detection of cyclops lesions, which may be

mimicked by a retained native ACL remnant ● the diffuse form is typically located throughout Hoffa’s fat pad (Figs 5.157c, d)

●● miscellaneous complications include: ●● intra-articular cartilaginous loose bodies ●● recurrent internal derangement resulting in a displaced meniscal tear ●● Hoffa’s disease: may occur following ACL reconstruction, possibly as an inflammatory response to debris

or due to fat pad injury: ● MRI demonstrates an oedematous and hypertrophic fatty mass within the infrapatellar fat pad

●● ganglion formation: related to the graft or tibial tunnel: ● graft ganglion cyst formation: is due to graft degeneration or partial rupture and is more common

with hamstring grafts, having the typical appearance of a ganglion cyst (lobular, well-defined and hypointense on T1W and hyperintense on T2W) (Fig. 5.158a)

● a tibial tunnel intraosseous ganglion cyst: may enlarge to enter the joint or produce a subcutaneous soft-tissue mass adjacent to the distal opening of the tunnel (Figs 5.158b, c)

● femoral and/or tibial tunnel enlargement: may be seen with the use of metallic or absorbable screws (Figs 5.158d, e), and fluid may normally be seen within the tunnels during the 1st year following hamstring grafts

●● occasionally mature bone may be seen within the graft (Fig. 5.158f) ●● hardware failure:176 is uncommon and includes fracture, migration and displacement, which may give rise

to instability or local impingement, particularly in the early post-operative period

●● donor-site complications: following ACL reconstruction may be related to the patellar tendon or rarely the hamstring harvest sites

●● patellar tendon: complications include patella baja during the 1st post-operative year, anterior knee pain, arthrofibrosis, patellar fracture, patellar tendinosis and tendon rupture: ●● the normal tendon initially appears thickened with low SI on T1W and intermediate SI on T2W, and

shows a central 5 mm gap at the site of tendon harvesting (Figs 5.159a, b), which fills with reparative tissue by 2 years

●● persistent hyperintensity and tendon thickness >10 mm after 12 months are consistent with patellar tendinosis (Figs 5.159c, d)

●● hamstring tendon: harvest site fluid may be seen at the site of tendon harvesting for the 1st 1-2 months: ●● in the following 6-12 months: a ‘neo-tendon’ develops at the site of the respective tendons to within

1-2 cm of the tibial insertions and by 1 year post-harvest, the hamstring muscles may have regained their original muscle bulk

●● complications: include persistent muscle atrophy and hamstring weakness at 1 year, and retraction of the semitendinosus muscle belly520

●● PCL reconstruction: may be performed for bony avulsion injuries, multi-ligament injury and symptomatic knees with grade 3 laxity, while the majority of isolated PCL ruptures are treated conservatively

●● BPTB and hamstring grafts are most commonly used as a single-bundle repair, reconstituting the anatomical course of the anterolateral bundle of the PCL, while double-bundle PCL repairs are rarely performed

●● the femoral tunnel: is located on the lateral aspect of the MFC, at the anterior half of the femoral PCL insertion site (Fig. 5.160a), into which the patellar end of the graft is fixed: ●● correct placement corresponds on coronal images to 1 o’clock for the right knee and 11 o’clock for the left

knee ●● a tibial tunnel: is used in the transtibial technique, in which it is located 15 mm distal to the articular surface

of the tibia (Fig. 5.160b), just below the normal tibial insertion of the native PCL ●● tibial fixation: can also be achieved with the tibial inlay technique, whereby a BPTB graft is directly fixed to

the posterior tibial cortex through a bone window ●● within the first 12 months: the graft appears thickened and relatively hyperintense due to revascularisation:

●● gradually, graft thickness diminishes and the graft returns to low SI on T1W/PDW (Fig. 5.160b) and T2W images (Fig. 5.160c)

●● arthrofibrosis, which is common after PCL grafts and appears as areas of reduced SI anterior (Fig. 5.160c) or posterior to the PCL

●● graft impingement: is more common with a transtibial technique employing a tibial tunnel: ● typically described at the opening of the tunnel where the graft makes a so-called ‘killer turn’, which

can predispose to focal stress and graft attrition with time ●● graft disruption: appears as regions of graft fibre discontinuity and high SI fluid crossing the graft on T2W

images ●● graft instability: may occur clinically despite normal MRI findings, or may reflect excessive posterior

placement of the femoral tunnel or an anteriorly placed tibial tunnel ●● ganglion cyst formation: is associated with tunnel widening and is more commonly reported with

hamstring grafts: ● they can be asymptomatic or may result in pain, a palpable mass or limitation in range of motion

●● graft donor site complications: are as for ACL reconstruction (see above)

