The Effect of Mechanical Varus on Anterior Cruciate Ligament and Lateral Collateral Ligament Stress: Finite Element Analyses

The current study analyzed changes in anterior cruciate ligament (ACL) and lateral collateral ligament stress as a result of mechanical varus. In an exploratory pilot study, progressive mechanical varus was introduced to a male finite element model of the lower limb at different knee flexion angles. Nine situations were analyzed (combinations of 0 degrees, 30 degrees, and 60 degrees knee flexion and 0 degrees, 5 degrees, and 10 degrees varus). The ACL stress was measured via changes in section force, von Mises stress, and fiber stress. Lateral collateral ligament stress was measured via changes in section force. For all 3 measures of the ACL, maximum stress values were found in extension, stress decreased with flexion, and the effect of varus introduction was most significant at 30 degrees flexion. With 60 degrees flexion, varus introduction produced a decrease in section force and von Mises stress and a small increase in fiber stress. In all situations and stress measures except fiber stress at 60 degrees flexion, stress was concentrated at the posterolateral bundle. For the lateral collateral ligament, the introduction of 5 degrees and 10 degrees varus caused an increase in section force at all degrees of flexion. Stress in the ligament decreased with flexion. Mechanical varus of less than 10 degrees was responsible for increased ACL stress, particularly at 0 degrees and 30 degrees knee flexion, and for increased lateral collateral ligament stress at all degrees of flexion. Stress was mostly concentrated on the posterolateral bundle of the ACL.