| The sagittal balance of the spine can make the human body fight against gravity with the minimum energy consumption,ensure that the human body upright and maintain the head up vision.Sagittal balance of spine is a whole concept.From the head to the pelvis,each segment is related to each other and affects each other.Any change of one segment will cause the change of the adjacent stage,and eventually lead to the imbalance of sagittal plane of spine.The cervical spine,which connects the head and trunk,bears the weight of the head,is the segment with the largest range of motion in the whole spine.Once the balance is lost,it will have an impact on the daily life of patients.It even affects the spinal sequence and the pelvic angle.Therefore,it is important to maintain sagittal balance.With the development of internal fixation technology,atlantoaxial instability caused by trauma,developmental deformity,inflammation and other reasons has been solved by more and more spine surgeons through posterior fusion.However,the fusion of the upper cervical spine will lead to the increase of the stress of the lower vertebral body and the disc,accelerate the degeneration,and easily lead to the imbalance of the sagittal position.In serious cases,even the kyphosis of the lower cervical spine occurs,which requires a revised operation.However,the evaluation indexes of sagittal balance of spine,especially cervical spine,are quite complicated.As an important parameter,thoracic 1 slope angle(T1S)can be used to evaluate the sagittal balance of the whole spine.The sagittal vertical axis of the spine(SVA)is also an important parameter for the evaluation of spinal sagittle balance.In the process of exploring the influence of correction of thoracolumbar kyphosis on the sagittal balance of the cervical spine,the researchers found that the sagittal vertical axis of the C7(C7SVA)is closely related to T1 S,but C7 SVA did not consider the influence of the head position on the sagittal balance of the cervical spine.In the study of the cervical spine,the sagittal vertical axis of the C2 has been taken into account(C2-7SVA).However,the stability and credibility of the system need further study and judgment.Objective:1.According to analysis and measurement of X-ray and CT imaging data of healthy volunteers,we seek different assessment methods to judge the differences in thecorrelation between T1 S and cervical curvature changes and their correlations.Verified the significance of T1 S for sagittal balance of the cervical spine.2.According to vitro biomechanical study of human cadaveric cervical spine specimens,the effects of different fusion angles of C1-2 and different T1 S angles on the range of motion(ROM)and intervertebral disc pressure(IDP)of each segment of the cervical spine were analyzed.To reveal the biomechanical mechanism of the changes in curvature of the lower cervical spine after atlantoaxial fusion.3.The effect of fusion angle on the sagittal balance of lower cervical spine was verified by case follow-up,and the best fusion angle of cervical spine 1-2 was found to maintain the normal sagittal balance,reduce cervical degeneration,enhance the postoperative effect and improve the quality of life of patients.Methods:1.C1-2,C2-4,C4-7,C2-7 Cobb angle and T1 S were measured the on the standard lateral cervical radiographs and CT imagins of 44 healthy volenteers.The measurement was conducted by two experienced orthopedists for twice,each time separated by at least2 weeks.Data of the measurement was recorded in separated document avoiding interaction.Intra-and inter-observer intraclass correlation coefficients(ICC)of the two methods were evaluated.Also,the results of the two groups were analyzed.2.Eight fresh cadaveric specimens(C0-T1)were used to establish posterior atlantoaxial fusion specimens(C1 posterior arch lateral mass screws + C2 pedicle screws were selected for internal fixation),and the T1 S value and C1-C2 angle of the specimens were changed.The three-dimensional movement of the cervical spine was simulated by using biomechanical experimental machine and strain measuring instrument.Different T1 S were selected by changing the atlantoaxial fusion angle,and hybrid was used.The test mode was used to detect the stress changes of the intervertebral disc and the range of motion of each vertebra under six conditions of flexion,extension,left and right lateral flexion,left and right rotation.3.We retrospectively analyzed the cases of 52 patients who underwent spinal surgery at the Long March Hospital in recent years due to atlantoaxial dislocation.The same experienced and skilled physician performed the surgery.Collect relevant imaging data,including lateral X-ray films of the cervical spine before,after,and 3 months after the operation until the last follow-up,and measure and analyze the C1-2 angle,C2-4angle,and C4-7 angle,C2-7 angle,T1 S angle and other parameters,to evaluate the degree of cervical spine curvature changes in patients with atlantoaxial fusion,to verify the important role of T1 S in cervical sagittal balance.Results:1.The cervical lordosis is an important basis for maintaining the sagittal balance of the cervical spine.T1 S,as an important parameter for judging the sagittal balance of the cervical spine,has a direct effect on the angle of the physiological lordosis of the cervical spine.The cervical lordosis is mainly