NEW YORK (Reuters Health) - 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.
Analyzing the medical records of nearly 1,000 knee-replacement surgery patients, researchers found that 30 percent of them gained five percent 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.
"After knee replacement we get them stronger and moving better, but they don't seem to take advantage of the functional gains" and become more active, 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," he added.
Hundreds of thousands of people in the U.S. undergo knee replacement surgery each year.
The goal of putting in a new knee is to alleviate pain and get people moving around more, but Daniel Riddle, lead author of the new study and a professor at Virginia Commonwealth University, said his team had noticed that patients tended to gain weight after surgery.
To see whether this was common, Riddle's group used a patient registry from the Mayo Clinic in Rochester, MN, which collected information on 917 knee replacement patients before and after their procedures.
The researchers found that five years after surgery, 30 percent of patients had gained at least five percent of their weight at the time of the surgery. That's 10 pounds or more on a 200-pound person, for example.
In contrast, less than 20 percent of those in a comparison group of similar people who had not had surgery gained equivalent amounts of weight in the same period.
Riddle's team explains in their report, published in Arthritis Care & Research, 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 the researchers observed could be the age at which patients get surgery, Riddle said. 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 among knee surgery patients.
"There's something going on in these patients that predisposes them above and beyond their peers to weight gain," Riddle told Reuters Health.
Indeed, the team also found that patients who had lost weight before their surgery were slightly more likely to gain weight afterward.
Riddle said that could be because when people lose weight in anticipation of an event, such as knee surgery, they are more likely to put on weight after they've achieved that goal.
Zeni said that to help people stave off the pounds after surgery, 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 told Reuters Health.
Riddle agreed that the habit of being sedentary before knee replacement may carry over after surgery.
He said it will be important to develop and study weight loss interventions for these patients, and to target them to those who are most at risk of getting heavier, like relatively younger patients in their 50s and 60s and those who have lost weight before knee surgery.
In the meantime, Riddle said, patients can also take some action on their own by talking with their doctors about maintaining a healthy weight and consulting with a nutritionist and physical therapist about lifestyle changes.