Study On The Mechanism Of Congenital Atlanto-axial Dislocation By 3D Nonlinear Finite Element Method

Objective: To construct and validate the effectiveness of a 3D nonlinear finite element model of occipito-atlanto-axial complex (OAAC) and investigate the mechanism of atlanto-axial dislocation (AAD) induced by congenital atlanto-occipital fusion (AOF).Methods: Reconstructed the 3-dimensional geometry of occipital bone, atlas and axis from cranio-cervical junction (CVJ) ultrathin CT scan data of a young male volunteer. After automatic element meshing, we defined material property of bones by CT value based grading method. We constructed the volume of transverse ligament and performed refined meshing by hand. Twelve groups of ligament elements were constructed in OAAC according to their anatomic research data, whose material property were defined by their force-displacement curve that modeled with 6 point staged linear method. The elements of cartilages and contact pairs were constructed on the joint surfaces of OAAC to simulate contact behavior of joints. After the 3D nonlinear finite element model of OAAC was constructed, this model was applied static moment load that equals to cadaver mechanic experiment, to simulate various movements of OAAC and validate the effectiveness of this model. After validation of this finite element model of OAAC, 40 link elements were constructed between occipital bone and atlas to simulate congenital AOF. The biomechanics of OAAC combined with AOF were analyzed under the conditions of physiological load, overload and ligaments injuries respectively.Results: This 3D nonlinear finite element model of OAAC included large portion of occipital bone, whole atlas, axis, transverse ligament, all joint cartilages and ligaments. By applying physiological load, the range of movement of flexion, extension and rotation was similar to results of cadaver mechanic experiment. This model gained large range of movement within small load range. With increasing load, the increment of range of movement trended to constant. This model showed apparent nonlinear mechanic feature. The movement between occipital bone and atlas nearly disappeared after constructed link elements between them. Simple AOF would not lead to increased range of movement of flexion under physiological load, but lead to change of stress-strain environment of atlanto-axial joint and increase of ligament force that limited flexion of atlanto-axial joint. This kind of change of stress-strain environment and increased ligament force would become more apparent under over load. Decrease of ligaments stiffness would increase range of movement of flexion of atlanto-axial joint and change stress-strain environment of atlanto-axial joint as well.Conclusion: The biomechanical feature of this 3D nonlinear finite element model of OAAC is mainly consistent with true human movement pattern. This model can be further used in biomechanical experiment of CVJ. There are two important biomechanical features after AOF: change of stress-strain environment of atlanto-axial joint and increase of ligament force. The synergistic effect of these two factors may results in the development of AAD.