Rheumatoid arthritis is an autoimmune disease. The joint process is triggered in the subsynovial tissues and then secondarily affects the articular cartilage. Inflammatory cells proliferate in the synovial lining. The synovium hypertrophies and forms a pannus, which is granulation tissue composed of inflammatory cells, fibroblasts, and blood vessels. The pannus dissolves collagen and proteoglycans.
Collagenase is produced in mass quantities.
The patients often present with hot, swollen joints and are extremely disabled. As time goes on, gross deformities develop, due to weakening of the supporting ligaments and tendons around the joints. X-rays show obliteration of the joint space. Periarticular osteopenia is often seen. Very little reactive bone is seen.