Role of inflammatory factors and adipose tissue in pathogenesis of rheumatoid arthritis and osteoarthritis. Part I: Rheumatoid adipose tissue.

For many years, it was thought that synovial cells and chondrocytes are the only sources of proinflammatory cytokines and growth factors found in the synovial fluid in patients suffering from osteoarthritis and rheumatoid arthritis. Currently, it is more and more frequently indicated that adipose tissue plays a significant role in the pathogenesis of these diseases as well as that a range of pathological processes that take place in the adipose tissue, synovial membrane and cartilage are interconnected. The adipose tissue is considered a specialized form of the connective tissue containing various types of cells which produce numerous biologically active factors. The latest studies reveal that, similarly to the synovial membrane, articular adipose tissue may take part in the local inflammatory response and affect the metabolism of the cartilage and subchondral osseous tissue. In in vitro conditions, the explants of this tissue obtained from patients suffering from osteoarthritis and rheumatoid arthritis produce similar pro- and anti-inflammatory cytokines to the explants of the synovial membrane. At this stage already, knowledge translates into imaging diagnostics. In radiological images, the shadowing of the periarticular soft tissues may not only reflect synovial membrane pathologies or joint effusion, but may also suggest inflammatory edema of the adipose tissue. On ultrasound examinations, abnormal presentation of the adipose tissue, i.e. increased echogenicity and hyperemia, may indicate its inflammation. Such images have frequently been obtained during ultrasound scanning and have been interpreted as inflammation, edema, hypertrophy or fibrosis of the adipose tissue. At present, when the knowledge concerning pathogenic mechanisms is taken into account, abnormal echogenicity and hyperemia of the adipose tissue may be considered as a proof of its inflammation. In the authors' own practice, the inflammation of the adipose tissue usually accompanies synovitis. However, we also diagnose cases in which the inflammatory process in the joint is no longer active, but abnormal vascularity still persists in the adipose tissue. There are also cases where abnormal adipose tissue is the only sign of inflammation. Therefore, ultrasound examination confirms the existence of the additional site of inflammation, i.e. the adipose tissue which should be evaluated at the stage of initial diagnosis and during follow-up, also in terms of remission.