Chapter 14 - Highly Crosslinked and Annealed UHMWPE

Abstract

This chapter explores the sequential irradiation and annealing process that is shown to eliminate free radicals more effectively and form crosslinks than a single cumulative radiation dose of the same magnitude followed by annealing. The sequential and annealing process produces a microstructure that is not significantly different from that of ultra-high molecular weight polyethylene (UHMWPE) gamma sterilized in nitrogen (N2/Vac). The use of sequential irradiation and annealing provides the same level of crosslinking at a lower radiation dose with a consequent improvement in mechanical properties that allows X3 to be used for total knee replacement (TKR). The experience with highly crosslinked UHMWPEs over the last decade shows a progressive development in understanding the relationship between UHMWPE structure (crosslink density and crystallinity) and properties (strength, ductility, and wear). Highly crosslinked UHMWPEs were introduced clinically in the 1970s by Oonishi and coworkers and by Grobbelaar and associates, and reintroduced by Wroblewski and colleagues in the 1980s. The crosslinking methods used were not based directly on knowledge of UHMWPE structure–property relationships, and clinical use was limited. There was no commercial introduction, but these materials are of historical interest.