Highly crosslinked, irradiated

The clinical introduction of a highly crosslinked, irradiated and thermal treated UHMWPE (HXLPE) in the late 1990s provided a significant... 

Under UV irradiation, the photoinitiator cleaves into radical fragments that react with the vinyl double bond and thus initiate the polymerization of the monomer. If the latter molecule contains at least two reactive sites, the polymerization will develop in three dimensions to yield a highly crosslinked polymer network. 

High Pressure. Irradiation under high pressure increases the crosslinking yield of some polymers (64-66). This has mechanistic implications which need to be further explored. 

Highly crosslinked UHMWPE can be produced by irradiation of a blank UHMWPE with ionizing radiation, in particular by X-rays, y-rays or electron beams, in order to produce radicals. The subsequent treatment of the irradiated material consists in exciting free radicals, which have not recombined, by means of microwave radiation or ultrasound. The process is claimed to ensure a substantially complete recombination of the free radicals. In addition, the crosslinking of the UHMWPE is also further optimized (30). 

High crosslink density at the irradiated surface (depth of penetration controlled by monomer diffusion)... 

Studying the decay of the usual nine-line spectrum in irradiated, highly crosslinked, poly(glycolmethacrylate), Lazar and Szocs [93] observed that the third, fourth and fifth lines decayed at different rates. This was attributed to the superposition of a doublet due to the more stable radical... 

In the late 1990s, highly crosslinked UHMWPE was found to have an effectively zero wear rate. Muratoglu et al. (2001) described how 40 mm thick discs of UHMWPE were irradiated in air using a lOMeV electron... 

Wang (2001) found that irradiation crosslinked UHMWPE had a significantly lower wear rate than un-crosslinked material. The radiation dose must be high to obtain the optimum effect (Fig. 15.19). Rieker et al. (2003) showed that the wear surfaces of highly crosslinked UHMWPE implants after 18 months in vivo, consisted of folds (Fig. 15.20). Such folds are also found in conventional UHMWPE, but fatigue leads to their detachment from the surface. The folds on the surface of the crosslinked polymer appear to stay in place. Crosslinking leads to a reduction in crystallinity, hence a... 

Chemical crosslinks may be obtained by randomly joining segments in already formed chains, by random copolymerization, or by end-linking functionally-terminated chains. Sulfur cures, peroxide cures, and high-energy irradiations are familiar methods of random crosslinking. Copolymerization monomers where at least one type has three or more reactive sites also lead to randomly crosslinked networks. Formation of networks by end-linking... 

Starting in 1998, orthopedic manufacturers introduced highly crosslinked UHMWPEs for THR. These materials are processed with a total dose ranging from 50 to 105 kGy, depending on the manufacturer. Besides choice of dosage, each manufacturer adopted a different route for production that includes a proprietary combination of three important factors 1) an irradiation step, 2) a postirradiation thermal processing step, and 3) a sterilization step (Figure 6.10). 

The first conventional UHMWPE material was a control, virgin, unirradiated material. The second conventional UHMWPE material was gamma-radiation sterilized in nitrogen with a dose of 30 kGy (referred to as 30 kGy, Y-N2). The two highly crosslinked UHMWPEs were both gamma irradiated with an absorbed dose of 100 kGy. One of the crosslinked materials was heat treated until the specimen center reached 110°C for 2 hours (referred to as 100 kGy, 110°G). The second was heat treated at 150°G for 2 hours (referred to as 100 kGy, 150°G). The degree of crystallinity of the four material types was determined by differential scanning calorimetry (DSC), see Table 14.2. 

Abt N.A., and W. Schneider. 2003. Influence of irradiation on the properties of UHMWPE. In Highly crosslinked and thermally treated ultra-high molecular weight polyethylene for joint replacements. S.M. Kurtz, R. Gsell, and J. Martell, Eds. West Conshohoken, PA American Society for Testing and Materials. 

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