With the fast development of computer technology and frequent updates of various finite element analysis software, finite element analysis has become a major method for biomechanical studies . The purpose of our study was to create a series of cervical C2-C7 segments 3D model for finite element analysis, in order to further probe into the screw positioning during ACDF surgery and the stress change in the segments adjacent to the fused segments.
ACDF is considered to be the most classical surgical method for treating cervical spondylosis, long term follow-up period for clinical studies after surgery showed that, ACDF could relieve the symptoms of neck-shoulder pain, and the relatively satisfactory fusion effect could attenuate the risk of second operation on the responsible levels as well. However, ASD caused by fusion surgery cannot be ignored . Hilibrand etc.  reported a 2.9% incidence of ASD one year after surgery, and the incidence rises to 25.6% 10 years after surgery, in addition, more than half of the patients with ASD presented neurologic symptoms. Studies conducted by Zhang et al. showed that, it is not necessary for ASD patients without positive symptoms to receive treatment, and only one third of the ASD patients presenting symptoms required a second surgery . Maldonado et al.  followed up 105 patients who underwent ACDF and reported an incidence of 10.5% for ASD in the patients who received their surgery three or more years ago. In addition, after a long term follow up study, Angelo et al.  concluded that the major cause for ASD after ACDF surgery was the increase in the activity in such segment. However, the exact mechanism of ASD after ACDF surgery remains unclear. Currently, scholars mainly concur to the theory that, the biomechanical characteristic alter ACDF or the natural degeneration of adjacent levels are responsible for ASD, which requires us to delve deeper into the actual mechanism . In theory, the responsible segments lose their ROM after ACDF surgery, and the stress transfers away, causing the concentration of stress on the adjacent segments, accelerating their process of degenerating. The model analysis indicated that the ROM of adjacent segments increased to compensate the loss of ROM on the responsible segments, moreover, the stress change of each segments are different because varied levels. The ROM of the cephalad adjacent segment to the operative levels increased significantly after single level ACDF surgery, while the ROM of the whole cervical spine decreased linearly. Similarly, during the IDP monitoring, the mean IDP of the adjacent segments to the fused segments was higher than that of normal people, and the most significant IDP increase was observed on the superior adjacent segment .
Our study specifically focused on the ASD due to change of internal fixation screw positioning, and we discovered that different screw positioning during fusion of the C4-C6 caused different change of IDP on the adjacent segments. As shown in Figure4, the IDP in C2/3 are not significantly affected by the 4 different screw positioning, and the anteroinferior screw positioning in group II as well as the posteroinferior positioning in group IV brought the least IDP, hence we could know that the IDP could be closer to natural pressure when the screws are inserted in the superior part of the vertebral body. From the IDP change in C3/4, we found that except the IDP was the lowest under the rotation setting in group IV, all other settings brought similar IDP change. While on the C6/7 level, it is obvious that the posteroinferior screw positioning under posterior extension, rotation and lateral flexion settings yielded the minimum IDP value, and when under the anterior flexion setting, it returned identical value as the anteroinferior screw positioning. Therefore, from the stress test, we found that screw positioning has limited influence on the IDP change, however, the inferior screw positioning in group II and group IV could bring the most natural IDP.
As shown in Figure 3,under anterior flexion and posterior extension settings, each segments in group II and IV achieved higher ROM, and the ROM of the segments in group I and III were closer to natural state. Under left and right rotation settings, ROM of segments were higher in group II and IV than in group I and III, and group I and III still achieved more natural state on ROM; under left and right rotation setting, the ROM of C3/4 decreased because of the fusion of C4/5, and ROM of C2/3 increased to compensate the ROM loss of C3/4, hence, the ROM were closer to normal state when the screws were inserted in the anterior part of the middle column. Under left and right lateral flexion settings, ROM in group IV were smaller than those in the other three groups, and closer to angels in normal activity. The analysis result above proved that, on the cephalad end of the model, the pressure stress on C2/3 segment significantly increased when the vertebral body underwent lateral/anterior flexion and posterior extension. When an axial rotation changes in the vertebral body, relatively prominent with respect to the cephalic segment changes, and may directly affect the fusion of the distal segment. The IDP in the adjacent segments to the operative levels underwent significant change after surgery due to the change of stress, besides, the increase of the adjacent segments ROM, the concentration of stress and the IDP increase in adjacent segments would all contribute to the degeneration of ASD, which is in good accordance of previous studies results concluded by Watanabe S etc .
The stress on nephogram (Figure 4) of the model with different simulated anterior flexion and posterior extension activities presented us that, after ACDF surgery, the pressure stress was shifted upward 2 levels to C2/3 level, and cause IDP in the disc, which was supposed to bear much less load physiologically, to rise significantly, and further expedites the degeneration of segments in cephalad end. In addition, from the stress nephogram of the model with different Flexion and straight rotation. It also can be concluded that, the pressure stresses exerted on the screw and titanium plate in group III and group IV were the maximum, which indicated that such fused segments possess the strongest.