Turn on the TV and there is golf legend Jack Nicklaus extolling the virtues of his hip replacement. Still competing on the links, Nicklaus is among the growing number of people in the United States each year who have a hip joint replaced. So is rock star Eddie Van Halen, who was 43 when he had his hip replaced in 1999, the same year as Nicklaus.

Cases like these are laying to rest the stereotype that only the aged and the inactive receive hip replacements. The same holds true for those who have knee joints replaced.

The American Academy of Orthopaedic Surgeons (AAOS) calls total hip replacement an orthopedic success story, "enabling hundreds of thousands of people to live fuller, more active lives." In 2001, about 165,000 hip joints were replaced in U.S. hospitals, according to the National Center for Health Statistics. The same year, 326,000 knees were replaced. Total knee replacement is "highly successful in relieving pain and restoring joint function," says the AAOS. And a hip or knee replacement lasts at least 20 years in about 80 percent of those who get them.

But despite their success, hip and knee joint replacements still have drawbacks. There may be complications. They don't always last a lifetime and when they fail, surgery may be needed.

As artificial joints and surgical techniques to implant them continue to evolve, the medical community and patients hold out hope for joint replacements that cause fewer problems, last longer, and move more like a healthy natural joint.

What is Joint Replacement?

Joints are formed by the ends of two or more bones connected by tissue called cartilage. Healthy cartilage serves as a protective cushion, allowing smooth, low-friction movement of the joint. If the cartilage becomes damaged by disease or injury, the tissues around the joint become inflamed, causing pain. With time, the cartilage wears away, allowing the rough edges of bone to rub against each other, causing more pain.

When only some of the joint is damaged, a surgeon may be able to repair or replace just the damaged parts. When the entire joint is damaged, a total joint replacement is done. To replace a total hip or knee joint, a surgeon removes the diseased or damaged parts and inserts artificial parts, called prostheses or implants. These prostheses are considered medical devices, which are regulated by the Food and Drug Administration.

Why Joint Replacement?

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.