The elimination of free radicals in irradiated UHMWPEs with and without vitamin E stabilization by annealing under pressure

Abstract

Radiation crosslinking of ultrahigh molecular weight polyethylene (UHMWPE) has been used to decrease the wear of joint implant bearing surfaces. While radiation crosslinking has been successful in decreasing femoral head penetration into UHMWPE acetabular liners in vivo, postirradiation thermal treatment of the polymer is required to ensure the oxidative stability of joint implants in the long term. Two types of thermal treatment have been used: (i) annealing below the melting point preserves the mechanical properties but the residual free radicals trapped in the crystalline regions are not completely eliminated, leading to oxidation in the long-term and (ii) annealing above the melting point (melting) eliminates the free radicals but leads to a decrease in mechanical properties through loss of crystallinity during the melting process. In this study, we hypothesized that free radicals could be reduced by annealing below the melting point under pressure effectively without melting due to the elevation of the melting point. By avoiding the complete melting of UHMWPE, mechanical properties would be preserved. Our hypothesis tested positive in that we found the radiation-induced free radicals to be markedly reduced (below the detection limit of state-of-the-art electron spin resonance) by thermal annealing under pressure in radiation crosslinked virgin UHMWPE and UHMWPE/vitamin-E blends without loss of mechanical properties.

Comparative micro‐Raman study on standard, cross‐linked and vitamin E‐blended polyethylene acetabular cups after long‐term in vitro testing and ageing

“…To solve these issues, scientists focused their efforts to improve these polymeric materials. Highly cross‐linked polyethylenes (XLPEs) were developed by using high‐dose irradiation (50–100 kGy) to decrease the wear rate of UHMWPE . Post‐irradiation re‐melting has been proposed to allow the recombination of free radicals trapped within the crystalline regions, with a consequent improvement in wear resistance, but with a decrease of PE crystallinity and thus with a loss of strength .…”

“…Highly cross-linked polyethylenes (XLPEs) were developed by using high-dose irradiation (50-100 kGy) to decrease the wear rate of UHMWPE. [7,8] Post-irradiation re-melting has been proposed to allow the recombination of free radicals trapped within the crystalline regions, with a consequent improvement in wear resistance, but with a decrease of PE crystallinity and thus with a loss of strength. [9,10] Recent studies have focused the attention on the delay of oxidation phenomena through the addition of suitable stabilizing additives, capable to interfere with the reactivity of the radical species, interrupting the oxidation cycle and thus degradation.…”

In vitro analysis of the cytotoxic and anti‐inflammatory effects of antioxidant compounds used as additives in ultra high‐molecular weight polyethylene in total joint replacement components

“…In the area of TJR, it should be pointed out that currently the concentrations or doses of antioxidant compounds being added experimentally to UHMWPE are based on oxidative degradation protection of the polymer25 and measurement of the oxidative index after inclusion of antioxidant compounds at various doses 9, 26–29. Doses chosen (generally 1000 ppm to 3000 ppm) are based on efficiency of resistance to oxidation and are not currently based on any biological consideration 9, 29.…”

In-vivo degradation of middle-term highly cross-linked and remelted polyethylene cups: Modification induced by creep, wear and oxidation

“…However, the results of the first studies on retrievals made of first generation HXLPE showed the unexpected appearance of mechanical and chemical degradation (Currier et al, 2007(Currier et al, , 2010. If from one hand a simple post-irradiation annealing step has proved to be insufficient to remove all the free radicals generated during cross-linking , on the other hand postirradiation remelting reduces the crystallinity of the final microstructure, which conspicuously decreases the mechanical strength of the polymer (Baker et al, 2003;Gillis et al, 1999;Gomoll et al, 2001;Oral et al, 2011), and can not protect the material from possible generation of free-radicals during invivo service. In the attempt to enhance wear and oxidation resistance, a second generation of HXLPEs was engineered by introducing two alternative methods.…”