determined by the cervical 1-2,but T1 S cannot directly affect the C1-2Cobb angle.As the main component of the cervical lordosis,the C2-7Cobb angle is greatly affected by T1 S.The T1 S angle was significantly positively correlated with the C2-7Cobb angle(r = 0.493,p = 0.001).Compared with C2-4,C4-7 is more directly affected by T1S(r = 0.430,p = 0.006).Neither the choice of X-ray examination or CT three-dimensional reconstruction examination will affect the evaluation of sagittal balance of the cervical spine,but the X-ray examination is blocked by the patient's shoulder joint,and sometimes the thoracic 1 vertebra is not clearly displayed,and the clinical evaluation is limited.Although the CT measurement is smaller than the X-ray(Cobb X = 0.6384×Cobb CT + 7.5663),it can clearly show the vertebral boundary of the cervical and thoracic vertebrae,and the measurement is more accurate.It is a good alternative to X-ray examination.2.In vitro biomechanical experiments show that after the atlantoaxial is fixed,the T1 S and the C1-2 cobb angle have little effect on the lower cervical spine in the flexion and extension direction.However,the larger T1 S will cause a significant decrease in the lateral bend activity of C4-7,which is significantly negatively correlated(r = 0.-967,P =0.007).In the rotation,a larger T1 S will cause a significant increase in the rotation activity around C4-7,and the two are significantly positively correlated(r = 0.981,P =0.007).In addition,the fixation of the atlantoaxial will cause increased pressure on the lower cervical disc.In the forward and backward flexion directions,the pressure of each disc increased significantly.And the degree of increase was significantly positively correlated with the size of T1S(r = 0.998,P <0.000).However,in the lateral flexion and axial rotation directions,only the C3-4 disc pressure increased after fixation(p = 0.017),and the other segments did not increase significantly.Moreover,different atlantoaxial fixation angles and T1 S had no effect on disc pressure changes..3.According to a three-year follow-up,the cervical curve of the patients changed to varying degrees after atlanto-axial fusion.T1 S is relatively fixed and can be used as an important evaluation index of sagittal balance of the cervical spine.Patients with larger T1 S tend to have a more cervical lordosis.Compared with the C2-4cobb angle,a larger T1 S tends to cause an increase in the C4-7Cobb angle.Because the C1-2Cobb angle has a negative correlation with the lordosis of the lower cervical spine,a larger C1-2 fixation angle usually results in accelerated degeneration of the lower cervical vertebra,and even kyphosis.Therefore,for patients with atlantoaxial dislocation,it is necessary to measure T1 S before surgery,evaluate the sagittal balance of the cervical spine,and select the appropriate atlantoaxial fusion angle according to its size.For patients with larger T1 S,the atlantoaxial vertebra needs to be fixed at a relatively small angle(15-20 °)relative to the lordosis,while for patients with smaller T1 S,the atlantoaxial vertebra should be fixed at a slightly larger angle of lordosis(25 °),in order to maintain the sagittal balance of the cervical spine,slow down the speed of cervical degeneration,and improve the quality of life of patients with atlantoaxial dislocation as much as possible.Conclusions:T1S is closely related to cervical sagittal balance.A larger T1 S tends to lead to a larger C2-7 angle,and the C2-7 angle has a clear negative correlation with the C1-2angle.Although the CT examination results are different from the X-ray examination results,the assessment of cervical sagittal balance is not different.It also has good reliability.For patients with X-ray obstruction,CT examination can be selected instead.After atlanto-axial fusion,the range of motion of the fixed segment was significantly reduced,and the compensation of the lower cervical spine increased.In the forward and backward directions,the size of the T1 S and the change of the C1-2 angle had little effect on the lower cervical spine.However,a larger T1 S will cause a significant decrease in the left and right lateral curvature of the C4-7 segment,and a significant increase in the rotational activity of the C3-5 segment.At the same time,it will also increase the pressure of the cervical disc in the flexion and extension direction,and accelerate its degeneration.For atlantoaxial dislocation,in order to achieve the ideal surgical effect,the sagittal balance of the cervical spine should be considered.As an important measure of the sagittal balance of the cervical spine,T1 S is positively related to the lordosis angle of thecervical spine,while the too large fusion angle of the atlantoaxial will lead to the disappearance of the lordosis of the cervical spine and destroy the sagittal balance of the cervical spine.Individualized atlantoaxial fusion angles are selected according to different T1 S to reduce cervical Disturbance of position balance.For patients with larger T1 S,the atlantoaxial vertebra needs to be fixed at a relatively small angle(15-20 °)relative to the lordosis,and for patients with smaller T1 S,the atlantoaxial vertebra should be fixed at a slightly larger angle of the lordosis(25 °),can maintain the sagittal balance of the cervical spine to the maximum extent. |
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