The early degenerative or pre osteoarthritis (OA) state of the joint remains little understood. Recently we have published details of a pre-OA model (Thambyah and Broom 2009), involving the patellae of mature cows, where a virtually unlimited supply of intact to moderately degenerate tissue is available. The aim of this study is to utilise this model and quantify the morphometric, mechanical, and elemental changes seen in early joint tissue degeneration in the three vital tissues of OA: articular cartilage, calcified cartilage, and subchondral bone.
Ten patellae from mature cows were used. Gross examination with India ink staining confirmed five patellae to contain completely intact cartilage and five to have moderate (Outerbridge Grade II) degeneration. A specialised three point bending test was used to quantify elastic moduli of calcified cartilage and subchondral bone. Energy dispersive X-ray spectroscopy (EDS) was used to quantify chemical composition of cartilage, calcified cartilage, and subchondral bone. Differential Interference Contrast (DIC) microscopy was used to quantify histomorphometric data.
While the full cartilage thickness of the degenerate group was significantly (p < 0.05) thicker, by 24% compared to the intact group, the upper matrix zone of cartilage in degenerate tissue was significantly thinner by 88%. The calcified layer was significantly thicker in the degenerate tissue group than the intact group by 36%. There were no significant differences in the elastic moduli between groups. There were no significant differences in elemental composition between groups but Calcium and Phosphorus content were consistently greater in the deep subchondral bone than the upper by 9.2 and 9.3% respectively.
Early degenerative changes in the bovine joint tissue system, although too subtle to produce macro-level mechanical effects, are detectable at the micro-structural level. The data suggests that OA not only involves cartilage ‘wear’ but also significant changes in the underlying mineralised tissues.