A validated non-linear 3-D finite element model of human tibiofemoral joint was utilized to investigate the effect of constraint on tibial coupled internal-external and varus-valgus rotations on the passive joint response and force in ligaments under 100N drawer loads at different flexion angles. The model consisted of two bony structures and their articular cartilage layers, menisci and four principal ligaments. For the cruciate ligaments, the results showed that, in the fully unconstrained joint, ACL force decreased with flexion but remained as the primary ligament to resist the posterior femoral load throughout the range of flexion considered. A further significant decrease in ACL force with flexion angle was computed as the joint coupled rotations were constrained. As for PCL ligament, a minor contribution was at full extension under 100N anterior femoral load which further decreased as the coupled rotations were constrained. With joint flexion up to 90°, PCL force, however in contrast to ACL force, substantially increased in both constrained and unconstrained joints. Collateral ligaments, in the unconstrained joint at full extension, were the primary structures to resist the anterior femoral load but had negligible role in posterior-directed load. With joint flexion up to 90°, however, forces in collateral ligaments diminished. Similar trends were computed after fixing coupled tibial rotations with the exception of much greater LCL force and smaller MCL force at full extension under femoral anterior load and larger MCL force in flexion.

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