The sacrum (os sacrum) is the triangular shaped bone (irregular bone), which is composed of five sacral vertebrae (S1 to S5, fused in adults). The fused sacral vertebrae form a central sacral body, and paired sacral alae, arising laterally from S1.
The fifth lumbar vertebra is occasionally fused with the top of the sacrum. This is called the sacralization of the fifth lumbar vertebra (Fig. 2-22). The first sacral vertebra could be separated from the remaining sacral bone. On X-ray this is seen as 6th lumbar vertebra. This is called the lumbalization of the first sacral vertebra. This is also known as a transition vertebra or assimilation vertebra.