Assessment of Human Vertebrae Stiffness Variability Under Anterior Wedge Compression

The most common vertebral fracture among the elderly and post-menopausal women is the anterior wedge-shape compression fracture and its major cause is the loss of bone mineral density (BMD) due to osteoporosis. Many surgical procedures aimed to manage this clinical condition can cause an alteration of the vertebral stiffness along the human spine, thus increasing the risk of subsequent fractures. The identification of vertebral stiffness and strength is important because they represent essential predictors of future fractures; moreover, it can be helpful for surgeons, to restore the physiological condition of the spine at time of surgery. Therefore, the major purpose of this thesis is to assess the stiffness of human vertebrae and its variability along the spine. In order to accomplish this goal, specimen-specific finite element models of thoracic vertebral bodies were developed from Computed Tomography (CT) scans of 4 subjects. Both the three-dimensional geometry and heterogeneous mechanical properties distribution were extracted from the CT data. Mathematical relationships taken from literature were used to assign elastic-perfectly plastic material properties to each finite element of the model. The boundary conditions defined in the simulations induced an anterior wedge-shape compression. Lastly, stiffness and failure load of each vertebra were computed. The average stiffness in the upper thoracic spine was found to be 73% of the stiffness in the mid thoracic and 64% of the stiffness in the lower thoracic. Moreover, it increased within the upper and mid thoracic region, while decreased in the lower region. On the other hand, the average failure loads increased nearly of 2400 N from the upper to the mid thoracic, as well as from the mid to the lower thoracic region. The failure load in the upper thoracic spine was 67% of the failure load in the mid thoracic and 75% of the stiffness in the lower thoracic. In conclusion, the present work is able to provide a first insight in the vertebral stiffness variability along the thoracic spine. This information has potential implications for the evaluation of the risks of compression fractures and the improvement of surgical procedures, such as vertebroplasty and kyphoplasty. Enhancement of the current study can be obtained through the validation of the proposed methodology with cadaveric experiments, and an increased sample size.