FRIDAY, Sept. 5 (HealthDay News) -- Almost half of all American adults will develop osteoarthritis of the knee by age 85, and their odds increase if they are obese in middle age, a new study says.

A person's risk of having the painful condition increased as his or her body-mass index (BMI) rose, according to the University of North Carolina at Chapel Hill study. People who were of normal weight at age 18 but were overweight or obese by 45 or older had the greatest risk.

"These results show how important weight management is for people throughout their lives," senior study author Joanne Jordan, principal investigator of the Johnston County Osteoarthritis Project, said in a university news release. "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."

The study, published in the Sept. 15 issue of Arthritis Care & Research, was based on analyzing data collected from more than 3,000 North Carolinians over a 13-year period. Participants were interviewed and given a clinical exam, including knee X-rays and BMI measurements.

Obese people had a significantly higher lifetime risk, 64.5 percent versus 34.9 percent for normal weight and 44.1 percent for overweight participants. Those with prior knee injuries in their lifetime also had a higher risk than those without (56.8 percent vs. 42.3 percent).

No notable risk differences were found based on a participant's sex, race or education level.

More information

The Arthritis Foundation has more about osteoarthritis.