Experimental and Computational Characterization of the Anterior Cruciate Ligament: Challenges and Considerations for Soft Tissue Biomechanics

Abstract

The anterior cruciate ligament (ACL) is one of several major stabilizing soft tissue structures in the articular knee joint. However, the ACL is often torn, and has a low ability to self-heal, with upwards of 300,000 ACL replacement surgeries performed in the United States alone each year. There have been detrimental long-term effects of these replacements, especially the occurrence of early-onset osteoarthritis. Much research has been performed in the area of knee biomechanics to understand the role of the ACL in knee function, tissue biomechanics, and graft replacements. The ACL is known to be hyperelastic, anisotropic, and heterogeneous, with complex insertion sites. This dissertation focuses on characterization and computational modeling of the intrinsic full-field mechanical properties of the ACL. Understanding these properties leads to insight on tissue failure mechanisms and could one day aid in the prevention of ACL failures.

In this thesis I have developed a novel method for individually aligning the distinct anteromedial (AM) and posterolateral (PL) bundles of the ACL, in order to perform mechanical testing of both bundles in a well-understood loading state. I have also performed entire ACL experiments with clinical applications. To obtain full-field data, I have incorporated the use of digital image correlation (DIC), a non-contact deformation measurement technique. A patterning method to adapt DIC to soft materials is developed and discussed. Using DIC, full-field surface displacements and strains of the ACL have been acquired. These contours provide new information on the spatial deformation of the ACL’s bundles, providing new information on its hyperelastic and anisotropic response. The experimental results have been further utilized to develop a finite element (FE) model of the AM bundle of the ACL. Within this computational model, a complex geometry and microstructurally-based constitutive model are used to simulate observed experimental behavior. The numerical model demonstrates strain responses in agreement with the full-field experimental results, as well as stress/strain behavior in line with that of experiments. Few studies have discussed the full-field experimental response and computational modeling of the AM and PL bundles; this thesis provides insight into the complexity of the ACL and considerations for its accurate characterization.