The sacrum is a bone at the base of the spinal column that is formed following the fusion of 5 vertebrae. It lies in the region of the buttocks and just below it is another small series of bones called the coccyx. The presence of the sacrum gives rise to the normal rounded contour of the buttocks and the shape of the pelvis. Nerves from the spinal cord pass through a bony canal within the sacrum and exit the sacrum in a number of places to provide nerve supply to the bowel and the anal sphincters, the bladder and the bladder sphincters and, also, to the muscles and sensory organs in the lower limbs.
Sacral agenesis is a condition that exists when either part or all of the sacrum is absent.
It is possible for two of the five sacral segments to be absent without causing problems with the nerve supply. However, if three or more of the sacral segments are absent, it is probable that there will be some abnormality of the nerves coming out of the sacrum. If there is an abnormality of the nerves coming out of the sacrum, this can have a number of different effects. It may affect the bladder and the urinary sphincters and this may give rise to urinary incontinence and urinary tract infections. It is possible that in patients with sacral agenesis the bladder will store urine at abnormally high pressure and thus may pose a risk of causing kidney damage.