The most common reason for having a hip or knee replaced is osteoarthritis, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). This degenerative joint disease, marked by the breakdown of the joint's cartilage, is not limited to older people. Although it most commonly affects people over age 45, younger men and women also can get this disease.
Some people are born with a deformed joint or defective cartilage, which leads to osteoarthritis. Excess weight, joint fracture, ligament tears, or other injury can damage cartilage and cause osteoarthritis.
Rheumatoid arthritis is another condition that may be alleviated by hip or knee joint replacement. This chronic inflammation of the joint lining causes pain, stiffness, and swelling. The inflamed lining can invade and damage bone and cartilage. Rheumatoid arthritis generally starts in middle age, but can also affect children and young adults.
Loss of bone caused by poor blood supply (avascular necrosis), which led to Van Halen's hip replacement, and bone tumors may be other reasons for joint replacement.
"All You Need to Know About Joint Surgery," © 2002, Arthritis Foundation