Biomedical Engineering Reference
In-Depth Information
As the use of ISPs has grown in the United States
following FDA approval of the X-STOP in
November 2005
[60]
, reports of complications,
including early revision and spinous process frac-
tures, are beginning to surface in the literature
[61
e
63]
. For example, Verhoof et al.
[63]
reported
poor results when the X-STOP was implanted in
patients with degenerative spondylolisthesis. Using
a biplanar radiographic and CT examination
protocol, Kim et al.
[62]
observed a higher spinous
process fracture rate (22% of treated levels) than was
previously reported (1%) in the randomized clinical
trial that was performed for the FDA
[59]
.
The recent reports of complications underscore the
importance of better understanding the indications
and contraindications for ISP implantation. At
present, it still remains under clinical debate which
symptomatic spine patients are most likely to obtain
lasting pain relief from the targeted benefits of ISPs.
Although patient selection criteria for ISP utilization
are currently open to debate, the role of PEEK as
a suitable biomaterial for this application remains
unquestioned.
(A)
(B)
13.7.2 Pedicle-Based Posterior
Stabilization (PEEK Rods)
Pedicle-based systems represent another major
design category for posterior dynamic stabilization
systems. In this motion-preserving design paradigm,
a flexible (or “semiflexible”) element is substituted
for the metallic rod that is fixed to the posterior spine
by pedicle screws. The shift away from rigid instru-
mentation using metallic rods was driven by the
desire to reduce increased stress on adjacent discs
and facet joints that were thought to contribute to the
degeneration of adjacent levels
[64]
. In addition,
stress shielding of the vertebral column at the level of
rigid instrumentation using metal rods was consid-
ered to result in decreased stimulus for bone growth
during fusion. As a result, there has been growing
interest in using flexible or semiflexible posterior
fusion constructs to provide increased mobility and
allow load sharing while stabilizing the spine
[64
e
68]
.
Figure 13.16
Extension-limiting, interspinous device,
incorporating a PEEK spacer, and an anterior
posterior
radiograph showing the device implanted in the lumbar
spine (X-STOP, Medtronic Spinal and Biologics, Sunny-
vale, CA).
e
Spine), and the DIAM device, fabricated from sili-
cone encased within a woven polyethylene tere-
pthalate fiber jacket (DIAM: Medtronic Spinal and
Biologics)
[11,55,56]
. Dynamic ISPs are currently
under clinical evaluation in IDE studies for PMA
applications for the US FDA
[11,55,56]
.
13.7.2.1 Early Pedicle-Based Systems:
Graf Ligaments and Dynesys
In the 1990s, flexible posterior stabilization
systems emerged in Europe. One such device,
