Articular cartilage is a thin layer of soft tissue that lines the ends of bones in diarthrodial joints and is essential for normal joint function during physiological activity. Degenerate changes to the cartilage caused by osteoarthritis limits the ability of cartilage to fulfil its functions, and this often has a negative impact on an individual’s quality of life. The mechanisms driving the onset and progression of osteoarthritis are not well understood. Since cartilage is associated with joint loading, improved knowledge of the mechanical properties of normal and osteoarthritic cartilage may offer greater insight into osteoarthritis and more effective treatment methods.
The focus of this dissertation was to improve understanding of the mechanical properties of normal and osteoarthritic human cartilage. This was achieved by two inter-related studies that employed experimental and computational techniques to determine the anisotropic and hyperelastic properties of cartilage under fast loading.
The aims of the first study were to (1) measure the anisotropic properties at the articular surface of normal and osteoarthritic cartilage for fast loading rates; (2) measure the structural parameters (zonal thicknesses and proteoglycan thickness) of articular cartilage and assess the severity of cartilage damage indicative of osteoarthritis; and (3) determine whether the structural parameters and/or severity of cartilage damage correlate with the anisotropic properties at the surface. Small osteochondral specimens retrieved from healthy and osteoarthritic human knees were tested in unconfined compression at fast loading rates and large strains representative of weight-bearing activity. Anisotropy at the surface was quantified by direction-dependent Poisson’s ratios measured using imaging techniques. Structural parameters were measured using histological techniques while the severity of cartilage damage was quantified using the Osteoarthritis Research Society International (OARSI) grading scale. For all specimens tested, anisotropy was observed at the articular surface, a behaviour that has not been previously observed for high loading rates. The measured Poisson’s ratios were elevated in specimens with a greater amount of tissue damage, suggesting that anisotropy across the entire articular surface of the knee may be altered as a result of significant tissue damage.
The specific aims of the second study were to (1) measure the material constants of normal and osteoarthritic human knee cartilage using isotropic hyperelastic models; (2) determine whether the material constants correlate with histological measures of structure and/or cartilage tissue damage; and (3) quantify the abilities of two common isotropic hyperelastic material models, the neo-Hookean and Yeoh models, to describe articular cartilage contact force, area, pressure. Specimen-specific finite element models were used to describe the unconfined compression experiments in the first study. The material constants were determined iteratively to minimise the difference between the experimental contact force and that predicted by the finite element simulations. The hyperelastic material constants correlated strongly with OARSI grade, indicating that the mechanical properties of cartilage for large strains and fast loading rates change with tissue damage. The Yeoh model described contact force and pressure more accurately than the neo-Hookean model, whereas both models under-predicted contact area and poorly described the anisotropy at the articular surface. These results identify the limits by which simple isotropic hyperelastic material models may be used to describe cartilage contact variables.
The results of this dissertation provide novel data for the mechanical properties of normal and osteoarthritic cartilage and enhance our ability to model this tissue using simple isotropic hyperelastic materials. These mechanical properties may be used in the development of improved osteoarthritis treatment methods, such as the design of artificial cartilage implants that better match the recipient tissue when it is subjected to fast loading rates. The measurements of anisotropy have applications in osteoarthritis diagnosis methods since direction-dependent properties of cartilage may be measured non-invasively using techniques such as magnetic resonance imaging.
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