The sacral canal results from the fusion of the laminae of the five sacral vertebrae in the midline. Failure of fusion of the S4 and S5 results in the formation of the sacral hiatus. The sacral hiatus is easily identifiable as a small depression between the sacral cornua, hence making the performance of the sacral block possible with the use of landmarks.

The posterior superior iliac spines and sacral hiatus form an equilateral triangle pointing downwards and can be felt in the lateral and prone positions.

The dural sac ends at the level of S2 in adults and S3 in children.