Mechanical & computational modeling of implants

• Κύριος συγγραφέας: Φράγκου, Κατερίνα
• Άλλοι συγγραφείς: Μουστάκας, Κωνσταντίνος
• Μορφή: Thesis
• Γλώσσα: English
• Έκδοση: 2018
• Θέματα: Spine; Intervertebral disc; Finite Element Model (FEM); Implants; Σπονδυλική στήλη; Μεσοσπονδύλιος δίσκος; Μοντέλο πεπερασμένων στοιχείων; Εμφυτεύματα; 617.560 592

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.