Distance from dura mater to spinal cord at thoracic vertebral level

Abstract

Introduction: Neurologic complications related to thoracic epidural analgesia are rare but devastating. It is important to understand the anatomy of the spinal canal to minimize the risk of needle-related neurologic injury. Methods: We retrospectively investigated T2-weighted spine magnetic resonance images of 346 patients. The vertical distance from the dura mater to the spinal cord (DTC) at all thoracic intervertebral levels was examined. The DTC and distance from the skin to the dura mater (STD) were evaluated at three different thoracic intervertebral levels (T1/2, T5/6, and T10/11) using three different pathways: the U, L, and M lines. The U and L lines contacted the upper and lower borders of the interspinous space, respectively. The line M represented a blind approach, passing the midpoint of two spinous process tips and the point bisecting the ligamentum flavum at each interspinous space. Results: The vertical DTC was longest at the T5/6 intervertebral level and shortest at the T11/12 level. The vertical DTC was positively correlated with height (p = 0.013) and negatively correlated with age (p < 0.001). The U line was more horizontal than the L line at the upper and middle thoracic regions, but the relationship was reversed at the lower thoracic level. Among the three lines, the STD and DTC were longest on the L line at the T1/2 and T5/6 intervertebral levels. The distances were the longest on the U line at the T10/11 level. The angle between the U and L lines was largest at the T1/2 level and the difference in DTC between the U and L lines was greatest at T5/6. The STD on the M line was longer in males than in females (p < 0.001) and was positively correlated with height (p = 0.016) and weight (p < 0.001). The DTC on the M line was also longer in males than in females (p = 0.037) and shortened with age (p = 0.001). Conclusions: Differences in the DTC were observed among thoracic intervertebral levels, mainly due to cervical and lumbar enlargement of the spinal cord. Among the three approaching lines, the dimensions implying a safety margin were longest on the L line at T1/2 and T5/6, and longest on the U line at T10/11. The variability of the safety margin according to the angle of needle insertion was largest at T5/6, and the angle between the upper and lower borders of the interspinous space was largest in the upper thoracic region.