The very success of hip replacement surgery has brought with it new problems. Hip replacements are being fitted in increasingly younger people because of the enhanced quality of life they can provide. However, this means that artificial joints are wearing out well before death (the average life of a new joint if ten to fifteen years), so there is a growing need for second joint replacements. Although a second hip replacement continues to leave the person involved free from pain and much more mobile than they would be without the operation, the results are never as good as for a first operation.
The second operation has to be performed on a patient who is older and more frail and, therefore, that much less suitable to face the slight but real risks of major surgery. Scientists and engineers are currently trying to produce a hip replacement that will last substantially longer than those in use today. Although some prostheses have been shown to be defective, until recently the main reason why joints failed was that the spike driven into the femur gradually became loose, improved cements have largely solved this problem. The color enhanced x-rays shows the metal spike and plastic socket of a hip replacement.
When the cartilage on the femur and the socket of the pelvic bone deteriorates, surgery may be required to repair the joint. A new, less invasive method uses a metal cup and metal head to resurface the socket and femur. The standard method uses a metal ball and plastic cup. The next sets of problems are those caused by wear of the joint, and especially wear of the surface of the polyethylene cup in which the ball rotates. The debris released as this plastic surface is gradually worn down causes a biological reaction, which weakens the surrounding bone and steadily weakens the joint.
Scientists are tackling this problem in different ways. A new experimental prosthesis uses only metal components to resurface the femur head and the socket in the pelvis. It is suitable for younger patients and the bone must be of good quality, so women with osteoporosis are not candidates for the procedure. Another promising procedure being researched by a British team involves a material similar to diamond, a hard form of carbon which could be used to coat the surfaces of artificial joints. This material has the remarkable property of being nearly as hard as diamond, and, therefore, resistance to wear but also of being slippery and a good lubricant, like graphite, another form of carbon.
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