We're getting heavier, living longer and, as a result, placing greater demands on our knees. Many more people are having knee replacements - 80,000 patients, with an average age of 69, have such surgery each year in England and Wales.
However, the success of surgery is far from guaranteed. Between 10 per cent and 15 per cent of patients are unhappy with the result because of pain and stiffness or because the surgery has failed to restore the full range of movement. And 5 per cent are worse after surgery as a result of complications.
'Knee replacement is a good treatment for arthritis, but it isn't foolproof,' says orthopaedic surgeon Tim Wilton of the Royal Derby Hospital, one of the country's leading knee surgeons. The knee is the most complicated joint in the body.
Five to ten times our body weight goes through the joint with every step. 'A key part of the surgeon's job is separating out those patients who might well benefit from knee replacement from those who almost certainly won't'.
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