Synovial infiltration by inflammatory cells such as macrophages, mast cells or lymphocytes occurs in around 50% of the patients. In MRI studies, synovitis shown with or without contrast agent correlates with pain and also radiological progression of OA. The inflammatory status of the joint is also reflected by increased pro-inflammatory cytokine levels such as TNF-alpha or IL-l. This is likely a result of the inflammatory cell infiltration in the joint and mast cell activation. Interestingly, recent findings indicate that mast cells, but also subchondral osteoblasts express mechanoreceptors which change their expression pattern upon mechanical stimulation, for example, by the expression of IL-8. But also synovial fibroblasts and even chondrocytes produce more proinflammatory cytokines and partcipate in inflammation.