IRRADIATION OF CHEMICALLY CROSS-LINKED ULTRAHIGH MOLECULAR-WEIGHT POLYETHYLENE

Acetabular cups for artificial hip joints were prepared by compression molding of ultrahigh molecular weight polyethylene in the presence of peroxide. Peroxide crosslinking led to a decrease in the degree of cr ystallinity, peak melting temperature, and recrystallization temperatu re, as well as decreased crystal perfection and size. Peroxide crossli nked cups were sterilized with gamma rays at room temperature in air a tmosphere to an average dose of 3.4 Mrad. Irradiation produced further crosslinking in amorphous regions plus extensive chain scission of ta ut tie molecules and led to increased crystallinity and crystal perfec tion. A significant increase in carbonyl concentration was determined for irradiated specimens. In general, peroxide crosslinking reduces th e effect of irradiation on the crosslinked network, because chemical c rosslinking stabilizes chain fragments resulting from radiolytic sciss ion and suppresses recrystallization of broken chains from amorphous r egions. Wear rates were much lower for chemically crosslinked cups, wh ich showed about one-fifth of the wear of control cups for the period from 0.5 to 1.0 million cycles. (C) 1996 John Wiley & Sons, Inc.