Tribological behaviour of artificial cartilage in thin film lubrication

In most of existing total replacement joints composed of ultra-high molecular polyethylene (UHMWPE) and metallic or bioceramic components, the mixed or boundary lubrication prevails in daily activities. The thin film condition with local direct contacts brings about considerable wear. Excessive production of wear particles can induce the osteolysis and then the loosening of the prosthesis components As a tribological design solution to reduce wear, the applications of compliant artificial cartilage materials have been proposed, in which the soft-elastohydrodynamic lubrication is expected to be effective. In this paper, the simulator tests for walking motion and the simplified reciprocating tests were conducted for sliding of artificial cartilage such as PVA (polyvinyl alcohol) hydrogel and two kinds of SIPN (semi-interpenetrating network) hydrogels against metal or ceramic specimen, to examine friction and wear of artificial cartilage materials in thin film mixed or boundary lubrication. In simulator tests under walking condition, the combination of cylindrical femoral component and flat tibial one with PVA hydrogel layer showed higher wear than concave tibial one. In simulator tests of various artificial cartilage flat specimens lubricated with hyaluronate solution, PVA hydrogel of high water content showed the lowest friction at the start of the test, but friction gradually increased with running. In contrast, the PVA hydrogel with low water content exhibited the highest friction throughout testing. SIPN hydrogel containing polyurethane showed lower friction than SIPN containing cellulose acetate. In reciprocating tests of the spherical zirconia stationary specimen and the flat reciprocating artificial cartilage at constant load, the interruptive test with 2 s interruption at stroke ends showed larger wear than the continuous reciprocating tests for the same sliding distance. The difference in wear of hydrogel materials was examined by the AFM (Atomic Force Microscopy) observation at non-contact fluid tapping mode.