Inflammatory features of infrapatellar fat pad in rheumatoid arthritis versus osteoarthritis reveal mostly qualitative differences

Rheumatoid arthritis (RA) and osteoarthritis (OA) are characterised by joint destruction. In both diseases, inflammation is implicated; however, RA is generally associated with more inflammation in synovium1 and synovial fluid (SF)2–4 compared with OA. The infrapatellar fat pad (IFP), an adipose tissue located in the knee joint, has been proposed to contribute to disease progression in OA. Although IFP of OA has been extensively characterised and identified as a source of inflammation in the joint,5–7 only scarce information is available on IFP in patients with RA. Therefore, our aim was to get a first insight into the possible contribution of IFP to the inflammatory processes in the RA joint. To this end, we compared leftover IFP and synovium from patients with RA (n=20: 80% women, mean (SD) age 64 (10.7) years, median (range) body mass index (BMI) 26.9 (18) kg/m2) and patients with primary knee OA (n=51: 62% women, mean (SD) age 66 (8.9) years, median (range) BMI 28.9 (15) kg/m2); all were undergoing knee joint replacement surgery. The study was approved by the local medical ethical committee. Fat-conditioned medium (FCM) and adipocyte-conditioned medium (ACM) were generated as previously described.5 Adipokines and cytokines were measured in FCM …