A fusion of vertebrae that helps humans stand and walk
Thesacrumis a single bone comprised of five separatevertebrae.
This robust bone can endure a lot of pressure and motion.
Along with thecoccyx(tailbone), the sacrum provides a stable platform to help you sit upright.
This article discusses the anatomy and function of the sacrum bone.
It also looks at some health conditions related to the sacrum bone.
Formation of the Sacrum
Humans are born with four to sixsacral vertebraerather than a single bone.
The process typically begins in the mid-teens and finishes sometime in the early to mid-twenties.
It is thought to start earlier in females than males.
The male sacrum is longer, more narrow, and flatter than the female sacrum.
There is other variability in the human sacrum, though it is not well understood.
A ridge across the front or anterior portion of the S1 vertebra is called thesacral promontory.
There are small holes calledforamenalong both sides of the sacrum that are left over when individual vertebrae fuse together.
Each anterior foramen is usually wider than the corresponding posterior or dorsal (on the backside) foramen.
Each sacral foramina (plural of foramen) is a channel for the sacral nerves and blood vessels.
There are small ridges that develop between each of the fused sacral vertebrae calledtransverse ridgesor transverse lines.
Along the dorsal midline of the sacrum is themedian sacral crest.
This is a ridge formed from the spinous processes of the sacral vertebrae.
The top of the sacrum is called thebase.
It is connected to the largest and lowest of the lumbar vertebrae, L5.
The bottom, which is connected to the tailbone (coccyx), is referred to as theapex.
Thesacral canalis a hollow space that runs from the base to the apex.
The sacral canal serves as a channel at the end of the spinal cord.
The sides of the sacrum connect to the right and left hip (iliac) bones.
The attachment point is called theauricular surface.
The sacrum is not the same as the tailbone.
The bottom of the sacrum connects to the tailbone and is called the apex.
The cleft starts at about the level of the tailbone or coccyx.
The sacrum is curved forward and ends at the coccyx.
The curvature is more pronounced in females than in males.
The base of the sacrum is the widest part.
Here, it connects to the L5 lumbar vertebra via thelumbosacral joint.
The disc that is between these two lumbar vertebrae is a common source of lower back pain.
Attached to either side of the sacrum are the iliac bones.
These wings of the pelvis provide stability and strength for walking and standing.
While the most common is five, anomalies documented in humans have included having four or six sacral vertebrae.
Other variations are related to the sacrum’s surface and curvature.
The curvature of the sacrum varies widely between individuals.
In some cases, the first and second sacral vertebrae do not fuse and instead remain separately articulated.
Associated Conditions
The sacrum is often implicated as a focal point for lower back pain.
Summary
The sacrum is a single bone located at the base of your spine.
It consists of five separate vertebrae that fuse during adulthood.
The sacrum helps support your upper body when you sit or stand.
It also gives the pelvic girdle flexibility during childbirth.
Problems with the sacrum account for a significant portion of lower back problems.
One of the most common of these is sacroiliitis.
(Anand N, ed.).
Elsevier Health Sciences; 2015:729.
Barros G, McGrath L, Gelfenbeyn M.Sacroiliac Joint Dysfunction in Patients With Low Back Pain.
2019 Aug;36(8):370-375.
PMID: 31456628; PMCID: PMC6707638.
2016;206(4):681-6. doi:10.2214/AJR.15.15095
Iyer S, Kim HJ, Theologis A, et.
al.Outcomes of fusions from the cervical spine to the pelvis.Global Spine J.
2019;31(2):9095. doi:10.4103/tcmj.tcmj_209_17
