Multi length scale structural investigation of articular cartilage: How structure influences the tissue swelling and compressive response

Abstract

The objective of this PhD was to understand how degenerative changes associated with collagen network destructuring in articular cartilage influence its swelling response. Two testing methods were employed. Firstly, free swelling experiments were carried out using full-thickness sections of cartilage excised from the bone. Secondly, cartilage-on-bone blocks were subject to direct compression using a channel indentation technique. The incorporation of a relief channel allowed the cartilage to swell into the channel during creep loading, restricted purely by its own structural interconnectivity. Both studies were conducted using cartilage at varying stages of degeneration. The first three studies of this thesis describe swelling studies employing both bovine and human cartilage. For the bovine studies, the swelling response resulted in varying degrees of curvature, that curvature correlating with microstructure. With the surface layer intact, the tangentially aligned surface fibrils tended to restrict significant expansion of the surface layer. Importantly, removal of the surface layer with a microtome and naturally occurring degeneration of the surface layer elicited different swelling responses. Hence, simulating degeneration through surface removal is not an accurate practice. An additional important finding was the role of the deep zone cartilage in the swelling response and how this deep zone changes with maturity and early degeneration. For the human swelling study, there was a noticeable difference in swelling response compared with bovine, with most samples exhibiting minimal shape change post equilibration. The implication is that some microstructural differences must exist, which explains the difference in swelling response. In the final study, human cartilage was channel indented. Here we observed the site-specific variation in the tissue response which, was then compared with an unpublished bovine dataset. The human tissue was less compliant than its bovine counterpart which, correlated with both micro and ultrastructural differences. The findings from this work, the author believes, provides new insight into the influence of micro to sub-micro structural changes in articular cartilage on the gross scale mechanical response. Notably, both swelling and compression mechanics were studied, and both the collagen network transverse fibrillar interconnectivity and the zonally differentiated cartilage structure were found to be critical mechanical factors.