The most common cause of hip and knee damage is osteoarthritis—the erosion of cartilage between the joints that allows adjoining bones to rub together. (Joints damaged by rheumatoid arthritis, a less common but more serious disorder, can be replaced as necessary when medical treatment has failed.) Often, you can delay or even eliminate the need for joint replacement by the judicious use of over-the-counter pain relievers, losing excess weight, and by doing regular low-impact exercises such as tai chi and water workouts. To learn what exercises help relieve arthritis pain, speak with a physical therapist or contact the Arthritis Foundation.
But when the damage is advanced, joint replacement becomes a good option. In knee replacement, surgeons remove damaged portions of the thighbone, shinbone, or kneecap and insert artificial parts, generally using cement to help keep them in place. In hip replacement, they cut off the head of the thighbone, or femur, and hollow out its shaft. Then they insert a new hip socket and a new femoral head, anchored by a stem that's wedged into the shaft, usually without cement. In either operation, the parts are usually made of metal and polyethylene,a plastic. They can last 20 years, but they wear out earlier in younger, heavier, and more active people.
The most common reason hip and knee implants fail is loosening. As metal and plastic rub together, the friction wears away the surfaces, creating tiny plastic particles around the joint. The body attempts to remove those particles but in the process removes bone as well, causing the implant to loosen. The joint then has to be replaced, a procedure known as revision surgery, which is usually less successful than the original operation because of the bone loss.