Three-dimensional finite element analysis of the injury mechanism of axis ring fractures

English Keywords:Axis ring fracture  Upper cervical spine  Biomechanics  Finite element

English Abstract:

  【Abstract】 Objectives: To observe the stress distribution of the second cervical vertebra(axis) under different forces and predict the injury morphology of axis ring fractures(ARF), and to reveal the relationship between fracture pattern and injury mechanism. Methods: A healthy adult male volunteer was selected for skull and cervical thin slice CT scan. The CT data were imported into the software, and a three-dimensional finite element model of the normal skull base to the third cervical vertebra(C0-C3) was established by using the finite element software. The validity of the finite element model was verified. Five force bearing points on the skull geometric model were selected: the middle of forehead(point A), the front middle of the top of head (point B), the left side of the upper forehead(point C), the front left of the top of the head(point D), and the back of the occipital(point E). The forces at points A and B simulated cervical vertebra subjected to different degrees of hyperextension and axial compression violence; the forces at points C and D simulated hyperextension and axial compression with different degrees of rotational violence; and the force at point E simulated hyperflexion and axial compression violence. Finite element method was used to simulate different kind of forces(1400N) applied to the upper cervical vertebra. Then, the stress of anatomical structure of the axis ring was analyzed, based on the results of finite element analysis. Results: A three-dimensional model of C0-C3 was successfully established in this experiment, which had 1,315,238 units and 305,789 nodes. The finite element model was also validated. The stress of axis ring was mainly distributed in the area between posterior 1/4 of bilateral superior articular process and pediculoisthmic components(PIC) while point A bearing 1400N force. The stress of axis ring was mainly distributed in the area between posterior 1/3 of bilateral superior articular process and PIC while point B bearing the force. The stress of axis ring was mainly distributed in the posterior 1/3 of the superior articular process to the posterior vertebral body of the odontoid process on the left side, and the PIC, inferior articular process and lamina on the right side while point C and D bearing the forces. The stress of axis ring was mainly distributed in the vertebral body and the edge of bilateral superior articular processes while point E bearing the force. Conclusions: When the cervical vertebra was subjected to hyperextension and axial compression force, the stress of axis ring was mainly distributed in the area between the posterior quarter of bilateral superior articular process and PIC, causing ARF pattern with bilateral or unilateral PIC fracture of C2 as the core feature. And this kind of force might also cause bilateral fractures of superior facet and/or posterior vertebral wall fracture of C2, with basically parallel or symmetrical fracture lines. When the cervical vertebra was subjected to hyperextension and axial compression accompanied by rotational force, the stress of the axis ring was concentrated between superior articular process on one side and the posterior vertebral body of the odontoid process and the contralateral PIC, inferior articular process and lamina, causing ARF with one fracture of superior facet and/or posterior wall of the vertebral body of C2 and another contralateral fracture of different structures as the core feature. And such force in some cases might result in bilateral fractures of superior facet and/or posterior vertebral wall fracture of C2, with longitudinal fracture line on one side and transverse fracture line on the other. In the case of hyperflexion and axial compression force, the stress of axis ring was mainly distributed in the vertebral body and the edge of bilateral superior articular processes, revealing a high incidence of body fractures of the axis or complex axis ring fractures.