Quantifying In Vivo Laxity in the ACL and Individual Knee Joint Structures: A Numerical Modeling Approach

The integrity of knee joint structures, such as ligaments, is typically assessed in a clinical setting through a measure of joint laxity [1]. Knee joint laxity has been studied extensively due to the high incidence of knee injuries, joint pain and degeneration [2]. Such pathological conditions can lead to significant functional loss, affecting the quality of life of the patient. Excessive joint laxity can predispose the joint to instability including dislocations or further injury. The link between laxity and instability is not fully understood. A novel tool has been recently developed to quantify in vivo joint laxity with a custom knee loading apparatus (KLA) and magnetic resonance (MR) imaging [3]. This tool enables measurement of the gross anterior laxity of the joint, and the use of MR imaging allows laxity to be quantified at the level of the bony geometry. In the present study, a numerical modeling approach was applied to extend the experimental work of the KLA to provide an understanding of the contribution of the individual joint structures such as ligaments and contact, and to estimate material stiffness parameters of the anterior cruciate ligament (ACL). Numerical models enable non-invasive predictions of joint function, within the constraints of the underlying assumptions. Extensive work exists in the development of numerical models of knee joint mechanics, which typically assume material parameters for the individual joint structures from in vitro data. However, the model developed in the present study works to estimate the material stiffness parameters of the ligaments as a model output. In this way, the model can be applied to subject populations across the spectrum of laxity and can increase the understanding of the link between laxity and instability. This thesis presents the development of the numerical modeling procedures, and application to two subject case studies. The approaches and results of this study provide the basis to develop a fully robust, and accurate measure of in vivo knee joint laxity and ligament stiffness characteristics.