Abstract
INTRODUCTION
Multiple sources have consistently reported oxidation indices less than 0.1 with Marathon® inserts implanted up to 10 years. Understanding effects of oxidation level on UHMWPE wear in vivo is of great value. The objective of this study is to characterize the wear performance of Marathon® acetabular inserts at various levels of artificially induced oxidation, quantified using Bulk Oxidation Index (BOI) as determined per ASTM F2102, and to ascertain if wear rate is affected by progressive polyethylene oxidation.
METHODS
GUR 1050 UHMWPE acetabular inserts, re-melted and cross-linked at 5.0Mrad (Marathon®, DePuy Synthes Joint Reconstruction, Warsaw, IN), were artificially aged per ASTM F-2003 in a stainless steel chamber at 5 atm. oxygen pressure and 70°C. Samples were maintained at temperature for 9, 10.4 and 11 weeks. After aging was completed, Fourier Transform Infra-Red (FTIR) spectroscopy was employed on one insert from each time point to evaluate the induced oxidation as a result of artificial aging. Resulting induced BOI values measured by FTIR were 0.195, 0.528 and 1.184. UHMWPE inserts had an inner diameter of 28mm and an outer diameter of 48mm and were articulated against 28mm diameter M-Spec® metal femoral heads (DePuy Synthes Joint Reconstruction, Warsaw, IN). Testing was conducted on a 12-station AMTI ADL hip simulator (AMTI, Watertown, MA) with load soak controls per ISO 14242-1:2014(E) in bovine serum (18mg/mL total protein concentration) supplemented with 0.056% sodium azide (preservative) and 5.56mM EDTA (calcium stabilizer). The UHMWPE inserts were removed from the machine, cleaned, and gravimetric wear determined per ISO 14242-2:2000(E) every 0.5 million cycles (MCyc) for 4.0 MCyc total. A two-tailed student's t-test was used (variance determined by F-test results) to analyze differences in wear rates between the three test groups.
RESULTS
After 4.0 MCyc of wear testing, the average wear rate of the Marathon® UHMWPE acetabular inserts with OI levels of 0.195, 0.528 and 1.184 articulated against the CoCrMo femoral heads were 6.0 ± 4.2 mg/MCyc (±95% confidence interval), 8.7 ± 0.4 mg/MCyc and 15.3 ± 2.2 mg/MCyc, respectively (Figure 1). As BOI level increased, the wear rate also increased. Wear rates of inserts with BOI of 0.195 compared to 0.528 were not significantly different (p=0.11). Wear rates of inserts with BOI of 0.195 were significantly lower (p=0.00) compared to 1.184. Wear rates of inserts with BOI levels of 0.528 were significantly lower (p=0.00) compared to 1.184.
DISCUSSION
This study aimed to characterize the wear performance of Marathon® acetabular inserts at three levels of oxidation created by accelerated aging. Artificially aged inserts with a BOI level similar to those reported from clinically retrieved Marathon® inserts (0.195) had a wear rate equivalent to non-aged inserts previously tested (data not shown), indicating the in vivo oxidation of this highly cross-linked UHMWPE has no effect on wear rate. Although not measured clinically, higher levels of oxidation may result in significantly higher wear rates in vivo.
