The effects of ACL deficiency on mediolateral translation and varus-valgus rotation.

Background: The anterior cruciate ligament (ACL) constrains the anterior translation and axial rotation of the tibia. However, the effect of ACL injury on the mediolateral translation and varus-valgus rotation of the tibia is unknown. Because of the oblique orientation of the ACL, we hypothesized that ACL deficiency alters mediolateral translation and varus-valgus rotation.

Methods: The kinematics of 9 cadavers from full extension to 90 degrees of flexion under various loading conditions were measured before and after ACL resection using a robotic testing system.

Results: ACL deficiency increased the medial translation of the tibia and valgus rotation, especially at 15 degrees and 30 degrees of flexion. For example, at 15 degrees, ACL deficiency increased the medial translation from 1.2 (SD 0.9) mm to 1.8 (SD 1.1) mm in response to a quadriceps load. The valgus rotation also increased from 0.8 degrees (SD 0.6) to 1.7 degrees (SD 0.8).

Interpretation: ACL deficiency altered both the mediolateral tibial translation and valgus-varus rotation under various loading conditions. The increased medial tibial translation could shift the contact in the medial compartment towards the medial tibial spine, a region where degeneration is observed in ACL-deficient patients. In addition to restoring anterior laxity, ACL reconstruction might need to restore the mediolateral translation of the tibia and varus-valgus rotation of the knee.