Abstracts Posters SICOT-SOF meeting Gothenburg 2010 _2_

Poster
Topic: General Orthopaedics
Abstract number: 25065
A NEW MECHANISM FOR ULTRAHIGH MOLECULAR WEIGHT POLYETHYLENE
(UHMWPE) OXIDATION IN THE ABSENCE OF FREE RADICALS
Orhun MURATOGLU, Ebru ORAL, Bassem GHALI, Keith WANNOMAE
Massachusetts General Hospital, Boston (UNITED STATES)
Our understanding of oxidative stability in UHMWPE relies on residual free radical
elimination after irradiation. However, unexplained oxidation and decrosslinking
found in shelf-stored explanted hip implants fabricated from UHMWPE without
detectable free radicals suggested that contact with bodily fluids made it prone to
oxidation. UHMWPE absorbs squalene in vivo. We hypothesized that absorption of
these lipids could induce oxidation in UHMWPE.GUR1050 UHMWPE was irradiated
to 100-kGy, then melted at 150°C. Samples were dope d with squalene and
accelerated aged under 5 atm. of oxygen at 70°C. O ne set of samples doped with
squalene for 2 hours were also aged in air at 80°C for 1 and 5 weeks. Infrared
spectroscopy was used to quantify oxidation in the aged samples and swelling in
xylene to quantify crosslink density. Despite having no free radicals, irradiated and
melted UHMWPE oxidized heavily after squalene absorption and accelerated aging.
Small amounts of oxidation caused substantial decrosslinking. We attribute these to
the highly active unsaturations of squalene, which form hydroperoxides during
squalene oxidation and may initiate oxidation through the abstraction of hydrogen
from polyethylene chains and free radical generation. Oxidation rate was dependent
on oxygen concentration, which suggests a slower rate of oxidation in vivo. We
discovered an oxidation mechanism for UHMWPE through the absorption of
unsaturated lipids in the absence of free radicals. This presents a paradigm shift in
our understanding of oxidative stability of UHMWPE implants and warrants further
investigation.
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