For Fixed ACL Lengths, the ACL Force Varies with Relative Bone Orientations: A Finite Element Model

摘要

The anterior cruciate ligament (ACL) is one of the major structures that stabilize the knee joint. Several methods have been developed to quantify ACL forces during a variety of activities. A basic premise of many techniques is that the load borne by ACL is a function only of the length of the ACL. This premise has been held for many years. However, our recent data question this assumption. We measured the in vivo gait kinematics of the ovine stifle joint, reproduced the recorded kinematics in the same joint using a highly accurate parallel robot, and estimated the loads on different knee joint structures, including the ACL, based on the principle of superposition. Our results indicated that a wide range of forces is possible in the ACL for any specified inter-insertional distance, where the inter-insertional distance is defined as the distance between centroids of the insertional areas of the ACL on the tibia and femur. We hypothesize that the different fibres and fibre bundles that make up the ACL can be taut at different inter-insertional distances and bone orientations. Thus, the main objective of this study is to confirm this hypothesis using a finite element (FE) simulation of the ACL. The geometry of the FE model is reconstructed from magnetic resonance images (MRI) of a joint with the knee in full extension. The bones are assumed to be rigid, and an isotropic, hyperelastic, nearly incompressible constitutive model is assumed for the ACL. Various displacements and rotations within in vivo kinematic ranges are used as input data for the FE model and the ranges of loads obtained are compared against each other.