Contributions of the posterolateral bundle of the anterior cruciate ligament to anterior-posterior knee laxity and ligament forces

Purpose: The purpose of this study was to measure changes in anterior-posterior (AP) laxity and graft forces after cutting the posterolateral (PL) bundle of the anterior cruciate ligament (ACL). Methods: Twelve fresh-frozen cadaveric knees underwent AP laxity testing at +/- 100 N of applied tibial force. Resultant forces in the ACL were recorded during passive extension from 120 degrees to 0 degrees with no tibial force, 100 N of anterior tibial force, 100 N of quadriceps force, and 5 Nm of internal tibial torque. The femoral origin of the PL bundle was identified, the ligament fibers were dissected from bone, and tests were repeated. Results: Cutting the PL bundle significantly increased mean laxity by + 1.3 mm (at 0 degrees), + 1. 1 mm (at 10 degrees), and + 0.5 mm (at 30 degrees). For the passive knee extension tests, cutting the PL bundle significantly decreased mean ACL force at 0 degrees for all loading modes; the mean decreases were 31 N (with no tibial force), 50 N (with 100 N of anterior force), 33 N (with 100 N of quadriceps force), and 40 N (with 5 Nm of internal torque). Conclusions: The decreases in ACL force at 0 degrees from cutting the PL bundle are consistent with the commonly accepted view that the PL bundle tightens with knee extension. Cutting the taut PL bundle did significantly increase AP laxity between 0 degrees and 30 degrees, but the increases were relatively small. Therefore we conclude that the PL bundle plays a relatively minor role in controlling anterior tibial translation. Clinical Relevance: In view of our findings, the need to reconstruct the PL bundle for better restoration of a normal AP laxity profile is questioned.