Development and validation of patient-specific finite element models of metastatic human spine segments

Cancer is one of the major causes of morbidity and mortality in the world and one possible complication is the spread of metastases in the spine. The presence of vertebral metastases strongly affects the anatomical structure and mechanical properties of the bone and can lead to fracture, pain, disability and impairment of quality of life. Evaluating the mechanical strength of the pathologic vertebrae is thus fundamental in order to provide an objective support to the clinic decision and treatment plan. The current clinical methods used for the diagnosis of vertebral fracture are mainly based on bone mineral density and qualitative assessment of radiological data; however, they do not provide an objective and accurate fracture risk estimation. In the last decades, numerical methods have been widely used for decision making in clinical care. In particular, Finite Element (FE) models represent a useful and promising tool to simulate biological tissue behaviour and investigate the effect of critical loading conditions which cannot be tested experimentally. Patient-specific FE models, that include information about the geometry and mechanical properties of the bone, can be developed starting from medical images acquired through non-destructive and non-invasive imaging techniques. Different studies investigated the possibility to use FE analyses to predict vertebral strength and FE models that consider at least two functional spinal units proved to better predict the mechanical response in vivo because the load is physiologically distributed between all vertebrae and intervertebral discs. In the literature there are several FE models of spine segments, developed with different techniques and modelling parameters, but only few studies focused on the validation aspects. One of the possible ways to validate FE models is through Digital Image Correlation (DIC), which is a non-contact optical technique that provides full-field displacement and strain distribution over the specimen surface during the experimental tests. The aim of this thesis is to develop patient-specific FE models of the spine segments and validate them comparing the surface displacements of vertebral bodies predicted by the models with those measured experimentally using the DIC technique. A validation protocol, recently developed by the In Silico Medicine research group at the Medical Technology Lab - Rizzoli Orthopaedic Institute and at the Industrial Engineering Department - University of Bologna, was studied and applied to two different case studies. In particular, patient-specific FE models of cadaveric thoracolumbar spine segments were first developed starting from CT data; after registration, loading conditions were then defined to simulate the experimental testing; a point-to-point surface displacement comparison was finally performed. Also, important verification studies were conducted on the definition and implementation of the boundary conditions, repeatability of the experimental measurements and mesh converge analysis. The results obtained in this work allowed to demonstrate the possibility to apply the developed methodology to different case studies and highlight strength and weakness of the validation pipeline.