New and novel stabilisation approach for radiation-crosslinked Ultrahigh Molecular Weight Polyethylene (XL-UHMWPE) targeted for use in orthopeadic implants

There is a substantial evidence to show that oxidative degradation has a negative impact on the properties and performance of radiation-crosslinked ultra high molecular weight polyethylene (XL-UHMWPE) used as articulating components in orthopaedic joint implants and is the primary cause for their premature failure. In general, high energy radiation is required not only for sterilising the articulating surfaces but also for crosslinking the polymer to improving its mechanical properties and wear resistance thus minimising the extent of formation of wear-particles which are implicated in the aseptic loosening and failure of the implant. To address the oxidative stability of the XL-UHMWPE bearing surfaces, the natural hindered phenol antioxidant, vitamin E, which has been approved by FDA for use in this application, is used to enhance the oxidation resistance of orthopaedic implants, e.g. in tibial bearings. Further, a purified grade of the commercial antioxidant Irganox 1010^Ⓡ (pentaerythritol tetrakis(3-[3,5-di tertiary butyl‑4‑hydroxy phenyl]propionate) has also been FDA approved and used in some knee-based systems. In the case of vitamin E, its effectiveness as a potent free radical scavenger, however, presents some issues in respect of the polymer crosslinking process and the homogeneity of its distribution especially during manufacturing processes that involve the infusion of the vitamin into the polymer. The novel approach adopted in this study for the stabilisation of XL-UHMWPE bearing surfaces makes use of graftable polymer-reactive antioxidants (r-AOs) bearing a hindered phenol and also hindered amine moieties. Potentially, these r-AOs can be chemicaly grafted in-situ on the polymer backbone during the radiation-crosslinking process without the need for any additional manufacturing step. The results obtained have demonstrated a substantive stabilisation which can be achieved with these r-AOs. This was shown to be the case in all the XL-UHMWPE samples tested here which were manufactured according to a methodology typically used in the commercial production and crosslinking (by gamma- or electron-beam) of UHMWPE-based implants.