The sacral spine, or sacrum, refers to the large irregular triangular shaped bone made up of the five fused vertebrae below the lumbar region. There is a wedge-shaped intervertebral disc between the base of the last lumbar vertebra (L5) and the sacrum (S1) called the lumbosacral disc (L5-S1). The spinal canal extends into the sacrum and the end of the cauda equina terminates into sacral nerves which exit the canal through bony foramina (small canal openings).

The sacrum is inserted like a wedge between the two iliac, or pelvis, bones and is held together by the two sacroiliac joints.   Many back problems occur where the lumbar and sacral region of the spine connect because this region of the spine is subjected to a large amount of stress with certain activities. (Sacroiliac Joint).  Most peope with sacral problems have a preceding injury (fall or car accident), sports injury, prior or current pregnancy or less common forms of arthritis.