Normal functional anatomy of joints

The current chapter has a functional anatomical focus on factors directly relevant to the inflammatory and degenerative rheumatic diseases. Given the immense importance of synovial inflammation in the rheumatic disorders we pay particular attention to synovial (diarthrodial) joints. Many subtypes are defined, according to the shape of the articulating bones and/or the type of movement permitted. The most defining characteristic of any synovial joint is the presence of a joint cavity containing fluid secreted by a synovial membrane. This fluid nourishes the articular cartilage. Some synovial joints contain fibrocartilaginous menisci and/or fat pads and others have cartilages lining the surfaces of ligaments or tendons that replace or reinforce the joint capsule. These cartilages form part of ’enthesis organs’ and contribute to the formation of ’synovio-entheseal complexes’. Such synovial membrane-synovial fluid-cartilage functional units are widespread and appear to be of key importance in the pathogenesis of erosion formation in inflammatory arthritis. Typically, they occur at diverse sites including fibrocartilaginous regions of extensor tendons at the interphalangeal joints and where tendons change directions on bony surfaces in the ankle joint. They are collectively termed functional entheses and contribute to a number of pathological processes including tendonitis, periostitis and osteitis.