Biomedical Engineering Reference
In-Depth Information
Fig. 16.8 [48,49]
. The most likely application for
such all-polymer PEEK bearings is currently thought
to be cervical total disc replacement (see Chapter 13).
Researchers have also evaluated other poly-
aryletherketone (PAEK) polymers (e.g., PEK), as
well as carbon nanotube-reinforced PEEK, for all-
polymer bearings
[48,49]
. Under comparable test
conditions, the PAN fiber CFR-PEEK self-
articulating couple has consistently yielded the
lowest wear among those studied thus far (
Fig. 16.8
).
Austin et al.
[48]
have also evaluated the wear
behavior of all-polymer PEEK bearings under more
aggressive loading conditions than typically used
in standardized pin-on-disc studies. After 2 million
cycles of testing under the “reference” conditions
described in
Table 16.1
, the authors systematically
increased the load exerted on the pin from 80 N to
320 N. They found that for the PEEK and CNF-
PEEK all-polymer bearings, increasing the load
resulted in localized surface failure and higher wear
rates (
Fig. 16.9
). Of the bearing combinations studied,
only PANfiber, CFR-PEEKexhibited relative stability
under aggressive loading conditions (
Fig. 16.9
).
Recently, tribological tests have been reported for
PEEK biomaterials using either a ring-on-plate
[50]
or using a unidirectional pin-on-disc tribometer
[51]
.
Interestingly, Xiong et al.
[50]
studied the friction
and wear behavior of PEEK matrix composites
reinforced with up to 20% UHMWPE powder.
Although these experiments included calf serum
lubrication,
Figure 16.8
Summary of wear factor results for all-
polymer PAEK articulations
[48,49]
. Each bar repre-
sents the average value for one independent test
series. CFR, carbon fiber reinforced; CNF, carbon
nanofiber reinforced; PAN, polyacrylonitrile fibers;
Pitch, pitch fibers.
As recounted in the previous section, early
experiments conducted by Wang et al.
[10]
in the late
1990s indicated that CFR-PEEK may damage CoCr
femoral heads and lead to higher wear rates than
observed articulating with ceramics. Since that time,
there has been increased appreciation for the metal-
lurgy of CoCr bearings, and the development of
newer “high carbon” (HC) and “low carbon” (LC)
grades of CoCr alloy. Scholes and Unsworth
[16]
compared the wear of CFR-PEEK composites with
these newer HC and LC grades of CoCr in a recent
study. As in the experiment by Wang et al.
[10]
, two
CFR-PEEK formulations were evaluated including
PAN and pitch fibers, but in Scholes and Unsworth's
case, medical grade PEEK-OPTIMA LT1 was used as
the polymer resin instead of industrial grade PEEK.
The wear factors for both PAN and pitch-based CFR-
PEEK OPTIMA grades was extremely low against
the CoCr alloys, ranging between 0.1 and 0.2
the
unidirectional motion
of
the
10
6
mm
3
/N m (
Table 16.2
). Scholes and Unsworth
were able to reproduce Wang's earlier finding, that
CFR-PEEK OPTIMA exhibited improved wear
resistance relative to unfilled, virgin PEEK when
articulating against CoCr (
Table 16.2
). In a second
series of tests, Scholes and Unsworth
[15]
evaluated
the wear resistance of PAN and pitch-based CFR-
PEEK OPTIMA against alumina and zirconia-
toughened alumina ceramics. The wear factors for
these ceramic-on-CFR-PEEK bearing configurations
were also extremely low, ranging between 0.1 and
0.2
Figure 16.9
Multidirectional pin-on-disc wear test
results for all-polymer unfilled PEEK, CFR-PEEK, and
CNF-PEEK articulations under constant and progres-
sively increasing loads
[48]
. Of the materials studied,
CFR-PEEK exhibited the lowest wear rates under
elevated loading conditions. Image provided courtesy
of Heather Austin, University of Waterloo.
10
6
mm
3
/Nm (
Table 16.2
), and comparable to
the CoCr-on-CFR-PEEK tests (
Table 16.2
).
The results of tribological testing of all-polymer,
medical grade PEEK articulations is summarized in
