Knee replacement operations have more than doubled in a decade

But the figures also show that those who are well off are more likely to have the surgery in the first place, irrespective of need.

The findings are based on an analysis of hospital treatment statistics in England between 1991 and 2000. The number of first time hip replacements increased by 18% during this period, while the need to revise the procedure more than doubled.

But first time knee replacements more than doubled, while the numbers of revision surgery rose by 300%, accounting for one in five of the total number of knee replacement operations, and one in five of those done in the National Health Service.

Rates of primary surgery were higher among women and among those aged between 65 to 79, but a substantial increase also occurred among the over 80s.

The authors point out that as techniques improve, and the population ages, very elderly people may be considered suitable for surgery. But health inequalities in access to the surgery were common, the figures showed. The most economically deprived 20% of the population had significantly lower rates of surgery than their wealthier peers - about 20% fewer operations per 1000 people.

It has been estimated that one in five joint replacement operations is carried out privately, say the authors, suggesting that the gap between the rich and poor may actually be greater.

The increases in revision surgery could be explained by a trend to operate earlier in life, leading to earlier wear and tear, the authors speculate. Alternatively, as has been suggested in some quarters, British surgeons enthusiastically adopted new surgical techniques before these had been tested in the long term.

Based on current treads, the authors conclude that knee replacements will overtake hips as the primary joint replacement in the UK, rising by a further projected 63% by 2010, while hip replacements increase by 22%. This has already happened in the US and Australia, they say.

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