Generating standing posture finite element models of the spine with radiation-free imaging

Finite element (FE) modelling is a powerful tool for investigating spinal biomechanics. Subject-specific finite element models of the spine are commonly built from supine data obtained from CT images, with harmful radiation and without soft-tissue structures. However, a more accurate evaluation of vertebrae and intervertebral disc loading requires the analysis of the subject in upright posture. In this study we propose a radiation-free alternative to create subject-specific standing FE models by combining supine MRI with ultrasound-based 3D spinal curvature assessment in standing posture. Subject-specific supine FE models of T3-L5 were created from synthetic CT (AI-generated from MRI) of 8 children. Dedicated spinal ultrasound was used to detect the positions of the vertebral laminae, which were used as input to convert the FE model from supine to standing. RMSE was found within 1 mm for the fitted vertebral levels. Validation against conventional radiographic measurements showed no statistical difference in sagittal alignment between the two imaging modalities in the lumbar and mid-to-lower thoracic parameters. In addition, the distance between the vertebral centroids measured from radiographs and from the upright FE model in sagittal and coronal plane was found to be less than 6 mm for the fitted levels. In conclusion, we demonstrate the feasibility of using completely radiation-free imaging as an alternative to CT and bi-planar X-ray to build subject-specific, upright, detailed FE models.