Biomedical Engineering Reference
In-Depth Information
Nevertheless, due to the loading demands in the
spine (especially in the lumbar region) and the
competing design goals of strength and thin implant
cross-sections to promote internal bone growth, the
literature suggests that implant fracture and debris
production could be considered important potential
failure modes for PEEK biomaterials in the spine.
Finite element analysis, in particular, has proven to
be an effective tool to evaluate the fracture risk of
PEEK implant designs in the spine
[45]
. The effect of
wear debris on the spinal cord has been investigated
in a rabbit model
[54]
. After injecting particle loads
into the spinal canal of rabbits, researchers have
concluded that PEEK particles appear “harmless” to
the spinal cord.
clinical evaluation
[11,55,56]
, but their gross
biomechanical function in the spine appears to be
fairly similar
[57]
. By distracting the posterior
elements to insert the ISP, the posterior intervertebral
disc is placed into flexion (or local kyphosis), the
spinal canal is locally widened, and the pressure on
the posterior disc is relieved
[57]
. Thus, ISPs are
intended to stabilize the spine in extension, but
provide little to no resistance to flexion, axial rota-
tion, and lateral bending
[57]
. The posterior distrac-
tion created by implanting an ISP also relieves nerve
root impingement, one of the causes of low back and
leg pain.
ISPs were initially designed and fabricated from
Ti alloy, serving as rigid or “static” spacers between
the interspinous processes. Examples of rigid ISPs
initially fabricated from Ti include the Wallis and
X-STOP posterior dynamic stabilization systems
[55,56]
. As PEEK gained widespread exposure to the
spine community through interbody fusion, ISP
implant designers developed second-generation
devices in which Ti was replaced with PEEK. For
example, Senegas
[58]
noted that the developers of
the Wallis devices converted their titanium inter-
spinous component to PEEK in 2004. The St Francis
developed X-STOP also converted from Ti alloy to
PEEK-OPTIMA as a spacer for its Interspinous
Process Decompression System (
Fig. 13.16
)
[11]
.
The X-STOP is currently produced by Medtronic
Spinal and Biologics (Memphis, TN). The biocom-
patibility and performance of PEEK in the X-STOP
is reviewed as a case study in Chapter 7 (see
Section 7.7).
Among ISPs, the X-STOP has the most extensive
track record in the clinical and biomechanical liter-
ature, as summarized recently by Kabir et al.
[56]
,
including a 2-year multicenter randomized control
trial that was performed as an Investigational Device
Exemption study in support of its Premarket
Approval (PMA) application for the FDA
[59]
. At the
time of this writing, the X-STOP is the only FDA-
approved posterior dynamic stabilization device that
can be legally marketed as such in the United States.
The approved indications for the X-STOP include
treatment of a confirmed diagnosis of lumbar spinal
stenosis in patients aged 50 years or older
[60]
.
The designs of ISPs vary from rigid or static
devices, like the X-STOP, to so-called “dynamic”
designs that are capable of deformation during
extension. Examples of dynamic ISPs include the
Co-Flex device, fabricated from Ti alloy (Paradigm
13.7 Posterior Dynamic
Stabilization Devices
The continued availability, radiolucency, and
biomechanical success of PEEK in spinal fusion
applications have stimulated interest in using the
biomaterial in posterior dynamic stabilization
devices. In contrast with fusion, which aims to
stabilize a painful and diseased spinal unit by elim-
inating motion, the general aim of dynamic stabili-
zation is to restrict motion and forces in directions
that cause pain, while allowing motion in the
asymptomatic directions
[11,55]
. Posterior dynamic
stabilization devices were developed to treat the
degenerated lumbar spine, but the precise indications
for treatment are extremely broadly advocated in the
literature, ranging from discogenic back pain, spon-
dylestheis, and spinal stenosis
[11,55]
. Although the
specific biomechanical mechanisms whereby poste-
rior dynamic spinal devices are intended to function
vary by design, the devices themselves fall into two
general categories depending on whether they are
implanted between the spinous processes or are
installed as flexible members using pedicle screws.
PEEK has been successfully incorporated into both
types of spinal dynamic stabilization devices, most
notably as a replacement for metallic interspinous
process spacers (ISPs) or metallic fusion rods.
13.7.1 Interspinous Process
Spacers
Several different designs of ISPs have been
developed in recent years and are currently under
