Biomedical Engineering Reference
In-Depth Information
such as glass-filled PTFE resulted in similar wear rates (Kurtz, 2009b). To date,
there have been attempts at decreasing the wear of joint implants by decreasing
friction, but the relationship is yet to be established.
TM
3.5.2 Carbon fiber-reinforced UHMWPE (Poly II
)
PolyII
TM
was a carbon fiber-reinforced UHMWPE composite, whose develop-
ment was based on improved wear properties of the composite. However, the
material was a clinical failure (Wright et al., 1988) due presumably to the lack of
integrity of the carbon fiber±polymer matrix interface under load among other
factors (Connelly et al., 1984).
3.5.3 Highly crystalline UHMWPE (Hylamer
TM
)
Hylamer
TM
was a highly crystalline UHMWPE with extended chain mor-
phology and increased yield and ultimate tensile strength as well as a higher
elastic modulus. Although in vitro wear tests initially showed wear behavior
similar to conventional gamma sterilized UHMWPE (McKellop et al., 2000),
clinical revision rates due to excessive wear were very high (Chmell et al., 1996;
Livingston et al., 1997). One reason could have been oxidation caused by
gamma irradiation and the possibly more severe effects of oxidation on this
highly crystalline material compared to conventional UHMWPE (Collier et al.,
1998). Hylamer was gamma sterilized in air for a period of time, after which it
was gamma sterilized in nitrogen and after 1995, it was sterilized using gas
plasma. Although the reasons remain unclear, it appeared that the clinical
performance of Hylamer
TM
was not an improvement over that of conventional
UHMWPE and the use of it was discontinued in 1997.
3.6 Joint-specific challenges and alternatives
3.6.1 Hip
A prevalent reason for revision surgery is dislocation of the implant in total hip
implants (22.5%; Bozic et al., 2009). Typically, larger femoral head sizes are
used to improve the stability of the joint and reduce dislocation rates. While this
is one of the main advantages of using metal-on-metal or ceramic-on-ceramic
implants, this presented a problem with conventional UHMWPE because the
wear rate was significantly increased with increasing head size (Livermore et al.,
1990). However, highly crosslinked UHMWPE wear appears to be largely
independent of femoral head size, possibly allowing the use of thinner
UHMWPE liners (Muratoglu et al., 2001b). This presents a new challenge, as
the use of thin liners increases the risk of fracture in these liners and improving
the fatigue and mechanical properties become essential.
