Biomedical Engineering Reference
In-Depth Information
(Blunn et al., 1997). However, the benefits of using vitamin E, the natural
antioxidant in preventing oxidation in polyethylene were demonstrated in vitro
(see Section 3.4.2) and a vitamin E-containing highly crosslinked UHMWPE
was introduced clinically in 2007 (E-Poly
TM
(now E1), Biomet, Inc.). Therefore,
there is a current trend towards using alternative antioxidants and processing
conditions in medical-grade UHMWPE; in fact, Ticona has announced a vitamin
E-containing GUR UHMWPE resin for use in the medical industry (http://
www.ticona.com/home/products/uhmw-pe/gur_uhmw-pe_premium.htm).
3.3.3
In vitro characterization methods
Physical characterization
The crystallinity of the polymer is determined by differential scanning calori-
metry (DSC). The sample is subjected through a heating cycle, usually at 10 ëC/
min. The limits of the run are 0 to 200 ëC and the integration limits are 50±160 ëC
(ASTM, 2007). The crystallinity is determined by normalizing the enthalpy of
fusion calculated in this manner to that of 100% crystalline UHMWPE. If the
limits of integration are extended to 20 ëC, then the crystallinity can increase by
approximately 3±5% and care should be taken when comparing results from
different sources. As mentioned before, crystallinity can be increased by high
pressure crystallizing UHMWPE, which can change its mechanical properties.
On the other hand, crystallinity can also increase due to oxidative chain scission
and local recrystallization of degraded chains. This crystallinity increase does not
result in an increase in mechanical properties because the molecular weight of the
polymer is decreased and its network structure is degraded. The DSC thermo-
gram can give important information on crystal populations and crystalline
lamellae size as well.
Since the introduction of highly crosslinked UHMWPE (see Section 3.4),
methods have been devised to measure the extent of crosslinking in UHMWPE
(Muratoglu et al., 1999; ASTM, 2006a, 2008c). These methods are based on the
inability of crosslinked UHMWPEs to dissolve in a good solvent such as xylene;
gel content of crosslinked UHMWPEs is often above 90%. Instead, they swell
until equilibrium is reached and this equilibrium swell ratio can be defined in
terms of a crosslink density, the Flory interaction parameter between the
polymer and the solvent, and the specific volume of the solvent (Flory, 1953).
The measurement is done either by measuring in situ linear expansion by
contacting a probe with the sample or gravimetrically. A typical crosslink
density for highly crosslinked UHMWPE is around 0.170±0.300 mol/dm
3
.
Wear
The abundance of UHMWPE wear particles has been linked to peri-prosthetic
osteolysis, the resorption of bone around implants (Dumbleton et al., 2002).
