Biomechanical characterization of human metastatic vertebrae: volumetric strain distribution in elastic regime

The spine is the most common site for bone metastases, which alter the internal microstructure increasing the risk of spinal instability. A comprehensive biomechanical characterization of human metastatic spine segments is essential to understand how the metastatic lesion impact on the mechanical behavior of vertebrae with and without metastases. This study aimed to measure and compare the volumetric strain field in elastic regime using Digital volume Correlation (DVC) in metastatic and healthy vertebrae to identify typical strain patterns. 12 human spine segments, consisting of one metastatic and one healthy vertebra, were in-situ tested in step-wise compressive loading within a micro-computed tomography (μCT). Volumetric strains were computed using a global DVC approach (BoneDVC). No statistical differences were found between metastatic (pooled data for lytic, blastic and mixed) and control vertebrae, for both medians maximum and minimum principal strains (-2614μ, -707μ, -1021μ and -1677μ for lytic, blastic, mixed metastases and control vertebrae, respectively). Lytic-metastatic vertebrae showed larger strains than their controls. Unexpectedly, also vertebrae with blastic metastases showed larger strains than their controls, highlighting the importance of considering also microstructural alterations and bone quality. Strain concentrations were found close to the lytic lesions and around the blastic one. Moreover, the strain pattern in physiological conditions reflected the one measured at failure and strengthens the hypothesis that those regions experiencing failure were those with the larger strains in physiological condition. These results highlighted the need of considering biomechanical factors in planning strategies to prevent fractures in metastatic patients. This method, once generalized on a larger sample, could provide useful knowledge that can inform clinical decision-making and guide future research efforts for assessing the spinal instability.