Biomedical Engineering Reference
In-Depth Information
hinder diagnostic assessment of the bone growth.
Carl McMillin, a polymer engineer at AcroMed,
was familiar with high-performance thermoplastics
and recommended PAEKs for the cage to overcome
both limitations [27,28] . The clinical and
commercial success of this medical device, which
came to be known as the Brantigan cage after its
primary surgeon champion, lay the foundation for
the current widespread use of PEEK in spine
implants.
CFR-PAEK cages were subjected to an extensive
in vitro biocompatibility and mechanical testing
program, including pull-out tests in cadaver bone,
static compression, static compressive shear, static
torsion, and dynamic compressive shear testing
( Fig. 13.6 ). The mechanical behavior of the cages
was found to be more than sufficient for the intended
application [29] . Indeed, the mechanical strength of
the CFR-PAEK cages, although not as strong as
a metal cage, could support the weight of a full-
grown elephant ( Fig. 13.6A ). Initially, the full suite
of biocompatibility testing, prescribed at the time
under the Tripartite agreement, was also performed
by the manufacturer; these tests were later repeated
for PEEK-OPTIMA after the ISO 10993 standard
was created [25] . The CFR-PAEK cages passed all of
the biocompatibility tests prescribed by the Tripartite
agreement and ISO 10993 [25] .
Starting in May 1989, PEEK and PEKEKK were
evaluated in a 2-year pilot clinical study of the spine
cage for PLIF in 26 human patients [30] . Both
implant materials were consolidated into plates with
continuous 68% by weight carbon fibers for rein-
forcement, and the cages were subsequently
machined from the plates into their final form. In the
PLIF procedure, in addition to a cage or bone spacer,
the spine is further stabilized by pedicle screws and
axial rods or plates. In the 26 patients, the initial
clinical trial evaluated 32 interbody cages (of two
designs and two materials) and 31 alternative inter-
body fusion therapies, for a total of 63 total fusion
levels. Thirty-one of the 32 cages survived 2 years of
follow-up. Clinical results were good or excellent in
21 of the 26 patients, and fair or poor results were
traced back to problems unrelated to the cage.
Because of the radiolucency of the cages, interbody
fusion could be identified in 100% (31/31) of the
cage levels. Despite the problematic clinical research
design, Brantigan's pilot clinical study [30] marks the
first implantation of carbon-reinforced PEEK and
PEKEKK in the human spine.
Figure 13.4 CFR-PEEK lumbar fusion cage, used in
concert with posterior screws and rods.
Figure 13.5 CFR-PEEK lumbar fusion cage loaded
with fragmented bone graft, prior to implantation.
Image courtesy of Bill Christianson, DePuy Spine.
develop the PLIF cage, Arthur Steffee M.D. and
John Brantigan M.D., initially conceived of a tita-
nium device that would allow bone to grow through
a columnar fenestration in the device [27] .There
were two perceived drawbacks with the initial
proposed design, the first being the stiffness of the
titanium device, which might promote stress
shielding and inhibit bone growth, and the second
being the radiopacity of the device, which would
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