WEAR RESISTANCE OF CROSSLINKED POLYETHYLENE AFTER AGING

Abstract

Highly cross linked polyethylenes have been shown to be substantially wear resistant. Typically, crosslinking is achieved by radiation in a low oxygen environment. While the early wear-simulation data is encouraging, concerns remain about the potential for aging and oxidative damage on exposure to oxygen during storage or in the body. This study measured wear rates in highly crosslinked liners that had been exposed to room air for up to 4 years.

Polyethylene liners were divided into four groups: two groups of highly crosslinked liners, XL (freshly opened) and XL-Aged (aged); and two groups of nominally crosslinked liners, N (freshly opened) and N-Aged (aged). The highly crosslinked liners were crosslinked with 9.5 Mrad of warm electron-beam irradiation, treated to a post-cross linking heat treatment to quench free radicals (WIAM), followed by ethylene oxide sterilization. The nominally cross linked liners were sterilized with 2.5 Mrad. The aged liners (XL-Aged and N-Aged) were stored in saline (at 37°C) exposed to room air for 4 years. Three liners from each group were tested in a hip-wear simulator (90% bovine serum) for 5 million cycles. Gravimetric wear measurements were made at 500,000 cycle intervals.

The N and N-Aged groups wore at rates of 14.76 ±3.1 and 15.58 ±1.21 mg/million cycles, respectively. The wear in both XL and XL-Aged groups was not measurable, resulting in weight gains of 2.73±0.5 and 2.17 ±1.1 mg/million cycles, respectively.

WIAM cross linked polyethylene has been reported to generate the least free radicals and has the least potential for oxidative damage. There have been concerns regarding the validity of artificial aging by the high-temperature oxidation. Aging in saline at body temperature while exposed to room air is more representative of in vivo aging. This data supports the results of artificial aging and the long-term durability of WIAM polyethylene.