Estimation of ACL Forces by Reproducing Average Knee Kinematics

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URL: http://d-scholarship.pitt.edu/8398/

Abstract

The force of the anterior cruciate ligament (ACL) in response to simple loads designed to simulate clinical exams, such as a pivot shift test or an anterior drawer test, have been quantified in vitro. In addition, force transducers have also been used to estimate the forces in the ACL in vivo in animal models, but to date there are few non-invasive noncontact methods for use in human subjects. The specific aim of this study is to evaluate the feasibility of a non-invasive, non-contact methodology for estimating force in the ACL by reproducing average kinematics in 6-degrees of freedom (DOF) from one set of porcine knees (source) onto a separate set of porcine knees (target). A non-invasive, non-contact methodology to estimate forces in the ACL in response to in vivo knee kinematics will allow surgical procedures and rehabilitation protocols to be improved.

Source kinematics were collected in response to an anterior load of 100 N and a valgus load of 5 Nm at 30°, 60°, and 90° of knee flexion. After kinematics were collected for eight source knees, the average of these kinematics were calculated. The in situ force in the ACL of the target knees in response to reproducing the average kinematics was compared to the in situ force in the ACL of the source knees from the applied loads.

A significant difference in the in situ force in the ACL between the source knees and the target knees was found for all flexion angles in response to an anterior load and at 60° of knee flexion for valgus loading. These differences can be attributed to the variations in the coupled motions of individual knees due to the applied loads compared to the average kinematics of the source knees. In other words, when average kinematics are computed, variations in knee laxity cause coupled motions to be eliminated or reduced, artificially constraining the motion of the knee. However, knees with similar anterior knee laxity (length of toe region from load-displacement curve) had similar coupled internal-external rotations. Therefore, this study provides promising evidence that this innovative methodology can be extended to account for knee laxity and coupled motions by matching cadaveric knees to groups of subjects with similar anterior and internal-external knee laxity throughout the range of flexion-extension. Thus, an accurate understanding of the forces in the ACL and ACL graft during in vivo activities may be obtained using this methodology.

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