Osteoarthritis is a progressive form of arthritis characterized by breakdown of the cartilage in joints. Osteoarthritis (OA), also known as degenerative joint disease and wear and tear arthritis, causes pain in the joints with activity. The knees and the hips are common locations for osteoarthritis. Osteoarthritis is common in people over 60 years of age, but it can affect younger people, particularly where they have had joint injury or joint surgery.