Mapping the Biomechanical Properties of Human Knee Cartilage.

Abstract

Knee osteoarthritis is one of the most common musculoskeletal pathologies. With no cure other than total joint replacement, and the incidence of osteoarthritis rising worldwide, the need to understand how the disease develops has reached a critical level. Alterations in the spatial loading pattern on the joint’s articular cartilage, for example, due to knee injury, have been hypothesized to trigger the onset of the disease. The theory presupposes that the mechanical properties of knee cartilage are non-uniform such that the underlying cartilage is unable to sustain the abnormal loading pattern and deteriorates. However, this tenet is challenging to test directly, in part because it requires detailed knowledge of spatial mechanical properties of the cartilage, which is currently unknown. Therefore, this dissertation sought to address current knowledge gaps by mapping the elastic modulus of healthy human knee articular cartilage across the joint surface. This work represented the first such mapping with fine spatial resolution and employing a physiologically relevant compressive strain rate. Significant variations in modulus were found across the femur and tibial cartilage. Moreover, these variations conformed to a consistent regional pattern across knees, which has not previously been demonstrated. These findings subsequently motivated the development of a constitutive relation that could successfully simulate spatially dependent, high strain rate mechanics. A transversely isotropic hyperelastic model was developed and compared with isotropic hyperelastic models to determine which constitutive relation best represented the natural cartilage mechanics. The transversely isotropic model replicated the spatial mechanical dependence of the tissue through variations in a single model parameter. The model is mechanistic, has a structure and parameters that are analogous to human cartilage physiology, is computationally efficient, and is straightforward to implement in commercial finite element packages. The transversely isotropic model, therefore, represents a novel method for implementing the non-uniform mechanics of knee cartilage that are critical to understanding the initiation and progression of knee osteoarthritis. The characterization of regional mechanical properties of human knee cartilage performed herein has contributed essential baseline knowledge that will fundamentally advance experimental and computational studies of knee osteoarthritis development toward widespread prevention of this devastating disease.