Biomechanics of the Human Triple-Bundle Anterior Cruciate Ligament

Purpose: To investigate the biomechanics of the intermediate (IM), anteromedial (AM), and posterolateral (PL) bundles in the human anterior cruciate ligament (ACL). Methods: Eighteen human cadaveric knees were tested with a robotic/universal force-moment sensor testing system. Anterior tibial translation (ATT) was determined under an 89-N anterior tibial load. Coupled ATT was determined under a combined rotatory load of 7-Nm valgus and 5-Nm internal rotation torque (pivot moment). Each bundle's in situ forces were measured under identical external loading conditions. Results: Under anterior load, the PL bundle's in situ force was highest at 0 degrees and decreased during flexion. Under the anterior load, the AM bundle's in situ force was significantly higher than the IM and PL bundles' force at 15 degrees, 30 degrees, and 60 degrees. Under the pivot moment, the AM bundle's in situ force was significantly higher than the PL and IM bundles' force at 0 degrees and 15 degrees, and the IM bundle had the lowest in situ force at 0 but higher in situ force than the AM and PL bundles at 30 degrees and 45 degrees. IM and AM bundle removal increased ATT under the anterior load at all angles. Cutting the PL bundle after IM and AM bundle removal (whole ACL removal) significantly increased ATT under the anterior load at 0 degrees, 15 degrees, and 30 degrees of knee flexion and increased coupled ATT under the pivot moment at 0 degrees and 15 degrees. Conclusions: The biomechanical role of each of the 3 ACL bundles (AM, IM, and PL) was measured with a robotic/universal force-moment sensor testing system. The AM bundle stabilized the knee against both the anterior and rotatory loads. The PL bundle stabilized the knee especially near full extension. The IM bundle supported the AM and PL bundles through all flexion angles, especially from 30 degrees to 45 degrees, against the rotatory load. Clinical Relevance: Knowledge of functions of the different ACL bundles will help improve ACL reconstruction techniques to enable restoration of normal knee function.