Biomedical Engineering Reference
In-Depth Information
referred to as the Graf ligament system, consisted of
braided polypropylene (PP) bands connected to Ti
pedicle screws [69] . Thus, the Graf ligament system
was a tension-only device; the PP bands could not
support compression. The long-term clinical results
of this system were mixed [69 e 72] , and Graf liga-
ments never gained widespread clinical acceptance,
nor were these devices approved for use in the United
States.
Another early pedicle-based design developed in
the 1990s was the Dynesys system, consisting of
polyethylene terepthalate (PET) cord and a cylin-
drical polycarbonate urethane bumper spanning Ti
alloy pedicle screws. In the Dynesys design, the PET
cord resists tension and the PCU bumper provides
resistance to compressive and lateral loading.
Initially commercialized by Sulzer Spine, Dynesys
was acquired by Zimmer Spine (Minneapolis, MN)
who currently markets the device. The Dynesys
system has been widely used in Europe and the
United States [73 e 76] .
Dynesys became available on the US market in
2005 after receiving FDA clearance as an adjunct to
fusion. However, this 510(k) clearance did not
include nonfusion applications of Dynesys, which
were included as a PMA application to the FDA. In
November 2009, an FDA Panel convened to evaluate
Zimmer Spine's PMA did not recommend approval
for a variety of reasons, including concern over the
proposed indications for use and complications
related to screw loosening and breakage. Since that
time, several studies in the clinical literature have
reported screw complications with Dynesys [77 e 80] .
Examination of retrieved Dynesys explants has also
demonstrated macroscopic creep of the PCU
components (on average, 4.3 of angular deforma-
tion), and beyond 5 years of implantation evidence of
surface degradation was observed on the PCU where
it was exposed to bodily fluids [79] . Zimmer Spine
developed a modification to Dynesys with the intro-
duction of hydroxyapatite-coated screws. As of this
writing, Dynesys has not been granted FDA approval
as a dynamic stabilization device, and therefore
cannot be legally marketed for nonfusion indications
in the United States.
with the polymers previously considered for poste-
rior spine implants, PEEK exhibits outstanding creep
and degradation resistance under in vivo conditions,
as we have seen in Chapter 6. During cadaver-based
biomechanical studies, PEEK rods have been shown
to offer comparable stability as Ti rods [65 e 67] .In
addition to radiolucency, the anticipated clinical
benefits of PEEK rods are hypothesized to include
improved load sharing with bone to promote fusion,
as well as decreased stresses at the pedicle screw
interface during bone healing [64,67] .
PEEK rods are currently available for fusion in the
United States from two major manufacturers (CD
HORIZON LEGACY: Medtronic Spinal and Bio-
logics, Memphis, TN; EXPEDIUM: DePuy Spine,
Raynham, MA). The rods differ in terms of their
PEEK formulation and design. EXPEDIUM rods are
fabricated from image contrast grade PEEK and have
a circular cross-section ( Fig. 13.17 ). The CD
HORIZON LEGACY is fabricated from unfilled
PEEK-OPTIMA and has an elliptical cross-section
( Fig. 13.18 ). The LEGACY rods have metallic end
caps for visualization in radiographs. If the physician
wants to directly visualize the rods, they can be
detected using CT scans [82] . A description of the
rationale, surgical technique, and case studies of the
CD HORIZON LEGACY system have been pub-
lished [64] .
Fatigue testing of PEEK rod systems is chal-
lenging because of their flexibility and notch sensi-
tivity [83,84] (see also Chapter 5 on fatigue). Care
should be taken in the design of the tulip and set
screw where the PEEK rod is connected to the
pedicle screw. In addition to the tulip and set screw
design, the screw tightening torque can also influence
the fatigue behavior of PEEK rods [85] . The inter-
action between the tulip, set screw, and PEEK rod
creates a geometric stress concentration and potential
initiation site for fatigue failure [67] .
Conventional standardized fatigue test methods
(i.e., ASTM F1717 [86] ) for metallic fusion rod
systems need to be modified for flexible fusion
constructs incorporating PEEK rods. Specifically,
ASTM F1717 provides a basis for both static and
dynamic characterization of fusion constructs, but the
fatigue test methods specified in the standard
recommend load-controlled testing based on static
failure. Previous studies suggest that displacement
control may be a more relevant method for testing
semirigid fusion constructs incorporating PEEK rods
[67,68] . Furthermore, while these studies have
13.7.2.2 PEEK Rods
PEEK rods were developed for both fusion and
nonfusion applications [64 e 68,81] , but today only
have FDA clearance as adjuncts to fusion. In contrast
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