Contrary to popular medical opinion and experience, carrying extra body weight may increase someone's likelihood of having a hip- or knee-joint replaced, but it's unlikely to affect how fast an artificial joint wears out, a study by the University of Utah has found.
It's a "totally unexpected finding," said Dr. Kurt Hegmann, associate professor of family and preventive medicine and director of the Rocky Mountain Center for Occupational and Environmental Health, who led the study published in the November issue of American Journal of Preventive Medicine.
"We always knew and there's a lot of research saying that obesity causes arthritis in the knees and the need to replace them," he said. "Studies on hips so far have not been consistent to our 100 percent satisfaction."
The U. researchers wanted to resolve that issue, so they embarked on a large study of weight as a factor in hip replacement, carefully adjusting out age as a factor and the often inherent osteoarthritis in the elderly, regardless of their weight.
They found that "sure enough, weight is a pretty strong risk factor" for needing hip replacement. They decided to take the research a step further to see if weight causes the artificial joint to wear out faster, as well, leading to what's called revision surgery, where the artificial joint must be replaced.
"I don't know a single doctor who doesn't think that's true," said Hegmann. "But that's not what the study showed. We were totally surprised."
They studied the cases of 840 hip- and 911 knee-joint replacement surgery patients who had operations at LDS Hospital from 1992-2000. The patient ages ranged from 55-74. As a control group, they used 5,578 healthy Utahns who enrolled in the National Cancer Institute's Prostate, Lung, Colorectal and Ovarian Cancer screening trial from 1996-2000. It's an ongoing study at the U.
Osteoarthritis — degenerative joint disease — is a common form of arthritis that often results in the need to replace either hip or knee joints. And obesity is a risk factor for developing osteoarthritis, but everywhere, not just in the hips and knees, which are weight-bearing joints. Obesity increases the risk of osteoarthritis in every joint, including fingers, toes, wrists, "so we know there's something systemic going on," Hegmann said.
Because arthritis is one of the leading causes of disability, he said it's important to understand what causes it so focus can be placed on prevention. Right now, researchers "have not the foggiest notion" of the causes of joint degeneration, though it's clear excess weight plays a role.
The study findings have a very practical prevention application right now, even before all the mechanisms of osteoarthritis are understood, Hegmann said. "Someone with mild hip arthritis really needs to lose weight, otherwise there'll be the risk of needing hip replacement."
When researchers looked at other types of surgeries, such as partial hip replacement, they did not find a link to obesity, he said.
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