Biomedical Engineering Reference
In-Depth Information
Table 3.2 First generation highly crosslinked UHMWPEs
Tradename Manufacturer
Radiation Post-irradiation Sterilization
dose
thermal
method
(kGy)
treatment
Marathon TM
Depuy
50
Melting
Gas plasma
Longevity TM
Zimmer
100
Melting
Gas plasma
Crossfire TM
Stryker
75
Annealing
Gamma irradiation
Prolong TM
Zimmer
65
Melting
Ethylene oxide
Durasul TM
Zimmer
95
Melting
Ethylene oxide
XLPE TM
Smith & Nephew
100
Melting
Ethylene oxide
ALTRX TM
DePuy
75
Melting
Gas plasma
method is melting the crystals to allow the recombination of the trapped free
radicals in the crystals. This method eliminates all detectable free radicals
(Muratoglu et al., 2001a), but it is detrimental to the mechanical properties and
fatigue resistance of UHMWPE (Baker et al., 2003). Radiation crosslinking
reduces mechanical properties with increasing radiation dose due to decreased
plasticity, and post-irradiation melting decreases these properties further due to
the loss of crystallinity during melting (Oral et al., 2006b). This UHMWPE has
been in clinical use for over 10 years and has shown good wear resistance in vivo
up to 5 years (Digas et al., 2007). However, the reduction in fatigue strength of
crosslinked UHMWPEs is a concern for increased risk of fracture, especially
under adverse loading conditions (Halley et al., 2004; Tower et al., 2007).
In contrast, the mechanical properties of crosslinked UHMWPE are maintained
when post-irradiation thermal annealing is used below the melting point.
However, annealing below the melting point reduced but did not eliminate the
residual free radicals, a problem exacerbated by the terminal gamma sterilization
of this first generation highly crosslinked material, which introduced new free
radicals. Thus, this material was not resistant to oxidation (Wannomae et al.,
2006; Currier et al., 2007).
￿ ￿ ￿ ￿ ￿
3.4.2
Second generation highly crosslinked UHMWPEs
Second generation highly crosslinked UHMWPEs (Table 3.3) were developed to
overcome the shortcomings of the first generation highly crosslinked
UHMWPEs; namely, the low mechanical properties for irradiated and melted
UHMWPEs and low oxidation resistance for
irradiated and annealed
UHMWPE.
The fatigue strength loss due to post-irradiation melting of crosslinked
UHMWPE could be eliminated by the introduction of the antioxidant vitamin E
into crosslinked UHMWPE (Oral et al., 2004). Vitamin E can be incorporated
into UHMWPE by two methods (Fig. 3.6): one is blending of the antioxidant
into UHMWPE resin powder
followed by consolidation and radiation
 
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