●EFFECTS OF A SADDLE JOINT CERVICAL DISC REPLACEMENT ON IMPLANTED LEVELS
*Awada, J; ***Ryan T; ***Carannante F; ****Louis R; +**Valdevit, A; *****Errico, JP; *Errico T
*NYU Medical Center, New York, NY, +**Lutheran Medical Center, Brooklyn, NY, *****Stryker Spine - MPU, Summit NJ
[email protected]
Introduction: While the engineering principles behind intervertebral
In lateral bending, while the prosthesis displayed over five times the
disc replacement devices may vary, all are designed to restore motion.
motion seen with the use of the cervical plate, this difference was not
An ovine animal model was employed to investigate the efficacy of a
statistically significant (P>0.05).
cervical intervertebral disc prosthesis based upon a saddle joint design.
Extension
The investigators hypothesized that implantation of the prosthesis would
restore motion comparable to that displayed by the intact specimen.
P<0.05
Specimen loading was accomplished using a materials testing machine
in displacement control at 1mm/s to a central deflection of 5mm
resulting in an angular deflection of approximately 4o at the C3-C4
implantation site. Intervertebral deflections were continuously recorded
for 6 cycles at C3-C4 with the aid of radially deflecting transducers
located anteriorly and laterally across the C3-C4 site (Figure1B).
150
Percent Intact
Materials and Methods: Five ovine specimens from C1 to C7 were
prepared and mounted in a custom fixture that permitted flexion,
extension and lateral bending without disruption of specimen alignment.
(Figure 1A)
P>0.05
P<0.05
125
100
75
50
25
0
Intact Extension CerviCore Extension Fusion Extension
Specimen Condition
Figure 3. Results of motion at C3-C4 in extension.
ACTUATOR
Flexion
LOAD CELL
YOKE
P<0.05
PIN
150
PIN
PIN
ADJUSTMENT
SLOT
BOVINE LUMBAR SPINE SEGMENT L1-S1
FIXED
BALL BEARING SLIDE
A
B
Test Fixture
Figure 1. A) Experimental testing configuration. B) The radially
deflecting transducer is capable of continuously recording deflection
across the arc.
The transducers were calibrated prior to use and mounted to metallic
posts using an adhesive. The maximum deflection at each of the
locations cited was extracted from the sixth loading cycle. Specimens
were testing in the intact condition and again following surgical
intervention that consisted of an intervertebral cervical disc prosthesis
implantation at C3-C4 (Figure 2). The prosthesis consists of a saddle
joint with varying radii in the transverse and sagittal direction so as to
permit relative motion between the proximal and distal component. A
subsequent surgery was performed that included a cervical plate (Reflex,
Stryker Spine, Allendale, NJ) across the C3-C4 site.
Deflection data in each loading mode and under each condition was
expressed as a percentage of the intact specimen deflection. Statistical
comparisons were performed using an ANOVA with a Newman-Keuls
post-hoc test.
Figure 2. The CerviCore
cervical
intervertebral
Results: In
disc prosthesis is based extension, no
on a saddle joint design. statistically
In this particular version, significant difference
two 3.5mm cervical (P<0.05) was found
screws in conjunction across the
with
metal
spikes intervertebral disc at
provide initial fixation.
the C3-C4 site due to
the implantation of
the prosthesis (108%
of intact) (Figure 3). However, the
implantation of the plate significantly reduced
motion (8.6% of intact). In flexion, the
implantation of the prosthesis did not
statistically significantly (103.6% of intact) affect the motion at C3-C4
as compared to the intact level (P>0.05). In this loading mode the
Reflex plate displayed statistically significantly reduced motion (21.2%
of intact) with respect to the intact level. (P<0.05) (Figure 4).
Percent Intact
PIN
P>0.05
P<0.05
125
100
75
50
25
0
Intact Flexion
CerviCore Flexion
Fusion Flexion
Specimen Condition
Figure 4. Results of motion at C3-C4 in flexion.
Discussion: In all three loading modes tested, the disc prosthesis
demonstrated comparable mobility (102.6%) relative to the intact
specimen while the cervical plate displayed a mean value of 16% of
intact (P<0.01). Unlike lumbar disc motion, cervical disc kinematics
cannot be approximated by a single center of motion. Current lumbar
devices encompass single centers of motion be they fixed or mobile. In
the case of the cervical spine the unco-vertebral joints necessitate not
only a mobile center of rotation in the anterior to posterior direction but
also a center that moves in the medial to lateral direction as well. The
saddle joint configuration in this prosthesis permits simultaneous rolling
and translation in each of the loading modes examined. The saddle
design permits full range of motion while retaining sufficient material
thickness (for strength and durability) and minimizing the overall height
of the device to permit implantation.
Conclusion: Under this testing regimen, a cervical intervertebral disc
prosthesis based on a saddle joint design is a viable alternative as a
motion restoring device in the spine. It should be noted that restoration
of motion at a surgical site should also consider reduction of mechanical
effects on adjacent levels.
***The Stevens Institute of Technology, Hoboken, NJ, ****The Cooper
Union for the Advancement of Arts and Science, New York, NY,
*****Stryker Spine - MPU, Summit NJ
52nd Annual Meeting of the Orthopaedic Research Society
Paper No: 1210