Coupled Motions Under Compressive Load in Intact and ACL-Deficient Knees: A Cadaveric Study

Abstract

Knowledge of the coupled motions, which develop under compressive loading of the knee, is useful to determine which degrees of freedom should be included in the study of tibiofemoral contact and also to understand the role of the anterior cruciate ligament (ACL) in coupled motions. The objectives of this study were to measure the coupled motions of the intact knee and ACL-deficient knee under compression and to compare the coupled motions of the ACL-deficient knee with those of the intact knee. Ten intact cadaveric knees were tested by applying a 1600N compressive load and measuring coupled internal-external and varus-valgus rotations and anterior-posterior and medial-lateral translations at 0deg, 15deg, and 30deg of flexion. Compressive loads were applied along the functional axis of axial rotation, which coincides approximately with the mechanical axis of the tibia. The ACL was excised and the knees were tested again. In the intact knee, the peak coupled motions were 3.8deg internal rotation at 0deg flexion changing to −4.9deg external rotation at 30deg of flexion, 1.4deg of varus rotation at 0deg flexion changing to −1.9deg valgus rotation at 30deg of flexion, 1.4mm of medial translation at 0deg flexion increasing to 2.3mm at 30deg of flexion, and 5.3mm of anterior translation at 0deg flexion increasing to 10.2mm at 30deg of flexion. All changes in the peak coupled motions from 0degto30deg flexion were statistically significant (p<0.05). In ACL-deficient knees, there was a strong trend (marginally not significant, p=0.07) toward greater anterior translation (12.7mm) than that in intact knees (8.0mm), whereas coupled motions in the other degrees of freedom were comparable. Because the coupled motions in all four degrees of freedom in the intact knee and ACL-deficient knee are sufficiently large to substantially affect the tibiofemoral contact area, all degrees of freedom should be included when either developing mathematical models or designing mechanical testing equipment for study of tibiofemoral contact. The increase in coupled anterior translation in ACL-deficient knees indicates the important role played by the ACL in constraining anterior translation during compressive loading.