Three dimensional strain analysis of vertebrae with artificial metastases through digital volume correlation

Bone is a common site for metastases and spine represent the most frequent site. Lytic lesions are associated with the loss of bone tissue, which can compromise the mechanical competence of the vertebra, leading to spine instability. Rigid stabilization is a solution, but it is a complex surgery, that can be very critical for oncologic patients; on the other hand, an untreated metastasis can lead to mechanical failure of the bone, leading to pain, immobilization and in the worst case, paralysis. In this study, a protocol to analyse the strain with simulated lytic metastasis under compressive loading has been developed and optimized using a porcine vertebra. The strain distribution has been measured experimentally using micro-computed tomography (micro-CT) and Digital Volume Correlation (DVC), which provided three-dimensional displacements and strains maps inside the specimen. The ideal parameters for the DVC have been found by analysing two repeated scans in constant strain condition and setting a target of 200 microstrain for the errors (one order of magnitude lower than typical strains in bone subjected to physiological loading conditions). An ideal nodal spacing of 50 voxels (approximately 2 mm) has been chosen and a voxel detection algorithm has been applied to all data to remove regions outside the bone. In order to understand how the presence of the defect could alter the strain distribution, the porcine vertebra has also been subjected to non-destructive compressive load before and after the preparation of a mechanically induced lytic metastasis in the vertebral body. An increase of the 40% of the compressive principal strain after the defect has been found in proximity of the lesion. This protocol will be used in future studies to analyse the effect of size and position of artificially metastatic lesions in the vertebral body of human spines.