| Objective:①To establish and validate a finite element model of axis for the further clinical study.②To discuss the stress patterns and predict fracture patterns under anterior odontoid load in different direction on the vertical plane.③To discuss the stress patterns and predict fracture patterns under anterior odontoid load in the different orientation on the horizontal plane.Methods:①We obtained the CT data from a healthy adult man for the regularly health examination. He was checked by radiative and clinical examination and was proved that he has not any spinal disease. Then his cervical spine was scaned by Siemens Somatom Sensation 64 with 0.6mm thinkness without any interval. We get the CT images and save the data with DICOM format. Then the CT images were inputed to the Materialise Mimics and product a 3D surface image of axis by selecting image, preconditioning and remeshing tools. The 3D image was inputed to the ANSYS ICEM CFD 10.0 with STL format and became a 3D entity, And the element type and material properties were definited at the same time. The 3D entity was inputed to the FEA software-ANSYS10.0,checked the element type and material properties and constrained in all degrees of freedom on the inferior aspect of the C-2 endplate, the inferior surfaces of the facet joints, and the inferior aspect of the spinous process,and an axial compressive load of 50 N was applied to the both sides of superior surfaces of the facet joints to imitate the quality of head. Load Cases 1 and Case 2 were created to mimic the exact loading conditions and constraints of Groups 1 and 2 of the biomechanical study conducted by Doherty, et al. (1)Load Case 1 consisted of 1736 N applied over a group of nodes on the anterior surface of the dens, to simulate pure extension, oriented in a posterior direction (2)Load Case 2 consisted of 1279N applied to a group of nodes on the anterior aspect of the dens, oriented 45°lateral of pure extension from the sagittal plane, oriented in a posterior and lateral direction Then the model was solved by ANSYS 10.0.②The load orientation was sequentially varied from -60°to 60°in 15°intervals from the coronal plane. The magnitude of the load was 200 N for each iteration.③The load orientation was sequentially varied from 0°to 45°in 15°intervals from the sagittal plane. The magnitude of the load was 200 N for each iteration.Result:①The result of present finite element model of axis closely correlate with the biomechanical testing of results obtained by Doherty et al. Pure extension loading produced a Type III stress pattern with maximum stress of 123 MPa. Loading at 45°produced a Type II stress distribution with a maximum stress of 121 MPa. These stresses are within 11% and 12%, respectively, of the reported yield stress of cortical bone (138 MPa).And the Von Mises stress patterns of the current model was consistent with fracture patterns,which made by Doherty et al.②Variations in the vertical orientation of the load produced changes in the stress patterns. A transition from a Type III to Type II pattern occurred with pure extension loading with the angle between -60°to 60°.The bigger the angle,the higher the stress.③Variations in the horizontal orientation of the load also produced changes in the stress patterns. An increasing the horizontal angle of the load vectors also resulted in a change from a Type III to a Type II stress pattern.Conclusion:①The current finite element model of axis can properly simulate the characteristic of axis in biomechanics testing. It can be used for the further study.②The FE model predicts that a transition from Type III to Type II odontoid fracture occurs when the orientation of vertical and horizontal vector is increasing under two loading pattern with different angles on the anterior surface of dens.
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