Establishment and Finite Element Analysis of a Three-dimensional Dynamic Model of Upper Cervical Spine Instability

Objectives To establish a dynamic three-dimensional (3D) model of upper cervical spine instability and to analyze its biomechanical characteristics. Methods A 3D geometrical model was established after CT scanning of the upper cervical spine specimen. The ligament of the specimen was fatigued to establish the upper cervical spine-instability model. A 100-N preloaded stress was applied to the upper surface of the occipital bone, and then a 1.5-Nm moment was applied in the occipital-sagittal direction to simulate upper cervical spine flexion and extension. Subsequently, the 3D dynamic model was established based on trajectory data that were measured using a motion-capture system. The stress on the main ligament and the relative motion angle of the joint were analyzed. Results The shape of the model grid was regular and the total number of its units was 627 000. After finite-element analysis was conducted, results of the ligament stress and relative movement angle were obtained. After the upper cervical spine instability, the pressure of the alar ligament during the upper cervical spine extension was increased from 2.85 to 8.12 MPa. The pressure of the flavum ligament was increased during the upper-cervical spine flexion, from 0.90 to 1.21 MPa. The pressure of the odontoid ligament was reduced during the upper cervical spine flexion and extension, from 10.46 to 6.67 MPa and 25.66 to 16.35 MPa, respectively. The pressure of the anterior longitudinal ligament and cruciate ligament was increased to a certain degree during upper cervical spine flexion and extension. The pressure of the anterior longitudinal ligament was increased during flexion and extension, from 7.70 to 10.10 MPa and 10.45 to 13.75 MPa, respectively. The pressure of the cruciate ligament was increased during flexion and extension, from 2.29 to 4.34 MPa and 2.32 to 4.40 MPa, respectively. In addition, after upper cervical spine instability, the articular-surface relative-movement angle of the atlanto-occipital joint and atlanto-axial joint had also changed. During upper cervical spine flexion, the angle of the atlanto-occipital joint was increased from 3.49 degrees to 5.51 degrees, and the angle of the atlanto-axial joint was increased from 8.84 degrees to 13.70 degrees. During upper cervical spine extension, the angle of the atlanto-occipital joint was increased from 11.16 degrees to 12.96 degrees, and the angle of the atlanto-axial joint was increased from 14.20 degrees to 17.20 degrees. Therefore, the movement angle of the atlanto-axial joint was most obvious after induction of instability. Conclusion The 3D dynamic finite-element model of the upper cervical spine can be used to analyze and summarize the relationship between the change of ligament stress and the degree of instability in cervical instability. Frequent or prolonged flexion activities are more likely to lead to instability of the upper cervical spine.