●● MCL reconstruction: may be considered for grade 3 injury associated with ACL disruption, while isolated MCL injuries are usually treated conservatively: ●● rarely, an isolated tear of the anterior superficial fibres of the distal MCL may displace superficial to the pes

anserine tendons, preventing healing and resulting in a Stener-like lesion, an injury which may require operative repair522

●● surgery includes stapling or suturing, and the MCL will appear thickened and hyperintense initially, eventually reverting to uniform low SI but remaining thickened indefinitely (Fig. 5.161a): ●● the repaired ligament may undergo ossification (Pellegrini-Stieda), with characteristic low SI on all pulse

sequences

●● posterolateral corner (PLC) surgery: includes primary repair or ligamentous reconstruction ●● PLC primary repair: is used following avulsion injuries of the popliteus or FCL and involves the use of suture

anchors or cancellous screws to achieve fixation ●● PLC reconstruction: includes repair of the FCL, popliteus tendon and/or the popliteofibular ligament in

the context of grade 3 injuries (full-thickness disruptions), which may be achieved with the use of graft augmentation, including patellar or hamstring tendons or an Achilles tendon allograft:523 ●● fixation often requires the use of osseous tunnels, which may be placed in the fibular head, posterior aspect

of the lateral femoral condyle or lateral tibia (Figs 5.161b, c) ●● posterolateral corner injuries: commonly occur in the setting of multiple ligament injuries including ACL,

MCL or PCL disruption, which may also require surgical repair

●● lateral patellar dislocation: is initially treated conservatively, but recurrent dislocation may require surgery, which includes various combinations of soft-tissue and bony procedures

●● soft-tissue procedures: involve the medial and lateral stabilisers (MPFL reconstruction, medial capsular plication, and lateral release) and the patellar tendon

●● bone procedures: include trochleoplasty to reconstruct a dysplastic trochlea, and tibial tuberosity transfer in patients with an abnormal TTD

●● MPFL reconstruction: is aimed at restoring the restraining function of the medial stabilisers, and either the gracilis or semimembranosus tendons may be used to restore the torn ligament

●● the Roux-Goldthwait procedure: is a realignment procedure where the patellar tendon is split vertically, the lateral half being pulled under the medial half and attached to the tibia, thus pulling the patella medially

●● trochleoplasty: is indicated for patients with types C and D trochlear dysplasia and aims to improve trochlear depth, sulcus angle and lateral inclination, thus ensuring more stable patellar tracking

●● tibial tuberosity transfer: the Elmslie-Trillat procedure involves medial displacement of the tibial tubercle, and may be modified by anteriorisation of the tubercle by ~15 mm

●● patellofemoral arthroplasty: may be considered in the setting of severe CMP and absence of OA involving the other knee compartments, while patellectomy is now rarely performed as a salvage procedure for severe fractures or unremitting pain

●● MRI findings: ●● MPFL reconstruction: fixation devices or transverse screws may be seen within the patella and MFC

(Figs 5.162a-c), with the graft running between the medial pole of the patella and MFC (Fig. 5.162d) ●● lateral release: may result in excessive lateral scar tissue ●● Roux-Goldthwait procedure: a double patellar tendon is demonstrated (Figs 5.162e-g) ●● tibial tuberosity osteotomy: medial displacement of the tuberosity, possibly with associated elevation

(Figs 5.163a, b) ●● patellectomy: absence of the patella with diffuse thickening of the quadriceps and patellar tendons

(Figs 5.163c, d) ●● recurrent injuries can also be demonstrated (Fig 5.163e)

●● cartilage repair procedures: can be grouped into 4 main categories, local marrow stimulation techniques, implanted synthetic scaffolds, biological tissue grafts and cell-based therapies

●● local stimulation techniques: are used less frequently than in the past and include abrasion arthroplasty, subchondral drilling and microfracture: ●● microfracture: is the most commonly performed of these techniques and is usually offered to patients

<40years of age ●● all of these techniques require the penetration of subchondral bone, with the resulting formation of a fibrin

clot that can differentiate and remodel to form fibrocartilage within the chondral defect ●● a successful treatment results in the production of complete lesion filling with smooth, congruent cartilage,

which is of similar SI to the native cartilage ●● oedema-like subchondral SI may be seen initially following the treatment, but this resolves after several

months ●● long-term failure of microfracture (Figs 5.164a, b) may sometimes be attributable to intra-articular

osteophyte formation

●● implanted synthetic scaffolds: are secured within the defect, designed to promote cartilage and/or bone growth and may be seeded with cells, growth factors or blood products, or may be acellular525

●● a number of acellular scaffolds are commercially available, including TruFit plugs, which are cylindrical scaffolds designed with two polymers, one to promote chondrocyte growth at their surface and another to facilitate bone growth at their base

●● the superficial repair cartilage typically appears irregular and hyperintense and there is often poor correlation between the extent of osseous integration on imaging and the clinical and functional outcomes (Figs 5.165a, b)

