Separate and combined influence of body mass index and waist circumference on arthritis and knee osteoarthritis.

OBJECTIVE: To determine if the combination of body mass index (BMI) and waist circumference (WC) explains an additional component of arthritis and knee osteoarthritis risk than is accounted for by either anthropometric measure alone.
DESIGN: Cross-sectional study. PARTICIPANTS: Participants were part of the Third National Health and Nutrition Examination Survey, a representative sample of Americans conducted from 1988 to 1994. The arthritis analyses included 15 570 participants aged 18 years and older. The knee osteoarthritis analyses included 2323 participants aged 60 years and older. Body mass index and WC were measured in all participants and classified into sex-specific tertiles. OUTCOME MEASURES: Arthritis (determined by self-report) and knee osteoarthritis (determined by radiograph).
RESULTS: Independent of sex, the likelihood of arthritis and knee osteoarthritis increased in a graded fashion when moving from the lowest to highest BMI tertile and when moving from the lowest to highest WC tertile (Ptrend < 0.01). In the next phase of analyses, subjects were divided into WC tertiles within each of the BMI tertiles. Within the lowest BMI tertile, the likelihood of arthritis and knee osteoarthritis was not different in the lowest, middle and highest WC tertiles (P > 0.1). Within the middle BMI tertile, the middle and high WC tertiles were more likely to have knee osteoarthritis compared to the lowest WC tertile (P < 0.05). Within the highest BMI tertile, the highest WC tertile was more likely to have arthritis and knee osteoarthritis compared to the lowest WC tertile (P < 0.05).
CONCLUSIONS: Both BMI and WC were strong predictors of arthritis and knee osteoarthritis. When a categorical approach was used for BMI and WC, similar to that in the clinical setting, independent effects of these anthropometric variables were observed.