| Backgrounds:(1)There are many morphological variations of bone structures under the condition of occipitalization of the atlas,making the screw placement for reduction and fixation dangerous.However,there is a paucity of literature pertaining to morphometric measurement of occipitalized C1 lateral mass;(2)At present,there have been many reports on the variation of vertebral artery in the case of craniocervical junction malformation.However,there is still a lack of study on the positional variation of the internal carotid artery that is anterior to the lateral mass of atlas;(3)The pathogenesis of atlantoaxial dislocation and basilar invagination under the condition of occipitalization of atlas remains unknown,and it is also difficult to study the pathogenesis by conventional means.The finite element model can simulate the biomechanics of craniocervical junction with occipitalization of atlas;(4)The final pathogenesis of craniocervical junction malformation is mainly attributed to the pathological changes of the medulla oblongata and the superior cervical spinal cord.The biomechanical factors such as excessive intratissue stress and strain are considered as the main pathogenic mechanism of this change.Making the mechanism clear would is beneficial to the choice of treatment and the judgment of prognosis for craniocervical junction malformations.Objectives:(1)In the first part of the study,we define the morphometric dimensions of occipitalized C1 lateral mass so as to provide surgeons with valuable information for preoperative planning and to guide the C1 screw placement intraoperatively;(2)Based on the computed tomography angiography(CTA),in this section,we perform a qualitative analysis and quantitative measurement of the relationship between internal carotid artery and the occipitalized atlas,so as to provide some useful anatomical information for preoperative evaluation;(3)In this research,we simulate the deformity of occipitalization of atlas with a finite element model to study the biomechanics in this pathological situation,and to explore the pathogenesis of atlantoaxial dislocation and basilar invagination;(4)In this section,we attempts to construct a finite element model of medulla oblongata and superior cervical spinal cord by means of three-dimensional reconstruction of continuous thin-slice pathological sections.With this model,we explore the effects of atlantoaxial dislocation and basilar invagination on the stress and strain within nerve tissue.Methods:(1)The cervical CT data of 131 patients with occipitalization of atlas and 50 patients with normal structure of craniocervical junction were collected.Then the two groups of data were reconstructed and resegmented by Mimics software,and related quantitative parameters of the atlas were measured respectively.(2)The cervical CTA data of 86 patients with occipitalization of atlas and patients with normal structure of craniocervical junction were collected.Then the two groups of data were reconstructed and resegmented by Mimics software,and the relationship between the internal carotid artery and the lateral mass of atlas was evaluated qualitatively and quantitatively.(3)A finite element model of occipitalatlantoaxial complex was constructed and verified.The condition of occipitalization of atlas was simulated by changing the model conditions.The range of motion in each segment,the stress on transverse ligament and articular surface of lateral atlantoaxial joint in the two models under different loading conditions were extracted.(4)The donated medulla oblongata and upper cervical spinal cord specimens were obtained,and a finite element model of medulla oblongata and upper cervical spinal cord was constructed by the methods of continuous pathological section,staining,scanning,depiction and 3-dimentional reconstruction.The compression and traction on medulla oblongata and upper cervical spinal cord which is caused mainly by atlantoaxial dislocation and basilar invagination respectively were simulated.The corresponding stress and strain in the neural tissue were extracted.Results:(1)Except for the medial height,all the other measurements of heights and widths in the occipitalization group were smaller than those in the control group(P<0.05).The ideal inward angle ?(20.1±8.3°)in the occipitalization group was higher than that in the control group(15.3±3.8°)(P<0.05).There was no significant difference in the maximum safe cephalic angle ? between the occipitalization group(33.5±9.9°)and the control group(32,9±6.6°),(2)There was significant statistical difference between the occipitalization group and the control group in the position relationship between the internal carotid artery and the lateral mass of atlas.The distance from the medial edge of the internal carotid artery to the midsagittal plane was 21.18±3.72 mm in the occipitalization group and 22.66±3.11 mm in the control group(P<0.05).The mean value of the shortest distance from the posterior edge of the internal carotid artery to the anterior edge of the lateral mass of atlas was 3.67±2.16 mm in the occipitalization group and 3.60±1.25 mm in the control group(P<0.05).(3)Compared to the intact model,at flexion,extension,lateral flexion and rotation in occipitalization model,the range of motion decreased by 39.2%,44.7%,18.8%and 8.5%respectively,the maximum stress on transverse ligament increased by 63.2%,6.1%,12.3%and 21.5%respectively,and the maximum stress on the superior articular surface of lateral atlantoaxial joint increased by 12.4%,22.6%,34.8%and 26.1%respectively.(4)With the gradual increase of compression degree,the maximum stress on white matter and gray matter of the model showed an increasing tendency.However,when the compression degree reached>25%,the stress on the white matter suddenly increased obviously.Similarly,with the gradual decrease of the clivus-canal angle,the maximum stress on the white matter and gray matter of the model showed an increasing tendency.However,when the clivus-canal angle decreased to 120°,the stress on the white matter would suddenly increase.Conclusions:(1)Though the hypoplastic Cl lateral mass brings limitation to the screw insertion in some extent,it is still broad enough to accommodate a screw safely both in female and male patients.Considering the irregularity of C1 lateral mass in patients with occipitalization of the atlas,preoperative imaging assessment is critical and C1 lateral mass screw placement should be performed individually.(2)The relative relationship between internal carotid artery and lateral mass of atlas in occipitalization group was significantly different from that in control group.During the C1 screw placement,the risk of internal carotid artery injury in patients with occipitalization of the atlas was significantly higher than that in patients without occipitalization of the atlas.Therefore,we recommend careful preoperative CTA evaluation in all patients before surgery.(3)The functional loss of bilateral atlantooccipital joint in patients with occipitalization of the atlas can lead to excessive motion of C1-C2 segments,which result in excessive stress on the transverse ligament and the articular surface of lateral atlantoaxial joint.For patients with occipitalization of the atlas,even if without symptoms,considering the dynamic development process of atlantoaxial dislocation and basilar invagination,regular follow-up should be carried out,and intervention should be considered at the early stage of the disease.(4)In this study,a finite element model of medulla oblongata and upper cervical spinal cord with fine anatomic structure was successfully constructed.With the increase of compression degree and the decrease of clivus-canal angle,the stress in nervous tissue increased,but the increasement in the white matter was more obvious than that in the grey matter.The white matter of medulla oblongata and upper cervical spinal cord may play a buffering and protective role for gray matter.The reduction of atlantoodontoid interval plays a leading role in the damage of neural tissue in craniocervical junction. |
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