Subject-specific finite element modeling of the adolescent thoracic spine for scoliosis research

Abstract

Adolescent Idiopathic Scoliosis (AIS) is a three-dimensional (3D) disorder of the thoracolumbar spine affecting 2-4 % of children between ages 10 and 16 years. Currently, finite element (FE) models of the spine and rib cage that have been used to study etiology of AIS to optimize bracing methods and plan corrective surgeries are limited by their adherence to a single patient-specific geometry with biomechanical inaccuracies. The objectives of this study are to (1) create a biomechanically accurate FE model of the osteo-ligamentous thoracic spine and ribcage of a 10 year old (10 YO) skeletally normal subject using multi-block and conventional meshing methods that will act as a baseline model in the development of subject-specific models, (2) validate the 10 YO thoracic spine FE model (baseline model) with and without the ribcage, (3) apply dual-kriging interpolation methods to morph the multi-blocks of 10 YO vertebrae (Multi-block template) to develop a subject-specific model for the 16 YO normative thoracic spine (4) validate the FE model of the 16 YO thoracic spine with and without the ribcage, (5) develop a 12 YO AIS subject-specific FE model of the thoracic spine using the semi-automated morphing approach and compare the range of motion (ROM) values of the scoliotic spine with the ROM of a12 YO normative thoracic spine and (6) Analyze the change in ROM on all the three validated FE models by varying the elastic modulus/stiffness of all the components except nucleus pulposus by ±25%. The Chest CT scans of a skeletally normal 10 YO male subject was digitally reconstructed using Mimics (Materialise Inc., Belgium) to obtain the geometry of the thoracic vertebrae, ribs and sternum, which were meshed using a multi-block approach in ANSYS ICEM-CFD 14.5 (ANSYS, Canonsburg, PA). Soft tissue including intervertebral discs, costal cartilage and spinal ligaments were modeled using Hypermesh 11 (Altair Inc, Troy, MI). Due to limited availability of experimental data on mechanical properties and range of motion dataset of pediatric and adolescent thoracic spine, data available for the adult spine were scaled using appropriate scaling factors for validation purposes. To develop age- and subject-specific FE models from the baseline FE model, ‘multi-block morphing’ approach was developed. The ‘Multi-block morphing’ technique is a semi-automated approach that uses the methods of dual-kriging and multi-block meshing to develop subject-specific FE models with high-quality solid hexahedral elements. The T1-T12 multi-blocks pertaining to the vertebrae of the 10 YO thoracic spine model (multi-block template) were morphed to create multi-blocks for the corresponding vertebrae of a 16 YO skeletally normal subject and a 12 YO AIS subject using the multi-block morphing method. Majority of the elements in the baseline FE model of the 10 YO thoracic spine, as well as the 16 and 12 YO created by morphing the template blocks (T1-T12) had the desired quality (Jacobian>0.5, aspect ratio<5, degree of skewness<60 and warpage<50). While true FE…