延髓及上段颈髓矢状径计算模型的建立

Setting-up of the calculation model for sagittal diameter of bulbo-medullary junction

目的建立正常枕颈联合部位延髓及上段颈髓(延脊髓)矢状径随距测量基线(C2椎体前下缘与C3椎体后上缘的连线)上方距离变化的计算模型(矢状径计算模型),并检验其误差.方法选取2018年4月至2018年8月在北京积水潭医院住院治疗并行颈椎MR检查的颈椎间盘突出症及颈椎管狭窄症患者164例进行影像学测量研究.将延脊髓分为上、下两段,定义其分界线(即平行于测量基线且经过寰椎前弓松质骨下缘的直线).选取100例正常枕颈联合部位的正中矢状位MRI,测量并计算延脊髓上、下段的矢状径随距测量基线上方距离的变化率,建立以个体枢椎下缘水平延脊髓矢状径和距测量基线上方距离为自变量的延脊髓矢状径计算模型.选取另64例正常枕颈联合部位的影像学资料,测量寰椎前弓下缘处及其上、下10 mm处的矢状径实际值,并与模型计算值进行比较,计算误差值及误差率.在分界线处采用替代估计法(以枢椎后下缘处实际矢状径作为分界线处拟正常矢状径)和均值估计法(以前100例的均值作为分界线处拟正常矢状径)估算延脊髓矢状径,与矢状径模型计算值进行比较,计算误差值及出现较大误差(定义为≥1 mm)的百分率.结果延脊髓矢状径计算模型的计算公式:(分界线下方)延脊髓矢状径=枢椎下缘水平矢状径+0.0472×测量基线上方距离,(分界线上方)矢状径=枢椎下缘水平矢状径+0.0472×分界线至测量基线距离+0.298×(测量基线上方距离-分界线至测量基线距离).计算模型的误差评估结果显示,矢状径模型计算值的误差随距测量基线上方距离的增加而增大,寰椎前弓下缘上方10 mm处误差为(1.06±0.72)mm,误差率10.52%±8.26%.矢状径计算模型相较于均值估计法,出现较大误差的百分率更低(Z=-3.527,P<0.001);而相较于替代估计法,矢状径计算模型的误差更小(Z=-4.88,P<0.001),出现较大误差的百分率更低(Chi-Square=6.015,P=0.024).结论延髓及上段颈髓(延脊髓)矢状径计算模型可较准确地估计延脊髓不同部位的矢状径,为延脊髓受压程度的定量评估和寰枢关节复位后是否需要减压的判断提供重要参数.

Objective To develop a calculation model for normal sagittal diameter(SD)of bulbo-medullary junction with the change of distance above the measurement baseline(the line connecting the anterior lower margin of the C2 vertebral body and the posterior upper margin of the C3 vertebral body),and to investigate its calculation error.Methods All of 164 patients with cervical disc herniation or cervical spinal stenosis who underwent cervical MRI between April 2018 and August 2018 in Beijing Ji-shuitan Hospital were included in this study.The normal bulbo-medullary junction was divided into two parts from top to bottom,and the dividing line was defined factitiously(the line parallel to the measurement baseline and through the lower margin of cancel-lous bone of the anterior arch of atlas).On the middle sagittal MRI images of 100 cases of normal bulbo-medullary junction,the change rate of the SD along the distance above the measurement baseline was counted on the upper and lower segments separately.The calculation model for SD of bulbo-medullary junction was established,with the SD of spinal cord at level of the lower margin of axis and the distance above the measurement baseline as independent variables.After setting-up of the calculation model,the actual SD at the lower margin of the C1 anterior arch and 10 mm above and below it was measured on other 64 cases of normal bul-bo-medullary junction.The actual SD and calculation value were compared for calculating the error and error rate.The SD at the dividing line was estimated using the substituted estimation(the actual SD at level of the lower margin of axis)and mean-value esti-mation(the mean SD of the first 100 cases).Calculation value,substituted estimation and mean-value estimation were compared,and their calculation error and the occurrence rate of significant error(no less than 1 mm)were also compared.Results Calcula-tion formula for SD of bulbo-medullary junction:(below the dividing line)SD=sagittal diameter at level of the lower margin of axis(SDA)+0.0472×height above the measurement baseline(HAB),(above the dividing line)SD=SDA+0.0472×height of divid-ing line above the measurement baseline(HDL)+0.298×(HAB-HDL).The error of calculation model increased with the dis-tance above the measurement baseline.The error at the topmost level was 1.06±0.72 mm,and the error rate was 10.52%±8.26%.Compared with the estimation method using the mean value,the calculation model was accompanied with a significant-ly lower ratio of significant error(Z=-3.527,P<0.001).Compared with the estimation method using a substitute,the error of the calculation model was significantly smaller(Z=-4.88,P<0.001)and the ratio of significant errors was significantly lower(Chi-Square=6.015,P=0.024).Conclusion The SD calculation model could accurately estimate the SD of a normal bulbo-medullary junction,and has great significance for the quantitative imaging assessment and decompression strategy in patients with atlantoaxial instability.