Biomechanical finite element analysis of different reduction degrees after posterior C2-3 fixation for unstable Hangman′s fractures

English Keywords:Hangman′s fracture  Finite element method  Stability  Biomechanical  Degree of reduction

English Abstract:

  【Abstract】 Objectives: To evaluate the biomechanical stability of different reduction degrees after posterior C2-3 fixation for unstable Hangman′s fracture, and provide theoretical support for the selection of clinical treatment options. Methods: A 27-year-old healthy male volunteer was selected, and a 256-slice thin-slice spiral CT scan was performed on the entire head and cervical spine segments with a slice thickness of 0.25mm. A finite element model of the C0-3 segment of the normal cervical spine was established by Mimics 19.0, Geomagic 2013, Hypermesh 2019 and Abaqus 2019 software, and its effectiveness was verified comparing the range of motion(ROM) of each segment with the previous finite element analysis and in vitro biomechanical experimental data. Typical Hangman′s fracture models with different degrees of postoperative reduction were simulated by re-segmenting, deleting, and weakening the strength of the grid elements, including four models with postoperative residual fracture lines of 0mm, 1mm, 2mm, and 3mm. And the posterior C2-3 pedicle screw rod system was implanted in the above models to obtain internal fixation models with different degrees of postoperative reduction. A vertical downward force of 75N was applied to the occipital condyle of those models to simulate the gravity of the skull, and a torque of 1.5N·m was applied to simulate the four working conditions of flexion, extension, lateral bending and rotation in real life, and the stress distribution of the screws between C2-3 and the displacement changes of the fractured end node along the C2 vertebral body were compared between these four models under different working conditions to judge its stability. Results: The ROM of the established normal cervical spine model under various working conditions was basically consistent with the data in previous literature, therefore the model was verified to be effective. Finite element analysis results indicated that during flexion and extension, the maximum stress of the screw appeared in the model with a fracture gap of 3mm. The maximum stress in forward flexion was 85.34MPa and in posterior extension was 46.34MPa. As the fracture gap increased, the maximum stress was increasing, mainly concentrated in the tail of the connecting rod and screw. During lateral bending and rotation, the maximum stress of the screw appeared in the model with a fracture gap of 2mm, the maximum stress of lateral bending was 57.26MPa, and the maximum stress of rotation was 59.78MPa. The stress was mainly concentrated in the connecting rod of the screw and the middle and rear section of the C2 screw. When the fracture gap was 0mm, the absolute displacement of each node on the fracture end node path of the axis vertebral body in flexion, lateral bending, and rotation was smaller than the node displacement of the other three models, and it was the most effective in limiting and fixing the displacement of the C2 fracture end. In extension condition, the displacement values of the four models were not much different. Conclusions: After fixation and reduction of Hangman fracture C2-3, the greatest stress on the screw is under flexion and extension when the fracture gap is 3mm, and it appears under lateral flexion and rotation when the fracture gap is 2mm. When the fracture gap is 0mm, the displacement along the fracture end node path is the smallest, and the effect of restricting fracture displacement is good.