Osteoarthritis (OA, osteoarthrosis, degenerative joint disease) represents failure of the diarthrodial joint. It has numerous causes. As the heart may fail due to disease of the myocardium, endocardium, or epicardium, the joint may fail due to an abnormality in the articular cartilage, underlying bone, synovial membrane, supporting ligaments, or neuromuscular system. Primary (idiopathic) OA represents the most common form of the disease. It is usually monoarticular and tends to progress slowly. It most commonly involves the small joints of the hands, the hips, knees, spine, and the metatarsophalangeal joint of the great toe.