Accelerated ageing experiments with crosslinked and conventional ultra-high molecular weight polyethylene (UHMW-PE) stabilised with α-tocopherol for total joint arthroplasty

Author： Wolf C. Macho C. Lederer K.

Publisher： Springer Publishing Company

ISSN： 0957-4530

Source： Journal of Materials Science: Materials in Medicine, Vol.17, Iss.12, 2006-12, pp. : 1333-1340

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Abstract

Samples of untreated ultra-high molecular weight polyethylene (UHMW-PE), UHMW-PE sterilized with γ-rays in nitrogen atmosphere (conventional UHMW-PE, widely used for articulating surfaces in endoprostheses) and UHMW-PE, which has been crosslinked by electron beam irradiation and annealed subsequently, were stabilized with α-tocopherol and aged in air at 120^∘C as well as in 10% aqueous hydrogenperoxide with 0.04 mg/ml FeCl₃ as catalyst at 50^∘C. The oxidative degradation was monitored with the help of infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), solubility measurements and size exclusion chromatography (SEC) and were compared to unstabilized samples. When aged in air at 120^∘C, the crosslinked UHMW-PE showed a slightly slower increase of the carbonyl (CO)-number (according to DIN 53383) in FTIR than conventional UHMW-PE. A stabilisation with 0.4% w/w α-tocopherol resulted in an increase of lifetime by a factor of approx. 40 for all samples. Ageing in 10% aqueous H₂O₂ at 50^∘C yielded similar results for all three unstabilised samples. The addition of the natural antioxidant α-tocopherol led to a prolongation of lifetime by a factor of approx. 2.5. A linear loss of α-tocopherol was detected during ageing. An increase of crystallinity as well as lamella thickness during ageing was observed with the help of DSC. The two-phase structure of crosslinked UHMW-PE with two melting endotherms at 114^∘C and 137^∘C was replaced very quickly by a single melting point at 130^∘C. This effect was delayed with the stabilized samples. In the solubility and SEC measurements, a severe molecular degradation and drop of molar mass of all materials could be observed after ageing in H₂O₂, leading to a complete destruction and, in case of crosslinked UHMW-PE, to a serious damage of the molecular network, respectively.