| Περίληψη: | The fast pace of life and the heavy work load in modern society, lead to several clinical problems.
Lower back pain is the most frequent medical disease affecting people at some point in their life.
Back pain often appears due to injuries, spinal instability or degeneration that cause malfunctions to
small cartilages inside the spine, called Intervertebral Discs. The intervertebral disc withstands the
body loads like in case of lying down or during standing upright, in which the disc bears almost 450-
600N. However, aging affects its functions. During daily activities or even worse in case of weight
lifting or extravagant exercising, the disc gets dehydrated and thus, loses its height and damage may
occur. This can cause impingement of neural structures resulting in pain, numbness and weakness.
Additionally, changes may appear in the structure of the vertebral bodies and the mechanical
properties of the disc. Unfortunately, in worst case scenario of Lower Back Pain and when the
physiotherapy is not enough, surgical treatment may be inevitable and even if it is not preferred, total
disc replacement may follow.
The spine is a structure bearing the applied loads with the assistance of the ligaments, tendons,
muscles, bones, intervertebral discs and provides a range of motions while protecting the spinal cord.
The vertebral bodies are joined by two bilateral facet joints in the posterior region and are separated
at each level by a cartilaginous intervertebral disc. The intervertebral disc consists of a central gel,
the Nucleus Pulposus which is surrounded by Collagen fibers embedded in Ground substance
composing the Annulus Fibrosus. Factors like trauma, tumors, infections, degenerative disorders can
contribute to spinal instability and may affect the bones, discs, joints, or ligaments.
Two of the most common clinical problems are degeneration and herniation. Degenerative changes
may affect the vertebral bodies and the intervertebral disc. Intervertebral disc works like a shock
absorber for the spine. As the disc degenerates, the ability for it to handle such stresses also changes.
Additionally, the peripheral collagen fibers of the disc are stressed resulting in herniation of the disc
material outwards and compressing the nerve roots or the spinal cord.
The aim of this research work is to provide an implant of the intervertebral disc in a natural shape
and size to replace the real one. A Finite Element Model was developed to represent the disc and
analyze its behavior in extreme situations. Specifically, the model was tested in simulations under
compression, full forward flexion and right lateral bending. Following, sixteen biomaterials were
also studied to replace the parts of the disc. These biomaterials were simulated under the same
loading conditions and their stress, strain and deformation distributions compared with the ones
produced by the human disc. The choice of biomaterials that would compose the implant, was made
by the similarity mechanisms.
Therefore, an artificial lumbar disc was developed by polystyrene, Teflon and rubber to replace the
nucleus pulposus, the collagen fibers and the ground substance, respectively. The implant was
simulated under the same loading conditions and compared with the real disc. The two models
showed similar behavior during full forward flexion and higher deviation during compression and
lateral bending. The stress applied on the artificial disc during lateral bending was higher compared
to the human disc, due to the higher stiffness and strength of the Teflon and polystyrene. Furthermore,
the parts of the human disc have been modelled as linear elastic materials, while the rubber has been
modified as a hyperelastic material that shows higher resistance to deformation and resulting in nonlinear
behavior compared to the real disc.
This research work contains an extensive literature review on spinal anatomy, mechanics and
therapy, a complete methodology procedure and a preliminary Finite Element Model of L4/5
vertebral bodies along with their intervertebral disc. Not only has the Finite Element model generated
very promising results about the behavior of the intervertebral disc during compression, flexion and
bending, but also reasonable and useful conclusions were produced regarding the biomaterials that
could be utilized in an artificial implant for total disc replacement. The specific analysis of this thesis
provides data about the flexible properties of the disc, the bulge regions, the stress, strain and
deformation distributions during the different loading conditions and comparisons with the artificial
disc.
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