Preliminary steps of validation of a vertebral finite element model including a subject-specific cortical compartment.

Vertebral metastases affect one third of metastatic patients. They compromise bone integrity, often leading to fracture and neurological damage. The risk of fracture of a metastatic vertebra is currently estimated by the Spinal Instability Neoplastic Score (SINS). However, SINS is mostly qualitative, and lacks assessment of vertebral mechanics. The present thesis is part of the qSINS (i.e. quantitative SINS) project, conceived at the Bioengineering and Computing (BIC) Laboratory of IRCSS Istituto Ortopedico Rizzoli and funded by the Italian Ministry of Health (RF-2016-02364359). The main challenge of qSINS is to develop subject-specific finite elements models (SSFE) based on CT images routinely collected to stage the disease. In fact, no reliable SSFE model is available in the literature to quantify the reduction of metastatic vertebral strength over intact conditions. Preliminary studies within the qSINS project identified the modelling of cortical compartment, and a more realistic load distribution over the vertebral endplate as two key modelling steps. Consequently: -??the first aim of this thesis was to develop a SSFE model with a patient-specific cortical layer (CBM-model), using estimates of cortical bone thickness and density from clinical CT images; to assess how a subject-specific cortical compartment influenced stiffness, strain and strength, the CBM-model was compared with a literature-based model that mimicked the cortical bone with a uniform shell; -??the second aim, linked to the currently starting qSINS validation experiment, was to verify the correctness of the model replication of boundary conditions; to exclude the effect of bone inhomogeneity, a polyurethane replica of the vertebra was built, tested and modelled; -??the third aim was to compare SSFE computed displacements and strains with those measured on cadaver vertebrae by Digital Image Correlation technique. Preliminary results on the first experimentally tested L5 vertebra are finally reported.