Sacrum

The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30.

joint

congenital disorder in humans caused by abnormal fetal development of the lower spine

Tarlov cysts, also known as perineural cysts, are cerebrospinal fluid-filled sacs that form within the nerve root sheath, most commonly at the sacral level of the spine.

joint between the last lumbar vertebra and the first sacral vertebra