Antioxidant Performance in UHMWPE Knee Bearings: A Mid-Term Retrieval Report

Joint arthroplasty bearing materials must maintain a balance between wear resistance, toughness, and oxidation resistance. Antioxidant-doped polyethylene has been introduced to stabilize free radicals resulting from the cross-linking process while avoiding mechanical property losses associated with previous generations of highly cross-linked polyethylene. Furthermore, the antioxidant should prevent or greatly reduce oxidation occurring in vivo. The purpose of this study is to understand the extent to which retrieved, antioxidant-doped UHMWPE devices exhibit chemical and microstructural signs of oxidation. A group of 261 antioxidant knee bearings from an IRB-approved retrieval database were assessed for oxidation and microstructural changes that would be expected with oxidation. Three different antioxidant materials were included in this study, including diffused vitamin E (VE-D), blended vitamin E (VE-B) and pentaerythritol tetrakis[3-(3,5- di-tert-butyl-4-hydroxyphenyl)] propionate (PBHP), with an emphasis on the latter. Ketone oxidation index (KOI) and crystallinity were assessed for all materials, while crosslink density was assessed for the PBHP materials. In vivo durations were 0-107 months, making this the largest and longest known study of antioxidant efficacy in retrieved devices. Increases to KOI with in vivo duration were minimal, with nearly all values remaining below 0.2 out to the maximum duration observed. These increases were largely attributed to the presence of absorbed species near the material surface, where maximum KOI occurred in most devices. Microstructural changes typically associated with oxidation did not yield any meaningful changes, indicating that polymer degradation is not occurring in these materials to any significant extent. Subsurface KOI peaks were noted in five devices, suggesting that small amounts of polymer oxidation may develop in these materials given the right conditions. However, unlike subsurface ketone peaks associated with oxidation in previous generations of UHMWPE, these were very small and pose no threat to the mechanical properties of the materials. In retrievals evaluated to date, all antioxidant formulations appear to be effectively controlling in vivo oxidation. Small amounts of polymer oxidation observed in several devices are not likely to have clinical relevance. Continued monitoring over the long term will be necessary to ensure this remains the case.