Advanced Biomechanical Modeling of Spine. In-vitro Testing and Finite Element Modelling

The focus of this thesis is spinal column, one of the most complex and fundamental anatomical structures in the entire human body with the main functions to support upright posture, provide movements and protect the central nervous system. The aim of the thesis was to investigate spinal anatomy and biomechanics through in-vitro experimental campaigns conducted on human cadavers and animal specimens and through a computational approach. The acquired anatomical, biomechanical and experimental knowledge were then exploited for seeking and employing a mathematical model that could simulate the biomechanical response of the intervertebral disc under prolonged loads. In addition, an infinite population of spinal geometries was created,i.e. a 3D atlas of the pathological lumbar spine, by using the advanced technology available today of finite element modeling. The goal was to create useful tool for the bioengineering field that could be exploited to have access to accurate geometries of spinal columns. These geometries can be used to perform computational simulations that can be used in clinical practice for the fabrication of medical devices and for the simulation of surgical procedures. Finally, the last research step was to create a finite element model of the spine to be validated through experimental data. This thesis has been divided into five main chapters, each one addressing one crucial aspect. The first two chapters provide a detailed background on the anatomy and biomechanics of the spine; while the last 3 chapters encapsulate the innovative research work conducted over the three years of doctoral studies, in which published works and those still in the process of publication have been included, those have enriched the literature with new concepts and instruments never before presented. Chapter 1 presents a detailed description of the anatomical structures of the spine in its major regions: cervical, thoracic, lumbar, sacral and coccygeal. Indeed, each region has been described as unique in its anatomy, morphology and function. Specifically, more attention has been placed on the intervertebral disc as it is a key element in spinal biomechanics, separating the vertebrae and acting as a shock absorber to distribute forces along the spine and providing spinal flexibility. It is crucial to analyze it from a biomechanical point of view as it is one of the first spinal components to develop degenerative diseases. Chapter 2 covers the study of the spinal biomechanics. Hence, several key biomechanical concepts were introduced such as: the Functional Spinal Unit (FSU), which is the smallest functional unit of the spine, consisting of two adjacent vertebrae and the intervertebral disc separating them; the global reference system for an FSU; and the major movements, i.e., flexion, extension, axial rotationl, and lateral bending. The remaining part of the chapter focused on experimental tests employed to study in spine biomechanics to measure key parameters such as flexibility, creep, and stress relaxation. Employing experimental tests enables the quantification of deformation phenomena which occur over time, offering new perspectives to better understand the long-term behaviour of the spine under repeated and chronic stress conditions. Chapter 3 focused on the main degenerative diseases of the spine, with special emphasis on the pathologies affecting the intervertebral disc. Degenerative disc diseases are generally related to aging and mechanical deterioration of the intervertebral discs, but they are also influenced by genetic and biomechanical factors. Indeed, disc herniation is characterized by the leakage of the nucleus pulposus across weakened annulus fibrosus fibers, leading to acute pain. It is one of the most common disorders afflicting a large portion of the world’s population. In addition, the final part of the chapter provide an overview of surgical treatments for spinal diseases, such as spinal fixation and spinal fusion, as well as innovations in the field of disc Nucleus Replacement (NR). Indeed, a schematic review on the nucleus replacement has been published, representing a major focus of the research work in this thesis. This schematic review was an historical one, and it was the very first in its kind presented in the literature. The reviews covered the nucleus replacement history from 1955 until today, presenting the nucleus replacement as a promising alternative to fusion, as it preserves vertebral mobility and reduces the risks associated with postoperative stiffness. Chapter 4 is the experimental section of this thesis work, in which two innovative research papers were included. In this chapter, the concepts learned in Chapter 2 were implemented to conduct in-vitro experimental campaigns on bovine tails and human cadavers, aiming to simulate the actual biomechanical conditions of the human spine. Specifically, the aim was to obtain experimental data which could be used to perform viscoelastic analysis of tissues, using prolonged tests to measure creep deformation, through which mathematical modelling of disc biomechanical behavior followed. A novel aspect of modelling intervertebral disc biomechanics was introduced with the fractional calculus matheamtical tool. Indeed, it was shown how fractional calculus, compared with classical models, can offer a more accurate description of nonlinear mechanical behavior of tissues. Finally, Chapter 5 is the computational section, where how Finite Element Modeling (FEM) can be used to simulate the biomechanical behavior of the spine is presented. The chapter contains four research papers. First, the main key aspects to consider in realizing a FEM of the spine were presented, aiming to replicate accurately and precisely the interactions between the different components of the spine under different loading conditions. The next step involved the creation of a three-dimensional atlas based on data from patients with spinal diseases. The greatness of these research papers are about of the creation of this 3-D atlas which offers the possibility to virtually simulate the effects of pathologies and to have available an infinite population of different anatomical geometries. Finally, the chapter ends with the presentation of a new finite element model that was built on a patient-specific basis, which was then validated through experimental data.