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Sacrum

The sacrum is a large, triangular bone at the base of the spine, formed by the fusion of five sacral vertebrae (S1–S5). It lies between the two iliac bones, articulating superiorly with the fifth lumbar vertebra (L5) at the lumbosacral joint, and inferiorly with the coccyx at the sacrococcygeal joint. Laterally, it articulates with the ilium of the pelvis at the sacroiliac joints.

The sacrum is concave anteriorly and convex posteriorly. Its anterior (pelvic) surface has four pairs of sacral foramina for the exit of ventral rami of sacral spinal nerves, while the posterior surface bears dorsal foramina for dorsal rami. The sacral canal, a continuation of the vertebral canal, transmits the cauda equina and ends at the sacral hiatus.

Embryologically, the sacrum develops from vertebral elements that fuse by adulthood. It serves as the keystone of the pelvis, transmitting body weight from the axial skeleton to the lower limbs via the hip bones.

Synonyms

  • Os sacrum

  • Sacral bone

Function

  • Provides structural support for the pelvis and spine

  • Transmits body weight from the vertebral column to the pelvic girdle

  • Protects sacral nerves within the sacral canal

  • Serves as attachment for muscles, ligaments, and fascia of the pelvis and lower back

Nerve Supply

  • Sacral spinal nerves (S1–S5) exit via anterior and posterior sacral foramina

  • Contribute to the sacral plexus, innervating pelvis, perineum, and lower limbs

  • Dorsal rami supply posterior muscles and skin; ventral rami contribute to pelvic and lower limb nerves

Arterial Supply

  • Lateral sacral arteries (branches of internal iliac artery)

  • Small branches from the median sacral artery (from abdominal aorta)

  • Anastomoses with superior and inferior gluteal arteries

Venous Drainage

  • Lateral sacral veins drain into internal iliac veins

  • Median sacral vein drains into the left common iliac vein or directly into the IVC

  • Extensive connections with vertebral venous plexuses (Batson’s plexus), allowing spread of pelvic malignancies to the spine

MRI Appearance

T1-weighted images:

  • Cortical bone: low signal intensity

  • Bone marrow: intermediate signal, variable with fatty content

  • Pathologies (fracture, tumor, infection) may alter marrow signal

T2-weighted images:

  • Cortical bone: low signal

  • Marrow: intermediate to high signal, especially if edematous

  • Fluid collections or cystic lesions appear bright hyperintense

STIR:

  • Suppresses fat, highlighting marrow edema, trauma, infection, or tumor infiltration

  • Sensitive for detecting occult sacral fractures

T1 Fat-Saturated (Pre-contrast):

  • Bone marrow shows intermediate signal, clearer against suppressed fat

  • Helps identify lesions or edema within marrow

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Enhances pathological lesions such as metastases, infections, or vascular tumors

  • Useful in evaluating sacral canal, foramina, and soft tissue involvement

MRI Non-Contrast 3D Imaging:

  • Provides multiplanar reconstructions of sacral canal, foramina, and articulations

  • Critical in pre-surgical planning, sacral nerve mapping, and trauma evaluation

CT Appearance

CT Pre-Contrast:

  • Excellent visualization of cortical bone, fractures, lytic lesions, and sclerosis

  • Identifies sacroiliac joint changes, congenital anomalies, or sacralization

CT Post-Contrast:

  • Highlights neoplastic, infectious, or vascular lesions involving bone and soft tissues

  • Demonstrates extension into sacral canal, foramina, or pelvic structures

  • 3D reconstructions useful for surgical and orthopedic planning

CT image

Sacrum CT sagittal anatomy image -img-00000-00000

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Sacrum CT axial image

CT VRT 3D image

Sacrum CT VRT coronal image

MRI image

Sacrum MRI axial anatomy image -img-00000-00000

MRI image

Sacrum MRI axial anatomy image -img-00000-00000