Load and displacement data for finite element models of human and pig vertebrae

Six porcine and four human cadaveric vertebrae were imaged using micro computed tomography (μCT) (Scanco μCT80, Scanco Medical, Switzerland) at a voxel size of 0.074 mm. Species-specific threshold values were then applied to segment the images of all of the specimens of each species. Each set of segmented images was then imported into an image processing software tool (ScanIP, Simpleware Ltd, Exeter) and downsampled to a resolution of 1 x 1 x 1 mm using an averaging method that allowed for partial volume effects. Each downsampled voxel represented the average of the binary segmented voxels within it and therefore the greyscale value of the downsampled voxel represented the BV/TV value of that region of the underlying bone. For all of the specimens, the regions were then imported into a FE meshing tool (ScanFE, Simpleware Ltd, Exeter) and a FE model of the vertebra and two end-caps was generated. An element size of approximately 1 mm was used because this had previously been shown to be sufficient for vertebral stiffness evaluation in specimens of a similar size and under similar conditions. A combination of hexahedral and tetrahedral linear elements were used to represent the vertebral geometry; in total each model contained between 200,000 and 400,000 elements. The cement region was assigned an elastic modulus of 2.45 GPa. Each element within the bone was assigned an elastic modulus based on BV/TV value derived from the downsampled voxel grayscale. All of the models were imported into a finite element software package (ABAQUS CAE version 6.9-1, Simulia Corp, USA). The models were solved and the specimen stiffness determined. To normalize for size, the ‘apparent modulus’ was also determined by multiplying the stiffness by the vertebral height and dividing by the cross sectional area. All the models were processed and the predicted vertebral stiffness was determined.