The influences of lubricant and material on polymer/CoCr sliding friction

Lubricant has recently been identified as one of major factors in in vitro wear testing of joint implants. In the present study, the influence of lubricant on polymer/CoCr sliding friction was investigated. Deionized (DI) water, diluted or undiluted bovine calf serum (BCS), bovine synovial fluid and human periprosthetic synovial fluid were compared as the lubricant. In addition, the effect of the type of polymeric material was also examined for four different polymers: non-irradiated GUR1050, GUR1050 gamma irradiated in nitrogen, GUR1050 electron-beam irradiated and non-irradiated Teflon. The sliding friction was measured on a standard pin-on-disk tribometer. It was found that: (a) due to the different rheological and chemical properties of various lubricants, the measured friction was significantly influenced by the lubricant; (b) changing from DI water to diluted BCS as the lubricant led to an increased friction for all of the tested polymeric materials, even though the viscosities of these two lubricants were nearly equal; (c) bovine synovial fluid did not simulate closely human synovial fluid, even though both fluids had nearly identical rheological properties; (d) human periprosthetic synovial fluid was associated with the lowest friction for all pin materials; (e) there was no statistical difference between the mean values of coefficients of friction of non-irradiated GUR1050, GUR1050 gamma irradiated in nitrogen and electron-beam irradiated highly crosslinked UHMWPE; and (f) Teflon had the lowest coefficient of friction among all the polymer pins tested.