Sacral Spine (Sacral Region)
The sacral spine, also known as the sacral region, includes the sacrum and coccyx which form the base of the vertebral spine.
The human spine begins at the base of the skull, where the cervical vertebrae receive the spinal canal, which houses the spinal nerves and its protective layers. The spinal cord continues through the thoracic spine and the lumbar spine, where the spinal cord actually ends near the top of the lumbar spine (in the lower back). The spinal nerves do continue downward through the same canal that housed the spinal cord, and continues through the structures of the sacral spine. In this article, we will examine the anatomy of the sacral spine, and how our backs and nerves may be affected by disease or injury.
The Sacral spine, or sacral region, begins below the last lumbar vertebrae (L5). The two main structures of the sacral spine - the sacrum and coccyx - together stabilize the spine at the base of the back and make up the pelvic curvature.
The Pelvic Curvature: The pelvic curvature is one of the two primary curves of the spine, meaning that it is present at birth. The pelvic curve is known as a Kyphotic Curve, meaning that it is concave anteriorly and convex posteriorly.
The Sacrum and the Coccyx: The sacrum and coccyx are both structures within the Sacral Region which existed as separate vertebrae at birth and become fused during development. For this reason the sacrum are also known as "false vertebra" or "fixed vertebra" The sacrum is an element of 5 fused vertebral bones. The coccyx is an element of 3-5 fused vertebral bones. Together, the sacrum has the shape of a downward-pointed arrowhead. The top of both structures is called the base, and the bottom called the apex, for both structures.
The Sacrum: The sacrum consists of 5 fused vertebral segments. In the vertebral spine, each individual vertebra has a vertebral foramen - a hollow canal for the spinal cord and spinal nerves. These spinal nerves continue in the sacrum, where each sacral segment has a posterior sacral canal for the passage of the spinal nerves. The sacrum also has 4 sacral foramen though which the spinal nerves exit the bottom of the spine.
In males, the sacrum is longer, narrower, and more vertical in position than in females. In females, the sacrum is more acutely curved on itself with the greatest curvature being in the lower half of the bone.
Articulations (Points of Connection): The points of connection between the top of the sacrum and the bottom of the lumbar spine include the last intervertebral disc and the last facet joint (zygapophyseal joint). The joint where the sacral and vertebral bodies meet, separated by the discs, is called the lumbosacral junction (vertebral junction). The disc in this joint, along with the next one above it, are the two discs most likely to degenerate due to wear and tear, and are the most commonly associated with back pain conditions. The sacrum also articulates with the pelvic, at the sacroiliac joints and the auricular surfaces. The auricular surfaces of the sacrum articulate with the auricular surface of the iliac bones of the hips.
The inferior surface of the apex of the sacrum has an oval facet for articulation with the coccyx and two processes called the sacral cornua, which project downward from the posteriolateral aspect of the last sacral segment to join the cornua of the coccyx. There are two processes called coccygeal cornua, which project upward from the 1st coccygeal segment to join the sacral cornua.
In males and females, pelvic pain or lower back pain may occur when a trauma has occurred with sufficient enough force to affect the joints or articulation in the sacral spine. Chiropractors take special care to test the mobility of the sacroiliac joints when looking for the potential causes of lower back pain. Sacroiliac joint dysfunction is a condition where there is too much or too little mobility of these SI joints (hypermobility and hypomobility).