ABSTRACT
Anterior tibio-femoral displacement (ATFD) is regulated essentially by the anterior cruciate ligament (ACL). The aim of this study was to document the intra-subject variability and reliability associated with the intra-day non-invasive instrumented measurement of ATFD and knee stiffness in normal and ACL-deficient knees. Such markers of function may aid the identification and prophyactic management of lesser sport-related ligamentous trauma which, without appropriate rehabilitative intervention, may form a precursor to subsequent and more orthopaedically serious musculoskeletal trauma.
Nineteen active adult male recreational team-game players (age 27.5 ± 4.9 years; height 1.78 ± 0.04 m; body mass 79.2 ± 4.0 kg [mean ± SD]) with unilateral complete ACL rupture (arthroscopically verified) gave their informed consent to participate in this study. Following habituation to procedures, each subject completed a single evaluation session. All subjects were tested as near to the same time of day as possible (±1 hour). The same test administrator performed all measurements. Within the evaluation session, the subjects completed a series of three assessments of the normal contralateral (NORM) and then ACL-deficient (ACL) knee using a computer-interfaced instrument constructed to measure ATFD and knee stiffness. Indices of ATFD were measured at anterior tibial displacement forces of 120 N and 200 N during incremental loading applied in the sagittal plane with a knee flexed to 0.44 rad (25°). An index of knee stiffness was calculated as the gradient of the resultant applied force-displacement curve between 120 N and 200 N. To localise the action of the tibial displacement forces, subjects were seated and securely strapped at the distal femur and tibia.
Repeated measures ANOVA of V% scores revealed that ATFD measured at increased levels of applied force (200 N: 5.8 ± 4.9 % [NORM]; 2.5 ± 1.8 % [ACL]) demonstrate greater reproducibility than measurements at lower applied forces (120 N: 8.7 ± 7.1 % [NORM]; 3.6 ± 3.2 % [ACL]) (p<0.005). Assessment of ATFD in the ACL knee demonstrated greater measurement reproducibility compared to the NORM knee (p<0.005). Similarly, the measurement of stiffness in the ACL knee (10.4 ± 10.2 %) demonstrates greater reproducibility compared to the NORM knee (17.3 ± 12.2 %) (p<0.05). Consideration of intra-class correlation coefficient (R1) scores for ATFD measurements suggests that ACL and NORM knees demonstrated similar levels of single measurement reliability across applied force conditions, ranging between 0.98 and 0.99. By contrast, the measurement of stiffness in the NORM knee (0.84) reveals reduced single measurement reliability compared to the ACL knee (0.97). The SEM% scores (95% confidence intervals) for ATFD demonstrated greater error in the NORM knee (± 10.6 % [200 N]; ± 15.8 % [ 120 N]) compared to the the ACL knee (± 7.0 % [200 N]; ± 7.5 % [120 N]). A similar trend was observed for the measurement of knee stiffness.
Measurements of ATFD undertaken at higher levels of applied force offer greater reproducibility and should be preferred on this basis, assuming subject tolerance. Results suggest that intra-day measurement of ligamentous function in the NORM knee requires greater kinanthropometric rigour compared to the ACL-deficient knee. Since the contralateral uninjured limb is used as a reference to appraise levels of dysfunction in contemporary clinical practice, then it should serve also to identify necessary multiple-trial rigour in the measurement protocol. Results suggest that the mean score of 5 trials is required to achieve an ATFD (200 N) measurement error of better than ± 5% in ACL-deficient and contralateral knee comparisons. By comparison, the mean score of 15 trials is required to achieve equivalent measurement sensitivity in the assessment of knee stiffness.
