Effect of Anteromedial and Posterolateral Anterior Cruciate Ligament Bundles on Resisting Medial and Lateral Tibiofemoral Compartment Subluxations

Abstract

This study analyzed the interaction of the anteromedial and posterolateral portions of the anterior cruciate ligament (ACL) in resisting medial and lateral tibiofemoral compartment subluxations under multiple loading conditions. By use of a 6-df robotic simulator, 10 human cadaveric knees were tested in 3 states: intact ACL, partial ACL (loss of either the anteromedial bundle [AMB] or posterolateral bundle [PLB]), and deficient ACL. The testing profile involved anterior-posterior translation and internal-external rotation, as well as 3 pivot-shift loading conditions with varying internal rotation torque (1- or 5-Nm) and coupled anterior force (35- or 100-N). Digitization of anatomic landmarks provided tibiofemoral compartment translations and centers of tibial rotation. During pivot-shift testing (100-N anterior force, 1-Nm internal rotation torque, and 7-Nm valgus), the lateral and medial compartment anterior translation increased by a mean of 2.5 ± 0.8 mm (P= .016) and 3.4 ± 2.0 mm (P= .001), respectively, on AMB sectioning and 1.3 ± 0.9 mm (P= .329) and 0.6 ± 0.7 mm (P= .544), respectively, on PLB sectioning. Higher internal rotation torque (5 Nm v 1 Nm) on pivot-shift testing reduced central and medial anterior translation after ACL sectioning. There was no change in internal rotation on AMB or PLB sectioning. During the Lachman test (100-N), AMB and PLB sectioning increased central translation by 3.6 ± 1.6 mm (P= .001) and 0.7 ± 0.6 mm (P= .498), respectively. Both ACL bundles function synergistically in resisting medial and lateral compartment subluxations on the Lachman and pivot-shift tests. The AMB provided more restraint to anterior tibial translation during both tests as compared with the PLB. PLB sectioning produced no statistically significant change in anterior translation on the Lachman or pivot-shift test. Neither bundle contributed to resisting internal rotation. An ACL graft designed to duplicate the AMB would theoretically resist medial and lateral compartment anterior subluxations under multiple loading conditions. The PLB provides a secondary restraint at low flexion angles. Neither ACL bundle resists internal tibial rotation or allows a positive pivot-shift subluxation.