The solvent extraction step applied in conventional oxidation measurement protocols for UHMWPE retrievals resulted in an elevated oxidation index (OI) in remelted highly cross-linked UHMWPE (RM-HXLPE). The present study seeks to confirm the effect of solvent extraction on OI measurement and to understand the relationships among soak-aging, fluid uptake, and resulting OI from various test protocols. Two materials were tested, representing legacy gamma-in-air sterilized (GammaAir-PE, GUR4150, 30 kGy) and remelted highly cross-linked (RM-HXLPE, GUR1050, 100 kGy, 147°C/5h) UHMWPE. Concave discs approximately 19 millimeters (mm) in diameter and 3 mm in dome thickness were machined from both materials prior to soak-aging. Soak-aging consisted of a combination of: (1) ASTM F2003 accelerated aging (5 atm O2, 70 °C for 14 days), and (2) either static soaking (SS, for 11.57 days) or dynamic load-soaking (LS, 2280 N at 1 Hz for 1 million cycles) in bovine synovial fluid at 37 °C to simulate the combination of shelf and in-vivo aging, respectively. Unsoaked samples were used as control (C) group. Thin films (150 μm) were harvested from cross-sections of all groups and were subjected to two solvent extraction protocols using Sohxlet (Heptane for 6 h (HEP6) or Hexane for 16 h (HEX16)) prior to be analyzed by two OI analyses using Fourier transform infrared spectroscopy (FTIR). FTIR analyses (128 scans/spectra, 4 cm−1 resolution) were carried out using both peak height at and peak area centering 1714 cm−1 for OI and 1734 for fluid uptake index (FI); carbon-carbon vibration at 1368 cm−1 was used for normalization. All GammaAir-PE data was further normalized using prewash control while RM-HXLPE data used computed results. The paired t-test was used with a significance level of p < 0.05.Introduction:
Materials and Methods:
