Background/Purpose: Obesity, a major risk factor for knee osteoarthritis (OA), is a state of systemic inflammation through elaboration of adipokines (pro and anti-inflammatory cytokines) from adipose tissue. Whether degree of adiposity or systemic level of pro-inflammatory adipokines is associated with presence of local inflammation (synovitis) in knee OA is not known. Thus, in this study, we evaluated the cross-sectional association between total body fat mass and serum leptin (a proinflammatory adipokine) level to the presence of synovitis of the knee on MRI in community-dwelling older adults, with and without knee OA.

Methods: We included participants from the Multicenter Osteoarthritis (MOST) study, which has whole body Dual Energy X-ray (DXA) for assessment of fat mass, serum leptin assay and knee MRI (fat-suppressed (FS) fast spin-echo intermediate-weighted (IW) sequences) for evaluation of synovitis available. Sex-specific tertiles were created for total body fat mass and serum leptin from baseline visit. Person-level synovitis was defined by WORMS score >=1 on knee MRI from the same visit separately at 3 specific sites: 1) Infrapatellar fat pad; 2) Intercondylar; and 3) whole knee Effusion. A sum score of synovitis was calculated by adding the scores at the above sites. To assess the relation of fat mass tertiles and serum leptin tertiles to knee synovitis, we performed logistic regression for the site specific synovitis and linear regression for sum of synovitis score, adjusting for age, sex, education, physical activity, smoking status and body weight (proxy for loading effect). Analyses were repeated, stratified by the presence of radiographic knee OA (Kellegren-Lawrence grade ≥2) status.

Results: Among 2871 subjects, 1015 subjects developed synovitis (162 infrapatellar, 419 intercondylar and 462 effusion). We did not find an association between tertiles of fat mass or serum leptin with site specific synovitis or sum of synovitis scores (Table 1). Results did not change when stratified by radiographic knee OA.

Conclusion: Our results suggest that presence of knee synovitis may not be associated with obesity-related systemic inflammation.  However, as our study is limited by power, larger studies are needed to comprehensively study this relation between obesity-related systemic inflammation and synovitis in knee OA as it may provide insight into the pathogenesis of knee OA.