Osteoarthritis is a chronic joint disease in which the main changes are degeneration of the cartilage surfaces of the joints and secondary osteophytes. The main manifestations are joint pain and immobility. X-rays show narrowing of the joint space, dense subchondral bone, broken trabeculae, sclerosis and cystic changes. There is lip-like hyperplasia at the edge of the joint. In the later stage, the bone ends are deformed and the joint surface is uneven. The cartilage in the joint peels off and the bone fragments into the joint, forming an intra-articular free body. Osteoarthritis, also known as degenerative arthritis, is not actually an inflammatory condition, but is primarily a degenerative condition that involves premature aging of the joints, particularly the articular cartilage. Osteoarthritis represents the aging of the joints and is therefore called age-related arthritis. The broader term osteoarthritis also includes a number of other aseptic arthritic disorders. Osteoarthritis of the knee is a chronic osteoarthritic disorder caused by degeneration and osteophytes of the knee cartilage, also known as proliferative arthritis of the knee, degenerative arthritis and osteoarthrosis. The disease occurs mostly in middle-aged and elderly people, but can also occur in young people; it can develop unilaterally or bilaterally.