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RESEARCH PAPERS

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

[+] Author and Article Information
David Liu-Barba

Biomedical Engineering Graduate Group, University of California, One Shields Avenue, Davis, CA 95616

M. L. Hull1

Department of Mechanical Engineering, and Biomedical Engineering Graduate Group, University of California, One Shields Avenue, Davis, CA 95616mlhull@ucdavis.edu

S. M. Howell

Department of Mechanical Engineering,  University of California, One Shields Avenue, Davis, CA 95616

1

Corresponding author.

J Biomech Eng 129(6), 818-824 (May 14, 2007) (7 pages) doi:10.1115/1.2800762 History: Received December 12, 2005; Revised May 14, 2007

## 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.

###### FIGURES IN THIS ARTICLE
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Copyright © 2007 by American Society of Mechanical Engineers
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## Figures

Figure 1

Diagram of six degree-of-freedom knee loading apparatus. The degrees of freedom follow the coordinate system of Grood and Suntay (22) so that the flexion-extension axis is fixed to the femur and the axial rotation axis is fixed to the tibia. Accordingly, the femur unit provides two degrees of freedom, flexion-extension (F-E) rotation and medial-lateral (M-L) translation. The tibia unit provides the remaining four degrees of freedom. Pneumatic actuators (omitted for clarity) develop loads in all degrees of freedom except F-E where loads are developed by a stepper motor. The loads developed by the actuators are measured with strain gage load cells. Motions in all degrees of freedom are enabled through the use of low-friction bearings. The motions are measured with linear variable differential transformers (LVDTs) for translations and rotational variable differential transformers (RVDTs) for rotations. Flexion angle is adjustable over the full physiologic range and the F-E counterbalance ensures that the weight of the specimen does not create an unwanted F-E moment. In the present study, compression loading was applied with the knee at a preset flexion angle and the coupled motions in the remaining degrees of freedom were measured.

Figure 2

Example load-displacement curves for coupled motions in four degrees of freedom under compressive load for one intact knee specimen at 30deg of flexion. Positive numbers represent movement in the first-listed direction of the respective degree of freedom (e.g., “internal” for I-E rotation). For V-V rotation, varus is the first rotation. Solid lines represent loading; dashed lines represent unloading. The knee was loaded from 0Nto1600N(2.1×BW) and unloaded in 100N increments.

Figure 3

Average I-E rotations for the intact knee and ACL-deficient knee for each flexion angle during loading referenced to the unloaded position of the intact knee. Positive numbers represent movement in the first-listed direction of the respective degree of freedom (e.g., “internal” for I-E rotation). For V-V rotation, varus is the first rotation. Error bars are not shown for clarity. The average rotations were comparable for the intact knee and ACL-deficient knee.

Figure 4

Average V-V rotations for the intact knee and ACL-deficient knee for each flexion angle during loading referenced to the unloaded position of the intact knee. Positive numbers represent movement in the first-listed direction of the respective degree of freedom (e.g., “internal” for I-E rotation). For V-V rotation, varus is the first rotation. Error bars are not shown for clarity. The average rotations were comparable for the intact knee and ACL-deficient knee.

Figure 5

Average M-L translations for the intact knee and ACL-deficient knee for each flexion angle during loading referenced to the unloaded position of the intact knee. Positive numbers represent movement in the first-listed direction of the respective degree of freedom (e.g., “internal” for I-E rotation). For V-V rotation, varus is the first rotation. Error bars are not shown for clarity. The average translations were comparable for the intact knee and ACL-deficient knee.

Figure 6

Average A-P translations for the intact knee and ACL-deficient knee for each flexion angle during loading referenced to the unloaded position of the intact knee. Positive numbers represent movement in the first-listed direction of the respective degree of freedom (e.g., “internal” for I-E rotation). For V-V rotation, varus is the first rotation. Error bars are not shown for clarity. The average translations significantly increased for the ACL-deficient knee compared to the intact knee.

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