sacrum and coccyx

The sacrum is formed by the fusion of five sacral vertebrae. It has a natural kyphosis. The median sacral crest is seen to bear the spinous tubercles and this crest is the fused spines of the sacral vertebrae. Below the spine of S4 is the sacral hiatus which is due to failure of fusion of the S5 laminae. It is through this hiatus that caudal epidural anaesthesia is performed. Note the four coccygeal segments which, in this case, are existing as separate entities, but are often fused together. The sacral spinal canal contains the cauda equina and the spinal meninges. These are best seen, however, on a lateral lumbosacral myelogram. The filum terminale from the pia mater emerges below the sacral hiatus and passes downwards to insert into the coccyx. Babies of mothers suffering from diabetes mellitus may occasionally have sacral agenesis.