Biomedical Engineering Reference
In-Depth Information
changes in UHMWPE materials in vivo are crucial to predict the stability of joint
implants in the long term. Commonly used accelerated aging techniques involve
high temperature and/or high oxygen concentration; for example, 80 ëC in air for
5 weeks or 70 ëC in 5 atm. of oxygen for 2 weeks (Kurtz et al., 1999). Although
neither method accurately simulates the subsurface oxidation peaks that are
commonly observed in explanted oxidized UHMWPEs, they have nevertheless
provided methods to compare the relative oxidative stability of UHMWPEs in
development to conventional UHMWPE. However, it appears that the use of
antioxidants will necessitate the revision of these aging methods into more
aggressive ones as it is very difficult to differentiate between these relatively
highly oxidation-resistant UHMWPEs (Rowell et al., 2008).
3.3.4
In vivo assessment tools
There are very few methods to measure the performance of total joint implants
in vivo. In vivo penetration (creep and wear) is determined by using two-
dimensional or three-dimensional radiographic methods (Dumbleton et al.,
2002). The most commonly used radiographic method is the Martell technique
(Martell and Berdia, 1997), which uses computer-assisted definition of boun-
daries and has been adapted to allow 3-D assessment (Hui et al., 2003). A more
accurate technique is radiostereometric analysis (RSA), which measures the
distance between tantalum beads placed in the implant and in the pelvic bone at
implantation to triangulate the exact position of the components in vivo. Both of
these methods appear to differentiate between the femoral head penetration rates
of gamma sterilized conventional UHMWPE liners into the acetabulum from
those of highly crosslinked UHMWPE liners (Manning et al., 2005; Digas et al.,
2007), which have shown 70±90% reduced in wear in hip simulator studies
(McKellop et al., 1999; Muratoglu et al., 2004). It is not clear if these methods
will be able to differentiate the in vivo performance of different highly
crosslinked UHMWPEs with very low wear rates.
3.3.5
Sterilization methods and `conventional' polyethylene
Gamma irradiation is the preferred method of sterilizing UHMWPE medical
implants. Unlike the alternative techniques of gas plasma or ethylene oxide gas
sterilization, radiation penetration through components of various thicknesses is
not limiting. Typically, a radiation dose of 25±40 kGy is used for sterilization.
Historically, gamma sterilization in air was used to sterilize total joint implants
until the mid-1990s, when it was widely acknowledged that radiation caused
oxidative embrittlement of implants in the long term.
Ionizing radiation induces free radicals by radiolytic cleavage of C±C and C±
H bonds on the polymer backbone. Owing to the relative immobility of chains of
this high molecular weight polymer, C±C bonds are mostly recombined and
