Modeling and Numerical Analysis of a Cervical Spine Unit - Biologically Responsive Biomaterials for Tissue Engineering - page 137

and cancellous bone, on the contrary, is a porous—spongy—material. Bone material
is inhomogeneous because it is composed of a crystallized mineral and a fibrous
organic component. When dealing with bone material, additional difficulties arise.
Due to changes in bone density and architecture, the mechanical properties of bone
change from one volume to the next. In addition, bone is non-linearly elastic. Thus,
the bone material can be considered like a two-phase hierarchical composite struc-
ture. In order to study the biomechanical behavior of a bone, at least two different
materials have to be attributed: cancellous bone for the core and cortical bone for
the shell.
The structure of the vertebrae varies from one segment of the spinal column
to the next, reflecting specific static and functional requirements. The cervical
bones are designed to allow flexion, extension, bending, and axial rotation of
the head. They are smaller than the other vertebrae, allowing a greater amount
of movement.
The spinal column performs a variety of mechanical functions like head, upper
limbs, and thorax sustaining; absorption, dampening, and transferring of the dynamic
loads; and complex movements in physiological ranges. The smallest functional
element of the spinal column is also known as a mobile segment or spine functional
unit. A spine functional unit consists of two neighboring vertebrae, the interverte-
bral disc between them, the facet joints, and the ligaments [ 26 ] .
Generally, the experimental testing of a biomechanical structure, including the
spinal column or some of its segments, implies many issues. Thus, a computer sim-
ulation approach is far more reliable. In order to perform a computer simulation, a
realistic 3D virtual model of the spine has to be developed. Because of the different
nature of the spinal elements, each one has to be individually created. The assem-
blage of those elements materializes in the desirable model which must be highly
realistic detailed life-size. The model should also be designed to answer specifically
the studied aspects. Its predictions are valid only within the boundaries of assump-
tions and limitations that it incorporates [ 40 ] .
A comprehensive classification of biomechanical models is presented by M. M.
Panjabi in Cervical spine models for biomechanical research [ 40 ] . “Biomechanical
models are widely used for the understanding of the basic normal function and
dysfunction of the cervical spine and for testing implants and devices.” Biomechanical
models can be broadly categorized into four groups [ 40 ], each of them having its
own recommendations and limitations:

• Physical models looking as natural specimens, made of nonorganic materials or

obtained by plastination, are often used as valuable tools for patient and student
education [ 27, 46, 53 ] .
In vitro models consisting of a cadaveric spine specimen are useful in providing •

basic understanding of spine behavior [ 40 ] .
In vivo animal models allow the modeling of living phenomena, such as fusion,

•

development of disc degeneration, instability, etc.
“Computer models are developed from mathematical equations that incorporate •

geometry and physical characteristics of the human spine and may be advantageously