Roles of adsorbed film and gel layer in hydration lubrication for articular cartilage

To maintain low friction and low wear in natural synovial joints, adsorbed film formation on articular cartilage surface appears to play an important role in mixed or boundary lubrication regime where local direct contact occurs. Furthermore, the proteoglycan gel layer at the uppermost superficial zone in articular cartilage appears to play an important role in preserving low friction and low wear by the hydration lubrication mechanism even after removal of adsorbed film. However, the interaction or synergistic action between adsorbed film and hydrated film/surface has not yet been clarified. To examine the roles of adsorbed film and gel layer on articular cartilage surface in hydration lubrication, the changes in friction were observed in the reciprocating test of articular cartilage against a glass plate, at repeated rubbing including restarting after interrupting-unloading process. It is noticed that at restarting immediately after loading, hydration lubrication is expected to become effective. The lubricating roles of protein-adsorbed film in hydration lubrication are discussed on the basis of experimental results in both cases with and without surface gel layer. The comparison of albumin and gamma-globulin is described in connection with the existence of a gel layer.