●● biological tissue grafts: include osteochondral autograft transplantation and allograft transplants, which aim to produce a hyaline-like cartilage repair tissue

●● osteochondral autograft transplantation (mosaicplasty): in this technique, osteochondral plugs are harvested from a relatively non-weight bearing part of the knee joint, such as the intercondylar notch or femoral trochlea, and implanted into the chondral defect

●● osteochondral autografts: are used for lesions <5 cm2 and <10 mm in depth, although typically lesions are <1 cm2:528 ●● a variety of features can be assessed with MRI, including graft incorporation, congruity of the articular

surface, appearance of the donor-site and post-operative complications ●● graft incorporation: oedema and enhancement at the transplantation site is indicative of graft

revascularisation, reparative and inflammatory reactive tissue, which may be seen within 4-6 weeks of transplantation:

● usually, this subsides between 6-12 months, by which time essentially normal marrow SI should be seen (Figs 5.166a, b), although mild increased T2W SI may persist

●● congruity of the articular surface: the osteochondral plugs should be flush with the native articular surface, which requires careful surgical technique such that the plugs are placed perpendicular to the articular surface:

● incongruity: may be due to poor surgical technique, subsequent graft subsidence or graft displacement/ rotation

●● SI characteristics of the repair tissue: this may either appear as normal hyaline cartilage or show heterogeneous increased SI, indicative of fibrocartilage repair tissue

●● the donor-site: is seen as cylindrical tubular defects in the subchondral bone and overlying cartilage, which contains low SI on T1W and high SI on T2W images in the early post-operative period, but fills in with cancellous bone and fibrocartilage-like material within 6-9 months

●● complications include: donor site pain, condylar fracture, donor site AVN, loose bodies, graft incongruence and graft migration/loosening

●● osteochondral allograft transplantation:525 utilises cadaveric donor bone and is typically a salvage procedure reserved for larger defects: ●● marrow elements are removed from the graft material, which contains limited bone, thus reducing the

immunological risks often associated with allografts ●● the graft can be tailored to the size and shape of the defect and may be secured with the aid of a bio-

absorbable screw ●● MRI findings:

●● are similar to those seen following osteochondral autografts: ● however, transplanted adult cartilage does not typically show integration with adjacent native cartilage

and a low SI demarcation at the margins of the transplanted cartilage may be present ● the interface between the grafted bone and the native bone may show increased SI on T2W images,

and may widen over time529

●● cell and tissue culture therapies: include autologous chondrocyte implantation (ACI) and matrix-assisted autologous chondrocyte implantation (MACI)

●● ACI: is a 2-stage procedure where chondrocytes are initially harvested via arthroscopy from the femoral trochlear area or intercondylar notch, grown in culture for 4-6 weeks until ~12,000,000 cells are present, and

subsequently re-implanted into the debrided defect under a periosteal covering, which is sutured into place, typically requiring a mini-arthrotomy:

●● following this a 3-phase period of growth and maturation of the cartilage graft is seen: ●● in the 1st 6 weeks the number of cells increases to form a soft, primitive repair tissue ●● in the next 20 weeks there is formation of type 2 collagen and proteoglycans, with expansion of the

extracellular matrix ●● in the final phase (which can last up to 3 years) there is remodelling of the extracellular matrix and

integration of the tissue into the underlying bone ●● by 9-18 months: ACI repair tissue should be as firm as native cartilage to arthroscopic probing ●● histological studies: have shown hyaline or hyaline-like cartilage in 75-80% of grafts ●● improved clinical function has been reported in ~75% of subjects following ACI after at least 10 years of

follow up532 ●● the first generation of this technique utilised a periosteal covering, while the second generation employs

a collagen-based patch to cover the repair (collagen-covered autologous chondrocyte implantation; CACI)

●● the third-generation technique: uses a collagen matrix as a scaffold in which the chondrocytes are grown ex-vivo (MACI) and is stabilised with a fibrin glue rather than sutures

●● MRI allows assessment of: ●● the status of repair tissue (fibrous or hyaline): in the early post-operative stage, repair tissue shows

intermediate SI on PDW and T1W images, is hyperintense on T2W images with variable enhancement following contrast:

● in the mature stage: the ACI should completely fill the defect to the level of the adjacent cartilage, with restoration of contour of the articular surface and SI characteristics similar to native hyaline cartilage, although it may appear somewhat heterogeneous (Figs 5.167a-d)

●● the interface of repair tissue and native articular cartilage may be indiscernible, or appear as a dark line or as a sudden transition of SI (Fig. 5.167e):

● fissures: are commonly identified at the graft-host cartilage junction, and if the fissure is not filled with fluid, the graft is usually intact

●● the junction between repair tissue and subchondral bone: is usually isointense or slightly hypointense to the remainder of the ACI (Fig. 5.167f):

