The effect of crosslinking UHMWPE on in vitro wear rates of fixed and mobile bearing knees

This study evaluated the wear performance of conventional (gamma irradiation at 4Mrads in low oxygen) and moderately crosslinked (gamma irradiation at 5Mrads and melt-annealed) ultra high molecular weight polyethylene (UHMWPE) materials in fixed and mobile knee bearing designs. Kinematic and load inputs corresponding to the gait cycle outlined in the ISO displacement controlled knee simulation draft standard, ISO/CD 14243-3, were maintained in all simulations. Wear rates were determined gravimetrically. Surface finish changes and worn surface characteristics were also noted. Moderately crosslinked fixed bearing knee component wear rate was 74% lower than conventional fixed bearing components. There was no discernable difference in wear rate for the mobile bearing knee for the two materials. However, the mobile bearing knee wear rates were 94% and 79% lower than fixed bearing conventional and fixed bearing moderately crosslinked materials, respectively. In the fixed bearing knee, the majority of the wear reduction of the moderately crosslinked material compared to the conventional material was attributed to enhanced crosslinking due to the melt-annealing process rather than the dose difference. The reduction in wear rate of the mobile bearing knee compared to the fixed bearing was attributed to the reduced cross shear motion at the tibio-femoral articulation since mobile bearing wear was independent of material.