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WHAT does an artificial hip joint have in common with a bicycle? Answer: the design was virtually perfected many years ago, and most so- called improvements show little if any advantage over earlier models.

Of course, modern bicycles look different and many of them are different; but a well-maintained lightweight bicycle dating back to the 1950s or 1960s will continue to perform on good roads as well as any new design. Mountain bikes make sense only for the minority of cyclists who use their bikes on rough, uneven terrain. They are not used by the professionals in the Tour de France, when they make their way up and down the steep mountain stages of that most gruelling of races. The mountain bike is a triumph of marketing in which style and appearance are emphasised over any marked improvement in function.

What has all this to do with hip joints? The early attempts at designing artificial hips were mostly disastrous, but in the Sixties the surgeon and engineer Sir John Charnley produced a tough, reliable metal and plastic joint which has stood the test of time. More than one-fifth of the hip replacement operations done in Britain use Charnley joints. Follow-up studies have shown that between 85 per cent and 90 per cent of Charnley joints are still functioning well after 20 years (though many patients fitted with artificial hips die from other causes well before 20 years have passed).

But the manufacturers of hip joints, like those of bicycles, want to sell new, more expensive designs. A survey by surgeons at the Nuffield Orthopaedic Centre in Oxford (current Journal of Bone and Joint Surgery, 1995, 77B, pp520-7) found that 62 different types of artificial hip joint are on the market in Britain at prices ranging from pounds 250-pounds 2,500. Half of these joints have come on to the market in the past five years. Fewer than one-third of the joints available have been assessed in clinical studies published in research journals which use independent reviewers. The verdict of the Oxford surgeons is that "there is little or no scientific evidence that the newer, more expensive implants are better than the old designs. Some will undoubtedly be worse."

Why are surgeons so easily seduced into trying new designs when the old Charnley model has such a good record? Part of the answer lies in the psychological make-up of surgeons: they want to innovate, come up with minor modifications to the design, do better than they did 10 years ago. But even with the best designs put in by the best surgeons, some joints do fail - they work loose or become infected. Surgeons who spend a lot of time dealing with the small proportion of failures will be tempted to believe that even a good design may be improvable. Certainly, changes in surgical technique have contributed to a progressive improvement in short-term results during the past few years.

As in so much of modern medicine, the way forward must lie with more, better, longer clinical trials. The Oxford surgeons suggest that colleagues who want to try out one of the newer designs should be allowed to do so only if the operations are part of a formal research study. At present, data on the results five years after operation are available for only eight of the 62 hip joints on the market. Results 10 years after surgery are available for only three models. Only for the Charnley design are there data covering 20 years.

Yet many people having hip joints replaced will be hoping to live 10 years, if not longer. The Oxford report suggests that those most closely concerned - the patients and the NHS purchasers who have to pay for the joints - should ask why the surgeon has chosen one design rather than another, and what evidence there is about the long-term results. After all, once the operation is done, no one can see whether the joint is a fashionable new model or a boring, reliable old one.