Biomedical Engineering Reference
In-Depth Information
16.2
model develoPment
16.2.1 f
finite
e
lement
m
odelinG
of
an
i
ntact
H
ealtHy
c
erVical
S
pine
A healthy male subject (32 years old, 65 kg, and 172 cm) without any radiographic evidence of
degeneration participated in the study. The subject was briefed on the research procedure and signed
a form of consent. The study plan was approved by the ethical committee of the corresponding insti-
tute. The geometry of the four vertebrae was reconstructed based on computer tomography (CT) scan
images taken from the subject. The CT images included the C3-C6 vertebrae and were obtained at
0.5-mm intervals and at a resolution of 0.6 mm (Brilliance iCT, Philips, Netherlands). Commercial
software (MIMICS 10.1, Materialise, Leuven, Belgium) was used to transform the planar CT scans
into solid vertebral volumes and exported as STL format files. The solid volume was then imported
into an inverse engineering software (Rapidform 2006, INUS Technology Inc., Korea) to locally
smooth the surface and convert into a Nonuniform rational basis spline (NURBS) surface geometry
structure. The vertebrae were imported into an FE package (ABAQUS 6.9.1, Simulia Inc, USA) and
meshed with tetrahedron elements. A lay of triangle shell elements were generated from the exter-
nal surface of the trabecular bone to represent the cortical bone and endplates with a thickness of
0.4 mm (Keaveny and Buckley 2006). The nodes on the shell element were shared with the external
tetrahedron element surface of the cancellous bone (Figure 16.2a).
The intervertebral disc was represented as a continuous solid structure occupying the interver-
tebral space and partitioned into an annulus and nucleus pulposus and meshed with hexahedron
elements (Figure 16.2b). The nucleus pulposus occupied 43% of the total disc volume and was
located slightly posterior to the center of the disc, which was consistent with anatomic measure-
ments (Denozière and Ku 2006), and the Poisson's ratio was set as 0.49, which represents an incom-
pressible material property. The modeled annulus fibrosus consisted of a composite material based
on clinical observations (Drake et al. 2008). A layer of annulus fiber was embedded in the matrix
of annulus ground substance with an inclination of between 15 and 30 degrees with respect to the
transverse plane. The content of the annulus fiber was approximately 19%, similar to the natural col-
lagen content of the annulus (Denozière and Ku 2006). The fibers were meshed into truss elements.
Spinal ligaments play an important role in spinal biomechanics and stability. In the present
study, the main cervical spinal intervertebral ligaments were incorporated into the model (Narayan,
Srirangam, and Frank 2001), including the anterior longitudinal ligament (ALL), posterior lon-
gitudinal ligament (PLL), flaval ligament (FL), and interspinous ligament (ISL). Their insertion
points were chosen to mimic anatomic observations as closely as possible (Drake et al. 2008).
Cartilage endplate
Flexion-extension
Axial rotation
Lateral bending
Cortical
bone
Cancellous
bone
ALL
(a)
Matrix of annulus fibrosus
Nucleus pulposus
SL
Annulus fiber
(b)
(c)
(d)
FIgure 16.2
(See color insert.)
Details of the finite element cervical model: (a) vertebra, (b) disc,
(c) ligament insertion point, (d) entire cervical segment and illustration of spinal motion.
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