Biomedical Engineering Reference
In-Depth Information
of the crystallinity loss during post-irradiation melting (Oral et al., 2006a). Its
crosslink density is comparable to that of 100 kGy irradiated and melted
UHMWPE; thus wear resistance is comparable as well (Oral et al., 2006a).
Another second generation method introduced in crosslinked UHMWPE was
sequential irradiation and annealing. This material is irradiated at 30 kGy and
subsequently annealed below the melting point and this process is repeated three
consecutive times. The rationale for using this approach was to enhance chain
mobility during thermal annealing by using lower doses of irradiation at a time
and eliminate free radicals more efficiently. The terminal gamma sterilization
was replaced by gas plasma sterilization so as not to induce new free radicals.
While in vitro wear resistance is high, the free radical concentration of X3
TM
is
low but still detectable at a level comparable to that of conventional, gamma
sterilized UHMWPE (Wang et al., 2006). It is too early to determine what the
effect of these free radicals will be on the long-term performance of this material
as a joint bearing surface.
Another second generation UHMWPE was ArComXL
TM
, which was prepared
by solid-state deformation following radiation crosslinking. The manufacturer
states that the tensile mechanical properties of the material are 30% improved in a
directional manner. This material also had 90% less free radicals than control
UHMWPE (Kurtz et al., 2006) as mechanical deformation is known to also induce
mobility in the crystalline lamellae and enhance free radical recombination
(Muratoglu et al., 2005a). It showed high oxidation resistance after accelerated
aging for 4 weeks, while conventional UHMWPE oxidized heavily.
The clinical introduction of these second generation crosslinked UHMWPEs
is based on the promise that they can improve the longevity of joint implants by
obtaining wear, oxidation and fatigue resistance simultaneously. There is a
continued effort in the community to improve the material properties to make it
more forgiving under more demanding conditions. This is especially important
as more young and active patients (age < 60) are undergoing total joint
replacement with the expectation of full functionality and long implant life.
As much as we, orthopedic researchers, would like to believe new materials
will solve more problems associated with the outcomes of total joint replace-
ment, some material modifications in UHMWPE have led to clinical failures. I
briefly discuss these in the next section.
3.5
Failures in material development
3.5.1
PTFE
Dr Charnley's rationale for using PTFE as part of the joint bearing couple was to
minimize friction between the articular surfaces. However, surprisingly, PTFE
showed tremendously high wear rate with most implants having to be revised
within a couple of years (Charnley, 1963). Similarly, modifications of PTFE
