Biomedical Engineering Reference
In-Depth Information
techniques have evolved considerably since 1911
when fusion was first suggested as a treatment for
Pott's disease
[13]
. During the first half of the 20th
century, posterior fusion techniques for treatment of
back pain included resection of the posterior
elements and implantation of bone fragments har-
vested from the patient's pelvis
[14]
. Dissatisfaction
with the stability and outcomes of these early
posterior surgical techniques led to the development
of interbody fusion between adjacent lumbar and
cervical vertebral bodies during the 1930s and 1950s
[15
e
17]
. Early interbody fusions were accomplished
by removal of the disc and implantation of bone graft,
either from the patient or from newly created bone
banks of cadaver bone
[16]
. However, the success of
early interbody fusion surgery also proved to be
variable
[18]
. A key limitation for interbody fusion
remained the strength and stability afforded by the
bone graft. Along with the development of posterior
instrumentation, the invention of interbody implants,
known as cages, would help expand the acceptance of
both cervical and lumbar fusion surgery starting in
the 1990s.
The first interbody implants were used in horses,
not humans. George Bagby M.D.
[19]
developed
a cylindrical stainless steel implant in 1982 known as
the “Bagby Basket” to fuse unstable spine segments
in race horses that had become paralyzed or other-
wise neurologically impaired. The concept of an
interbody implant for humans was developed in the
1980s and 1990s as the “Bagby and Kuslich” (BAK)
technique. The BAK cage of the 1990s was a threa-
ded, fenestrated titanium cylinder that could be filled
with bone graft. The BAK cage was manufactured by
Spine-Tech, Inc., Minneapolis, MN. The stand-alone
threaded cage was approved by the FDA in 1996 after
a rigorous multicenter clinical trial for implantation
by a direct anterior approach, a posterior lumbar
interbody fusion (PLIF), and a laproscopic approach
[20]
. In the FDA investigational device exemption
(IDE) clinical trial, Kuslich et al.
[20]
reported that
91.7% of patients were fused at 2 years and 95.1% of
patients were successfully fused at 4 years.
For a short time, the stand-alone BAK cage was
widely used. However, the clinical results of the
threaded cage after FDA approval did not match high
expectations raised by the prospective clinical trial
[21
e
23]
. The poor results of the BAK cage that were
reported following FDA approval are now thought to
be largely due to poor patient selection by surgeons,
technical difficulties with implantation, and its
stand-alone indication
[24]
. Consequently, the period
of the late 1990s is sometimes referred to as the
“Cage Rage” by some members of the spine
community. Regardless, Bagby and Kuslich are
credited for developing the first FDA-approved
interbody fusion cage as a stand-alone device in
combination with bone graft. The second-generation
threaded cage, the LT-Cage (Medtronic Spinal and
Biologics, Memphis, TN), was approved in 2002 in
combination with BMP. In retrospect, the Cage Rage
of the late 1990s set the stage for the introduction of
PEEK into this dynamic and growing field of spine
surgery.
13.3 CFR-PEEK Lumbar Cages:
The Brantigan Cage
In parallel with the development of threaded tita-
nium cages, PAEK biomaterials for interbody spinal
cages were also developed for posterior lumbar
interbody fusion (PLIF) in the 1980s and 1990s by
AcroMed (Cleveland, OH, now DePuy Spine,
Raynham, MA). The CFR-PEEK I/F lumbar fusion
cage was approved by the FDA in 2001, following the
aftermath of the Cage Rage. By this time, interbody
fusion surgery had evolved from the experience with
the stand-alone threaded cages. The CFR-PEEK cage
has been described in a monograph by John
Brantigan M.D., the lead surgeon inventor
[25]
.In
this section, we provide an overview of the devel-
opment history for the CFR-PEEK cage. Readers
interested in a more detailed account are encouraged
to read Brantigan's treatise
[25]
.
In contrast with the initial stand-alone design of
the threaded cage, the Brantigan cage was typically
intended to be used in concert with posterior instru-
mentation for added stability. A model of a PLIF
instrumentation system is illustrated in
Fig. 13.4
. The
PLIF procedure is conducted in two phases. In one
phase of the surgery, a cage is placed anterolaterally
between the vertebral bodies of the treated level. The
cage may be packed with bone graft (
Fig. 13.5
)or
a sponge containing BMP (e.g., INFUSE bone graft
[26]
). The cage may be reinforced with an anterior
plate, to prevent extrusion. In another phase of the
surgery, posterior screws and rods are also typically
placed to provide additional stability to the treated
level (
Fig. 13.4
).
Due to the mechanical loading requirements for
these permanent implants, the surgeons who helped
