Cartilage damage in osteoarthritis and rheumatoid arthritis--two unequal siblings.

Cartilage damage is a key feature of degenerative joint disorders-primarily osteoarthritis (OA)-and chronic inflammatory joint diseases, such as rheumatoid arthritis (RA). Substantial progress has been made towards understanding the mechanisms that lead to degradation of the cartilage matrix in either condition, which ultimately results in the progressive remodelling of affected joints. The available data have shown that the molecular steps in cartilage matrix breakdown overlap in OA and RA. However, they have also, to a great extent, changed our view of the roles of cartilage in the pathogenesis of these disorders. In OA, cartilage loss occurs as part of a complex programme that resembles aspects of embryonic bone formation through endochondral ossification. In RA, early cartilage damage is a key trigger of cellular reactions in the synovium. In a proposed model of RA as a site-specific manifestation of a systemic autoimmune disorder, early cartilage damage in the context of immune activation leads to a specific cellular response within articular joints that could explain not only the organ specificity of RA, but also the chronic nature and perpetuation of the disease.