Combined numerical and morphological study of the lumbar spine: parametric finite element model and evaluation of dynamic implants

Low back pain is a major cause of disability and requires the development of new devices to treat pathologies and improve prognosis following surgery. Finite Element (FE) Methods represent an appealing solution to provide mechanical evaluations of new devices speeding up the design process, as well as evaluating several anatomical scenarios. The aim of this thesis was to develop an accurate FE of the lumbar spine and the evaluation of the variability introduced by morphological and material parameters. The generation of the geometrical model were implemented in a toolbox, the LMG (Lumbar Model Generator), with dimensions based on correlation analyses or subject-specific measurements. It allows the automatic preparation of the FE model, performing the mesh generation and evaluation, assigning material properties, boundary conditions and analysing the results. The FE model of a functional unit (L1-L2) was evaluated and the FE results were in agreement with studies available in literature. Sensitivity analyses on the material properties and morphological parameters were performed and the most influential parameters identified. Moreover, the mechanical behaviour of two devices, the BDyn (S14 Implants (Pessac, France)) and the GsDyn (a device for the paediatric scoliosis developed as part of the Spinal Implant Design project) were evaluated

NUMERICAL ESTIMATION OF MECHANICAL PROPERTIES OF HUMAN TRABECULAR BONE TISSUE: A NEW METHOD BASED ON DIGITAL IMAGE CORRELATION.

The estimation of tissue level mechanical properties of trabecular bone is still a hard task. A widely used technique is based on the combination of mathematical models, obtained through µCT images, with displacements measurements registered during uni-axial compression tests. One of the main limitation of this approach is linked to the method used to measure the displacements which considers punctual acquisition for an inhomogeneous material. Moreover, the measuring instruments require the contact with the sample surface exposed to the mechanical tests, creating local artefacts which may produce significant differences between the numerical model and physical system. Recently, techniques based on the image correlation allowed the measurement of the displacements field on a surface portion during the experimental test. This thesis develops a method to integrate the DIC measurements with the numerical models obtained through micro-CT images, in order to estimate the mechanical properties of trabecular bone tissue. An accurate registration between experimental measures and numerical model is developed and qualified through sensitivity studies on the characterizing parameters. The comparison of the displacements measured values with the foretold ones, allows to quantify the coherence between numerical model and physical model. Finally, the acquired experimental information are used to estimate the mechanical properties at the tissue level, leading to values coherent with the estimations realized with the punctual methods, assumed as reference