BIOCOMPATIBILITY OF A PEROXIDE CROSS-LINKED UHMWPE VITAMIN E BLEND

Introduction We have previously demonstrated that peroxide crosslinked vitamin E-blended UHMWPE maintains its clinically-required wear and mechanical properties [1]. This material can potentially be used as an irradiation-free bearing surface for TJA. However, using organic peroxides in medical devices requires a thorough examination of tissues in contact with the implant. For this study we crosslinked polyethylene using five times the needed concentration of peroxide (2,5-Dimethyl-2,5-di(t-butylperoxy)-hexyne-3 or P130), followed by implantation to determine implant biocompatibility, and pre and post implant peroxide residual contents. Methods The study was performed after institutional approval following ISO standard 10993–6. Study groups: not crosslinked (0.2 (1050) VE), crosslinked (0.2 VE (1050)/5% P130) and crosslinked-high temperature melted (HTM) (0.2 VE (1050)/5% P130). Materials were blended and consolidated, machined (2.5 diameter × 2.5 cm height), sterilized and implanted in the dorsum New Zealand white rabbits. Pre and post implantation FTIR was performed. Two samples were implanted in each rabbit; n =6 samples were included for each group. After 4 weeks, samples were explanted, analyzed using FTIR, and subcutaneous tissues processed for histological analysis. Results FTIR absorbances at 914cm−1, 1169cm−1, and the OH absorbance at 3450cm−1 showed differences between materials ( Fig 1A ). There was a significant increase in the absorbance at 914 for the non-crosslinked and crosslinked samples after explantation (p = 2.77E–17, p = 4.22E–23, Fig 1B ). There was a significant decrease in all peroxide related absorbances after explantation for the crosslinked and HTM samples (p < 0.05, Fig 1B ). Before implantation, these absorbances were significantly higher in the crosslinked and crosslinked/HTM samples than those in the non crosslinked sample (p<0.05, Fig 2A ). Peroxide related absorbances of the crosslinked sample were also significantly higher than those of the crosslinked/HTM sample (p<0.05, Fig 2A ). After explantation, the crosslinked samples had significantly higher absorbances than both the non crosslinked and crosslinked/HTM samples (p < 0.05, Fig 2A ). All peroxide related absorbances of the crosslinked/HTM samples were significantly higher than those of the non crosslinked sample (p < 0.05, Fig 2A ). The non crosslinked sample showed no significant differential between these absorbances at implantation and after retrieval. The crosslinked sample had the largest differential between the total peak absorbances before implantation and retrieval at 914cm−1. The crosslinked/HTM samples had the largest differential between the total peak absorbances before and after implantation for both 1169cm−1 and the OH absorbances ( Fig 2B ). All explants were recovered after four weeks in vivo ( Fig 3A ). No difference was found in the histological analysis of the tissue characterized by a synovial-like lining with signs of fibrosis around the implants ( Fig 3B ). Discussion The main challenge of this study was identifying pre and postoperative implant peroxide residual peaks via FTIR. We wanted to ensure that peroxide was present in implants before implantation, to ensure their elution into tissues. Conclusions Peroxide crosslinked polyethylene stabilized with vitamin E can potentially be used as an alternate bearing surface. Irradiation-free processing could result in cost-effectiveness and more accurate cross-linking of polyethylene implants.