In Situ Forces in the Anteromedial and Posterolateral Bundles of the Anterior Cruciate Ligament under Simulated Functional Loading Conditions

Background: The in situ forces of the anteromedial (AM) and posterolateral bundles (PL) of the anterior cruciate ligament (ACL) under simulated functional loads such as simulated muscle loads have not been reported. These data are instrumental for improvement of the anatomical double-bundle ACL reconstruction. Hypothesis: The load-sharing patterns of the 2 bundles are complementary under simulated muscle loads. Study Design: Descriptive laboratory study. Methods: Eight cadaveric knees in this study were sequentially studied using a robotic testing system. Each knee was tested under 3 external loading conditions including (1) a 134-N anterior tibial load; (2) combined rotational loads of 10 N·m of valgus and 5 N·m internal tibial torques; and (3) a 400-N quadriceps muscle load with the knee at 0°, 15°, 30°, 60°, and 90° of flexion. The in situ forces of the 2 bundles of ACL were determined using the principle of superposition. Results: Under the anterior tibial load, the PL bundle carried peak loads at full extension and concurrently had significantly lower force than the AM bundle throughout the range of flexion (P <.05). Under the combined rotational loads, the PL bundle contributed to carrying the load between 0° and 30°, although less than the AM bundle. Under simulated muscle loads, both bundles carried loads between 0° and 30°. There was no significant difference between the 2 bundle forces at all flexion angles (P >.05). Conclusion: Under externally applied loads, in general, the AM bundle carried a greater portion of the load at all flexion angles, whereas the PL bundle only shared the load at low flexion angles. The bundles functioned in a complementary rather than a reciprocal manner to each other. Clinical Relevance: The data appear to support the concept that both bundles function in a complementary manner. Thus, how to re-create the 2 bundle functions in an ACL reconstruction should be further investigated.