Chapter 13 - Highly Crosslinked and Melted UHMWPE

Abstract

This chapter reviews the development of the irradiation and melting method and presents the preclinical and clinical evidence supporting the use of this technology. Crosslinking is advanced as a means to impede the formation of surface fibrils and reduce the wear of ultra-high molecular weight polyethylene (UHMWPE). Crosslinking of UHMWPE molecules would decrease the chain mobility needed for the large-scale plastic deformation and hence slow down the surface fibril formation. Crosslinking of UHMWPE can be achieved by subjecting the polymer to ionizing radiation or by using peroxide or silane chemistries. Silane crosslinking received limited attention. UHMWPE crosslinks when exposed to ionizing radiation through free radical recombination reactions. Thermal treatment of radiation-crosslinked UHMWPE is proposed to avoid the oxidation that is observed in terminally gamma sterilized UHMWPEs. Two main forms of thermal treatment are used—annealing below the melting point of the irradiated polymer or melting after irradiation. It summarizes that crosslinking, as previously mentioned, occurs primarily in the amorphous phase of UHMWPE. In the crystalline domains, the chain segments are not in close enough proximity to allow the recombination of the free radicals—in fact the lattice spacing is smaller than the carbon–carbon bond length. Therefore, for a crosslink to form in the crystalline region, the chain has to kink, which is energetically not favorable.