Knees bearing the weight of obesity

(CNN) - Obesity isn’t just about waistlines or clothing sizes. The toll of carrying all that excess weight affects your joints, and your knees can take a pounding.

Four studies presented at the 2011 Annual Meeting of the American Academy of Orthopaedic Surgeons, examined how the heavier Americans have gotten, the more stress they’ve put on their knees.

A Harvard Medical School research team found that the number of total knee replacement surgeries has doubled from 1997 to 2007.

During that same period, the proportion of obese people increased by about 15 percent. This could be a factor why more people are getting their knees replaced, the study speculates.

But having such surgeries isn't easy for obese people.

They experience more complications after hip and knee replacement surgeries, according to another study.

Researchers compared 12,355 normal weight, overweight and obese patients. The number of complications such as readmissions, leg swelling, bacterial infections, respiratory disorders, neurologic and gastrointestinal problems, were higher for the obese.

One way to relieve those risks of complications is weight loss.

In a study from Emory University, a group of 10 morbidly obese patients had bariatric surgery. A year later, their knee function improved and pain decreased. It didn’t completely clear their knee osteoarthritis - there is “probably some element of irreparable damage from being morbidly obese,” according to the researchers.

Those findings were echoed by the Penn State College of Medicine. After bariatric surgery, patients reported improvements in knee pain, stiffness and function.