Abstract
Introduction:
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.
Materials and Methods:
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.
Results:
Fluid uptake was evident in soak-aged GammaAir-PE and RM-HXLPE samples prior to extraction. FI of RM-HXLPE significantly increased after HEP6 and HEX16 extractions while FI of GammaAir-PE slightly increased after HEP6 but remained unchanged after HEX16 extraction (Fig. 1). Solvent extraction using either HEP6 or HEX16 increased OI measurement in all RM-HXLPE groups. A similar trend was observed in SS GammaAir-PE but not in LS samples (Fig. 2). Furthermore, oxidation measurement using peak area (A) yielded higher OI than that obtained from peak height (H), likely due to the proximity of fluid ketone vibration at 1734 cm−1 to oxidation ketone vibration at 1714 cm−1 (Fig. 3).
Discussion:
Aged GammaAir-PE and RM-HXLPE absorbed synovial fluid after a short period soaking at 37°C. The fluid uptake appeared to contribute to the elevated OI measurement when peak area analysis was used. Solvent extractions used in contemporary retrieval analysis increased OI measurement in RM-HXLPE significantly. These results confirmed the artificial impact of synovial fluid, solvent extraction, and conventional OI analysis on oxidation measurement of PE. Investigation of alternative methodology is necessary to improve OI measurement in UHMWPE retrievals.
