**3 Anatomy of a joint**

A brief review of the basic anatomy of a typical synovial joint is presented here to help understand the mechanisms involved in OA-induced damages of the involved joint that culminate in pain and other symptoms of OA.

Fig. 1. A typical synovial joint

A joint is where two bones meet. Articular cartilage covers the bone ends which are lubricated by synovial fluid. Seventy to eighty per cent of the cartilage is made up of water and a type II collagen with proteoglycans and glycosaminoglycans produced by chrondrocytes. The collagen fibres in the cartilage offer tensile strength to the cartilage because of its architectural makeup. The cartilage, however, contains no intrinsic blood vessels. It receives its nutrition from the synovial fluid. The synovial fluid, which is secreted by the synovial membrane lining the inner surface of the joint, facilitates not only movement but also provides nutrients, phagocytosis and other immunologic functions within the joint. The integrity of a joint is therefore dependent upon its architecture, the cartilage, bone and the supporting structures enclosing the joint. OA in simple terms is a result of alterations in the aforementioned architectural structures within the joint with resultant pain, loss of function and instability in the involved joint. Figure 1 shows the diagram of a typical synovial joint.
