Objective  To establish, with finite element technology, a three-dimensional nonlinear finite element model of the normal occipital bone, atlas and axis and a three-dimensional nonlinear finite element model of concomitant atlanto-occipital fusion and atlantoaxial dislocation, providing a biomechanical method for clinical research on the upper cervical spine.

  Methods  Finite element analysis was conducted with the CT data of a 27-year-old male volunteer, and a three-dimensional nonlinear finite element model, i.e., the normal model, of the normal occipital bone, atlas and axis was established accordingly. Finite element analysis was conducted with the CT data of a 35-year-old male patient with concomitant atlanto-occipital fusion and atlantoaxial dislocation. Then, the ideal state of a simple ligament rupture under high load was generated by computer simulation, and a three-dimensional nonlinear finite element model of concomitant atlanto-occipital fusion and atlantoaxial dislocation was established, i.e., the atlanto-occipital fusion with atlantoaxial dislocation model. For both models, a vertical upward torque of 1.5 N·m was applied on the upper surface of the occipital bone. Through comparative analysis of the two models under stress, the data of the range of motion (ROM) for flexion, extension, lateral bending, and rotation were examined. In addition, stress and deformation analysis with 1.5 N·m torque load was conducted to validate the effectiveness of the two three-dimensional nonlinear finite element models established in the study.

  Results  When the normal model established in the study was under 1.5 N·m torque load, it exhibited a maximum ROM for each unit of flexion, extension, and the ROM approximated the experimental measurement results of human mechanics, confirming the validity of the simulation. The stress and deformation results of the model were consistent with the basic principles of mechanics. The moment-angular displacement of the model showed obvious nonlinear characteristics. Compared with the normal model, the atlanto-occipital fusion with atlantoaxial dislocation model showed reduced ROM of the atlanto-occipital joint under a torque of 1.5 N·m, while the ROM of the C1-C2 joint for the four conditions of flexion, posterior extention, lateral bending, and rotation under load, with the exception of rotating motion, was greatly increased compared with that of the normal model, which was in line with the actual clinical performance of the patient.

  Conclusion  The atlanto-occipital fusion with atlantoaxial dislocation model and the three-dimensional nonlinear finite element model of the normal occipital bone, atlas and axis were successfully established by finite element technology. The models had valid simulation and reliable kinematic characteristics, and could be used as a reliable tool to simulate clinical diseases.