ABSTRACT

Recurrent dislocation of the patella may follow an isolated injury but is most often the consequence of pre-existing anatomical abnormality.

Instability may be caused by deficiencies in either the dynamic or static stabilizing mechanisms of the patella. The dynamic component comprises mainly the vastus medialis obliquus muscle (VMO) which has relatively horizontal fibres and acts as a medial stabilizer. The static components include the shape of the patella and the shape of the femoral condyles. The Q-angle is the angle formed by the line of pull of the quadriceps muscle and the patellar tendon through the mid-point of the patella. This is a valgus angle and is greater in females than males (10° vs. 15°). The resultant lateral force vector when the knee is actively extended is therefore directed laterally. The angle is increased in genu valgum, persistent femoral anteversion, external tibial torsion or with a laterally positioned tibial tuberosity. A dysplastic patella, patella alta or hypoplastic femoral condyles may contribute to instability.