Passive Laxity With Functional Stability Reveals Potential Bilateral Dynamic Compensatory Mechanisms in Patients Undergoing ACL Reconstruction at Long-term Follow-up

Background: Knee stability can be conferred passively by ligaments and menisci and actively by the neuromuscular system. We sought to determine the relationship between passive tibiofemoral alignment and dynamic constraint in patients undergoing anterior cruciate ligament (ACL) reconstruction (ACLR) and matched control participants who have been followed for more than a decade. Purpose/Hypothesis: It was hypothesized that (1) anterior tibial position would be greater in the surgical knee compared to the contralateral knee and when compared to knees of control participants, and (2) the surgical limb differences would be greater in the dynamic state during a 1-leg hop-for-distance landing task. Study Design: Controlled laboratory study. Methods: A total of 21 participants were recruited from a recently completed longitudinal clinical trial (NCT00434837): 10 patients who had undergone ACLR 10 to 15 years earlier and 11 matched control participants without knee injury. The 3-dimensional (3D) tibiofemoral position was extracted from each participant's computed tomography images as a measure of passive alignment. Dynamic 3D knee kinematics were recorded using biplane videoradiography during the landing of a 1-leg hop-for-distance activity. Side-to-side differences in knee kinematics between limbs were used as a measure of dynamic constraint. Peak anterior tibial position was the primary outcome measure, and peak anterior tibial position as a function of flexion angle was the secondary outcome measure. Results: The passive tibial position of patients with ACLR was 7.5 ± 2.3 mm more anterior compared to that of uninjured participants and 3.1 ± 1.1 mm more anterior than their contralateral limb ( P < .05). The mean peak dynamic anterior position was not different between surgical and contralateral limbs in ACLR patients ( P = .83). When anterior position was explored as a function of flexion angle, peak anterior tibial position was up to 10.3 mm greater in the ACLR surgical limbs ( P = .01) and 7.5 mm in the contralateral limbs ( P = .001) compared to the limbs of control participants. Conclusion: Passive alignment is abnormal long after ACLR, whereas side-to-side dynamic constraint is largely restored, but with a persistent bias toward greater anterior tibial position that is present bilaterally. Clinical Relevance: Compared with similar studies at earlier postoperative time points, the results at long-term follow-up suggest that ACL graft function deteriorates with time, which can be compensated for to some degree by the neuromuscular system.