The goal of putting in a new knee is to alleviate pain and get people moving around more. But that's not what's happening for everyone.
Looking at the medical records of nearly 1,000 patients, researchers found that 30 percent of them gained 5 percent or more of their body weight after surgery.
One possible explanation 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.
