Study objective: To determine whether obesity preceded knee osteoarthritis and was thus a possible cause.
Design: Cohort study with weight and other important variables measured in 1948 to 1951 (mean age of subjects, 37 years) and knee arthritis evaluated in 1983 to 1985 (mean age of subjects, 73 years).
Setting: Population-based participants; a subset (n = 1420) of the Framingham Heart Study cohort.
Methods: For those subjects in the Framingham Study having knee radiographs taken as part of the 18th biennial examination (1983 to 1985), we examined Metropolitan Relative Weight, a measure of weight adjusted for height at the onset of the study (1948 to 1951). Relative risks were computed as the cumulative incidence rate of radiographic knee osteoarthritis in the heaviest weight groups at examination 1 divided by the cumulative rate in the lightest 60% weight groups at examination 1. Relative risks were adjusted for age, physical activity level, and uric acid level.
Results: In 1983 to 1985, 468 subjects (33%) had radiographic knee osteoarthritis. For men, the risk of knee osteoarthritis was increased in those in the heaviest quintile of weight at examination 1 compared with those in the lightest three quintiles (age-adjusted relative risk, 1.51; 95% confidence interval [CI], 1.14 to 1.98); risk was not increased for those in the second heaviest quintile (relative risk, 1.0). The association between weight and knee osteoarthritis was stronger in women than in men; for women in the most overweight quintile at examination 1, relative risk was 2.07 (95% CI, 1.67 to 2.55), and for those in the second heaviest group, relative risk was 1.44 (95% CI, 1.11 to 1.86). This link between obesity and subsequent osteoarthritis persisted after controlling for serum uric acid level and physical activity level, and was strongest for persons with severest radiographic disease. Obesity at examination 1 was associated with the risk of developing both symptomatic and asymptomatic osteoarthritis.
Conclusions: These results and other corroborative cross-sectional data show that obesity or as yet unknown factors associated with obesity cause knee osteoarthritis.