Current-Generation Crosslinked Polyethylenes for Total Hip Arthroplasty: Mechanistic Insights Into Oxidative Stability and Antioxidant Efficiency.

Abstract

Highly crosslinked polyethylene (XLPE) has revolutionized total hip arthroplasty (THA), but increased radiation doses and complex crosslink structures may increase oxidation susceptibility. The long-term oxidative stability of XLPE depends on manufacturing factors, including crosslink density, antioxidant (vitamin E) concentration, incorporation method, and radiation parameters. This study examined their interplay in current-generation XLPE acetabular liners produced by sequential irradiation and annealing, vitamin E diffusion, or grafting under extended accelerated aging. XLPE liners were thermally aged at 70°C in 5 atm oxygen for up to 18 weeks. Raman and Fourier-transform infrared spectroscopy were used to assess oxidative degradation via oxidation index (OI), trans-vinylene index (TVI), vitamin E index (VEI), and phase composition. Sequentially irradiated and annealed XLPE showed a sigmoidal OI increase with post-aging crystallization, indicating structural reorganization and potential embrittlement. In contrast, all vitamin E-stabilized XLPEs (0.1-0.7 wt%) maintained negligible OI values. Although an inverse correlation between VEI and OI was not strictly observed, OI increased with TVI, suggesting that higher crosslink density may increase oxidation susceptibility despite overall negligible oxidation. Vitamin E-diffused XLPEs exhibited greater antioxidant depletion than grafted variants but demonstrated comparable oxidation resistance. These findings highlight the inherent limitations of sequentially irradiated and annealed XLPE in resisting oxidation due to persistent residual free radicals, whereas vitamin E stabilization, either by diffusion or grafting, effectively suppressed oxidation and preserved XLPE integrity. Although formulations differed in vitamin E concentration, incorporation route, and radiation source, these factors exerted only secondary influence once approximately 0.1 wt% vitamin E was incorporated.