Oxidation In Vivo And In Vitro

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Download or read book In Vitro Aging Method Yields Clinically Relevant Oxidation In Joint Bearing Materials written by and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Introduction: It is important to understand oxidative degradation of joint bearing materials to predict in vivo wear, cracking, and delamination due to stress and cycles of use [1]. Traditionally, oxidative degradation is thought to occur when free radicals in the polymer react with oxidative species on the shelf or in vivo. Pre-implantation oxidation of free-radical containing materials was eliminated with gamma-barrier sterilization and oxygen-impermeable packaging, but oxidation has been shown to increase exponentially with time in vivo. Retrieval studies have revealed a threshold ketone oxidation index of 1.0-1.5 (KOI, 1715 cm-1 peak normalized to the 1368 cm-1 peak) for maintaining mechanical integrity of polyethylene. Many bearings are shown to reach this level before 12 years in vivo [1]. Device manufacturers have tackled this problem by removing free-radicals with post irradiation thermal-treatments [2] or by adding radical stabilizing antioxidants [3]. Because the current ASTM artificial aging method (F2003) states u201cthis practice is not intended to simulate any change that may occur in UHMWPE following implantationu201d, studying long-term stability of these materials is limited to retrieval studies with small sample sets, short in vivo times, and uncontrollable surgeon- and patient-specific variables. The current objective is to map a more gentle (relative to ASTM F2003) in vitro artificial aging of gamma barrier devices to published in vivo oxidation trendlines, and to use this aging system to predict stability of an antioxidant material.Methods and Materials: Five rectangular prisms (5 x 5 x 8 cm) were cut from GUR 1020 resin stock materials prepared as follows: u201cVirginu201d non-cross- linked (TVI = -0.001), u201cAntioxidantu201d PHBP-containing, 85 kGY u03b3-irradiation, EtO (TVI = 0.031); u201cRemeltedu201d 75 kGy u03b3-irradiation, melted in argon, gas plasma treated (TVI = 0.024); u201cGVF-Lowu201d conventional UHMWPE, gamma vacuum foil (GVF) (Mean TVI = 0.014); u201cGVF-Highu201d conventional UHMWPE, GVF (TVI = 0.016). The blocks were artificially aged in a pressure vessel under 45 PSI (3 atm) O2 at 63oC for 10 weeks, with sampling at intermediate timepoints. Oxidation at 0, 4, and 6 weeks are reported as the maximum KOI measured from test coupons collected at each time point. Uniaxial tensile testing was conducted with an Intron 5544 load frame with a 2-kN load cell, pneumatic sample grips, and a video extensometer on ~200 micron thick dogbone specimens stamped from thin sections microtomed from the longitudinal sides of the prisms at each time point (ASTM spec D638). Oxidation profile and trend line data for retrievals presented in Figures 1 and 2 come from an IRB-approved retrieval database, queried for all u03b3-barrier sterilized tibial components. In total, 216 retrievals were analyzed, with in vivo duration ranging from 0 to 190 months and an average duration of 54 months. Literature- based retrieval maximum oxidation and mechanical data [1] were shown for comparison in Figure 3.Results: The oxidation profile of the in vitro-aged GVF materials showed a sub-surface oxidation peak after 6 weeks (Figure 1). The oxidation profile of a tibial bearing retrieval implanted for 6.8 years is shown for comparison. KOI measured in the GVF materials increased exponentially with time (Figure 2) Oxidation rate data from retrieved u03b3-barrier knee retrievals is shown for comparison. To achieve clinically relevant oxidation in GVF-Low and GVF-High (KOI 1.0-1.5), the aging process presented in this study takes approximately 6 weeks (Figure 2). Remelted, Antioxidant, and Virgin are chemically stable for at least 6 weeks in vitro (Figure 1, 2). Increasing KOI negatively correlated with Ultimate Tensile Strength (UTS) (Figure 3).Discussion: In vitro oxidation of GVF materials yielded oxidation profiles similar to those observed in gamma-sterilized retrievals, as characterized by the subsurface oxidation peak and identity of chemical species. While the oxidation peaks appear at approximately the same depth, the breadth of the peak is greater in the artificially aged samples, likely owing to increased oxygen diffusivity in the high-pressure/high-temperature environment. In vivo aging can be mapped to an accelerated aging process by a conversion factor of ~1.2 years in vivo to 1 week in vitro. The effects of this oxidation on UTS and other tensile properties were analogous to those observed in retrieval studies (Figure 3). The relative chemical inertness of virgin, remelted, and antioxidant materials in the absence an initiator (e.g. stress, lipids, etc.) is consistent with a free-radical mediated oxidation mechanism but does not suggest an explanation for in vivo oxidation of remelted highly cross-linked polyethylene [3]. Antioxidant material appears stable to this aging method for 6 weeks.References: [1]JOA Vol. 22 No. 5 2007. [2]JBMR-B 106.1 (2018): 353-359. [3]JBJS-A. 2010;92:2409-18.Abstract Significance: In vivo oxidation is mimicked in oxidation profile and oxidation rate by an in vitro aging environment. This technique has promisein evaluation of materials against the free-radical oxidation pathway. Future work must be done to assess other (e.g. lipid-related) pathways.