Factors Affecting the Wear of Irradiated UHMWPE

Abstract

Effect of sliding directionality on wear of irradiated UHMWPE (ultra-high molecular weight polyethylene) was studied under serum-lubricated conditions. When exposed to unidirectional sliding, by reciprocation along a line, steady-state wear rates were negligibly small against polished countersurfaces, only reaching measurable levels against roughened countersurfaces. In unidirectional cases, more rapid wear existed during the first million sliding cycles. If instead subjected to multidirectional sliding, by translating countersurfaces through closed paths, unirradiated UHMWPE experienced rapid wear of 8.3 × 10−7 mm3/Nm against polished CoCr. Crosslinked UHMWPE, produced by gamma irradiation to 4 Mrad dose with subsequent 200 °C (melt) vacuum storage, reduced wear three-fold. Vacuum storage allows crosslinking between radiation-induced radicals without competitive oxidative scission, while increased temperature augments crosslinking kinetics. Wear resistance was similarly enhanced using 70 °C (sub-melt) vacuum storage by increasing dose to 10 Mrad. Under multidirectional sliding against polished CoCr, all UHMWPE adopted steady-state wear rates from the onset of sliding. Multidirectional sliding produced higher steady-state wear rates than unidirectional sliding, regardless of whether rectangular or circular translation paths were employed. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Seattle, Washington, October 14, 2000