The lubricating performance of total knee prosthesis models with compliant layer as artificial cartilage was evaluated and discussed from the viewpoint of adaptive multimode lubrication. The minimum film thickness and inclination of rubbing surface during walking were estimated by numerical analysis based on the plane inclined surface model for polyvinylalcohol (PVA) hydrogel and polyurethane. The numerical results show the effectiveness of softer material on fluid film formation during walking, although surface inclination during swing phase is larger for polyurethane than PVA hydrogel. The actual fluid film formation in knee prostheses during walking was examined by measurement of degree of separation by electric resistance method and frictional force in simulator tests. Under walking condition lubricated with lubricants of appropriate viscosity, considerable elastohydrodynamic film was formed corresponding to numerical analysis. Under thin film conditions lubricated with low viscosity lubricants where significant local direct contact occurred between rubbing surfaces due to breakdown of fluid film, the addition of proteins remarkably decreased friction and suppressed stick-slip for PVA, but increased friction for polyurethane. The addition of phospholipid liposomes had an effect in reducing of friction after repetition of rubbing process in walking motion. The adsorbed film formation of synovia constituents on stainless steel plate was observed by atomic force microscopy.
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