Many people who have joint replacement surgery to fix worn out knees gain significant amounts of weight after their procedures, according to a surprising study.

It has long been assumed that joint replacements help people waylaid by pain to be more active, increasing the odds of weight loss.

But researchers at the University of Delaware followed more than 100 knee replacement patients for two years and found that a majority gained an average of 14 more pounds than a control group who did not have surgery.  

That’s even more than the 10 pounds most people gain when they quit smoking.

“Typically patients with end stage arthritis will say, 'Once I get my knee replaced the pain will go away and I’ll be able to exercise and I will lose weight.' So we were surprised there was any weight gain after surgery at all,” says Joseph Zeni, Jr., PhD, a physical therapist and research assistant professor at University of Delaware in Newark.

For the study, researchers followed 106 adults with severe, painful knee osteoarthritis (OA) and 31 people without any knee pain.

Participants with osteoarthritis underwent knee replacement surgery and received outpatient physical therapy. 

Two years later, two-thirds of the knee replacement patients had gained weight, regardless of their education level, marital status, income level or activity level prior to surgery. The control group did not experience any weight gain over that two-year period, suggesting that the extra pounds can’t be attributed to aging. The study was published in the journal Osteoarthritis and Cartilage.

And it doesn't appear to be an isolated result. 

One 2006 study, for example, published in the journal Surgeon, found that a majority of people who were not obese before surgery had gained an average of 10 pounds three years after their joint replacement, and people who were obese before their joints were replaced gained even more – an average of 14 pounds in three years.

Weight gain is particularly dangerous for joint replacement patients because higher body mass has been identified as a risk factor for loosening of joint components, which can lead to the need for a revision surgery. 

“People who had arthritis were much heavier to begin with and after surgery stayed heavier and continued to gain weight,” Zeni says.

Researchers did not track what participants were eating. But they did determine that functional ability improved, even though weight loss did not follow.

“We were surprised. Especially with the magnitude and really the percentage of the population that ends up gaining – which is most of them. It’s surprising,” says Lynn Snyder-Mackler, PhD, who co-authored the study.

She says one explanation for the findings may be the way doctors currently recommend knee replacements.

“The way total knee is approached right now is to tell patients to wait until you can’t stand it anymore to have your knee replaced and many times this means people are very sedentary for a very long time,” Snyder-Mackler says. “The curve to get back on the wagon and start exercising again takes a lot of time.”

She thinks it may be that patients stop exercising because of knee pain and then never get back into the habit after surgery.

Researchers say weight gain after knee surgery needs to be treated as an independent issue that requires nutrition and weight management education and patients need to be told that treating knee pain won’t automatically lead to weight loss. 

“Surgeons think pain is the main problem and once it’s done, the knee doesn’t hurt so it’s fixed. But in fact there are lots of other things still wrong after that, largely as a consequence of having the disease for so long,” Snyder-Mackler says.

Dr. Lawrence Cheskin is the director of the Johns Hopkins Weight Management Center at the Bloomberg School of Public Health in Baltimore. He says this research would have even more of an impact if the scientists had looked at a different comparison group.

“Compare it to people who didn’t get their knee fixed,” Dr. Cheskin says. “Instead they’re comparing it to a group who is healthy and doesn’t have knee problems. Those groups are different because it may be there’s something about people with arthritis that they are more prone to gain weight whether their arthritis is treated with knee surgery or not.”

He does like that this study makes it clear that the medical community can’t expect that knee surgery will be the cure-all for weight problems among this group.

“It’s surprising that you have this fairly substantial weight gain. Fourteen pounds over two years sounds like too much. The average American adult gains a pound or two a year in middle age,” Dr. Cheskin says. “So this is clearly too much and it doesn’t make sense because you would think if your knee is better you’d be more active. But perhaps it indicates that these are people who won’t be doing exercise even if their knee doesn’t hurt.”