Hip, hip, hooray

July 9, 1999

Two new techniques will improve the lives of those with hip replacements. Geoff Watts reports.

Materials scientist John Metcalf uses an apt comparison to illustrate one of his projects. "You can think of this stuff as a pudding with raisins in it," he says. "You want the raisins to be a good size. And, most important, you want them evenly distributed."

The material he's talking about - tougher than any pudding mixture - is actually a recipe for the steel (the matrix of his "pudding") used to make a particular type of hip joint. His "raisins" are the particles of chrome and cobalt carbide that improve the steel's resistance to wear.

Given that doctors now have scores of different hip joints to choose from, is there room in the market for another? Orthopaedic surgeon Derek McMinn of Birmingham Nuffield Hospital believes so. He has resurrected two old ideas and is giving both a new future. What triggered his interest were a couple of all-metal hip joints that had continued to work well for 25 years. Most joints of this type - where a metal ball wedged into the thigh bone rotates in a metal cup fitted into the hip bone - would have long since worn out or seized up. Metal-on-metal, for these purposes, has been considered a destructive combination, which is why joints where a metal ball rotates in a cup lined with plastic were first introduced and are now standard.

Any number of people whose lives have been blighted by arthritis will testify to the success of plastic-lined implants. But in ten years, a joint might go through ten million cycles of movement. Even the toughest plastic will not last forever, and debris from worn linings may cause problems. Sooner or later the joint will fail and another operation will be needed.

The all-metal joints that set Dr McMinn thinking showed few signs of wear. He asked the manufacturers what made them different from others fitted around the same time. The manufacturers admitted they did not know and passed the problem over to the Materials Research Institute at Sheffield Hallam University. By measuring the precise dimensions of the ball and socket, studying the hardness of the metal used to make them and using electron microscopy to probe its microstructure, John Metcalf came up with two explanations.

The joints that lasted longest, he says, were those that were the most perfectly spherical and were made of metal with the most even distribution of carbide particles. The companies that had commissioned Metcalf's report, Centaur Precision and Midland Medical Technologies, set out to cast and finish a new generation of all-metal joints.

Spherical surfaces can now be routinely manufactured to a tolerance of just one micron. By adjusting the temperature, rate of cooling and other such features of the casting process, it is possible to get the optimum distribution of carbide particles: Metcalf's ideal "raisin pudding".

But surgeon Dr McMinn has also been trying another, less invasive, method of creating the new joint. In conventional hip replacement, the surgeon saws off the head of the thigh bone and the new ball is mounted on a tapering metal shaft that is driven into the bone's core. Dr McMinn, by contrast, simply grinds sufficient material off the surface of the bone's ball and caps it with a steel hemisphere held with a short locating pin. In effect, he is resurfacing rather than replacing the joint.

The principal beneficiaries of the new joint are likely to be young arthritis sufferers. "There's a need, especially in active young people, for something smaller and more durable than conventional hip replacement," says Dr McMinn. "That's been obvious for 50 years."

Resurfacing and using metal-on-metal artificial joints are ideas that have been around for decades. But to work well in hip joints, manufacturing technology had to catch up - which has now happened.

Dr McMinn and his colleagues have fitted about 1,500 of the new joints. Surgeons in Britain carry out 50,000 hip replacements annually, and Tim Band, Centaur Precision's medical development manager, sees a bright future for the new joint. "It needs healthy bone, so older patients with weaker bones may not be suitable. But probably half the patients having hip replacements would be."

This may be the salesman speaking. Dr McMinn's estimate is nearer 10 per cent. But whatever the joint's true potential, it seems to have several advantages. The operation is slightly quicker and patients may be able to leave hospital earlier. Cost is no real barrier. The new joints are more expensive than some conventional implants, but cheaper than the Rolls-Royce end of the market. Most important, resurfacing avoids imposing a new set of stress patterns on the femur, stress that can lead to resorption of its bone and consequent weakening.

If all goes well, younger people needing surgery for an arthritic hip will no longer face the likelihood of eventual joint failure, followed by more discomfort and the prospect of another operation.

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