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ACL anatomy

The ACL consists of individual fiber bundles, named for their insertion points on the tibial footprint. The larger anteromedial (AM) bundle inserts anteromedially on the tibial side and originates more proximally on the femoral side than the posterolateral (PL) bundle, which is posterolateral relative to AM bundle.

Studies indicate that the AM bundle tightens in flexion while the PL bundle relaxes; in extension, the PL bundle tightens while the AM bundle relaxes. What is unusual about the workings of the two bundles is the way their insertion points on the femur relate to each other in flexion and extension. In extension, their insertion points are vertical to each other, but with the knee in 90 degrees flexion, the insertion points are horizontal to each other. Likewise, the bundles themselves shift from parallel orientation in extension to crossed orientation in flexion.

Arthroscopic view of the double bundles of the anterior cruciate ligament, showing both the anteromedial (AM) and the posterolateral (PL) bundles, as well as the septum separating the two bundles.

In traditional (single-bundle) ACL reconstruction, a graft is used to basically replicate the positioning of the AM bundle. The double-bundle technique uses two separate grafts to replicate the positioning of both the AM and the PL bundles. Because the AM bundle makes a greater contribution to anteroposterior knee stability, while the PL bundle makes a greater contribution to rotational stability, a double-bundle technique may be better able to restore normal knee kinematics.

Jan/Feb 2007 AAOS Now
http://www.aaos.org/news/bulletin/janfeb07/clinical4.asp