Want healthy knees? Then you better floss your teeth.

Yes, you read that right. Scientists have found traces of gum bacteria in the knees of people with rheumatoid arthritis and osteoarthritis. The study, published in the Journal of Clinical Rheumatology, adds more evidence of the link between poor oral health and poor health in general.

Researchers from Case Western Reserve University in Cleveland essentially traced the passage of bacteria in the mouth to the fluid surrounded the kneecap, called synovial fluid. By analyzing the DNA of the bacteria, the researchers could determine that the progeny of the gum bacteria entered into the bloodstream and settled in the synovial fluid, which was in a weakened state as a result of arthritis.

"In healthy people, the synovial fluid is more or less sterile," said study leader Nabil Bissada, chairman of the department of periodontics at the Case Western School of Dental Medicine. "Bacteria can make the diseased area much, much worse."

Although Bissada's group could not conclude that the gum bacteria caused or worsened the arthritis in the patients it studied, the finding provides a new wrinkle in the still controversial theory of gum disease's role in a host of ailments. [7 Medical Myths Even Doctors Believe]

Knee and mouth disease?

Advanced gum disease, or periodontitis, has long been associated with heart disease. One example of this link is the accumulation of plaque in the arteries, called atherosclerosis, which may be exacerbated by the same bacteria causing plaque on the teeth. Another example is an inflamed heart valve, irritated by gum bacteria. That's the theory, anyway.

In April this year, however, the American Heart Association (AHA) issued a statement, published in the journal Circulation, downplaying the role that gum disease may play in heart health. The AHA said that periodontitis and cardiovascular disease share common risk factors — smoking, diabetes, and advanced age, for example — but that there is no convincing evidence that one causes the other. Nor is there evidence that treating periodontitis can improve heart health, the AHA concluded.

The lead doctor on the statement was Peter Lockhart of Carolinas Medical Center, in Charlotte, N.C. "If cause and effect is someday proven, it will probably be fairly minor," Lockhart explained on MedlinePlus, a health website sponsored by the U.S. government.

Bissada, however, thinks the link is far from minor; people with periodontitis are twice as likely to have cardiovascular disease, he said. Bissada also said his group has unpublished data showing how periodontitis is more of a risk factor for heart attacks than LDL "bad" cholesterol levels or C-reactive protein levels, a measure of inflammation.

And now his group has data tying gum disease to arthritis.

They laughed at the ulcer guys, too

The Case Western Reserve study on knee arthritis is small, only 36 patients, none with advanced periodontitis. Of these patients, five had gum bacteria in their synovial fluid. Most convincing, though, was the fact that for two of the patients, the bacteria found in the mouth and in the synovial fluid were genetically nearly identical, providing smoking-gun evidence of bacteria relocating.

Piecing the evidence together, Bissada said it is well known that advanced periodontitis can lead to harmful bacteria entering the bloodstream. "Once it gets in, it can go anywhere," he said. The target is often a site of existing inflammation, such as the arteries or, in this case, the knee.

Even for patients without advanced periodontitis, there is a possibility of bacteria migrating to a weakened knee joint, he said.

Bissada recommends that people with osteoarthritis or rheumatoid arthritis be examined for periodontitis and treated accordingly. The AHA does not recommend such measures for heart health. But considering how it took years for scientists Barry Marshall and Robin Warren to convince skeptical colleagues of the crazy idea that ulcers are caused by the bacteria H. pylori, not spicy food and stress, it wouldn't be such a bad idea to start brushing and flossing.

Christopher Wanjek is the author of the books "Bad Medicine" and "Food At Work." His column, Bad Medicine, appears regularly on LiveScience.

Copyright 2012 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.