Effectiveness of adsorbed film and gel layer in hydration lubrication as adaptive multimode lubrication mechanism for articular cartilage

With various daily activities, the effectiveness of adsorbed film formed on a gel-like layer at the uppermost superficial articular cartilage in natural synovial joints becomes important to control the friction and wear of articular cartilage in mixed or boundary lubrication regime as an adaptive multimode lubrication mechanism. Furthermore, in the case where the adsorbed film has been removed, the proteoglycan gel layer is expected to preserve low friction and protect against the wear of bulk cartilage tissue with an effective hydration lubrication mechanism. Besides, it is indicated that the biphasic lubrication plays an important role in lowering of friction in articular cartilage containing a large amount of water. At the present stage, however, the detailed relationship between adsorbed films and hydrated gel layers has not yet been elucidated. In this article, the frictional behaviours of articular cartilage on a glass plate were observed in the reciprocating tests with the restarting process after interruption and unloading. The lubricating effectiveness of adsorbed films in hyaluronate (HA) solutions was examined using intact and partially damaged cartilage specimens. The role of albumin and g-globulin in relation to the surface conditions of gel layer is discussed. The restarting friction immediately after reloading became lower as a result of recovery of the effective interstitial fluid pressurization and hydration and adsorbed films have a significant effect on the frictional behaviour at local contacts. To clarify the molecular phenomena taking place under rubbing condition, in situ observations of the forming adsorbed film were conducted. The measurements were performed using the fluorescent staining method for protein and HA molecules at low contact pressures.