Abstract
INTRODUCTION
Electron-beam-irradiated dl-α-Tocopherol (Vitamin E)-blended UHMWPE is now being considered as a potential new bearing surface material for hip prosthesis [1]. However, Vitamin E stabilizes some of the primary free-radicals required for crosslinking, thereby reducing the material's crosslink density [2]. Additionally, some biological-stabilization effects of Vitamin E may also be reduced by oxidation. In this study, Vitamin E radicals in electron-beam-irradiated UHMWPE were measured and identified using Electron Spin Resonance (ESR), and the effects of annealing on radical stabilization and crosslink density were examined.
MATERIALS & METHODS
Both pure UHMWPE and Vitamin E added (0.3% w/w) resin was used to produce bulk specimens via vacuum direct compression molding at 220°C under 25 MPa for 30 min. Cylindrical pins (3.5 mm diameter, 40 mm length) for ESR measurement were then machined and placed in vacuum packaging. The pins were irradiated at 300 kGy, with half of each test group annealed at 80°C for 24 hours. Free radical measurements were made using a high-sensitive X-band ESR operating at 9.44 GHz. Detection of Vitamin E radicals was performed by comparing the characteristic symmetrical spectrum of oxidized Vitamin E to the spectra observed for the pins using both g-value and linewidth as references. Crosslink density was measured via gel fraction analysis and was performed in accordance with ASTM D2765. Thin sections (20 × 40 mm2, 200 μm) were machined from the bulk specimens, which were then placed in vacuum packaging, irradiated and annealed at the same conditions as those for the ESR measurements. Two of these thin sections were then placed in a stainless-steel cage (200 µm pore diameter) and were immersed in decahydronaphtalene at 200°C for 24 hours. These specimens were then extracted using soxhlet extractor at 100°C for 24 hours and dried in vacuum at 150°C for 12 hours.
RESULTS
The characteristic symmetrical spectrum of oxidized Vitamin E was measured and identified in the electron-beam-irradiated Vitamin E-blended specimens [Fig. 1]. For the annealed samples, this spectrum was reduced [Fig. 2]. The annealing treatments increased gel fraction [Fig. 3] and decreased the total amount of primary free-radicals [Fig. 4] in the electron-beam-irradiated Vitamin E-blended specimens at the same rate as that for the Virgin specimens.
DISCUSSION & CONCLUSIONS
Electron-beam-irradiated Vitamin E-blended specimens showed the same characteristic symmetrical spectrum as that of oxidized Vitamin E. Thus, measurement and identification of Vitamin E radicals in electron-beam-irradiated UHMWPE was confirmed. Also, annealing treatment at 80°C for 24 hours was effective in stabilizing Vitamin E radicals. The results showed in [Fig. 3] and [Fig. 4] suggest that the treatments increased gel fraction by accelerating the reaction between primary free-radicals. The results also suggest that Vitamin E radical stabilization was achieved through the interaction of Vitamin E radicals amongst themselves, and not through the interchange between Vitamin E radicals and primary free-radicals.
