2.1 Experimental data
In 2019, in the Digital Medicine Center of Inner Mongolia Medical University, the finite element analysis of 4 different methods of internal fixation of the child's anterior cervical approach was completed. The experimental specimen came from the corpse of a 6-year-old girl from the Anatomy Laboratory of Inner Mongolia Medical University. Height 115cm, weight 41kg. X-ray of cervical spine was taken to exclude cervical vertebra deformity, trauma, tumor, etc. The CT(United States, GE, Lightspeed dual-source 64-slice spiral CT) scan range was a spiral axial scan of the whole cervical spine from top to bottom. Scanning parameters: tube voltage 120KV, tube current 125mA, layer thickness 0.625mm, no interval scanning. Complete cervical spine CT image data was obtained and saved in DICOM format. The experiment was approved by the Ethics Committee of Inner Mongolia Medical University.
2.2 Experimental method
2.2.1 Establish a finite element model for the removal of the C4/5 disc of the lower cervical spine in children
CT data was constructed into a normal child C3-C7 three-dimensional finite element model by Mimics 21.0, Pro/E 5.0, Geomagic studio 2015, HyperMesh 14.0, Abaqus 6.14. Multiple unit types were used to build the model, 361012 units and 509161 nodes were contained. The C4/5 disc removal of the model was modified into an unstable finite element model. The boundary conditions and loading conditions were the same as the full model loading conditions. In the experiment, the ligament was hidden for view convenience, but the finite element analysis results were not affected.
2.2.2 Establish 4 kinds of internal fixed finite element models
According to the child’s cervical instability model, a three-dimensional model of the internal fixation steel plate and fixed locking screw was constructed. Four kinds of internal fixation plate screws were used to construct geometric solid models to build four personalized anterior cervical internal fixation models:
(1) Anterior cervical Orion plate internal fixation (ACOP) for children’s anterior cervical locking internal fixation system, which consisted of 4 ordinary vertebral screws with connected and fixed steel plates, and the screws were directly driven into the vertebral body.
(2) Anterior cervical vertebral locking plate (ACVLP) included 4 vertebral screws with threaded heads and corresponding locking plates, which inserted into the vertebral body in the same way as ACOP.
(3) Anterior cervical single cortical pedicle locking plate (ACSLP), included 2 vertebral screws and 2 pedicle screws. Vertebral screws and pedicle screws were ross-locked of in the vertebral body.
(4) This study designed an anterior cervical bicortical pedicle locking plate (ACBLP) combined with the commonly used anterior cervical ORION internal fixation system. It composed of 2 full-thread vertebral screws and 2 full-thread bicortical pedicle screw penetrating side block through pedicle. The vertebral screws and full-thread bicortical pedicle screw were cross-locked in the vertebral body. At the same time, the two screw caps were double fused and locked with the steel plate.
The elastic modulus, Poisson’s ratio, element type and characteristic value of the implant material were inputted into the model [7] (implant titanium alloy E = 110000MPa, μ = 0.3, tetrahedral element). The three-dimensional finite element models of the four internal fixation systems after cervical C4/5 discectomy were established, as shown in Figure 1.
2.2.3 Loading calculation of 3D finite element model
The cervical vertebra screw and the vertebral screw were defined as close contact without sliding and compression deformation. Constraint boundary: The degrees of freedom of all nodes on the lower edge of the C7 cone were restricted in all directions, C3 was not subject to any restrictions, and the center of the upper edge of the vertebral body received a load that simulated the weight of the head. Assumptions: The material properties of the biological materials involved in this experiment were assumed to be continuous, homogeneous and isotropic; there was no mutual sliding between the sections of the model under force; there was enough stability between the units; the stress and deformation of each part of the material during the loading process were excluded. Loading conditions: A 27.42N simulated head weight preload is applied to C3, and the additional pure moment of movement is 1.8Nm[8]. ABAQUS finite element software was used for finite element analysis. According to the experimental methods of cervical spine movement characteristics and specimens, simulated cervical spine movement was divided into six types of movement: anteflexion, backward flexion left flexion, right flexion, left rotation and right rotation.
2.2.4 Setting and operation of different working conditions
The stress value of the model was set. A 27.42N•m torque load was applied to simulate cervical spine movement to ACOP, ACVLP, ACSLP, and ACBLP after cervical C4/5 discectomy internal fixation model respectively under 6 working conditions. Five points were randomly selected from C4, C5's superior articular process, contralateral superior articular process, bilateral pedicle and vertebral body, the corresponding values were obtained to compare and analyze the biomechanical activity and stress changes of the four internal fixation models. At the same time, the stress distribution and strain of the steel plate and screw unit of the four kinds of internal fixation systems of the lower cervical spine in children were observed.
2.2.5 Statistical analysis
The data were inputted into SPSS 22.0 for analysis. The data is expressed by x¯±s. The same measurement parameter was compared in different internal fixation models by one-way analysis of variance (One-way ANOVA) with multi-sample means comparison, and P<0.05 was considered as significant difference.