Do First-generation Highly Crosslinked Polyethylenes Oxidize In Vivo?

Abstract

Background Highly crosslinked and thermally treated polyethylenes were clinically introduced to reduce wear and osteolysis. Although the crosslinking process improves the wear performance, it also introduces free radicals into the polymer that can subsequently oxidize. Thermal treatments have been implemented to reduce oxidation; however, the efficacy of these methods with regard to reducing in vivo oxidative degradation remains to be seen. Polyethylene oxidation is a concern because it can compromise the ultimate strength and ductility of the material. Questions/purposes We analyzed the oxidation, oxidation potential, and mechanical behavior of thermally treated highly crosslinked polyethylene retrieved acetabular liners. Methods Three hundred seven acetabular liners were collected from consecutive revision surgeries at six institutions over a 10-year period. Twenty-four were sterilized using nonionizing methods, 43 were sterilized in an inert environment, 80 were highly crosslinked and annealed, and 160 were highly crosslinked and remelted. Oxidation and oxidation potential were assessed by Fourier transmission infrared spectroscopy. Mechanical behavior was assessed by the small punch test. Results Oxidation and hydroperoxide (oxidation potential) indices were elevated in the annealed and gamma inert sterilized groups compared with those of the remelted liners and uncrosslinked gas sterilized controls, particularly at the rim. We also detected an increase in oxidation over time at the bearing surface of the remelted group. Ultimate strength of the polyethylene at the bearing surface was negatively correlated with implantation time for the annealed liners. Conclusions Within the first decade of implantation, the clinical outlook for first-generation highly crosslinked polyethylene remains promising. However, ongoing research continues to be warranted for first-generation highly crosslinked polyethylene bearings to monitor the implications of elevated oxidation at the rim of annealed liners as well as to better understand the subtle changes in oxidation at the bearing surface of remelted liners that occur in vivo.

Does Cyclic Stress Play a Role in Highly Crosslinked Polyethylene Oxidation?

“…In the other cases, namely remelted HXLPE subjected to aging alone, or to aging and cyclic loading, comparatively lower OIs were registered. Our results generally agree with previous retrieval studies that confirmed ex vivo stability loss and measurable oxidation levels for remelted HXLPE explants [9,15,16,22,24]. In this sense, Oral and colleagues recently demonstrated that absorbed squalene, a biological prooxidant, could initiate and accelerate oxidation in remelted crosslinked polyethylene [24].…”

“…However, retrieval studies have recently reported measurable oxidation levels in remelted HXLPE explants [8,15,22]. The mechanisms of in vivo oxidation of remelted HXLPE implants remain unclear.…”

“…Annealing treatments are generally less aggressive to the original crystalline microstructure, and, therefore, the prethermal stabilization mechanical properties are largely preserved [13]. However, annealing stabilization leaves residual (measurable) free radicals, and, in this sense, researchers soon found evidence that annealed HXLPE implants oxidized in vivo [11,12,15].…”

Effect of CoCr Counterface Roughness on the Wear of UHMWPE in the Noncyclic RandomPOD Simulation

“…There are indications that any type of UHMWPE that has been irradiated, including the two studied in the present paper, may show oxidation in vivo, which is known to be detrimental for the strength and the wear resistance [23]. Therefore the fact that only fresh UHMWPE specimens were tested may be considered a limitation of the study.…”

Multidirectional Wear and Impact-to-wear Tests of Phospholipid-polymer-grafted and Vitamin E-blended Crosslinked Polyethylene: A Pilot Study

“…Oxidative degradation of some of the first generation of CLPE formulations has been considered a potential limiting factor for the long-term performance of THA [7,28]. During gamma irradiation (for crosslinking or for sterilization), free radicals formed in the PE molecular structure cause embrittlement through a cascading oxidation reaction [7].…”