The Biomechanical Study Of Spinous Process Screw Unilateral Fixation Combined With Pedicle Screw Fixation On The Contralateral Side In Axis:A Finite Element Analysis

A variety of degenerative conditions,congenital anomalies,trauma,and tumor may result in upper cervical instability.The upper cervical instability often needs posterior operation and atlantoaxial or occipitocervical fixation.The techniques for fixation to restore upper cervical stability have potentially neurovascular risks and are the key point of the surgery.The C2 screw fixation is the most commonly used posterior fixation technique for the occipitocervical junction because of its biomechanical properties.Several screw fixation techniques have been described for C2 fixation.These techniques include pedicle screw,translaminar screw,pars screw,and transarticular screw.These techniques have demonstrated to be effective method for cervical stabilization in most patients.However,these techniques are not applicable in all patients and appropriate C2 fixation method for a patient who has thin laminas and VA anomalies together was controversial.The spinous process screws technique is thought to be advantageous because of the large size of the C2 spinous process and the ability to directly visualize all relevant structures.A C2 spinous process screw seems a useful alternative for fixation as a salvage technique when there is anomalous anatomy.The purpose of this study is to perform a finite element analysis of spinous process screws in C2 as unilateral fixation in three different anatomic finite element models of atlantoaxial and occipital-cervical regions and to compare with bilateral pedicle screw fixation.This study has been divided into two parts:(1)Development and validation of a finite element model of the occipito-atlantoaxial region with three different anatomic conditions;(2)A finite element analysis of the biomechanical feasibility of spinous process screw unilateral fixation combined with pedicle screw fixation on the contralateral side in axis;(3)A case report of the outcomes of spinous process screw fixation in axis.Chapter Ⅰ Development and validation of a finite element model of the occipito-atlantoaxial region with three different anatomic conditionsObjective:This study aimed to establish a complete FEM of the intact occipito-atlantoaxial region and validate the model.Methods:Three models of the upper cervical spine(C0-C3)supplied by three people with different anatomic conditions of the axis were reconstructed using computed tomography images.The images were processed using commercially-available software,such as Mimics,Hypermesh and Abaqus software.A nonlinear finite element model of C0-C3 had been developed and validated.The inferior surface of the C3 vertebra was constrained completely.Vertical load of 50 N was applied on the C0,to simulate head weight and 1.5 Nm torque was applied to the CO to simulate flexion,extension,lateral bending,and axial rotation.The ROM of the C0-C1,C1-C2 and C2-C3 segments of the models was measured.Results:There are 83862 nodes,184486 elements and 143 springs in the finite element models.Since the boundary and loading conditions in these studies were comparable,we compared the ROM of the C0-C1,C1-C2 and C2-C3 segments of the intact finite element models witih the results of the in vitro test to validate our model.Conclusion:The finite element models of C0-C3 can effectively simulate the geometric and biomechanical characteristics of the occipitocervical junction and be applied into the biomechanical study of upper cervical and occipitocervical spine.Chapter Ⅱ A finite element analysis of the biomechanical feasibility of spinous process screw unilateral fixation combined with pedicle screw fixation on the contralateral side of axis in atlantoaxial and occipitocervical fixation.Objective:This study aimed to determine the biomechanical feasibility of spinous process screw unilateral fixation combined with pedicle screw fixation on the contralateral side of axis in atlantoaxial and occipitocervical fixation by a finite element analysis.Methods:The nonlinear finite element models of the intact upper cervical spine(C0-3)had been developed and validated,including normal anatomic,a high-riding vertebral artery and thin lamina of axis.Then the Anderson Ⅱ odontoid and Jefferson fracture models were simulated.Atlantoaxial fixation was completed by C1 lateral mass screws combined with C2 pedicle screw on one side and spinous process screw on the contralateral