The biomechanical study of different type of instruments for atlas and axis complex fracture: a finite element analysis

English Keywords:Atlas  Axis  Fracture  Internal fixation  Biomechanics  Finite element

English Abstract:

  【Abstract】 Objectives: To analyze the biomechanical stability of different instruments for atlas and axis complex fracture by finite element method, and to provide theoretical reference for the clinical application. Methods: The geometries from occiput(C0) to C3 were performed from CT images of a healthy volunteer. A three-dimensional finite element model of intact upper cervical spine(FEM/intact) was established by software of Simpleware 3.0, Hypermesh 10.0, Abaqus 6.9, and the model was validated through Panjabi′s data. Then, the bottom of the odontoid process and the junction between the anterior and posterior arch and lateral mass were removed to simulate FEM of Jefferson type Ⅱ odontoid fracture(FEM/fracture). A computer-aided software according to a fixed system specification was used to build the FEM of atlantoaxial pedicle screw fixation(FEM/PSF) and occipital-cervical fusion(FEM/OCF). Flexion, extension, lateral bending and rotation were imposed on the intact, fracture and two fixation models respectively. The Von Mises stress distribution and range of motion(ROM) of each segment in different models was compared. Results: (1)FEM/intact was established clearly with good geometric similarity, which was validated by Panjabi′s work. The ROM of FEM/fracture increased obviously than FEM/intact under same loading conditions, especially a increase of 72.1% in flexion and extension, and an increase of 43.7% in left and right rotation. (2)Each of the FEM of upper cervical spine loading in fixation system matched clinical practice. Compared with FEM/intact, the ROM of C0-C1 in flexion/extension and rotation in FEM/PSF increased by 59.2% and 68.3% respectively. The ROM of C1-C2 in flexion/extension, lateral bending and rotation decreased by 92.2%, 31.3% and 99.6% respectively. The ROM of C2-C3 in flexion/extension, lateral bending decreased by 13.6%, 0.6% and the ROM in rotation increased by 0.7%. Compared with FEM/intact, the ROM of C0-C1 in flexion/extension, lateral bending and rotation in FEM/OCF decreased by 93.8%, 90.4% and 90.2% respectively. The ROM of C1-C2 decreased by 89%, 55.7%, 97.4% and the ROM of C2-C3 decreased by 92.8%, 95.2% and 90.5% in flexion/extension, lateral bending and rotation respectively. (3)The maximal Von Mises stress in the internal fixations was 321.19MPa in atlantoaxial pedicle screws in left rotation, and 228.84MPa in occipital-cervical fusion system in left bending. Stress was mainly concentrated in the junction of rods and screws. Conclusions: Two surgical methods are effective for the atlas and axis complex fracture, both procedures can ensure good stability. C1-C2 pedicle screws can increase the ROM of C0-C1.