Changes in Knee Laxity and Ligament Force After Sectioning the Posteromedial Bundle of the Posterior Cruciate Ligament

Purpose: Our purpose was to evaluate the role of the posteromedial (PM) bundle of the native posterior cruciate ligament (PCL) in restraining posterior tibial translation and the effects of sectioning of the PM bundle on PCL forces. Methods: The PCL’s femoral origin was mechanically isolated by use of a cylindrical coring cutter, and a cap of bone containing the ligament fibers was attached to a load cell that recorded resultant force in the ligament as the knee was passively extended from 120° to 0° without and with simulated tibial loading conditions. Anteroposterior laxity was also measured after load cell installation. The PM bundle was cut at its femoral origin, and all tests were repeated. Results: Cutting the PM bundle produced small but statistically significant increases in mean laxity at 0° (+1.06 mm) and 10° (+0.83 mm) of flexion; mean laxities at 30°, 45°, 70°, and 90° were unchanged. Forces in the remaining anterolateral bundle were not significantly different from those in the intact ligament for any mode of tibial loading, with the exception of the valgus moment, where sectioning of the PM bundle significantly reduced the PCL force at 0° and 5° of flexion. Conclusions: The relatively small increases in mean laxity after cutting of the PM bundle show that it plays a minor role in restraining posterior tibial translation. The minor changes in ligament force profiles after cutting of the PM bundle indicate that the remaining anterolateral bundle fibers continued to be loaded in a near-normal fashion. Clinical Relevance: This study helps to elucidate the function of the PM bundle in the native PCL. Because only small changes were seen in the biomechanical parameters tested, the rationale for reconstructing this bundle of the PCL could be questioned.

Key Words: Posterior cruciate ligament, Knee biomechanics, Posteromedial bundle