The in vivo kinematics of the anteromedial and posterolateral bundles of the anterior cruciate ligament during weightbearing knee flexion.

Background: Recently, double-bundle anterior cruciate ligament reconstruction has been advocated. However, there are little data on the in vivo biomechanics of the anteromedial and posterolateral bundles of the anterior cruciate ligament. Our objective was to measure the kinematics of the 2 bundles during weightbearing flexion.

Study Design: Descriptive laboratory study.

Hypothesis: The bundles of the anterior cruciate ligament are longest at low flexion angles during in vivo weightbearing flexion.

Methods: Magnetic resonance images from 7 healthy subjects were used to create 3-dimensional models of the knee. The attachments of the anteromedial and posterolateral bundles were outlined on each model. Next, the subjects performed a quasi-static lunge from full extension to 135 degrees while being imaged using a dual orthogonal fluoroscopic system. The models and fluoroscopic images were used to reproduce the motion of the knee. The length, elevation, deviation, and twist of the functional bundles were measured.

Results: The anteromedial and posterolateral bundles were longest at low flexion angles and shortened significantly with increasing flexion. The elevation and deviation angles of both bundles were similar at low flexion angles ( < 45 degrees ). The twist of the bundles was minimal ( < 5 degrees ) at low flexion.

Conclusion: With in vivo flexion, the anteromedial and posterolateral bundles did not demonstrate the reciprocal behavior noted in previous cadaveric studies. Both bundles were parallel and maximally elongated at low flexion angles. Our data suggest that if a double-bundle reconstruction is performed, 2 tunnels might need to be drilled in the femur and tibia to reproduce the orientation of the anterior cruciate ligament. Both anteromedial and posterolateral grafts should be fixed at low flexion angles to prevent over-constraint.