ABSTRACT
The impact of g-irradiation on retrieved liners was first discussed in an early paper by Eyererand Ke [769], in which a density increase was found for a large number of explanted UHMWPE liners. This phenomenon was explained by invoking the occurrence of a post-crystallization process as a result of oxidative chain scission. The phenomenon led to a reduction in average mole-cular weight of the polyethylene material and to an increased extractability of its constituents. These microstructural changes are nowadays generally recognized as the main reason for aging (and failing) of g-irradiated UHMWPE liners. An increase of crystallinity during the mechanical and chemical loading of the liner in vivo can be thus seen as a clear symptom of the presence of residual free radicals in the pristine material and, thus, of the associated occurrence of a time-dependent oxidation process. In this context, a recent study by Fouad [770] investigated the effects of natural aging up to 6 years in air on the thermal, mechanical, and viscoelastic properties of a UHMWPE used in total joint replacement. The results show that the lamellar thickness and degree of crystallinity of UHMWPE specimens increase by 38% and 12% due to aging, with clear indications that the formation of new crystalline fractions occurs within their amorphous region. The tensile properties also show a significant decrease in the elastic modulus, yield, fracture stresses, and strain to failure due to aging. The key to interpret these negative results can be found in a recent study by Oral et al. [771]. Those researchers hypothesized and proved that an increased cross-link density might hinder crystallization during processing due to decreased chain mobility. There are two main consequences to the decrease of chain mobility: (i) the fraction of “inhibited” crystallinity during processing can be considered as a measure of the expected in vivo instability of the polymeric structure; and, (ii) when crystallization forcedly occurs (e.g., by in vivo loading) under conditions of a lack of chain mobility, the result is an embrittlement of the material. In order to circumvent the problem and to achieve high crystallinity without embrittlement, Oral et al. [771] proposed a remelting process under high pressure (200°C under 380 MPa for 5 h)
to be performed after substantial cross-linking irradiation. According to this process, high oxidation resistance is also expected because the material is molten after crystallization at high temperature, therefore allowing the recombination of the residual free radicals from irradiation. The strategy followed in the most advanced UHMWPE liners nowadays manufactured aims at the same final result as in the study by Oral et al. [771], namely obtaining a relatively high degree of crystallization and of an in vivo stable microstructure with the concurrent annihilation of free radicals. However, this aim is not pursued by means of a high-pressure remelting process but by sequentially operating on the UHMWPE microstructure several partial steps of irradiation and annealing.
