Problems and paradigms in joint pathology.

This short review outlines aspects of joints relevant to current problems in articular, connective tissue disease and describes the pathology of rheumatoid arthritis and osteoarthrosis. The synovial joints display greatly varying degrees of anatomical specialisation. There is also heterogeneity of microscopic structure, to illustrate which the synovial components of the sacroiliac joints are considered. The chondron is regarded as a functional unit of hyaline articular cartilage but the responses of this tissue in disease are strongly influenced by its avascularity and by the need for chondrocytes to communicate with each other and with their local and systemic environments. Hyaline cartilage is capable of molecular replacement or substitution but not of repair by regeneration; it can, however, be replaced by fibrocartilage. The bearing surfaces of hyaline articular cartilage are never planar or smooth. Rheumatoid arthritis is a paradigm of connective tissue disease. It is not only a systemic disorder which may abbreviate life but, characteristically, is an aseptic form of symmetric polyarthritis. The inheritance of HLA-DR beta 1 and of female sex predispose to rheumatoid arthritis but the cause is unknown; it may be viral. Central to the disease is destruction of articular cartilage by sustained inflammation in which activated macrophages and TH cells, possibly of restricted clonality, combine to release cytokines, proteinases and the mediators of inflammation. Osteoarthrosis is a synovial joint syndrome, not a single disease. It is characterised by a loss of and change in the composition of cartilage proteoglycans leading to failure of normal responses to stress. The results include cartilage fibrillation and loss, bone exposure and a clinical syndrome of pain and disability. Rare forms of heritable chondrodysplasia lead to premature osteoarthrosis but, in most instances, the cause of osteoarthrosis appears to be either excess, inappropriate or insufficient mechanical demand, or traumatic, infective, inflammatory, endocrine or metabolic disease. There remain idiopathic ('primary') cases in which no cause is demonstrable.