Being overweight is known to increase the risk of needing a knee replacement, but a new study finds that knee replacement surgery may also raise a person's risk of gaining weight, according to a U.S. study.

Researchers, whose findings appeared in the journal Arthritis Care & Research, analyzed the medical records of nearly 1,000 knee-replacement surgery patients, and found that 30 per cent of them gained five per cent or more of their body weight in the five years following surgery.

One possible explanation for the counter-intuitive results, experts said, is that if people have spent years adapting to knee pain by taking it easy, they don't automatically change their habits when the pain is reduced.

"Patients who undergo knee arthroplasty are at increased risk of clinically important weight gain following surgery," wrote study leader Daniel Riddle, a professor at Virginia Commonwealth University.

The researchers found that five years after surgery, 30 per cent of patients had gained at least five per-cent of their weight at the time of the surgery - at least 5 kg or more on a 100kg person, for example.

In contrast, fewer than 20 per cent of those in a comparison group of similar people who had not had surgery gained equivalent amounts of weight in the same period.

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