DYNAMIC THERMOMECHANICAL PROPERTIES AND CRYSTALLINITY OF ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE TIBIAL INSERTS

A dynamic thermomechanical analysis method was used to determine the v alues of the viscoelastic parameters, storage modulus (E') and the tan gent of the loss angle (tan delta) of samples taken from the near surf ace and core regions of ultrahigh molecular weight polyethylene (UHMWP E) bar stock specimens that had been aged in ambient laboratory air fo r 12 months (the control group) and UHMWPE tibial inserts that had bee n subjected to the following treatments: gamma sterilization followed by shelf aging in ambient laboratory air for between? 56 and 112 month s or ethylene oxide (EtO) gas sterilization followed by shelf aging in ambient laboratory air for 73 months. Differential scanning calorimet ry was used to determine the melt temperature and percentage crystalli nity (%C) of these samples. There were three main findings. First, EtO sterilization produced no statistically significant effect on the val ues of E' or tan delta (relative to those for the control group), but the values of these parameters were markedly affected by gamma sterili zation, Second, for gamma-irradiated samples, E' increased with an inc rease ha the shelf aging time. Third, there was a strong direct associ ation between E' and %C. The various potential uses of this associatio n are described, paging special attention to its likely role in predic ting the in vivo wear of UHMWPE tibial inserts. (C) 1998 John Wiley & Sons, Inc.