Europium stearate additives delay oxidation of UHMWPE for orthopaedic applications: a pilot study

Ultrahigh-molecular-weight polyethylene (UHMWPE) is used as an articulating surface in prosthetic devices. Its failure under various mechanisms after oxidation is of utmost concern. Free radicals formed during the sterilization process using high-energy irradiation result in oxidation. Europium, an element of the lanthanide family, has a unique electron configuration with an unusual lack of preference for directional bonding and notable bonding to oxygen. Because of this, it currently is used in studies for stabilization of polymers such as polyvinyl chloride.

We asked whether europium stearate could enhance the oxidation resistance after irradiation in nitrogen of UHMWPE.

Conventional nonirradiated and gamma-irradiated in nitrogen UHMWPE were compared with polyethylene doped with 375 ppm and 3750 ppm europium(III) stearate under the same treatment conditions. Chemical characterization was performed by Fourier transform infrared (FTIR) microspectroscopy using 200-μm thin films. The oxidation of doped samples with time was compared with that of conventional samples using accelerated oven aging. The types of oxidation products were identified by FTIR and quantified per material and treatment condition as indications of the oxidation level and mechanism.

The generation rate of hydroperoxides and ketones was decelerated proportionally with concentration of europium stearates. The oxidative mechanism appeared similar to that of conventional polyethylene with the same types of measurable end products as ketones and hydroperoxides. Yet, the rate of generation of the latter appeared to be slowed down by the action of europium stearate.

Europium stearate mixed in UHMWPE decelerated the oxidation reactions triggered by gamma irradiation in nitrogen, seemingly without major alteration of the oxidation mechanism.