Biomedical Engineering Reference
In-Depth Information
the stress on the posterior facet joints. In summary, the constraints of ProDisc-C are more rigorous
than those of Mobi-C.
In the present study, these two artificial discs were sized so as to restore the intervertebral height
to 5 mm at the index segment. The primary dimensions of both artificial discs were 14 mm × 12 mm
(width × length). The metal endplates in both the ProDisc-C and Mobi-C were cobalt alloy (CoCrMo).
The Young's modulus and Poisson's ratio of the CoCrMo were 114 GPa and 0.3, respectively, and the
corresponding material properties of the UHMWPE were 1 GPa and 0.49, respectively.
The anterior longitudinal ligament and the whole intervertebral disc were completely removed,
and the intervertebral space was replaced with ProDisc-C (Figure 16.4b) or Mobi-C (Figure 16.4c).
Since the endplate is coated with a porous layer, the implant endplate was considered to be totally
integrated with the vertebral bone. A frictionless finite sliding contact was applied between the
interaction surface of the UHMWPE inlay and the CoCrMo alloy endplates in both ProDisc-C
and Mobi-C. The loading and boundary conditions were the same as the intact healthy cervical
spine.
16.4.2 S
pinal
m
otion
after
t
otal
d
iSc
r
eplacement
The rotational motions increased at the operative level (C4-C5) after total disc replacement with
either ProDisc-C or Mobi-C (Figure 16.5). With the ProDisc-C, the ROM increased by between
24% and 438% during flexion, extension, axial rotation, and lateral bending. With the Mobi-C, the
corresponding value increased by between 72% and 408%. The ROMs of ProDisc-C were lower
than Mobi-C during all spinal movements, except in extension.
As shown in Figure 16.5, both ProDisc-C and Mobi-C models allowed for up to five times greater
extension than the fusion model. The anterior longitudinal ligament and the disc play an important
role in constraining extension. During distraction, the disc supported 70% of the load. After exci-
sion of the anterior longitudinal ligament and intervertebral disc, the index site lost its ability to
constrain extension. Although the metal endplate in both ProDisc-C and Mobi-C was integrated
with the bony structure, the artificial disc component was separated and not constrained in the axial
direction.
At the adjacent segment, the ROM increased by 10% at C5-C6 during extension with ProDisc-C,
and increased by 20% during flexion with Mobi-C. Compared to the operative level, the ROM at the
adjacent level remained almost unaffected with both ProDisc-C and Mobi-C. Due to the increased
motion at the operative level and the limited compensatory motion at the adjacent segment, the
entire cervical segmental motion (C3-C6) increased by between 11% and 135% after the total disc
replacement surgery. During flexion, motion of the ProDisc-C more closely resembled the intact
healthy cervical spine than Mobi-C.
16.4.3 i
influence
of
t
otal
c
erVical
d
iSc
r
eplacement
on
f
acet
J
oint
Similar to the intact healthy cervical spine, the maximum facet joint contact force was seen dur-
ing extension (Figure 16.6). At the operative segment (C4-C5), the facet joint force in extension
increased by 60% and 63% in ProDisc-C and Mobi-C, respectively. In the ProDisc-C model, the
values increased by 20% at C5-C6 but decreased by 14% at C3-C4 in extension. In the Mobi-C
model, the forces increased by 20% at C5-C6 but only 1% at C3-C4. Compared to the increase at the
operative level, the change in contact force at the adjacent segment was relatively small.
The maximum tension in the capsular ligament was observed during lateral bending in the intact
healthy cervical spine (Figure 16.7). However, the maximum ligament tension was observed during
extension and axial rotation after both total cervical disc replacement surgeries. Because of the exci-
sion of the anterior longitudinal ligament and intervertebral disc, the cervical spine lost its ability to
restrict the transfer motion in the transverse plane, which would lead to relative sliding between the
superior and inferior vertebrae. In extension, the cervical spine displayed hypermobility (more than
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