Arthroplasty Bearing Surfaces - PEEK Biomaterials - page 254

improved wear performance [21] . Second-generation
metal-on-metal (MOM) and ceramic-on-ceramic
(COC) were also carefully studied in the 1990s as
alternate bearings for hip arthroplasty [22] . Thus,
highly cross-linked UHMWPE, MOM, and COC
bearings had already emerged in clinical use during
the late 1990s as alternatives to conventional
UHMWPE [22] , at around the same time as CFR-
PEEK came to be first considered for similar appli-
cations. In this section, we first review the clinical
experience with carbon-UHMWPE composites,
including CFR-UHMWPE (Poly II), as the back-
ground for understanding the development of CFR-
PEEK for total joint replacements.

[29] examined a composite containing 25 wt%
graphite powder in UHMWPE and, during prelimi-
nary pin-on-disc testing in water, they found it to
have one-seventh to one-thirtieth of the wear of
UHMWPE when tested against CoCr alloy. Simi-
larly encouraging results were initially predicted for
Poly II. A study by Ainsworth et al. [23] found CFR
Poly II to have greater compressive and flexural
yield strengths and elastic moduli, in addition to
a lower wear rate and higher stiffness. Rostoker and
Galante [30] observed that carbon fiber reinforcement
was responsible for a significant decrease in creep
strain in the composite. A study by Wright et al. [31]
reported that CFR-UHMWPE was 88% stiffer than
regular UHMWPE and was capable of withstanding
17% greater compressive loads.
Biocompatibility studies of CFR-UHMWPE con-
ducted during the 1970s and 1980s also determined
the response to be satisfactory [32] . Groth and
Shilling [33] found the carbon in Poly II to exhibit
minimal foreign body reactions after clinical
implantation of 11 to 61 months. Rushton and Rae
[34] investigated the intra-articular response to
particulate Poly II in mice and found it to be similar
to that of neat UHMWPE.
Other studies, conversely, cast doubt on the ultimate
viability of carbon-UHMWPE composite usage in hip
and knee joint replacement, especially after clinical
exposure. After longer duration pin-on-disc studies
using 25% graphite powder-filled UHMWPE, Ros-
toker and Galante [35] reported that the short-term
improvement in wear resistance appeared to be lost.
Sclippa and Piekarski [36] observed poor performance
of the reinforced composite in tension due to fiber
damage during injection molding as well as noting
generally poor interfacial fiber e matrix bond strength.
Others also reported issues with polymer molding and
incomplete particle fusion d Wright et al. [26,27]
examined clinical results from retrieved components
and found that the presence of carbon fibers prevented
a firm locking of the polyethylene particles that
constituted the matrix of Poly II knee-replacement
components. Connelly et al. [24] also found little
fiber e matrix adhesion in the composite, such that the
reinforced UHMWPE had approximately an eight
times greater fatigue crack growth rate compared to
plain UHMWPE. An example of poor adhesion of
carbon fibers to the UHMWPE matrix is illustrated
by scanning electron microscopy of the worn
bearing surface of a retrieved patellar component
( Fig. 16.1 ).

16.2.1 Carbon-UHMWPE
Composites in Joint Replacement
As was the case prior to the introduction of
composite femoral stems, attempts to modify
UHMWPE in the 1970s and 1980s using carbon
powder or carbon fiber to improve its clinical wear
performance were not successful [20] . One carbon-
UHMWPE composite, commonly known as Poly II
(Zimmer Inc., Warsaw, IN), was developed commer-
cially and used clinically. The CFR-UHMWPE was
reinforced by chopped, randomly oriented carbon
fibers in a direct compression-molded UHMWPE
matrix. The carbon fiber reinforcement was initially
considered responsible for improved wear behavior
relative to UHMWPE during initial experimental
testing conducted by the manufacturer [23] . However,
further studies ultimately revealed that such perfor-
mance came at the expense of the ductility, crack
resistance, and the fiber e matrix interface of the
composite [24] . Subsequent wear studies also showed
evidence of fiber disruption at the surface and abrasive
wear of the metallic counterface [25] . Thus, after its
clinical introduction, Poly II was found to exhibit wear,
fracture, and extensive delamination [26 e 28] ,and,as
a result, the material was eventually withdrawn [20] .
The clinical failure modes of carbon-UHMWPE
composites were not well simulated by the standard
testing and analysis methods available at the
time the materials were developed. Indeed, initial
evaluations of carbon powder-reinforced and CFR-
UHMWPE composites provided generally encour-
aging results. Several studies indicated enhanced
tribological and mechanical properties of carbon-
UHMWPE composites compared to neat UHMWPE
[23,29 e 31] . For example, Galante and Rostoker