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Topic

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S (Sacral spine)

The sacral spine consists of the five fused sacral vertebrae (S1–S5) forming the sacrum, a large triangular bone located at the base of the vertebral column. It forms the posterior wall of the pelvis and serves as a critical junction between the spine and the pelvic girdle, transmitting axial body weight to the lower limbs.

The sacral spine is an essential region in musculoskeletal anatomy, neuroanatomy, trauma imaging, and evaluation of degenerative and developmental spinal conditions.

Synonyms

  • Sacrum

  • Sacral vertebral segment

  • S1–S5 vertebrae

Location

  • Located between the lumbar spine and the coccyx

  • Inferior to the L5 vertebra

  • Superior to the coccyx

  • Posterior to the pelvic cavity

  • Articulates laterally with the iliac bones at the sacroiliac joints

  • Forms the posterior boundary of the pelvis

Anatomical components

  • Five fused vertebrae (S1–S5)

  • Sacral promontory:

    • Anterior projecting margin of S1

  • Sacral canal:

    • Continuation of the spinal canal

  • Anterior sacral foramina:

    • Four pairs transmitting ventral sacral nerve roots

  • Posterior sacral foramina:

    • Four pairs transmitting dorsal sacral nerve roots

  • Median sacral crest:

    • Fused spinous processes

  • Ala of the sacrum:

    • Lateral expansions of S1

Relations

Anteriorly:

  • Rectum

  • Presacral vessels and fascia

Posteriorly:

  • Paraspinal muscles

  • Subcutaneous tissues of the lower back

Laterally:

  • Iliac bones

  • Sacroiliac joints

Superiorly:

  • L5 vertebra and L5–S1 intervertebral disc

Inferiorly:

  • Coccyx

Neural structures related to the sacral spine

  • Sacral spinal cord termination (conus medullaris above S1 level)

  • Cauda equina nerve roots

  • Sacral nerve roots (S1–S5) exiting through sacral foramina

  • Filum terminale within the sacral canal

X-ray appearance

Plain radiographs (AP pelvis / lateral sacrum):

  • Sacral outline: Triangular bony structure with smooth contours

  • Fusion lines: Horizontal ridges marking fused sacral segments

  • Anterior sacral foramina: Symmetric paired lucencies

  • Sacroiliac joints: Visible lateral articulations with ilium

  • Sacral curvature: Gentle kyphotic contour on lateral view

CT appearance (pre-contrast)

Non-contrast CT:

  • Cortical bone: High-density outer cortex of sacral vertebrae

  • Trabecular bone: Lower-density cancellous bone centrally

  • Sacral canal: Well-defined central osseous canal

  • Anterior and posterior sacral foramina: Clearly delineated openings

  • Sacroiliac joints: Sharp cortical margins and joint spaces

  • Neural foramina: Symmetric bony channels

MRI appearance

T1-weighted images:

  • Sacral marrow: High signal intensity from fatty marrow (age dependent)

  • Cortical bone: Low signal intensity

  • Sacral canal: Low-signal CSF rim with visible nerve roots

  • Intervertebral disc at L5–S1: Intermediate-to-low signal

T2-weighted images:

  • CSF in sacral canal: High signal intensity

  • Nerve roots: Intermediate signal within bright CSF

  • Bone marrow: Intermediate signal

  • Sacroiliac joint fluid: High signal if present

STIR:

  • Bone marrow: Suppressed fat signal with homogeneous low background

  • Neural structures: Clearly outlined against suppressed marrow

  • Ligaments and soft tissues: Intermediate-to-low signal

  • Sensitive for detecting marrow and soft-tissue signal changes

MRI image

Sacral Spine MRI SAG anatomy Location and Imaging Appearance -img-00000-00000

CT images

Sacral Spine CT SAG anatomy Location and Imaging Appearance -img-00000-00000

X Ray image

X Ray Sacral Spine