Role of hyaluronic acid in joint lubrication.

Hyaluronic acid is the macromolecule which endows synovial fluid with its viscoelastic properties and it is often assumed these properties are important for the lubrication of the tissue surfaces in diarthrodial joints (Barnett, Davies, and MacConaill, 1961). The relationships, if any, that exist between the chemical and physical properties ofsynovial fluid and its lubricating ability, however, have not been determined. Recent in vitro studies concerned with the types of molecules present in synovial fluid which are responsible for the lubrication of the joint tissues have indicated that hyaluronic acid is a good lubricant for the synovial membrane (Radin, Paul, Swann, and Schottstaedt, 1971), but showed that this constituent was not essential for the lubrication of the articular cartilage (Radin, Swann, and Weisser, 1970). After the fractionation of synovial fluid by sedimentation equilibrium centrifugation in a cesium chloride density gradient, it was shown that the articular cartilage lubricating moiety was present in the protein fraction. More recent experiments have confirmed these data and shown that the lubricating moiety is complex and contains peptide and glycopeptide constituents (Swann and Radin, 1972). It thus appears from these in vitro studies that the different types of macromolecular constituents in synovial fluid can function independently in the lubrication of the different types of tissues in the joint. However, these constituents in vivo are intimately associated and the articular lubricating moiety interacts with hyaluronic acid and is retained as a component of the ultrafiltrate residue after the ultrafiltration of the synovial fluid (Swann and Radin, 1972). For this reason it is important to find out if hyaluronic acid has any effect on the lubricating ability of the protein fraction in an articular cartilage system. A well known fact is that the concentration and intrinsic viscosity of hyaluronic acid are both lower in synovial fluid from patients with rheumatoid arthritis (Balazs, Watson, Duff, and Roseman, 1967), but it is not clear whether these changes modify the lubricating properties of the fluid. A characteristic feature of rheumatoid disease is morning stiffness, and a suggestion was made recently that this is caused by a failure in periarticular soft tissue lubrication (Radin and others, 1971). It is also important, therefore, to determine whether the changes in the structure of hyaluronic acid and the composition of synovial fluid in patients with rheumatoid arthritis alters its ability to lubricate synovial tissue. The present experiments were performed in an attempt to answer some of these questions.