The sacrum is a large bone located at the terminal part of the vertebral canal, where it forms the posterior aspect of the pelvis. It is remarkably thick, which aids in supporting and transmitting the weight of the body.
- The sacrum is made of five fused vertebrae configured as an inverted triangular bone that is concave anteriorly and convex posteriorly.
- Within the sacrum lies the sacral canal, which is a continuation of the vertebral canal. It terminates as the sacral hiatus.
- Though the spinal cord ends in the lumbar region (L1-L2), the dura mater extends to S2.
- Also contained within the canal is the filum terminale, a ligament that extends from the conus medullaris and attaches to the base of the coccyx (serving as an anchor for the spinal cord).
- Four bilateral sacral foramina can be appreciated both anteriorly and posteriorly, which give rise to the S1-S4 spinal nerves.
Surfaces of the Sacrum
The posterior aspect of the sacrum exhibits numerous bony landmarks.
- The median sacral crest forms from the fusion of the first three sacral spinous processes. It serves as the attachment point for the supraspinous ligament.
- The intermediate sacral crests form by the fusion of the articular processes. These serve as attachment sites for the posterior sacroiliac ligaments.
- The lateral sacral crests form by the fusion of the transverse processes. These are the attachment sites for the sacroiliac and sacrotuberous ligaments.
The anterior aspect of the sacrum is less coarse than the posterior aspect.
- Once the fusion of the sacral vertebrae is complete, four transverse ridges can be appreciated.
- These ridges represent the remnants of the fused intervertebral discs.
- The sacral promontory projects anteriorly, forming the posterior ridge of the pelvic inlet.
- The remainder of the pelvic inlet forms by the margins of the ala, the arcuate line of the ilium, the pectin pubic, and the pubic crest.
The fusion of the vertebrae and the large size of the sacrum forms an ideal base that allows it to support the weight of the entire body.
- Lumbosacral joint - Were the superior aspect of the sacrum (ie the sacral promontory) articulates superiorly with the L5 vertebral body of the lumbar spine. Reinforced by iliolumbar and lumbosacral ligaments.
- Sacroiliac joints - The alae (wings) of the sacrum articulate bilaterally with the ilium (known as sacroiliac joints). They form an essential connection between the axial skeleton and the appendicular skeleton, and function to transfer force from the lower limbs to the vertebral column. The anterior sacroiliac ligament, the posterior sacroiliac ligament, and the interosseous sacroiliac ligament all provide joint stabilization.
- Sacrococcygeal joint - The inferior aspect of the sacrum articulates with the coccyx via the sacral and coccygeal cornua.
The sacrum serves as the attachment point for numerous muscles. These muscles can group into those that attach to the anterior sacrum, and those that attach to the posterior sacrum.
The anterior surface
- Piriformis - originates from sacral segments 2 through 4
- Coccygeus (pelvic floor muscle) - inserts on the border of lower sacrum and coccyx
- Iliacus - some fibers originate from the ala of the sacrum
The posterior surface
- elderly (see below). This trauma may lead to S2-S5 sacral nerve root injury that results in sexual dysfunction and bowel or bladder incontinence, which may present as peripheral numbness and decreased rectal tone in an acute setting.
- Sacral insufficiency fracture - usually vertically through the sacral ala, parallelling the sacroiliac joint, often with a transverse component - this pattern resembles the capital letter H, leading to the designation H sign (or Honda sign) (zone 2 as classified by Denis)
- The venous plexuses that reside around the vertebral column allow for metastasis of cancer from the pelvis. The sacrum can also be the site for primary sarcomas. Sacral tumors can result in lumbosacral plexopathy.
- The sacrum contributes to the sexual dimorphism of the pelvis. Firstly, the sacral promontory is less prominent in females than in males. This results in an oval-shaped pelvic inlet in females and a heart-shaped pelvic inlet in males.
- Additionally, a smaller promontory results in a wider anteroposterior – both true conjugate (from the top of the symphysis pubis to the promontory) and diagonal conjugate (from subpubic angle to promontory) – measurement. The wider space permits the passage of child during delivery.
- Teach me anatomy Sacrum Available from:https://teachmeanatomy.info/pelvis/bones/sacrum/ (last accessed 11.6.2020)
- Sattar MH, Guthrie ST. Anatomy, Back, Sacral Vertebrae. InStatPearls [Internet] 2019 Nov 28. StatPearls Publishing.Available from:https://www.statpearls.com/kb/viewarticle/32534 (last accessed 11.6.2020)
- Radiopedia Sacrum Available from:https://radiopaedia.org/articles/sacrum (last accessed 11.6.2020)
- Radiopedia pelvic-insufficiency-fractures https://radiopaedia.org/articles/pelvic-insufficiency-fractures-1?lang=gb (last accessed 11.6.2020)