Add the likelihood of developing arthritic knees to reasons for maintaining a healthy weight.

A CDC study predicts that more than four out of 10 Americans will develop osteoarthritis (OA) in their knees before age 85. Those odds rise with weight gained after adolescence, reaching almost 60% among obese adults.

“These results show how important weight management is for people throughout their lives,” observes senior study author Joanne M. Jordan, MD, MPH, professor of orthopedics at the University of North Carolina School of Medicine in Chapel Hill. “People who keep their weight within the normal range are much less likely to develop symptomatic knee OA as they get older and thus much less likely to face the need for major surgical procedures.”

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Dr. Jordan is the principal investigator of the Johnston County Arthritis Project. The longitudinal study analyzed data collected over 13 years from 3,068 residents of a rural county near Chapel Hill.

All the participants were aged 45 years or older (mean age 61); 42.6% were men, and 18.4% were black. Patients were examined at baseline and asked about their weight at age 18. Follow-up exams were performed an average of six years later.

Researchers found an overall lifetime risk of 44.7% for developing knee OA by age 85. There was no significant difference by sex, race, or education, but obese participants had the highest lifetime risk (60.5%) compared with those who were of normal weight (30.2%) or overweight (46.9%).

When BMI was tracked over their lifetimes, participants who maintained a normal weight at 18 years, baseline, and follow-up had the lowest lifetime risk (29.2%). Those who reported normal weight at age 18 but were overweight or obese at baseline or follow-up had a risk almost twice as high (59.9%). History of knee injury was another significant variable, yielding a risk of 56.8% vs. 42.3% (Arthritis Rheum. 2008; 59:1207-1213).

Researchers believe their analysis can be applied to the general U.S. population because the most significant risk factors—aging and obesity—affect everyone.

The findings send an important message to clinicians, Dr. Jordan states. “They need to include the risk of knee OA in the discussion when counseling patients about weight management, and they need to factor that risk into their treatment plans.”