Radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is the bearing of choice in joint arthroplasty. The demands on the longevity of this polymer are likely to increase with the recently advancing deterioration of the performance of alternative metal-on-metal implants. Vitamin E-stabilized, cross-linked UHMWPEs are considered the next generation of improved UHMWPE bearing surfaces for improving the oxidation resistance of the polymer. It was recently discovered that in the absence of radiation-induced free radicals, lipids absorbed into UHMWPE from the synovial fluid can initiate oxidation and result in new free radical-mediated oxidation mechanisms. In the presence of radiation-induced free radicals, it is possible for the polymer to oxidize through both existing free radicals at the time of implantation and through newly formed free radicals in vivo. Thus, we showed that reducing the radiation-induced free radicals in vitamin E-stabilized UHMWPE would increase its oxidative stability and presumably lead to improved longevity. We describe mechanical annealing, low pressure annealing, and warm irradiation of irradiated vitamin E blends as novel methods to eliminate 99% of radiation-induced free radicals without sacrificing crystallinity. These are significant improvements in the processing of highly cross-linked UHMWPE for joint implants with improved longevity.

Recently, highly crosslinked polyethylenes have emerged as an alternative bearing surface with tremendous potential for clinical success. However, the term highly cross-linked polyethylene refers to a great many materials, each manufactured under drastically different processing parameters, such as type of irradiation, dose, and warm versus cold state. It has been widely shown in laboratory hip simulator testing, that the wear resistance of UHMWPE improves significantly with increasing cross-link density, but the measurement of this parameter is somewhat controversial. While both swell testing of the polyethylene (direct) and trans-vinylene content (indirect) both yield information regarding the actual degree to which the material is crosslinked, no study to date has examined the exact relationship between these two tests. In evaluating the clinical performance of highly crosslinked polyethylenes, it is crucial that they be characterized according to the specific parameters by which they were manufactured. onship. Micro-Fourier Transform Infrared Spectroscopy (FTIR) and swelling measurements were performed on samples irradiated by either electron beam or gamma sources at varying doses, in both the cold and warm state. The trans-vinylene content was obtained from the ratio of the peaks at 965 cm-1 and 2022 cm-1, while the crosslink density was computed from Flory network theory.

The information for crosslink density was plotted versus trans-vinylene content to obtain the precise relationship between these two highly sensitive tests. This information can be used to aid in the clinical evaluation of commercially available highly crosslinked polyethylenes, and to improve our understanding of the very complex relationship between wear and the physical and chemical properties of UHMWPE.