Anatomic and biomechanical study of the lateral collateral and popliteofibular ligaments.

The position of the fibula around the proximal tibia varies in different people; it was therefore hypothesized that this variation would affect the orientation of the lateral collateral ligament and the popliteofibular ligament complex. This hypothesis was studied in 10 cadaveric knees by measuring the orientation and length changes in these structures as the knee was flexed. The data were correlated with tibiofibular joint position. The strength of the ligaments was also tested. There were significant correlations between fibular head position and ligament orientation with the knee extended. The lateral collateral ligament slackened significantly with knee flexion, whereas the popliteofibular ligament complex did not. The structures became significantly steeper in the sagittal plane as the knee was flexed. The lateral collateral ligament passed through vertical at 70 degrees of knee flexion and was thus poorly oriented to withstand tibial external rotation. The lateral collateral and popliteofibular ligaments had tensile strengths of 309 and 186 N, respectively. The popliteofibular ligament is dominant when the knee is flexed, because of the slackening of the lateral collateral ligament, and so it should always be reconstructed. The anatomic variation causes some knees to have better ligament orientations to withstand posterolateral tibial displacements and, conversely, other knees may be inherently more difficult to stabilize by reconstruction.