Hydrostatic and boundary lubrication of joints--nature of boundary lubricant.

A very low coefficient of friction in joints makes it difficult to define clearly the mechanism of cartilage lubrication. The present paper describes the two currently predominant and mutually complementary views aiming to elucidate this mechanism. The first mechanism, referred to as hydrostatic lubrication, involves interstitial fluid pressurization from the cartilage and its importance for the formation of a layer separating the weight-bearing surfaces. The second mechanism, called boundary lubrication, assumes the existence of a substance that binds to the cartilage surface, permanently separating the friction elements. It has not been clearly determined which substances occurring in the synovial fluid function as boundary lubricants. The authors briefly describe the physicochemical properties of lubricin, surface-active phospholipids and hyaluronic acid, including their role in boundary lubrication.