Learn · Structure
The sacrum and coccyx
The fused bones at the base of the column. The sacrum carries the weight of the trunk into the pelvis and lets the sacral nerves out through its foramina; the coccyx is the tailbone.
Updated 2026-10-022 sources
Why it matters
The sacrum joins the spine to the pelvis: its weight-bearing base takes the load of the column, and its foramina are the exits for the sacral nerves [1].
What it is
The sacrum is a single bone, also part of the pelvis, formed by the fusion of five sacral vertebrae [1]. The coccyx, or tailbone, forms from the fusion of four small coccygeal vertebrae and articulates with the inferior tip of the sacrum [1].
The sacrum is triangular: thick and wide across its superior base, where it bears weight, and tapering to an inferior apex that bears no weight [1].
How it works
- The sacral promontory is the anterior lip of the superior base [1].
- Beside it, the roughened auricular surface joins the ilium to form the immobile sacroiliac joints of the pelvis [1].
- The superior articular processes of the sacrum articulate with the inferior articular processes of L5 [1].
- The sacral canal runs down through the sacrum and ends at the sacral hiatus near its tip [1].
- Paired anterior and posterior sacral foramina connect to the sacral canal and let the branches of the sacral spinal nerves out of the sacrum [1].
On the back of the sacrum, the median sacral crest is the remnant of the fused spinous processes, and the lateral sacral crest forms from the fused transverse processes [1]. On the front of an older adult's sacrum, the lines of fusion can be seen as four transverse ridges [1].
When
The sacral and coccygeal fusions do not start until about age 20 and are not completed until middle age [1].
Normal behaviour
The number of lumbar and sacral pieces is not the same in everyone. In radiographs of 5941 people, 9.9 %[2] had a lumbosacral transitional vertebra: 5.8 %[2] a lumbarised S1 and 4.1 %[2] a sacralised L5 [2].
Common misconceptions
Misconception: The sacrum is one vertebra.
It is five sacral vertebrae fused into one bone; the fusion begins after about age 20 [1].
Check yourself
Where do the sacral spinal nerves leave the sacrum?
Through the anterior and posterior sacral foramina, which connect to the sacral canal [1].
What forms the sacroiliac joint?
The auricular surface of the sacrum joined to the ilium of the hip bone [1].
Read next
References
- Betts JG, Young KA, Wise JA, Johnson E, Poe B, Kruse DH, et al.. 7.3 The Vertebral Column. Anatomy and Physiology 2e. OpenStax. 2022. https://openstax.org/books/anatomy-and-physiology-2e/pages/7-3-the-vertebral-column
- French HD, Somasundaram AJ, Schaefer NR, Laherty RW. Lumbosacral Transitional Vertebrae and Its Prevalence in the Australian Population. Global Spine Journal. 2014;4(4):229-232. doi:10.1055/s-0034-1387808
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