Morphometry of sacral hiatus and its clinical relevance in caudal epidural block

Sacral approach to epidural space produces reliable and effective block of sacral nerves. It is necessary to have a detailed knowledge of sacral hiatus (SH) for optimal access into sacral epidural space. This study was undertaken to evaluate various landmarks of SH.

One hundred and fourteen adult dry human sacral bones were examined for morphometric analysis using vernier caliper. SH was categorized on the basis of shape.

Most commonly encountered shape of hiatus was inverted U (40.35%). Its apex and base were most commonly observed against fourth and fifth sacral vertebrae, respectively. Various defects in dorsal wall of sacral canal were recorded. Height and anteroposterior depth at the apex of hiatus were ranged 4.30-38.60 and 1.90-10.40 mm, respectively. Mean intercornual distance at base was 11.95 +/- 2.78 mm. The triangle formed by right and left posterior superior iliac spines and apex of SH was found equilateral in 45% cases only. Sacral cornua were marked by their bilateral presence in 55.26% and impalpable in 21.05% cases. Minimum distance between S2 and apex was 7.25 mm which suggested that it would not be safe to push the needle beyond 7 mm into sacral canal so as to avoid dural puncture. In 8.77% cases, depth of hiatus was less than 3 mm.

Single bony landmark may not help in locating SH because of anatomical variations. Depth of hiatus less than 3 mm may be one of the causes for failure of needle insertion. Surrounding bony irregularities, different shapes of hiatus and defects in dorsal wall of sacral canal should be taken into consideration before undertaking caudal epidural block so as to avoid its failure.