Biomedical Engineering Reference
In-Depth Information
As younger patients require joint replacements, hip resurfacing is becoming
more popular (Daniel et al., 2004) due to the conservation of bone stock and the
potential reduction rate in dislocation. The improvement of the mechanical
properties of highly crosslinked UHMWPEs may present a feasible UHMWPE
to be used in this more demanding application.
3.6.2 Knee
Fatigue-induced delamination wear is a major concern in total knees. The lower
fatigue resistance of highly crosslinked UHMWPEs compared with unoxidized
conventional UHMWPE is more of a concern due to increased contact stresses
in the knee. This can be especially important for adverse cases such as cam
impingement of the post, which is a geometrical design feature in some total
knee implants to provide stability to the joint in the absence of a posterior
cruciate ligament. Highly crosslinked UHMWPEs have been used in the knee
for close to a decade and there is growing interest in them (Durasul
TM
, Zimmer
since 2001, Prolong
TM
, Zimmer since 2002, X3
TM
, Stryker since 2005, XLK
TM
,
DePuy since 2005, XLPE
TM
, Smith and Nephew since 2008 and E-Poly
TM
,
Biomet since 2008), but there are currently no clinical studies reporting on their
performance.
3.6.3
Spine
I have discussed the development of UHMWPE mainly in total hip and knee
replacements because these large volume operations and the patient outcomes
associated with them have been the driving force behind these developments. At
the same time, the same articular pair, cobalt±chromium against UHMWPE, is
used in the replacement of other joints. Most notably, two aritifical discs
(CHARIT
Â
and ProDisc), which are used in total (intervertebral) disc replace-
ment (CHARIT
Â
was discontinued as of March 2010 and will be replaced by In
Motion, DePuy Spine) have been approved for single level replacement in the
US in 2004 and there are several others in development (Kurtz, 2006). The
driving force behind this development was the belief that `dynamic stabilization'
of the vertebrae following the degeneration of the supporting disc would
perform better functionally than the fusion of the adjacent segments, the most
common alternative.
The total disc replacement operation is relatively new and the interpretation
of its outcomes is complex as the problems surrounding the spine are less well
established especially in the presence of a synthetic device. The expected benefit
of these devices is the reduction in the operation rate at the adjacent levels,
which is known to be a progression associated with fusion (Lee, 1988).
One concern in spine implants is the use of conventional, gamma sterilized
UHMWPE based on the long history of the occurrence of wear and oxidation of
