Biomedical Engineering Reference
In-Depth Information
reported that “good pain relief and improvement in
function” was achieved for up to 2 years
[91]
. At the
time of publication, only 15 of the 225 devices
(6.7%) had reached the 2-year milestone of the
ongoing clinical study
[91]
.
A cervical all-PEEK TDR, known as the NUNEC,
was also developed by Pioneer Surgical Technology
[97,98]
. According to a recent conference abstract,
the cervical TDR was tested for 10 million cycles in
accordance with ASTM F2423, followed by an
additional 10 million cycles in accordance with ISO
18192
[98]
. The wear rates were found to be, on
average, 0.3 mm
3
per million cycles for both test
methods, and these rates were judged to be compa-
rable with other cervical TDRs. The clinical results
of the NUNEC are currently being evaluated by
a pilot study. At the 2009 SAS annual meeting,
clinical data were reported for 17 patients at 3
e
6
months of follow-up
[97]
.
In the first full-length journal article dedicated to
wear in PEEK cervical discs, researchers from Aes-
culap AG compared contemporary CoCr/UHMWPE
couples with PEEK-OPTIMA LT1, CFR-PEEK (30%
PAN fibers), and polyaryletherketone (PEK) all-
polymer articulations
[89]
. The study by Grupp et al.
[89]
provides the most extensive and detailed peer-
reviewed data available to date on the wear test
methods, rates, and wear particle characteristics for
three groups of all-polymer PEEK and PEK articula-
tions intended for the cervical spine. All the polymer
bearings used in this study were based on the design of
the Activ-C cervical artificial disc. The UHMWPE
was GUR 1020 gamma sterilized in nitrogen (30 kGy).
Testing was conducted in accordance with ISO 18192.
The PEEK and PEK bearings exhibited comparable
wear rates to CoCr/UHMWPE, whereas the CFR-
PEEK bearings had significantly lower wear rates
(
Fig. 13.20
). The size and morphology of the PEEK
and PEK wear particles fell within the range previ-
ously observed for CoCr/UHMWPE articulations.
Considered together, recent studies suggest that
PEEK biomaterials provide new design opportunities
for all-polymer disc arthroplasties. Both cervical and
lumbar PEEK artificial discs are currently in clinical
trials; however, these studies are still in progress, and
final data have yet to be published in peer-reviewed
journals. Although the first designs in clinical use
were fabricated from unfilled PEEK, there is not yet
consensus on the optimal PEEK biomaterial for all-
polymer disc bearings. Basic science pin-on-disc
studies
[99,100]
suggest that all polymer CFR-PEEK
Figure 13.20
Comparison of volumetric wear rates for
all-polymer PEEK and PEK articulations with conven-
tional, gamma nitrogen-sterilized UHMWPE/CoCr
cervical TDRs. Adapted from Ref.
[89]
.
*
CFR-PEEK
bearings were significantly lower than the other cervical
disc combinations tested (
p
<
0.05).
articulations fabricated with PAN carbon fibers
generate less wear than unfilled PEEK articulations
(e.g., Chapter 16), and these findings are corrobo-
rated by recent cervical TDR tests conducted in
a multidirectional spine wear simulator (
Fig. 13.20
)
[89]
. Similarly, whether evaluated on a pin-on-disc
experiment or in a spine wear simulator (
Fig. 13.20
)
[89]
, all-polymer PEK bearings appear to offer rela-
tively modest improvements relative to PEEK. It
remains to be seen if all-polymer PEEK bearing
performance is design specific, or whether all-poly-
mer disc replacements are equally well suited to the
cervical and lumbar spine. Although scientific debate
is sure to continue on these topics for years to come,
currently available data would suggest that PEEK
biomaterials are strong candidates for disc arthro-
plasty applications.
13.9 Summary
In summary, PEEK biomaterials have over
a decade and a half of successful clinical history in
load-sharing, fusion applications in the spine. Thus,
the clinical history of PEEK biomaterials is heavily
weighted by the cage experience from the spine
fusion literature. PEEK is playing a growing role in
spine implants for a combination of reasons,
including the clinical need for improved treatments
for operative treatment of persistent low back pain.
The radiolucency of PEEK is another key advantage
over metals, allowing visualization of the critical soft
