Background Obesity is a risk factor for osteoarthritis (OA) in weight-bearing and non-weight-bearing joints. Both increased mechanical stress and systemic processes seem to be of importance, although it is unclear which mechanisms play a role in certain joints.
Objectives To gain more insight into the relative contribution of mechanical stress and systemic processes to OA of weight-bearing and non-weight-bearing joints, we examined the association of surrogates for both mechanisms with OA of knees, hands or both.
Methods The Netherlands Epidemiology of Obesity (NEO) study is a population-based cohort including 6673 lean, overweight and obese participants aged 45-65 years. Weight (kg) and fat mass (kg) were measured, fat free mass (FFM) (kg) was calculated. The metabolic syndrome (MetS) was defined following the ATPIII criteria. Knee and hand OA were defined according to the ACR clinical criteria; based on physical examination, pain and stiffness (measured by questionnaire).
Odds ratios (OR) with 95% confidence intervals (CI) were calculated to associate surrogates for mechanical stress (weight, FFM) and systemic processes (MetS) with OA in knees alone, both knees and hands or hands alone, using individuals without knee or hand OA as reference. Analyses were adjusted for age, sex, height, smoking, education and ethnicity, and either metabolic factors or weight.
Finally, adjusted ORs were calculated for each OA type in three weight categories (<75kg, 75-90kg, >90kg), stratified by MetS. The lowest weight category without MetS served as reference.
Results After exclusion of participants with missing data (n=45), data from 6628 participants were analyzed (median (IQR) age 56 years (50-61), BMI 26 kg/m2 (23-28), 56% women). The estimated population prevalence of knee, both knee and hand, and hand OA were 10%, 4% and 8%, respectively. After adjustment for metabolic factors, knee OA was associated with weight (OR 1.49 (95%CI 1.32,1.68) and FFM (2.05 (1.60,2.62). Similar results were observed for OA in both knees and hands. Neither knee OA nor OA in knees and hands were associated with MetS after adjustment for weight.
In hand OA the opposite was observed; whereas no associations with weight and FFM were observed after adjustment for metabolic factors, hand OA was associated with MetS, independent of weight (OR 1.46 (1.06,2.02)).
The figure illustrates the relative contribution of weight and MetS to OA of the knees (a) and hands (b). The adjusted ORs for knee OA were higher in higher categories of weight than the lowest weight category. The adjusted OR of the highest weight category in individuals without MetS was 2.62 (1.77,3.88) (figure). The adjusted OR of highest versus lowest weight category in individuals with MetS was 2.30 (1.29,4.12). The presence of MetS, adjusted for the weight categories, did not result in a higher OR for knee OA (1.16 (0.91,1.47)) (figure). The same was observed in relation to OA in both knees and hands.
In hand OA on the contrary, the ORs did not increase with weight. MetS on the other hand was associated with hand OA, adjusted for the weight categories; OR 1.52 (1.10,2.09) (figure).
Conclusions In knee OA, whether or not in co-occurrence with hand OA, mechanical stress seems the most important underlying mechanism, whereas in hand OA alone, systemic processes might contribute most.
Disclosure of Interest None declared