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

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.