The hip joint is a ball-and-socket joint. At least 50,000 hip replacements are done in Britain each year. The operation is highly successful, as it replaces both the natural socket (the acetabulum) and the natural ball, the head of the thigh-bone (femur). Materials science plays a major part in the design of artificial hip and other joints. The early models, made of stainless steel and Teflon, eventually disintegrated, and many had to be removed. The preference now is for high-density polythene for the socket, titanium alloys for the shaft, and sometimes a separate ball made of an alloy of cobalt, chromium and molybdenum. Some surgeons use a ceramic head.

Unfortunately, advances are slow, because new ideas, however promising, cannot be tried out on patients unless the probability of success is very high. Some of the factors can be tested in machines that simulate many years of use in a short time, but others can be investigated only in a living subject.

Why it should be done

Hip replacement is the only effective treatment for a hip joint that can’t function adequately and painlessly. The most common causes of pain and reduced mobility are:

• Osteoarthritis and rheumatoid arthritis
• Septic arthritis
• Fracture of the neck of the thigh-bone (femur), with loss of the blood supply to the rounded head of the bone and resultant crumbling (avascular necrosis)
• Other injury to the joint
• Paget's disease of bone
• The late effects of congenital dislocation of the hips

Osteoporosis in women can lead to hip-joint fractures.

How it is performed

In the operation, done under general or spinal anaesthesia, the joint is completely removed. The upper part of the femur is sawn off and the natural hollow for the head of the femur (the acetabulum) is reamed out. A plastic socket, usually of high-density polyethylene, is fitted into the hollow in the pelvis. A short, angled metal shaft, with a smooth ball on its upper end to fit into the socket, is forced down into the hollow of the thigh-bone. The plastic cup and the artificial bone-head may be a press-fit or they may be fixed with acrylic cement. The hip replacement operation has come to be thought of by many as a routine and simple procedure. This is not so. Hip replacement is major surgery requiring a skilled surgeon. The results are usually very good.

Complications do occur, the most important being infection, which is always serious. The infection rate, in spite of the most elaborate precautions, is usually at least one per cent. A much more common complication is that the shaft of the prosthesis becomes loose in the hollow of the thigh-bone. Loosening, especially after about ten years, has been a major problem and has so far defied solution. Re-operation is often necessary in such cases.

Because of the tendency to loosening, and because this is more likely if the joint is heavily stressed, hip replacement is not commonly performed in young and physically active patients. Some surgeons reserve total hip-joint replacement for patients over 60.