● a high SI interface suggests poor integration between repair tissue and the subchondral bone ●● the subchondral bone marrow: MRI normally shows oedema for some months following ACI

(Figs 5.168a, b): ● however, persistence of marrow oedema 1 year post-ACI suggests a problem with the repair

●● subchondral cyst formation: has been reported in 10% of cases (Figs 5.168c, d), and is associated with a fibrocartilage-like appearance of the graft

●● complications include: ●● arthrofibrosis, which presents with joint stiffness and occurs in ~10% of cases ●● MRI findings

● hypointense bands of tissue, usually within Hoffa’s fat pad, the suprapatellar pouch and parapatellar recesses, which may extend to the surface of the ACI

●● hypertrophy of periosteal cover: can occur in 28% of cases and is usually asymptomatic:533 ● when symptomatic, it is treated by shaving of the overgrown tissue, but repeat surgery is usually

required unless the extent of hypertrophy exceeds 150% of the depth of the graft itself ●● MRI findings:

● protrusion of repair tissue above the level of the adjacent native articular cartilage by >1 mm (Fig. 5.168e)

● hypertrophied repair tissue may protrude into intercondylar notch and affect the function of the ACL ● MACI is associated with a reduced incidence of graft hypertrophy

●● underfilling of the defect is usually asymptomatic and is seen more commonly with MACI, particularly initially, but over 2 years the MACI tissue frequently fills the defect534

●● detachment (delamination) of all or a portion of the repair tissue, which has a reported incidence of 5-14% and usually occurs within 6-9 months of surgery

●● MRI findings: ● delamination in situ appears as the presence of joint fluid between the repair tissue and subchondral

bone simulating a cartilage flap (Figs 5.168f, g), while displaced delamination appears as a focal cartilage defect, which may result in an intra-articular loose body

●● detachment of the periosteal cover may also occur (periosteal delamination)

●● Hoffa’s fat pad: arthroscopy results in a horizontal, linear scar in the fat pad extending from the arthroscopy portal (Fig. 5.169a) and multiple foci of signal void on GRE sequences due to residual, radiographically occult metallic fragments (Fig. 5.169b)

●● fibrosis of the medial or lateral patellar retinacula or deep Hoffa’s fat pad is significantly associated with prior arthroscopy, medial retinacular fibrosis having a mean sensitivity of 82%, specificity of 72%, PPV of 75%, NPV of 81%, and accuracy of 77%

●● thickening of the medial/lateral retinaculum (Fig. 5.169a) or the patellar tendon (Fig. 5.169c) may be seen depending upon the entry site

●● ON: is a recognised complication of arthroscopic meniscectomy, always occurring on the side of meniscectomy and affecting the femoral condyle, tibial plateau or both with a reported prevalence of ~7%536

●● oedema-like marrow changes: may also be identified following arthroscopy, being reported in 34% of patients undergoing meniscectomy, even in the absence of post-operative symptoms537

●● MRI findings: ●● typical features of marrow infarction adjacent to the resected meniscus (Figs 5.170a, b) ●● oedema-like marrow changes adjacent to the resected meniscus (Fig. 5.170c)

●● MR techniques: that enable reduction of artefact around prosthetic knee joints are similar to those employed for total hip arthroplasty assessment (see Chapter 4)

●● complications of knee arthroplasty: include aseptic loosening, infection, foreign body granulomatosis (osteolysis), component failure, instability, periprosthetic fracture, bursitis, tendinopathy and extensor mechanism problems

●● MRI findings: ●● aseptic loosening: may be detected on MRI and optimisation of conventional sequences is needed to aid

assessment of the implant-bone interface542 ●● infection:543 manifests as peripherally enhancing fluid collections or possibly as abnormal increased SI

extending into the soft tissues: ● synovitis may also be a feature of periprosthetic infection and is optimally appreciated on axial images

adjacent to the patella as lamellated hyperintense synovitis on PDW MR images with a sensitivity of 86-92% and a specificity of 85-87%544

●● foreign-body granulomatosis (osteolysis):539,545 manifests as a well-defined, expansile periprosthetic intraosseous mass, typically with intermediate-high SI on T1W and fluid sensitive sequences:

● however, rapid osteolysis may be indicative of periprosthetic infection ● contrast enhancement may be useful to distinguish from neoplastic lesions but the enhancement

pattern may be indistinguishable from infection, with rim enhancement having been reported ●● extensor mechanism injuries:538 are reported in up to 12% of total knee arthroplasties and include patellar

and quadriceps tendon rupture, soft-tissue impingement, patellar fracture and ON: ● a medial parapatellar arthrotomy may propagate superiorly and inferiorly as tearing of the quadriceps or

patellar tendons in the early post-operative period ● patellar fracture is more common with patellar resurfacing, with a larger central plug and following

lateral release546