Dynamic thermomechanical properties and crystallinity of ultrahigh molecular weight polyethylene tibial inserts

A dynamic thermomechanical analysis method was used to determine the values of the viscoelastic parameters, storage modulus (E′) and the tangent of the loss angle (tan δ) of samples taken from the near surface and core regions of ultrahigh molecular weight polyethylene (UHMWPE) bar stock specimens that had been aged in ambient laboratory air for 12 months (the control group) and UHMWPE tibial inserts that had been subjected to the following treatments: γ sterilization followed by shelf aging in ambient laboratory air for between 56 and 112 months or ethylene oxide (EtO) gas sterilization followed by shelf aging in ambient laboratory air for 73 months. Differential scanning calorimetry was used to determine the melt temperature and percentage crystallinity (%C) of these samples. There were three main findings. First, EtO sterilization produced no statistically significant effect on the values of E′ or tan δ (relative to those for the control group), but the values of these parameters were markedly affected by γ sterilization. Second, for γ-irradiated samples, E′ increased with an increase in the shelf aging time. Third, there was a strong direct association between E′ and %C. The various potential uses of this association are described, paying special attention to its likely role in predicting the in vivo wear of UHMWPE tibial inserts.