side,compared with bilateral C2 pedicle screw fixation.Occipitocervical fixation was also completed by occipital screws combined with C2 pedicle screw on one side and spinous process screw on the contralateral side,compared with bilateral C2 pedicle screw fixation.Three different insertion techniques of spinous process screw were applied.Vertical load of 50 N was applied on the CO to simulate head weight and 1.5 Nm torque was applied to the CO to simulate flexion,extension,lateral bending,and axial rotation.The ROM of C0-C3 and von Mises Stress on the internal fixation were calculated and analyzed.Results:(1)Compared with the intact model,both the bilateral C2 pedicle screw group and the C2 spinous process screw group were statistically reduced the ROM of C1-C2 in the atlantoaxial fixation(p<0.05).Compared with bilateral C2 pedicle screw group,the spinous process screw group had bigger ROM of C1-C2(p<0.05).However,there was no statistical difference of ROM of C1-C2 between three insertion techniques of spinous process screw in the atlantoaxial fixation(p>0.05).(2)Compared with the intact model,both the bilateral C2 pedicle screw group and the C2 spinous process screw group were statistically reduced the ROM of C0-C2 in the occipitocervical fixation(p<0.05).However,there was no statistical difference between bilateral C2 pedicle screw group and spinous process screw group(p>0.05).There was also no statistical difference between three insertion techniques of spinous process screw in the occipitocervical fixation(p>0.05).(3)Compared with C2 spinous process screw group,the bilateral pedicle screw group were slightly reduced the von Mises Stress of the internal fixation,but without statistical difference(p>0.05).There was also no statistical difference of the von Mises Stress between those three insertion techniques of spinous process screw(p>0.05).Conclusion:(1)In the atlantoaxial fixation,both the bilateral C2 pedicle screw fixation and spinous process screw unilateral fixation combined with pedicle screw fixation on the contralateral side of axis in atlantoaxial fixation could provide good stability and low von Mises Stress of internal fixation.Compared with the latter,bilateral C2 pedicle screw fixation had better stability of C1-C2.(2)In the occipitocervical fixation,both the bilateral C2 pedicle screw fixation and spinous process screw unilateral fixation combined with pedicle screw fixation on the contralateral side of axis in atlantoaxial fixation could provide good stability and low von Mises Stress of internal fixation.The latter had comparable stability of C1-C2 to bilateral C2 pedicle screw fixation.(3)In atlantoaxial or occipitocervical fixation,all three insertion techniques of C2 spinous process screw can be used,especially when there is a high-riding vertebral artery or thin lamina of axis.Chapter Ⅲ A retrospective case study of the outcomes of spinous process screw fixation in axis.Objective:The purpose of this study is to report the outcomes of spinous process screw fixation in axis.Methods:we present two case of spinous process screw fixation in axis.The first case was a woman who had thin laminas and high-riding vertebral artery together with C1 fracture.CT revealed that her C2 laminas were thin and C2 pedicles were narrow with erosion.CT angiography showed high-riding vertebral artery was on the one side.This patient was treated by open reduction,internal fixation and fusion.Atlantoaxial fixation was completed by C1 lateral mass screws combined with C2 pedicle screw on one side and spinous process screw on the contralateral side,compared with bilateral C2 pedicle screw fixation.The second case was a patient with congenital craniovertebral anomalies and spinal cord edema.This patient was treated by posterior decompression,internal fixation and fusion.Occipitocervical fixation was completed by occipital screws combined with bilateral C2 pedicle screw and C3 lateral mass screw fixation.C2 spinous process screw fixation as a third anchor point in C2 was also inserted.Results:Patients underwent operative treatment without neurovascular complications.Bone union was achieved in all two patients.There was no loss of instruments.The neck pain was decreased after operation.The neuro function was great improved on discharge and good decompression was achieved on the postoperative magnetic resonance imaging for the second patient.Conclusion:A C2 spinous process screw seems a useful altrnative for fixation as a salvage technique when there is anomalous anatomy.