S516 Poster P-143 Spine

CREATION OF A FINITE ELEMENT MODEL OF THE RAT CERVICAL SPINE FROM MAGNETIC RESONANCE IMAGES

Colin Russell (1), Tae-Eun Chung (2), Thomas Oxland (1)

1. Division of Orthopaedic Engineering Research, University of British Columbia, Canada; 2. Department of Info-Mechatronics, Induk Institute of Technology, Korea

Introduction In order to better understand and treat spinal cord injury (SCI), especially within the cervical spine where it is most debilitating, further investigation of the spinal column and cord is necessary. Experiments in SCI research are often carried out on rat subjects; therefore a finite element (FE) model of the rat cervical spine has much potential utility in linking external injury conditions with cord damage. The purpose of this project was to derive surface models of the rat cervical spinal geometry from MRI data, for use in the creation of an FE model. Methods In vivo high resolution MRI scanning of a single rat cervical spine was performed at the UBC High Field Animal MRI Centre. Images were obtained from the 7 Tesla scanner with in-plane resolution of 0.156x0.156 mm, and 1 mm through-plane resolution, axial to the cord. Two scans, perpendicular to the upper and lower cervical spinal cord, respectively, were interpolated and registered using Mayo Clinic's Analyze software to form the base data. Surface models were extracted structure by structure from the base data using the open source ITK-SNAP segmentation software. Polygonal surface data for each of the nine vertebrae (C1-T2), seven intervertebral discs, and the spinal cord's white and grey matter were output from ITK-SNAP. Analytical NURBS surfaces were created from the polygonal models using INUS Rapidform, to prepare the surfaces for FE meshing.

Figure 1: FE mesh of the cervical rat spine. Altair HyperMesh was used to create FE meshes from the surface data, as well as 1D elements

representing relevant spinal ligaments (Fig. 1). The FE model was then imported into ESI Pam-Crash, and linear elastic material properties were assigned to the elements. Initial material property values have been adapted from Greaves’ human spinal FE model previously developed [Greaves, in press]. Spotweld elements were also created to fuse the intervertebral discs to their adjacent vertebrae. Results A preliminary simulation of a weight-hanging distraction test was run to verify correct function of the connections and contact conditions between model components. Translation of the centre of gravity (COG) of the C1 vertebra was constrained and a ramp up to 20 N downward force over 20 ms was applied to the COG of T2 (Fig. 2). The resulting component interactions were as expected.

Figure 2: Distraction FE simulation results. Discussion While achieving an operable FE model is encouraging, several steps are necessary to refine and validate the model. The global kinematics of the model will first be validated against experimental results of a simple weight-hanging distraction test. Further research will aim to refine the spinal cord model to incorporate nonlinear material properties, and to specifically validate the model against experimental SCI data collected by Choo et al [Choo, 2007]. Once sufficiently validated, the FE model will be of great utility in simulating varying injury mechanisms and comparing internal stresses and strains within the cord to observed damage patterns. References Choo et al, J Neurosurgery, 6:255–266, 2007. Greaves et al, Annals of Biomed Eng, (in press).

Journal of Biomechanics 41(S1) 16th ESB Congress, Posters