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 US 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.

“Future research should develop weight loss/maintenance interventions particularly for younger patients who have lost a substantial amount of weight prior to surgery, as they are most at risk for substantial post surgical weight gain."

Riddle's group used a patient registry from the Mayo Clinic in Rochester, Minnesota, which collected information on 917 knee replacement patients before and after their procedures.

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

“After knee replacement we get them stronger and moving better, but they don't seem to take advantage of the functional gains,” said Joseph Zeni, a physical therapy professor at the University of Delaware, who was not part of the study.

“I think that has to do with the fact that we don't address the behavioral modifications that have happened during the course of arthritis before the surgery.”

Riddle's team explained that this degree of weight gain can lead to “meaningful effects on cardiovascular and diabetes related risk as well as pain and function.”

Part of the explanation for the weight gain could be the age at which patients get surgery. People in their 50s and 60s tend to gain weight anyway. Still, in light of the lower rates of weight gain in the comparison group, which was also middle aged and older, Riddle said something else may also be at work.

Indeed, the team found that patients who had lost weight before their surgery were slightly more likely to gain weight afterwards - perhaps because when people lose weight in anticipation of an event, such as surgery, they are more likely to put it back on after they're achieved the goal. Zeni said that to help people stave off weight gain after surgey, health care providers need to address the sedentary lifestyle people often adopt to accommodate their arthritis.

“We need to encourage patients to take advantage of their ability to function better and get them to take on a more active lifestyle,” he said.