Osteoarthritis is a form of disease that features the breakdown and eventual loss of the cartilage of one or more joints. Cartilage is a protein substance that serves as a "cushion" between the bones of the joints. Among the over 100 different types of arthritis conditions, osteoarthritis is the most common joint disease. OA occurs more frequently as we age. Before age 45, osteoarthritis occurs more frequently in males. After 55 years of age, it occurs more frequently in females. In the United States, all races appear equally affected. Hand osteoarthritis, hip osteoarthritis, and knee osteoarthritis are much more common in seniors than