Distal femur morphology affects rotatory knee instability in patients with anterior cruciate ligament ruptures

PurposeDistal femur morphology has been shown to influence knee joint kinematics and may affect rotatory knee laxity. The purpose of this study was to determine the relationship between rotatory knee laxity and distal femoral morphology in patients with complete anterior cruciate ligament (ACL) rupture. It was hypothesized that increased posterior femoral condylar depth on standard lateral radiographs, quantified as the lateral femoral condyle ratio would correlate with increased rotatory knee laxity, measured by a quantitative pivot shift test.MethodsConsecutive patients who underwent ACL reconstruction from 2014 to 2016 were retrospectively reviewed. A standardized pivot shift test was performed preoperatively on both knees and quantified using tablet technology. Using standard lateral radiographs of the knee, the ratio of posterior condylar distance over total condylar distance was defined as the lateral femoral condyle ratio.ResultsData sets were obtained for 57 patients. The mean anterior translation of the lateral knee compartment during a quantitative pivot shift test was found to be 4.02.4mm and 1.30.9mm for the injured and uninjured knees, respectively. The mean lateral femoral condyle ratio on X-ray was 63.2 +/- 4.5%. There were significant correlations between the lateral femoral condyle ratio and the absolute quantitative (=0.370, p<0.05) and side-to-side differences in anterior translation of the lateral knee compartment (=0.419, p<0.05).Conclusion The most important finding from this study is that increased posterior femoral condylar depth, quantified as a lateral femoral condyle ratio, is associated with increased rotatory knee laxity in ACL-deficient patients. This suggests that distal femur morphology may influence rotatory knee laxity. This study may assist clinicians in evaluating ACL injuries and identifying patients at greater risk for persistent increased rotatory knee laxity after ACL reconstruction.Level of evidence III.