Women are three times more likely than men to be affected. The peak incidence is between the fourth and sixth decades. Genetic predisposition may occur especially with the HLA-DR4 haplotype. 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.