Conventional Total Hip Replacement (THR) is a very successful procedure for the treatment of arthritis of the hip, a condition causing considerable pain and loss of movement. The hip is a ball and socket joint, which allows the upper leg to move from side to side, back to front, and to rotate. The joint is made up of the head of the femur (the ball), which fits into the acetabulum (the socket). In the healthy hip, both the head of the femur and the acetabulum are covered with cartilage, a smooth surface which allows the joint to move freely.         The very earliest THR, in 1938, involved an implant with both surfaces made from metal – known as a metal-on-metal bearing. During the 1950’s and early 1960’s, a number of surgeons developed hip implants using this type of bearing, although these were not resurfacing devices. Many of these designs were not very successful, but the implants that did not loosen early have generally lasted very well.       In the early 1960's, a British Surgeon, Sir John Charnley, developed a new type of THR which is still in use today. This procedure involves cutting the worn head off the femur and replacing it with a metal ball and stem in the shaft of the femur, and a plastic cup in the pelvis, as shown in the diagram below. Both are typically anchored to the bone by “bone cement”. This has become a very common surgical procedure, with some 45,000 hip replacements being carried out in the UK every year – about 200 every working day.         The main problem with Total Hip Replacement using polyethylene cups is that the polyethylene gradually wears, eventually reaching a point where the replacement joint no longer functions properly. In older people, with a lower activity level, this may not happen for 20 or more years, but in younger, more active patients, this may happen much sooner.   The other problem is that although the plastic itself is inert (well tolerated by the body) as the metal rubs against the plastic, tiny particles of the plastic are worn away. This plastic debris can cause an irritation. As the particles get between the bone and the artificial joint, this irritation causes surrounding bone to be absorbed by the body, leading to loosening of the artificial joint.         To overcome these problems in younger, more active patients requiring hip replacement, a different type of implant was needed and this was the reason Corin developed Hip Resurfacing.