Polymers - Orthopaedic Biomaterials in Research and Practice - page 154

Although beneficial from the perspective of wear resistance, cross-
linking also reduces molecular mobility and ductility and, as a result,
leaves the material with an overall reduced fatigue and fracture resis-
tance. This is due to a reduction in chain mobility and stretch. Further,
not all free radicals are sufficiently mobile to recombine into cross-links,
and many end up trapped within the crystalline regions of the mate-
rial. These residual free radicals may eventually react with oxygen and
form unstable hydroperoxides, which will successively decay, and lead
to embrittlement of the material via chain scission and recrystallization.
Thermal treatment of irradiated UHMWPE may be employed to increase
oxidation resistance, accomplished through either melting or annealing
the material below the melt point. Heating the polymer to above its melt-
ing point can reduce crystallinity and provide enough energy for the
trapped free radicals to recombine with one another. Melting also has
the side effect of reducing the stiffness and fatigue strength of the mate-
rial, a result of the reduction in polymer crystallinity.
Highly cross-linked and thermally treated UHMWPE is now widely
accepted as the preferential form for THA. Since the introduction of
first-generation, highly cross-linked implants, clinical performance is
considered generally successful, though there have been a small number
of reports of rim fractures on some of the thinner liners implanted at high
abduction angles. On the other hand, highly cross-linked UHMWPE is
currently less used in the knee owing to the inherent reduction in ductil-
ity and fracture resistance associated with the irradiation necessary to
initiate the cross-links, along with questions pertaining to its clinical
necessity in this application.
Currently, there are three main options for sterilization of UHMWPE,
including gamma sterilization in barrier packaging, ethylene oxide gas
sterilization, or gas plasma sterilization. Gamma sterilization was a
controversial topic in the '90s because of findings showing that gamma
sterilization in air, when followed by extended shelf storage, increased
the risk for oxidation (including an increase in density and crystallinity
of the material) and mechanical degradation of UHMWPE owing to the
material becoming more brittle through the chemical reactions. Barrier
packaging has the ability to minimize oxidative degradation during stor-
age by limiting the access of oxygen to the polymer. The in vivo per-
formance of UHMWPE sterilized with barrier packaging still remains
poorly understood, though limited data suggest that oxidation may be
limited for 5 to 10 years. One advantage gamma sterilization has over
gas sterilization is that radiation-induced cross-linking is a by-product
of gamma irradiation and can increase the tribological (wear) perfor-
mance of the material. Ethylene oxide and gas plasma sterilization, on
the other hand, generate no free radicals that may subsequently oxidize
during shelf storage. Gas methods are currently considered the meth-
ods of choice for highly cross-linked UHMWPE materials. Gas plasma,
especially, is attractive as it does not leave toxic residues or incorporate
by-products that are environmentally hazardous
Despite its superior mechanical performance, UHMWPE is not a
perfect material. It can absorb small amounts (<0.1 v/o) of fluids, as can