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Lateral part of sacrum

The lateral part of the sacrum refers to the broad, expanded side portions of the sacrum situated on either side of the sacral body and canal. These regions contribute to the sacral ala superiorly and articulate with the ilium at the auricular surface to form the sacroiliac joint. They also provide attachment sites for important ligaments and muscles that stabilize the pelvis.

The lateral parts of the sacrum play a crucial role in weight transmission from the vertebral column to the pelvic girdle. They are key landmarks in spine and pelvic imaging and are frequently involved in trauma, degenerative sacroiliac disease, and tumor evaluation.

Synonyms

  • Sacral ala (superior portion of lateral sacrum)

  • Lateral sacral mass

  • Auricular surface of sacrum (articular region with ilium)

Surface Features and Anatomy

  • Superiorly: Forms the sacral ala, a broad wing-like surface lateral to the sacral promontory

  • Laterally: Possesses the auricular surface, ear-shaped, for articulation with the ilium in the sacroiliac joint

  • Posteriorly: Contains roughened areas for attachment of strong posterior sacroiliac ligaments

  • Inferiorly: Narrows toward the sacral apex, continuous with the coccyx and lateral pelvic structures

  • Anteriorly: Related to sacral foramina, through which ventral rami of sacral nerves exit

Relations

  • Anteriorly: Sacral plexus branches, pelvic viscera, and internal iliac vessels

  • Posteriorly: Posterior sacroiliac ligaments, gluteal muscles

  • Laterally: Iliac bone (forming the sacroiliac joint)

  • Superiorly: Fifth lumbar vertebra at lumbosacral junction

  • Inferiorly: Coccyx and sacrococcygeal junction

Function

  • Transmits body weight from spine to pelvis via sacroiliac joints

  • Provides articulation with ilium (sacroiliac joint), enabling limited but crucial stability and load transfer

  • Serves as attachment site for ligaments (anterior/posterior sacroiliac, sacrotuberous, sacrospinous)

  • Contributes to pelvic stability during standing, walking, and lifting

Clinical Significance

  • Trauma: Lateral sacral fractures are common in pelvic trauma

  • Degenerative disease: Sacroiliitis and sacroiliac joint arthritis often affect the auricular surface

  • Tumors: Site of metastases, primary bone tumors, or invasion from pelvic masses

  • Surgery: Key landmark in sacroiliac joint fusion procedures, lumbosacral fixation, and pelvic instrumentation

  • Imaging relevance: Differentiating sacral ala from iliac bone is crucial in cross-sectional imaging

MRI Appearance

T1-weighted images:

  • Normal lateral sacrum (bone marrow): intermediate to bright signal

  • Cortical bone: dark (low signal)

  • Fat in marrow: bright signal, especially in older patients

T2-weighted images:

  • Bone marrow: intermediate to bright signal

  • Cortical bone: dark signal

  • Pathology (edema, fracture, tumor): bright hyperintense areas within marrow

STIR (Short Tau Inversion Recovery):

  • Normal marrow: suppressed low-to-intermediate signal

  • Edema, inflammation, or tumor infiltration: bright hyperintense signal

T1 Fat-Sat Post-Contrast:

  • Normal marrow: mild enhancement

  • Pathologic marrow: focal or diffuse strong enhancement (infection, tumor, inflammation)

  • Cortical bone: remains dark

3D T2 SPACE / CISS:

  • Cortical outline: dark, sharply defined

  • Marrow: intermediate signal

  • Provides excellent delineation of sacroiliac joint surface, foramina, and nerve exits

CT Appearance

Non-Contrast CT:

  • Cortical bone: dense, bright white

  • Marrow: intermediate density

  • Clear visualization of sacroiliac joint, fractures, and cortical margins

Post-Contrast CT:

  • Bone does not enhance

  • Pathology (tumor, infection) appears as enhancing soft tissue infiltrating marrow or crossing into adjacent pelvic structures

  • Helpful in differentiating sclerotic vs lytic bone lesions

MRI image

Lateral part of sacrum MRI coronal anatomy image-img-00000-00000

MRI image

Lateral part of sacrum axial

CT image

Lateral part of sacrum CT axial anatomy image-img-00000-00000