Impacts of medial collateral ligament (MCL) stiffness adjustment on knee joint mechanics in mechanically aligned posterior-substituting (PS) total knee arthroplasty (TKA).

To investigate the biomechanical effects of medial collateral ligament (MCL) stiffness adjustments on knee kinematics-medial femoral rollback, femoral rotation, and joint contact forces-in mechanically aligned posterior-substituting (PS) total knee arthroplasty (TKA). A musculoskeletal model simulating squatting was developed using the AnyBody modeling system. A PS-TKA prosthesis was implanted, and MCL stiffness was modified in 20% increments. The effects on femoral rollback, femoral rotation, and joint forces were evaluated. Medial femoral rollback was not significantly affected by changes in MCL stiffness. However, when MCL stiffness exceeded 20% above normal, the pattern and magnitude of lateral femoral rollback were altered compared to other conditions. Increased MCL stiffness also altered internal-external femoral rotation and raised joint contact forces in the medial compartment. Muscle activity was largely unaffected by changes in MCL stiffness, although hamstring activity increased slightly during early flexion (0°-5°) when MCL stiffness exceeded 20%. Excessive MCL stiffness (over 20% above normal) affects lateral femoral rollback and increases joint contact forces, potentially elevating the risk of prosthetic wear. Maintaining MCL stiffness within physiological limits is critical for optimizing outcomes in varus knee TKA.