Behaviour of synovial fluid on surfaces of articular cartilage. A scanning electron microscope study.

Much attention has been given to the mechanics of synovial joints from a descriptive aspect and to studies relating to joint lubrication. The synovial fluid has been analysed and details of its composition are known (Ropes and Bauer, 1953). The principal constituent is hyaluronic acid-protein complex, the acid molecule having been shown to be a non-branched chain of repeating units with hydroxyl end-groups on each unit (Meyer, 1931). The acid-protein complex is responsible for the viscous properties of the synovial fluid (Negami, 1964; Davies, 1966) and it is fairly certain that it plays an important function in protecting the cartilage surfaces. A reasonably good picture has been obtained of the structure of articular cartilage, which is understood to consist of bundles of collagen fibres in a ground substance, chondroitin sulphate, with a high water content. The collagen bundles on the surface are believed to lie parallel to it. Those below are randomly or perpendicularly orientated to the surface depending upon the age and condition of the cartilage. (Benninghoff, 1930; Martin, 1953; Little, Pimm, and Trueta, 1958.) The remarkable lubrication characteristics of joints have been found to be due to the combination of properties of synovial fluid and articular cartilage. From experiments using a reciprocating friction machine and by measuring the topography of the surfaces of cartilage with stylus equipment, it has been concluded that the cartilage surfaces are prevented from contacting primarily because of the formation of trapped pools of lubricant (Dowson, Longfield, Walker, and Wright, 1969). Furthermore, it has been shown that, under some conditions,