Changes occurring in the surface morphology of articular cartilage during wear

Articular cartilage is an extremely complex material; it exhibits unique properties, unmatched by man-made materials. It is widely thought that articular cartilage plays a major role in controlling both the lubrication and, more importantly, wear of synovial joints. Degenerative joint diseases, such as osteoarthritis, are thought to be related to the wear of articular cartilage. As the wear rate accelerates, the cartilage is completely worn away, leading to direct bone-to-bone contact, eventually requiring joint replacement surgery. Intensive research has been focused on the lubrication properties of synovial joints, while their wear characteristics and mechanisms still remain poorly researched. In this paper, preliminary results obtained from wear studies conducted on articular cartilage samples are described and discussed. Sheep knee joints were worn in a joint simulator for different periods of time. Two criteria, i.e. changes in surface morphology and in wear particle morphology, were used in wear assessment. The worn joints were compared to control (unworn) joints to determine the changes occurring. The surface morphology of articular cartilage was imaged using both scanning electron microscopy (SEM) and environmental SEM, with surface damage found to increase as the duration of wear tests increased. Environmental scanning electron microscopy (ESEM) has allowed the true surface of articular cartilage to be imaged in its natural state, without the need for fixation. ESEM imaging has revealed a surface layer that lies on top of the collagen matrix. The function of this surface layer is found to be disrupted by wear. Wear particles were also collected and characterised using numerical descriptors. The wear particles were observed to change shape and size as wear progresses.