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The Effect of Crosslinking UHMWPE on In Vitro Wear Rates of Fixed and Mobile Bearing Knees
D. McNulty
Introduction
Significant wear reduction has been demonstrated in laboratoryhip wear simulations of highly crosslinked UHMWPE's comparedto historical clinical materials. Similar data is being accruedto predict the benefits of highly crosslinked materials inTKA. Factors to consider for simulated knee wear studies arekinematic and load inputs, and the effect of total knee systemdesigns. The purpose of this study was to evaluate highlyand moderately crosslinked UHMWPE materials in fixed and mobileknee bearing designs while maintaining the kinematic and loadinputs.
Materials and Methods
Multiple knee simulations were conducted using kinematic andload inputs in accordance with the ISO displacement controlledknee simulation draft standard ISO/CD 14243-3. Moderatelycrosslinked tibial insert components were machined from 1020CMS, vacuum packaged and dosed to ~4 Mrad using gamma irradiation.Highly crosslinked components were machined from 1050 REB,also vacuum packaged, gamma dosed to 5Mrad and subsequentlyquenched above the melt. Wear rates were determined from gravimetricweight loss. Wear morphology for the components was also noted.
Results
For fixed bearing knee components, highly crosslinked materialswore at approximately 1/3 the wear rate of moderately crosslinkedmaterials. There was no discernable difference in wear ratefor the mobile bearing knee for the two materials. However,the mobile bearing knee wear rates were approximately 1/10and 1/3 of the fixed bearing knee wear rates for highly andmoderately crosslinked materials, respectively. This reductionin wear rate of the mobile bearing knee was surmised to resultfrom a reduction of cross shear motion at the tibio-femoralarticulation. The cross shear motion was converted to reciprocatingmotion at the tibial insert/tibial tray interface which isrecognized as a lower wearing mode for UHMWPE.
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