The sacral spine, or sacrum, is the fused triangular bone at the base of the spine. It is located just below the lumbar region and just above the coccyx or tailbone, where it anchors the pelvis to the spine. The sacrum does not fuse together until early adulthood, and when it does, it is very difficult to fracture.
The sacral spine fits into the space between the two hip bones at the lower end of the spine. It connects to the last lumbar vertebra at the top of the sacrum, the coccyx below and the ilium section of the hip bones on either side. The sacral spine consists of five vertebrae, designated S1 through S5.
At birth and throughout childhood, the sacral vertebrae are distinct from one another. Between the ages of 16 and 18, however, these vertebrae begin to fuse together. Usually by the time a person reaches 26 years old, the sacral vertebrae have coalesced into a single fused bone structure.
Even after the sacral spine's fusion in early adulthood, four openings remain where the joints bonded together. These openings are called sacral foramina. The five sacral nerves travel out of the spine via the sacral foramina to the bowels, bladder and genitals.
The sacral spine plays a key role in supporting the upper body and connecting it to the lower body. From its position wedged between the hip bones, the sacrum stabilizes the entire pelvic girdle. In this way, the sacral spine acts much like a keystone in an arch, transferring the weight from the structure of the human body above and dispersing it out through the pelvis and into the legs below.
As an anchor, the sacral spine is dense and does not break easily. Isolated sacral injuries are especially rare. Usually, the cause of a sacral fracture also leads to fracturing in the bones of the surrounding pelvic girdle. The three most common types of sacral fractures are stress fractures caused by intense repetitive use, insufficiency fractures cause by osteoporosis and traumatic fractures caused by sudden traumatic events, such as car accidents.
Sacral fractures present as pain, tenderness or numbness in the lower back or buttocks. Bed rest is the most common treatment for sacral fractures. A healthcare professional might also prescribe medications to relieve pain or a sacral corset to stabilize the injury and provide comfort while it heals. In severe fractures where pieces of the broken sacral spine have moved away from each other, surgery might be necessary.