Joint replacement surgery has revolutionized the treatment of late-stage arthritis for most patients. Enhanced surgical techniques and modern care during and after the hospital stay have helped speed patients to quick recoveries back to near-normal function.

Designing engineers and orthopedic surgeons hope improvements of implants will allow longer lasting function of the replacements. These improvements may allow the replacement of joints in younger patients and provide an alternative to patients who fail to get adequate relief from more conservative treatments.

The hip is a good example of a large weight-bearing joint that is frequently replaced. It is made of two main parts: The ball at the top of the thighbone that fits perfectly into the round socket in the pelvis; and the shape of the bones and “ropes” of tissue between the bones, called ligaments that provide stability to the joint. The ball and socket are both covered with a smooth durable layer of cartilage that cushions the ends of the bones and enables them to move easily. It is this layer that “goes bad” in most forms of arthritis. Doctors and researchers continually try to find a way to replace healthy living cartilage effectively, but success is likely many years away.

Varied designs
At this time, the best medical science has to offer is to replace the entire joint. Many different designs and materials are currently used in artificial hip joints. All of them consist of two basic components: the ball component (usually made of polished metal) and the socket component (usually made of strong plastic). Both components are attached to the normal bone with either a rough surface (into which bone will grow) or bone cement.

The “weak link,” according to most studies of joint replacement, is the contact surface between the ball and socket. Just as the surface that nature makes can wear out with time (arthritis), the plastic surface of the joint slowly wears away with time and can eventually cause the metal parts to loosen from the bone. Even the most polished ball’s surface has tiny scratches that scrape against the plastic cup and cause the loss of tiny pieces of polyethylene plastic. These tiny pieces are recognized as foreign and stimulate the formation of cells that clean up the body. The cleanup cells in bone dissolve the bone (osteolysis) near the metal pieces of a joint replacement and can cause them to become completely loose and in need of rereplacement.

The second operation, or revision, is much more difficult and has higher risks of complications and lower rates of patient satisfaction.