Function of posterior cruciate ligament bundles during in vivo knee flexion.

Background: The biomechanical functions of the anterolateral and posteromedial bundles of the posterior cruciate ligament over the range of flexion of the knee joint remain unclear.

Hypothesis: The posterior cruciate ligament bundles have minimal length at low flexion angles and maximal length at high flexion angles.

Study Design: Descriptive laboratory study.

Methods: Seven knees from normal, healthy subjects were scanned with magnetic resonance, and 3-dimensional models of the femur, tibia, and posterior cruciate ligament attachment sites were created. The lines connecting the centroids of the corresponding bundle attachment sites on the femur and tibia represented the anterolateral and posteromedial bundles of the posterior cruciate ligament. Each knee was imaged during weightbearing flexion (from 0 degrees to maximal flexion) using a dual-orthogonal fluoroscopic system. The length, elevation, deviation, and twist of the posterior cruciate ligament bundles were measured as a function of flexion.

Results: The lengths of the anterolateral and posteromedial bundles increased with flexion from 0 degrees to 120 degrees and decreased beyond 120 degrees of flexion. The posteromedial bundle had a lower elevation angle than the anterolateral bundle beyond 60 degrees of flexion. The anterolateral bundle had a larger deviation angle than the posteromedial bundle beyond 75 degrees of flexion. The femoral attachment of the posterior cruciate ligament twisted externally with increasing flexion and reached a maximum of 86.4 degrees +/- 14.7 degrees at 135 degrees of flexion (P < .05).

Conclusion: These data suggest that there is no reciprocal function of the bundles with flexion, which is contrary to previous findings. The orientation of the anterolateral and posteromedial bundles suggests that at high flexion, the anterolateral bundle might play an important role in constraining the mediolateral translation, whereas the posteromedial bundle might play an important role in constraining the anteroposterior translation of the tibia.

Clinical Relevance: These data provide a better understanding of the biomechanical function of the posterior cruciate ligament bundles and may help to improve the design of the 2-bundle reconstruction techniques of the ruptured posterior cruciate ligament.