VITAMIN E STABILIZED HIGHLY CROSSLINKED UHMWPE – MATERIAL PROPERTIES, OXIDATION RESISTANCE AND WEAR PROPERITIES

Abstract

Vitamin E stabilized highly crosslinked UHMWPE (E-Poly^™) was developed to improve upon the properties of first generation highly crosslinked UHMWPE’s. The post-crosslinking processing for E-Poly^™ maximizes the strength of the material while at the same time stabilizing residual free radicals that remain after irradiation. E-Poly^™ is crosslinked with 100 kGy gamma irradiation prior to infusion of vitamin E. The infusion process involves diffusing vitamin E into the crosslinked material at temperatures beneath the melt temperature.

Small punch testing (ASTM F2183-02) was completed to evaluate strength of E-Poly^™ compared to gamma-inert sterilized UHMWPE. The results showed that the E-Poly^™ material had equivalent or better properties before and after accelerated aging than the gamma-inert sterilized UHMWPE (96–105 N vs. 75–88 N unaged; 100–115 N vs. 42–56 N 2-week aged).

Environmental stress crack testing evaluated the resistance to oxidation while the material was subjected to fatigue testing. A constant stress beam was tested for 5 weeks at 80C. Failure was defined as the appearance of cracks or fracture of the specimen. All 4 specimens of gamma sterilized components showed evidence of cracking prior to the completion of the test. 2 of 4 sequentially crosslinked and annealed specimens fractured prior to completion. None of the E-Poly^™ specimens showed cracks during testing. An examination of the amount of oxidation induced during this testing showed that the addition of fatigue loading increased the oxidation index for UHWMPE’s that had unstabilized free radicals. The surface oxidation index for gamma sterilized UHWMPE increased from ~0.3 to 1.1 and for sequentially crosslinked UHMWPE from ~0.3 to 0.7; the oxidation index for E-Poly^™ was negligible for all test condition.

Hip simulator testing (ISO 14242-1) showed that the volumetric wear rates for E-Poly^™ were 95–99 % less than that of ArCom. For 28mm head diameters the rates were 53.3 mm3/Mc for ArCom and 0.24 mm3/Mc for E-Poly^™. Wear particle morphology analysis showed that the E-Poly^™ wear particles were similar to ArCom. Qualitatively, there appeared to be fewer E-Poly^™ particles.

Knee simulator testing (ISO14243-1) was performed on both cruciate retaining and posterior stabilized Vanguard^® knees. The E-Poly^™ tibial bearing, CR and PS, showed 86% less wear than direct compression molded UHMWPE, the current gold standard. Fatigue testing of the PS post before and after accelerated aging (ASTM F2003) loaded to 1300lbs showed no degradation or failure of the post following 3 million cycles.

Vitamin E stabilized highly crosslinked UHMWPE has demonstrated excellent material properties, wear properties, and resistance to oxidation. These properties have been optimized through the combination of cross-linking, processing below the melt temperature subsequent to crosslinking, and the stabilization effect of vitamin E. These properties provide rational support to the utilization of vitamin E stabilized highly crosslinked UHMWPE for hip and knee applications.