Strain-induced microstructural rearrangement in ultra-high molecular weight polyethylene for hip joints: A comparison between conventional and vitamin E-infused highly-crosslinked liners

Infusion of vitamin E (α-tocopherol) in highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) liners has been conceived to achieve superior oxidation stability while preserving enhanced mechanical properties as compared to post-irradiation remelted liners. However, the presence of an antioxidant in the material microstructure brings an issue of concern in whether a “foreign substance” might reduce radiation crosslinking efficiency and/or change microstructural characteristics by diffusing into UHMWPE. The key to clarify this fundamental issue resides in performing a quantitative evaluation of the obtained material structure and its polymeric chain mobility on the molecular scale. In this paper, a Raman spectroscopic examination is presented of molecular orientation and phase fractions in as-processed vitamin E-infused UHMWPE acetabular liners in comparison with a model (undoped and unirradiated/uncrosslinked) and a conventional (undoped and 33 kGy-sterilized by gamma-irradiation) UHMWPE liners. The microstructural responses of structurally different liners to externally applied compressive strain were also monitored. The main results of the spectroscopic analyses can be summarized as follows: (i) preliminary gamma irradiation reduced the fraction of amorphous phase and increased the degree of molecular alignment, the vitamin E-infused liner preserving the crystallinity level achieved by the 100-kGy irradiation injected before infusion; (ii) the presence of vitamin E significantly promoted orientational randomness, which increased with increasing compressive strain magnitude, a phenomenon beneficial to minimize strain-softening-assisted wear phenomena.