Sept. 5, 2008 -- Nearly half of Americans are likely to develop arthritis in at least one knee by age 85. For people who are obese, the risk is greater.

Researchers from the CDC and the University of North Carolina studied data on 3,068 people in Johnston County, N.C. Participants, all 45 years old or older, were evaluated twice over an average follow-up of six years. Participants received physical exams, X-rays, and also were interviewed.

Researchers were able to use the data to estimate risk of developing symptomatic knee osteoarthritis (OA) by their mid-80s. OA is the most common form of arthritis.

At the start of the study, the average age of participants in the study was 61 with 15% having symptomatic knee OA. About six in 10 of the participants were overweight or obese. Though the study focused on one small geographic area, the researchers say the data are applicable to Americans as a whole.

Researchers found that the lifetime risk of having symptomatic knee osteoarthritis was 44.7%. Sex, education level, and race didn't seem to affect a person's chances of developing arthritis. The two factors that do have an effect: whether or not someone has a past knee injury and whether or not the person is overweight or obese.

Participants with a history of knee injury have a 56.8 % risk, compared with a 42.3 % risk for those without past knee injury. People of normal weight have a risk of 30.2% of developing knee osteoarthritis with symptoms, compared with 46.9% for overweight participants and 60.5% for obese participants.

The researchers, who write that arthritis is a leading cause of disability in the U.S., call for more focus on weight management by both individuals and their doctors.

"Simply put, people who keep their weight within the normal range are much less likely to develop symptomatic knee osteoarthritis as they get older and thus much less likely to face the need for major surgical procedures, such as knee replacement surgery," says Joanne Jordan, MD, in a news release. Jordan is principal investigator of the Johnston County Osteoarthritis Project and a senior study researcher. She is also a professor of medicine and orthopaedics in the School of Medicine at the University of North Carolina.