Loading of the two bundles of the posterior cruciate ligament: an analysis of bundle function in a-P drawer.

The purpose of this study was to clarify the role of the posterior cruciate ligament (PCL) in posterior drawer and to test the hypothesis that the anterolateral (aPC) and the posteromedial (pPC) bundles have different functions over the range of flexion. Selective cutting was utilised to determine percentage contributions to resisting posterior drawer. At lower flexion angles the primary restraint to posterior drawer was not the PCL but combined secondary structures. The percentage contribution of the combined secondary structures dropped below 50% at 30, 20 and 15 degrees of flexion for 2, 4 and 6 mm displacement respectively. In the midrange of flexion (40-120 degrees) the aPC was the primary restraint as it resisted between 50 and 75% of posterior drawer force. Towards full flexion the contribution of the aPC reduced, while the contribution of the pPC approximately doubled from extension to full flexion. This resulted in equal contributions from the aPC and pPC at full flexion for 2 and 4 mm displacement. For 6 mm displacement the percentage contribution of the pPC rose from 23% at extension to 57% of the total restraint at 130 degrees of flexion when it exceeded the 36% contribution of the aPC, although not significantly (p = 0.053).