Biomedical Engineering Reference
In-Depth Information
Table 3.3 Second generation highly crosslinked UHMWPEs
Tradename
Manufacturer Radiation Post-irradiation
Sterilization
dose
treatment
method
(kGy)
E-Poly (E1)
TM
Biomet
100
Vitamin E diffusion
Gamma
irradiation
X3
TM
Stryker
90
Sequential annealing
Gas plasma
(330)
ArComXL
TM
Biomet
50
Mechanical deformation Gas plasma
crosslinking, the other is the diffusion of the antioxidant into radiation cross-
linked UHMWPE. In addition to protecting UHMWPE against oxidation
initiated by radiation-induced free radicals (Oral et al., 2004, 2006c), vitamin E
reacts with primary free radicals induced on the polymer chains during radiation
and prevents crosslinking with increasing concentration (Parth et al., 2002; Oral
et al., 2005). In fact, crosslink density reaches a saturation level and at a vitamin
E concentration of 0.3 wt% (3000 ppm) or above, it is not possible to crosslink
UHMWPE to a level equivalent to that of 100 kGy irradiated virgin UHMWPE
(Oral et al., 2008). Therefore, if blending followed by radiation crosslinking is to
be used, the antioxidant concentration and the radiation dose have to be carefully
optimized to obtain good wear resistance and enough vitamin E to protect the
UHMWPE against oxidation in the long term. The vitamin E-containing
UHMWPE currently in clinical use (introduced in hips in 2007 and in knees in
2008) is prepared by post-irradiation diffusion of vitamin E using a two-step
doping and homogenization process to ensure the penetration of vitamin E
throughout the entire thickness of components (Oral et al., 2007). This material
has a uniform vitamin E concentration profile.
Vitamin E-stabilized, highly crosslinked UHMWPE has about 35% more
fatigue resistance than irradiated and melted UHMWPE due to the elimination
3.6 Vitamin E incorporation methods in highly crosslinked UHMWPE.
