A new study finds that knee replacement surgery may raise a person's risk of gaining weight, says a Reuters News  article based on a study published in Arthritis Care & Research.   

For this study, lead investigator Daniel Riddle, PT, PhD, FAPTA, and his 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 5 years after surgery, 30% of patients had gained at least 5% of their weight at the time of the surgery.

In contrast, fewer than 20% 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 said that this degree of weight gain can lead to "meaningful effects on cardiovascular and diabetes-related risk as well as pain and function."

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, reports Reuters.

"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, PT, PhD, 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."

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.

In fact, 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, says the article.