In natural synovial joint with compliant articular cartilage, the elastohydrodynamic film are likely to preserve low friction and protect the articular surfaces during walking. Under very thin film condition with local direct contacts at the start-up or in arthritis joints with low viscosity synovial fluid, other protective lubrication mechanisms such as mixed, weeping, boundary and gel film lubrication are expected to perform lubricating actions. In this paper, the role of surface layers of natural articular cartilage in thin film lubrication has been examined in reciprocating friction tests and the observation of the changes in rubbing surfaces by atomic force microscopy(AFM). Furthermore, the frictional behaviour of polyvinylalcohol (PVA) hydrogel as artificial cartilage was compared. The experimental observation for natural articular cartilage showed that the low friction was maintained for a considerable period at the initial stage in not a lubricated but an unlubricated condition. This fact suggests that the lubricating surface layer probably composed of adsorbed molecules and gel films existed in the rubbing surfaces. With repetition of rubbing, the friction gradually increased from the initial low value. AFM images of the intact articular cartilage and the rubbed cartilage surfaces at definite sliding distances indicated that the smooth surfaces were preserved for the low friction stage and the fibrous structure appeared for the higher friction stage. In friction test, PVA showed higher friction even at the initial stage under thin film condition. This discrepancy might be derived from the deficiency of lubricating surface layer.
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