Biomedical Engineering Reference
In-Depth Information
Figure 16.2
CFR-UHMWPE (Poly II) tibial component
retrieved after 24 years of implantation. The total knee
replacement was revised in 2007 for loosening. Image
provided courtesy of Francisco Medel, Ph.D., Drexel
University.
Figure 16.1
Exposed carbon fibers at the worn bearing
surface of a retrieved Poly II patellar component
showing fiber pull-out and poor adhesion to the
UHMWPE matrix
[37]
. The scanning electron micros-
copy image was taken at 350 times magnification and
provided courtesy of Francisco Medel, Ph.D., Drexel
University.
polymer matrix are both substantially improved in
PEEK biomaterials as compared with historical
UHMWPE.
A wear study by McKellop et al.
[38]
indicated
that Poly II exhibited a 10 times greater wear rate
than UHMWPE against a variety of common coun-
terfaces, such as 316 stainless steel and CoCr alloy.
Since the major mechanism of wear in CFR
polyethylene is abrasive wear that induces the
drawing out of fibers from wear surfaces
[39]
,
the interface strength is of critical importance in the
overall performance of Poly II and similar compos-
ites. Although Poly II fell out of clinical use by the
1990s, and no long-term clinical studies of the
material have yet been published, some of these
CFR-UHMWPE tibial components have managed to
survive long-term implantation in patients (
Fig. 16.2
)
[37]
. These recently revised, long-term Poly II
explants generally show wear, fatigue damage, and
delamination consistent with the short-term retrieval
reports by Wright et al.
[26,27]
.
The historical clinical experience with Poly II is
didactic in several respects for modern researchers
[37]
. Historical factors limiting the clinical perfor-
mance of CFR-UHMWPE are now thought to
include incomplete consolidation of the polymer
matrix, insufficient fiber
e
matrix interfacial strength,
and postirradiation degradation of the UHMWPE
matrix following gamma sterilization in air
[37]
.In
addition, proper control of manufacturing and pro-
cessing can be achieved with contemporary tech-
niques; fiber
e
matrix strength and stability of the
16.2.2 CFR-PEEK in Joint
Replacements
Although PEEK has been shown in the fatigue
literature to have excellent compatibility with carbon
fibers
[40]
, the prior history with CFR-UHMWPE
called for a conservative tribological evaluation
program for PEEK and its composites. The differ-
ences in matrix stability and fiber
e
matrix interfacial
strength between gamma
e
air sterilized UHMWPE
and PEEK are perhaps best illustrated by their
respective wear surfaces (
Figs 16.1 and 16.3
). In
contrast with historical CFR-UHMWPE (
Fig. 16.1
),
the worn surface of contemporary CFR-PEEK bear-
ings appears burnished and polished (
Fig. 16.3
). The
fiber
e
matrix interface of CFR-PEEK also remains
generally intact during the wear process, resulting in
relatively uniform wear of the fibers and PEEK
matrix and a smooth wear surface (
Fig. 16.3
).
The biotribology of PEEK was first reported by
researchers from Howmedica (Rutherford, NJ), who
were aware of not only the previous industrial wear
testing of composite PEEK but also the poor clinical
performance of Poly II
[11]
. Hip cups for wear
testing were initially produced by injection molding
30% discontinuous, pitch carbon fiber (Amoco,
Grade VMX-12)-reinforced 150 G PEEK resin. The
System 12 acetabular liners had a 28mm inner
