A multi-scale investigation of the joint tissue response to impact induced injury

Abstract

A bovine patellae model of early osteoarthritis was used to investigate the multiscale response of articular cartilage to impact induced injury, and how this response differed in tissue showing early signs of degeneration. An impact testing rig was designed, constructed and validated. Following this, the mechanical response of articular cartilage with increasing levels of degeneration was investigated. The resultant reaction force from the cartilage decreased as the amount of degeneration present increased. The elasticity of the collision also decreased with level of degeneration, indicating that collisions with more degenerate cartilage lose more energy. It was determined that articular cartilage with only mild degeneration was more vulnerable to high levels of macro-scale damage than healthy tissue. There were no significant changes in mechanical properties during earlystage degeneration, but there are subtle structural changes such a decreased brillar interconnectivity which lead to a large increase in tissue vulnerability. In healthy articular cartilage, the matrix contains a highly interconnected network of collagen brils which are effective in halting the progression of fissures. In mild degeneration, there is a loss of transverse connectivity between brils, leading to a reduced crack arresting ability. These nano-level structural changes have a large influence on the propagation of fissures through the matrix, allowing them to travel through to the sub-chondral bone. In mildly degenerate articular cartilage the peri-cellular matrix is disrupted, reducing its ability to provide a mechanical transition between chondrocyte and surrounding matrix. This lead to ~260% more dead cells in mildly degenerate G1 cartilage after impact injury compared to healthy G0. In a linear regression model, the coeffcient of restitution, percent cell death and macroscopic Outerbridge level of degeneration were able to account for 94.5% of the variation in cartilage tissue vulnerability to impact damage. This research has contributed to a deeper understanding of the link between biologically mediated micro-anatomical and physiological changes, vulnerability to injury and potential initiation of post-traumatic osteoarthritis. The earliest stages of disease initiation cause the tissue to have a sharp increase in vulnerability to severe damage with a high possibility to hasten development of full depth osteoarthritis.