Distance from Dura mater to spinal cord at the thoracic vertebral level: An introductory study on local subdural geometry for thoracic epidural block

OBJECTIVE: To evaluate the anatomical safety margins in relation to thoracic epidural block by analysing magnetic resonance (MR) images. METHODS: This retrospective study identified consecutive patients who underwent MR imaging of the thoracic vertebral spine. The distance from the dura mater to the spinal cord (DTC) was measured at different thoracic intervertebral levels using three different pathways as references: the ‘U’, ‘L’ and ‘M’ lines. RESULTS: A total of 346 patients provided MR images for analysis. The vertical DTC was the longest at the T5/6 intervertebral level (mean ± SD: 4.22 ± 1.43 mm) and the shortest at the T11/12 intervertebral level (mean ± SD: 2.51 ± 0.87 mm). The DTC was the longest on the ‘L’ line at the T1/2 and T5/6 intervertebral levels and on the ‘U’ line at the T10/11 intervertebral level. The difference in DTC between the ‘U’ and ‘L’ lines was the greatest at the T5/6 intervertebral level. CONCLUSION: Differences in the DTC were observed among the thoracic intervertebral levels. The variability of the safety margin according to the angle of needle insertion was the largest at the T5/6 intervertebral level.