Biomedical Engineering Reference
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simulation of the mechanical behavior is more accessible. For this reason, the solid
models of the anatomical spine segment and the mechanical stabilization system
were created.
Using CAD techniques ( feature base design ), a second model of a functional unit
of human cervical spine, both implanted and non-implanted, was developed. The
developed model guarantees that the functional unit geometry can be imported and
easily handled within ANSYS software.
The second model of the cervical unit is anatomically detailed and partially dif-
ferentiates between the cervical tissues. Structures implemented in the FE model
are the cancellous and cortical bone of the vertebra, intervertebral disc, cervical
ligaments, and facet joints. The neck muscles were not considered.
The mechanical elements used in implantation of the functional unit consist in a
stabilization system, which has the role of rigidly fixing two vertebrae (C2-C3).
This stabilization system is composed of the following elements: one bone plate,
four bone screws, and four locking screws [ 22 ] .
In order to complete the task, a good fundament concerning the behavior of the
materials under the stress condition was made. The FEM was used to determine the
stresses and strains acting at the bone-screws interfaces, during a flexion movement
of the head. In this way, the implanted cervical unit (ICU) was analyzed in three
cases. For the three units, the bone screw type is responsible for the differences.
7.2
Solid Reconstruction Modeling of a Cervical Vertebra
A functional spinal unit or a spinal motion segment is the smallest physiological
motion unit of the spine to exhibit biomechanical characteristics similar to those of
the entire spine [ 63 ]. The cervical functional unit is a structure (virtual or real) com-
posed of two adjacent vertebrae, the intervertebral disc and all adjoining ligaments.
The objective of the solid reconstruction modeling was to create a multi-solid
model of a cervical functional unit. The central idea was to build a model of cervi-
cal vertebra as an assembly of two different solid bodies: one for the spongy core
of the vertebra and another one for the cortical shell [ 50 ]. In this way, each consti-
tutive element has its own material features. The inhomogeneous material of the
vertebra is modeled as two homogeneous materials describing the two main
components of the bone tissue: the hard outer layer of bone composed of compact
bone tissue and the inner spongy structure that resembles honeycomb composed of
cancellous bone tissue.
7.2.1
Image Acquisition
In order to acquire the slice images of a cadaveric cervical vertebra, a Computer
Tomography (CT) system was used. The CT scanning is a painless imagistic proce-
dure that allows visualization of the living anatomical structures by exposing the
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