Infrapatellar fat pad features in osteoarthritis: a histopathological and molecular study

Background and aims: Knee osteoarthritis (KOA) is a common cause of disability and pain in adults. Obesity is increasingly recognized as the primary modifiable risk factor for the onset and progression of KOA. The mechanisms that link the two conditions are still not fully clarified but recent data suggest an important role mediated by the infrapatellar fat pad (IFP). Accordingly, we aimed to determine the histomorphological characteristics of IFP in individuals affected by KOA and compare them to those obtained from lean healthy individuals using a histological scoring system. Moreover, we determined the expression of certain adipocytokines in IFP of KOA subjects. Materials and Methods: We enrolled 28 subjects (BMI 35.5±5 kg/m2) undergoing total knee replacement for OA. The controls were represented by IFP and adjacent synovial membrane specimens sampled from bodies or bodies part of 8 healthy subjects without history of osteoarthritis involved in the Body Donation Program ‘Donation to Science’ held by the University of Padova. The microscopic anatomy of IFP was analyzed through histological and morphometrical methods. The histology of the synovial membrane adjacent to the IFP was also analyzed. We determined mRNA expression using Quantitative real time PCR for adipokines (leptin, adiponectin, Peroxisome proliferator-activated receptor gamm, fatty acid binding protein4) and cytokines (Interleukin 6 [IL-6], Tumor necrosis factor alfa[TNF-α], Monocyte chemoattractant protein-1[MCP-1], Vascular endothelial growth factor [VEGF]) in the IFP specimens of KOA subjects. Results: All the evaluated IFPs showed microscopical characteristics similar to white adipose tissue. IFPs were organized in adipose lobuli separated by fibrous septa. No differences were detected in the mean diameter of the adipose lobuli of the IFP in KOA patients and controls. Mononuclear infiltration was present in 22 KOA patients while it was not observed in any of the IFP used as control (p = 0.001). The average number of vessels and the thickness of interlobular septa were significantly higher in KOA patients compared to controls (p < 0.0001 and p = 0.004 respectively). BMI correlated positively with the thickness of the interlobular septa in IFP (p= 0.02, r= 0.42). Concerning synovial membrane, the presence of lymphocytic infiltration and hyperplasia was statistically higher in KOA compared to controls (p