Association of Persistent Interlimb Kinetic Asymmetry at Return to Sport With Increased Risk of Contralateral Anterior Cruciate Ligament Injury.

BACKGROUND: Return-to-sport (RTS) decision-making after anterior cruciate ligament (ACL) reconstruction (ACLR) is primarily based on clinical and functional criteria. Despite meeting these criteria, a substantial proportion of athletes sustain a second ACL injury, most commonly involving the contralateral knee, suggesting a persistent gap between clinical clearance and biomechanical recovery. PURPOSE: To determine whether interlimb kinetic asymmetry at the time of RTS is associated with an increased risk of subsequent contralateral ACL injury and whether kinetic asymmetry provides incremental prognostic value beyond conventional RTS criteria. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: A total of 500 patients who underwent primary unilateral ACLR and completed biomechanical testing at RTS clearance were included. Interlimb kinetic asymmetry was assessed during a standardized single-leg landing task using force-platform-derived peak vertical ground-reaction force (vGRF) and loading rate. Patients were followed for a mean of 36 months. The primary outcome was contralateral ACL injury. Associations were evaluated using Cox proportional hazards regression. Prognostic performance was assessed using the Harrell concordance index (C-index), time-dependent receiver operating characteristic analysis, calibration plots, and decision curve analysis. RESULTS: During follow-up, 120 patients (24%) sustained a contralateral ACL injury, corresponding to an incidence rate of 8.1 injuries per 100 person-years.Greater interlimb asymmetry in peak vGRF at RTS was independently associated with increased contralateral injury risk (adjusted hazard ratio per 1-SD increase, 1.47 [95% CI, 1.23-1.75]; P < .001). The combined model incorporating conventional RTS variables and kinetic asymmetry demonstrated improved discrimination (C-index, 0.76) compared with the conventional RTS model alone (C-index, 0.64; P < .001). Decision curve analysis showed greater net benefit for the combined model across clinically relevant risk thresholds. CONCLUSION: Persistent interlimb kinetic asymmetry at RTS is independently associated with an increased risk of subsequent contralateral ACL injury. Athletes demonstrating greater asymmetry at the time of RTS appear to be at increased risk despite successful clinical recovery, suggesting that persistent loading asymmetry may represent a clinically relevant marker of incomplete biomechanical restoration after ACLR.