Biomedical Engineering Reference
In-Depth Information
(A)
Figure 13.17
PEEK rod system for posterior lumbar
fusion (EXPEDIUMSystem: DePuySpine, Rayhnam,MA).
(B)
documented that a PEEK rod construct assembled
with traditional titanium polyaxial pedicle screws can
survive up to 5 million cycles of loading, the failure
location for subendurance limit tests appears to be at
the set screw
e
rod interface.
Thus, current test methods based on ASTM F1717
for PEEK rods appear to challenge the set screw
e
rod
interface and provide limited insight into the fatigue
strength of the PEEK rods themselves. PEEK rod
systems currently under development for spinal
applications differ considerably in terms of the
biomaterials employed (e.g., unfilled vs. radio-
opaque grades of PEEK), implant dimensions, and
rod cross-sectional geometry. Test method develop-
ment is currently under way under the auspices of
ASTM to create a general test method for evaluating
the fatigue strength of PEEK rods that would allow
for comparisons of the properties of the rod rather
than the set screw
e
rod interface. By applying greater
rotational constraint to the rods at the set screw
e
rod
interface, it is possible to shift the fatigue failure
location during the test away from the set screw to the
center of the rod
[87]
.
PEEK rod systems are currently evaluated not
only in terms of their static and fatigue performance
in standardized experimental protocols but also in
more clinically relevant biomechanical models, such
as cadaver spine and finite element models
[65
e
68]
.
Both experimental and analytical techniques can thus
be used to evaluate load sharing between a PEEK rod
system and the thoracolumbar spine. These experi-
mental and analytical techniques are also essential to
better understand the ability of the PEEK implant
system to resist motion and stabilize the spine.
(C)
Figure 13.18
(A, B) PEEK rod system for posterior
lumbar fusion (CD HORIZON LEGACY System: Med-
tronic Spinal and Biologics, Memphis, TN). (C) Radio-
graph of a CD HORIZON LEGACY PEEK rod system,
courtesy of Dr. Todd Lanman. Note that PEEK rods
are radiolucent and not visible on X-ray, except for
the metal end caps.
