Measurement/analysis of deformation and strain distribution of the anterior cruciate ligament in the human knee

Highly variable deformations of the anterior cruciate ligaments (ACL) cannot be adequately quantified by one-dimensional and/or localized measurements. Furthermore it is impossible to directly measure in-situ, non-uniform distribution of biaxial strain over the entire surface of the intra-articular ligaments. We employed an alternative approach; Instead of using an actual knee and ACL, a knee motion simulator and a pseudo ligament were prepared. The pseudo ligament was manipulated so as to simulate the deformation and stress the actual ACL undergoes during knee flexion. Two hundred and ten points drawn on the pseudo ligament at regular intervals, were measured by the stereo-photogrammetric method, and components of the Green strain tensor were computed. Next, using the finite element method, in which the ligament was treated as an incompressive hyperelastic membrane, the finite deformation of the ligament was theoretically analyzed. The results demonstrated that strain distribution varied, even along the fiber run, and large strain gradients were observed in the regions near the insertions. Futher, the following values were obtained as a function of knee flexion: three-dimensional change in shape, resultant pull force on the tibial insertion, and change of strain pattern by anterior-posterior displacement of the tibia.