Biomechanical Condition Changes Of The Adjacent Disc Followed The Correction Of Intervertebral Height In Single Level ACDF Surgery

Objective To establish an effective finite element model of the cervical spine and using the finite element method, to investigate the effect of single-level anterior cervical discectomy and fusion with different intervertebral height on the adjacent-level segments and to provide biomechanical reference.Materials and methods C3-C7 finite element model was established by CT image of normal adult cervical spine. After passing the normal cervical spine finite element model validation, the postoperative finite element model of the cervical spine was established by changing the height of the intervertebral disc and the material properties. C4-C5 fusion model and C5-C6 fusion model were obtained. The height of intervertebral disc type I: 2mm, type II: mm, type III: 2mm, type IV: 4mm. Under 45 N axial pressure, the models were tested for the following four parts: 1)displacement and bulge of adjacent-level disc(X axis direction,Y axis direction, Z axis direction) 2)compressive pressure on adjacent-level disc annulus and distribution 3.)pressure on adjacent-level nucleus pulpous 4) compressive stress on adjacent-level endplate.The differences of biomechanical parameters between the postoperative model and the normal model were comparedResults The finite element model of C3-C7 segment of normal cervical spine was established and verified by the method. The finite element model of the cervical spine and the experimental model accord with the degree of more than 90%. A variety of the postoperotive finite element models of the cervical spine were established by changing the height of the intervertebral disc and the material properties. The models were tested undering 45 N axial pressure. 1)The maximum value of the disc axial displacement was located in the front-middle part of the disc;the maximum value of the disc protrusion was located in the posterior-lateral part of the disc, there was statistical difference between I types and normal types. 2) The maximum value of the disc annulus axial pressure was located in the front-middle part of the disc annulus; the maximum value of the disc annulus axial pressure of I types was located in the posterior-lateral part of the outer disc annulus.; there was statistical difference between I types and normal types 3) Compared with other types, the pressure of I types nucleus pulpous was increased; there was no statistical difference between II types, III types, IV types and normal types. 4) With the increase of the intervertebral height, cervical curvature increased gradually and there was post shift trend of endplates compressive stress distribution in II types, III types, IV types. There was statistical difference between I types and other types on parameters of the upper adjacent segments and partial parameters of the lower adjacent segments.Conclusions Established a reliable normal adult C3-C7 cervical spine finite element model and a variety of C3-C7 fusion model followed single level ACDF. By finite element simulation, the stress distribution in the adjacent segment of the type IV, type III and type II models were more close to the normal physiological condition; The stress distribution of the adjacent intervertebral disc and the end plate were greatly different between typeⅠmodel and normal model. After ACDF, the recovery of intervertebral height can not only improve the cervical curvature and the adjacent segment biomechanical environment, but also slow down the speed of adjacent segment pathological changes due to abnormal force. In this study, we explored the biomechanical environment changes of intervertebral height correction degree followed single level ACDF surgery on the adjacent disc, biomechanical mechanism of cervical intervertebral disc degeneration and provided biomechanical reference for prevention of cervical disease, surgical treatment, postoperative rehabilitation.