Abstract:
This thesis presents an investigation of the load spreading mechanism of articular cartilage at the micro to the ultra-structural levels with detailed analysis of the influence of structural degeneration. Bovine patellar cartilage with varying degree of degeneration was classified into three experimental groups of healthy cartilage, mild to moderately degenerate and moderate to severely degenerate cartilage. These were then mechanically compressed using a channel indenter, in which a channel relief zone of non-directly loaded region of cartilage was created, after which microstructural analysis using the DIC microscope and ultrastructural analysis using SEM was conducted in order to examine the responses of cartilage tissue to compression in the directly loaded, unloaded and nondirectly loaded regions. Measurements of axial strain, channel relief bulge height and bulge angle have been found to suggest a progression of increased compliance of the cartilage to deformation with increased severity of degeneration. These results are consistent with the findings of previous studies which have suggested reductions in mechanical properties of cartilage with progressive degeneration. Analysis of channel bulge height and bulge angle measurements also imply that these two parameters are more sensitive in detecting subtle changes in tissue behaviour of cartilage exhibiting mild degeneration. The strain-limiting surface layer and extensive fibrillar interconnectivity observed inside the collagen network of healthy cartilage were found to serve important functions in reducing load penetration into the deep zone of cartilage and preserving tissue integrity to withstand against direct compressive forces. These features were found to be degraded in degenerate cartilage and consequently led to reductions in tissue stiffness, response and integrity. Zonal differentiation in cartilage was found to be important as the boundaries, which consisted of the strain-limiting surface layer and osteochondral junction, provided attachment areas for the intermediate collagen fibrillar network that confined fluid flow and load dissipation within middle layers of the cartilage tissue. The author believes that this thesis provides an improved methodology and loading protocol for compressing degenerate cartilage tissue using the channel indenter and also provides both micro- and ultrastructural evidence of the importance of tissue structure and the collagen fibrillar network to cartilage response and behaviour.