Clinical relevance of conus medullaris and dural sac termination level with special reference to sacral hiatus apex: anatomical and MRI radiologic study

The aim of this study was to determine the vertebral level and variations in the position of the termination of both conus medullaris (CMT) and dural sac (DST) with special reference to the apex of the sacral hiatus (SHA) using magnetic resonance (MR) images and human cadavers. Different measurements were made on 200 MR sagittal T1- and T2-weighted lumbosacral images and 60 formalin-fixed adult human cadavers. The linear distances between the CMT, DST and SHA, the anteroposterior diameter at SHA, delete length and thickness of the sacrococcygeal membrane were also measured. These measurements were correlated with age and sex. In the MRI study, the mean vertebral level of CMT and its highest incidence were seen at the lower third of L1 (L1L) in male patients and at the L1-2 disc in female patients; that of DST was observed at the upper third of S2 (S2U) in male patients and the middle third of S2 (S2M) in female patients and the mean vertebral level of the SHA was noted at the middle third of S4 (S4M) in both males and females with no significant age or sex differences (P < 0.05). In the cadaveric specimens, the mean vertebral levels of CMT, DST and SHA were seen at L1L, S2M and S4U, respectively in both male and female specimens, with no sex difference. Also in the MRI study, all linear distances in the male patients were significantly different (P < 0.05) from those in female patients with exception of the thickness of SCM and the AP diameter at SHA. Accurate knowledge of these levels and the distances in-between are important for safe and successful spinal and caudal anaesthesia.