Knee hyperextension and a small lateral condyle are associated with greater quantified antero-lateral rotatory instability in the patients with a complete anterior cruciate ligament (ACL) rupture.

PURPOSE: To identify factors associated with quantified rotatory stability (pivot-shift phenomenon) in the anterior cruciate ligament (ACL)-injured knee joint.

METHODS: A consecutive sample of 54 patients who were diagnosed with an ACL injury and admitted to our hospital to undergo ACL reconstruction were enrolled in this study. Antero-lateral rotatory laxity of the knee joint was quantified using a Kinematic Rapid Assessment device (KiRA; Orthokey LTD) under spinal block before initiating reconstruction of the ACL. Univariate and multivariate regressions were performed assuming relationships between patient characteristics (independent variables) and quantified antero-lateral rotatory stability (a dependent variable).

RESULTS: It was observed that a low BMI (t = - 1.659, n.s.), greater passive knee extension angle (t = 2.374, P = 0.023), and a narrower lateral femoral condyle width index (t = - 1.712, n.s.) could be candidates associated with the antero-lateral rotatory instability, using univariate analysis. Employing multivariate analysis controlling for these three variables, that the range of passive knee extension was found to be significantly associated with antero-lateral rotatory instability in the ACL-injured knee joint (t = 2.21, P = 0.035). Patients were then divided into two groups (pivot-shift negative versus positive groups) based on the KiRA-documented quantified pivot-shift test. Interestingly, 23.3% of patients were pivot-shift negative, even though their ACL was confirmed as a complete rupture by arthroscopic observations. The degree of passive knee extension was 2.3 ± 4.5 (mean ± SD) in the pivot-shift negative group, while it was 6.8 ± 6.6 in the pivot-shift positive group (n.s.). The lateral femoral condyle width index was 36.6 ± 2.0% in the pivot-shift negative group, and it was significantly wider than in the pivot-shift positive group (33.8 ± 2.6%, P = 0.0046). Finally, we estimated that the risk of positive pivot-shift depends on the degree of knee extension. The logistic regression analysis revealed that genu recurvatum significantly increased the odds ratio for positive pivot-shift (OR = 3.08, P = 0.047, 95% CI = 1.017-9.350).

CONCLUSIONS: This study revealed that greater antero-lateral rotatory instability in patients with a complete ACL rupture was associated with genu recurvatum and small lateral femoral condyle. These factors should be considered as predictors of a poor outcome from an ACL reconstruction due to a higher load on the ACL graft, and therefore, the attending physicians should modify the treatment strategies accordingly. This study indicates that joint hyperlaxity and bone morphology contribute to the rotational stability of the knee joint, in addition to the ACL and antero-lateral complex (ALC).

LEVEL OF EVIDENCE: IV.