Synovium in osteoarthritis & inflammatory arthritis: is OA an inflammatory arthritis?

There is considerable published evidence now to support the claim that there is an inflammatory reaction in the synovial membrane lining the joints of patients with osteoarthritis. Macroscopically, this synovitis is often localised to areas of the synovial membrane immediately adjacent to areas of chondral damage and can resemble the synovitis of rheumatoid arthritis. Microscopically, the inflammatory changes in the OA synovial membrane appear to be less than that seen in RA synovial membranes although there are examples of OA synovitis which looks identical to that seen in RA. Overall the differences in cytokine and cell adhesion molecule expression between RA and OA synovial membranes appear to be quantitative rather than qualitative.
It has now been established that these inflammatory changes in OA synovial membranes are not a feature only seen in late disease in the presence of significant chondral damage, but can be seen in early OA when chondral damage is limited in extent. However, it is still not clear whether the synovitis seen in OA has a primary pathological role or is a reaction to biochemical changes within articular cartilage. This has obvious implications for the treatment of OA.