On the mechanisms of the use of nitroxide as antioxidant for ultra high molecular weight polyethylene

Purpose: Ultra-high molecular weight polyethylene (UHMWPE) is commonly used as a lining material in total joint arthroplasty; however, irradiation used for sterilization and crosslinking generates free radicals that increase the material’s susceptibility to oxidative aging. This study explores the efficacy of nitroxides in scavenging free radicals in UHMWPE and their impact on crosslink density. Material and Methods: The irradiation and crosslinking of UHMWPE polymer specimens were performed using a ⁶⁰Co γ-ray source and a 7 MeV electron accelerator at the University of Maryland. The scavenging of alkyl radicals and the degree of crosslinking in UHMWPE were assessed using Electron Paramagnetic Resonance (EPR) spectroscopy and gel swelling in decalin solvent, as measured by a Thermomechanical Analyzer (TMA). Results: The nitroxide radicals TEMPO and TEMPOL demonstrated high efficiency in scavenging alkyl radicals. It was noted that without oxygen and antioxidants, most alkyl radicals undergo bimolecular reactions, resulting in crosslinks. When nitroxides are present, residual radicals combine with nitroxides or form long-lasting allyl radicals. The addition of nitroxide antioxidants, which efficiently scavenge free radicals, can prevent crosslinking if they diffuse into the polymer before irradiation, scavenging residual radicals within a crosslinked polymer. While it is possible to create highly crosslinked UHMWPE with nitroxides by adjusting factors such as dose, dose rate, irradiation temperature, and the duration of nitroxide doping, it was found that at low gamma dose rates, nitroxides nearly entirely inhibited crosslinking. Conclusions: When added before or after irradiation, nitroxides effectively scavenge residual free radicals while maintaining crosslink density. However, specific scenarios may inhibit crosslinking in the presence of nitroxide.