Reporter 444, 6 December 1999
More than one in ten people with an artificial hip will need to have it replaced. The body's own defences are the culprits, breaking down surrounding bone and loosening the implant. With the number of younger people receiving synthetic joints on the increase, an unusual University partnership is working to keep artificial knees and hips working as long as they need to.
About one in a hundred people in the UK has a hip that started life in a laboratory. These artificial joints have a useful lifetime of about twenty years, before the biological reactions which defend the body from harmful infections cause too much damage and a replacement for the replacement is needed.
When the artificial joint rubs against itself, tiny particles of polyethylene or metal are produced. These bits of 'wear debris' are the same size as bacteria and provoke the automatic defences carried in the blood into aggressive action.
"The friction doesn’t do enough damage to the joints to affect their performance," said project leader Professor John Fisher. "We're talking, literally, about microscopic particles being generated. But they accumulate in surrounding tissue and that's when the problems start."
Professor Fisher, of mechanical engineering, has teamed up with immunologist Dr Eileen Ingham and senior clinical lecturer in orthopaedic surgery Martin Stone to develop new implant materials that do not produce this problematic debris.
"For years researchers have been trying to overcome this problem by producing materials that wear less and produce less debris. In this project we are taking a different approach and looking at the biological interactions taking place and how these can be minimised," said Professor Fisher.
Dr Ingham said the problems begin when blood cells (called macrophages) target the tiny debris material. In their fruitless attempts to break the metal and plastic down they release cytokines - signal chemicals that call other cells to help fight off the intruders - and the whole region becomes inflamed. The cytokines also trigger a different kind of activity that eats away at patients' bones.
Osteoclast cells continually replace and repair bone, but the constant call to action issued by the overworked defences forces this reaction too far. The bones that surround the implanted hip are eaten away. The resulting cavities loosen the artificial joint, leaving it wobbly, painful and in need of removal. It can take up to twenty-five years for the situation to deteriorate to that stage, but the researchers can mimic it in a few days.
Professor Fisher is developing metal, polymer and ceramic materials that wear at different rates, and others that produce different shaped and sized particles when they rub against each other.
Dr Ingham then analyses the variety of biological reactions taking place in cultures of tissue cells grown in the laboratory.
Paradoxically, the solution may lie in materials that produce larger chunks of debris.
"The bone destruction is a direct result of the similarity in size between the particles and bacteria. The macrophage cells might not attack larger pieces as they would not appear to pose the same threat," said Dr Ingham.
The collaborative project is supported by one of the first of a brand new type of grant from the Engineering and Physical Sciences Research Council.
The £400,000 platform grant is intended to provide underpinning scientific support for a whole series of associated projects. Professor Fisher said a portfolio of twenty projects would be built on this foundation.
The researchers aim to produce a material that can be used to manufacture the next generation of longer-lasting artificial joints within the project's four-year lifetime. It will take more time, however, until they definitely know if they have cracked the problem.
"We are aiming to reduce the failure rate of joints that have been in people's bodies for twenty-five years," said Professor Fisher. "But, of course, it will take a quarter of a century for us to see the results."
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