By Amy Norton
NEW YORK (Reuters Health) - Older Americans are increasingly complaining of aching knees, and getting those knees replaced, even though X-ray evidence of knee arthritis is not on the rise, a new study finds.
Since the 1970s and 1980s, the percentage of older Americans reporting chronic knee pain has risen as much as three-fold -- both among people with visible signs of arthritic joints and those without any apparent arthritis.
Despite stable rates of knee arthritis, knee replacement surgeries rose almost nine-fold in 26 years, researchers report in the Annals of Internal Medicine.
The nation's growing obesity problem and an aging population seemed to partly account for the increase in knee pain, the study found. But that did not tell the whole story.
"We don't really know what the other reasons could be," said lead researcher Uyen-Sa D.T. Nguyen, of Boston University School of Medicine.
One possibility, she told Reuters Health, is that older adults these days are more likely to admit to having pain.
"I suspect that what happened in the past is that people often wouldn't complain," agreed senior researcher Dr. David T. Felson, who is also with Boston University.
But it's hard to know for sure, the researchers say.
Regardless, it's possible that the rising rate of knee pain is one factor in the surge in knee replacement surgeries in recent decades, Felson said.
Knee replacement is one treatment option for severe osteoarthritis -- the "wear-and-tear" form of arthritis, in which cartilage cushioning the joints gradually breaks down. That allows bones to rub together, which can cause pain, swelling and limited range of motion. Over time, the joint may become permanently misshapen.
In 1979, the rate of knee replacements among Americans age 65 and older was 10 per 10,000. By 2006, it had risen to 87 per 10,000.
"We're seeing an explosion in knee replacements," Felson said.
"And if knee replacements are to be limited in some way," he added, "it would probably be wise for us to find some way of preventing knee pain."
People with knee pain but no apparent osteoarthritis would not be the ones getting knee replacements, according to Felson.
But the study also found that a growing proportion of older adults with objective osteoarthritis are describing the condition as painful -- and chronic pain is a major reason that people with arthritis opt for a knee replacement, the researchers point out.
The findings are based on data from two long-running health studies. One was an ongoing government health survey that asked Americans ages 60 to 74 about knee pain.
Overall, Nguyen's team found, the percentage who said they'd had knee pain for at least one month rose by two-thirds between 1974 and 1994. And there was a further increase among white women over the following decade.
The second study involved 1,800 adults age 70 and older who were asked about knee pain and had X-ray imaging of their knees.
In 1983-85, eight percent of women and four percent of men with no X-ray evidence of arthritis reported knee pain. But by 2005, those figures were 25 percent and 16.5 percent, respectively.
There was a similar pattern among people who did have arthritis on X-rays. In the 1980s, 26 percent of women and 19 percent of men complained of knee pain. Twenty years later, nearly half of both men and women with arthritis said they were in pain.
When it came to actual X-ray evidence of arthritis in the two study groups, though, there was little change.
Over the same period, that rate was consistently around one-third of men. And the rate even dipped somewhat among women -- from 42 percent to 35 percent.
All of that suggests an increase in knee osteoarthritis is not responsible for the surge in knee replacements, said Dr. Allan Gelber, of Johns Hopkins University School of Medicine in Baltimore.
So what is? "One possibility," Gelber told Reuters Health, "is that Americans are undergoing knee replacements at an earlier age, or at lesser severity grade (of knee arthritis)."
Future studies could look into that question, according to Gelber, who wrote an editorial published with the study.
For middle-aged and older adults with knee pain, Gelber said it's important not to "fall into the reflexive response of, 'I have knee pain, I'm getting older, I must have knee arthritis."
He advised getting a thorough evaluation of knee pain -- which includes talking with your doctor about your symptoms and history, along with any imaging tests.
Gelber also advised fellow doctors in his editorial to remember, "Not all knee pain in middle-aged and older adults is the result of osteoarthritis."
Other possible causes of knee pain include injuries to soft tissue like ligaments and tendons, and other forms of arthritis, like gout. Gelber noted that knee pain can also be part of more "global" chronic pain problems that affect other parts of the body too (such as fibromyalgia).
And those causes, Gelber said, might help explain why knee pain increased in the current study, even though X-ray evidence of osteoarthritis did not.
SOURCE: http://bit.ly/vpU9CN Annals of Internal Medicine, December 6, 2011.