Mechanical functions of the three bundles consisting of the human anterior cruciate ligament.

PURPOSE: The reconstruction technique to individually reconstruct multi-bundles of the anterior cruciate ligament (ACL) has been improved in the last decade. For further improvement of the technique, the present study was conducted to determine the force sharing among the three bundles (the medial and lateral bundles (AMM and AML) of the anteromedial (AM) bundle and the posterlateral (PL) bundle) of the human ACL in response to hyperextension, passive flexion-extension and anterior force to the knee. METHODS: Using a 6-DOF robotic system, the human cadaveric knee specimens were subjected to hyperextension, passive flexion-extension and anterior-posterior tests, while recording the 6-DOF motion and force/moment of the knees. The intact knee motions recorded during the tests were reproduced after sequential bundle transection to determine the bundle forces. RESULTS: The bundle forces were around 10 N at 5 N-m of hyperextension and remained less than 5 N during passive flexion-extension. In response to 100 N of anterior force, the AMM and PL bundle forces were slightly higher than the AML bundle force at full extension. The AMM bundle force remained at a high level up to 90° of flexion, with significant differences versus the AML bundle force at 15°, 30° and 60° of flexion and the PL bundle force at 90° of flexion. CONCLUSION: The AMM bundle is the primary stabilizer to tibial anterior drawer through wide range of motion, while the AML bundle is the secondary stabilizer in deep flexion angles. The PL bundle is the crucial stabilizer to hyperextension as well as tibial anterior drawer at full extension.