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Posterior inferior iliac spine

The posterior inferior iliac spine (PIIS) is a bony projection of the ilium located just inferior to the posterior superior iliac spine (PSIS), separated by a shallow notch. It marks the upper end of the greater sciatic notch, forming part of the posterior pelvic border.

The PIIS serves as an attachment site for the posterior sacroiliac ligaments and contributes to the posterior boundary of the sacroiliac joint. Unlike the PSIS, the PIIS is not easily palpable externally because of overlying musculature and fat. It is a crucial anatomical landmark in pelvic imaging and surgery, especially for sacroiliac joint pathology, pelvic trauma, and spinal procedures.

Synonyms

  • PIIS

  • Iliac spine posterior inferior

  • Spina iliaca posterior inferior

Function

  • Provides ligamentous attachment to posterior sacroiliac ligaments

  • Contributes to the posterior pelvic stability by reinforcing sacroiliac joint

  • Forms the superior boundary of the greater sciatic notch

  • Acts as a radiological landmark in CT/MRI of pelvis and sacroiliac joints

Nerve Supply

  • Sensory innervation of overlying skin: dorsal rami of lumbar and sacral spinal nerves

  • Indirect muscular stabilization via gluteal and paraspinal musculature

Arterial Supply

  • Superior gluteal artery (branch of internal iliac artery)

  • Contributions from iliolumbar artery

Venous Drainage

  • Superior gluteal vein and iliolumbar vein → internal iliac vein

MRI Appearance

T1-weighted images:

  • Cortical bone appears as low signal intensity

  • Underlying cancellous bone marrow shows intermediate signal

T2-weighted images:

  • Cortical bone: low signal

  • Marrow: intermediate signal

  • Adjacent edema, inflammation, or trauma: hyperintense signal

STIR:

  • Fat suppression highlights bone marrow edema, sacroiliac inflammation, or fractures

  • Useful in sacroiliitis or early inflammatory arthropathies

T1 Fat-Saturated (Pre-contrast):

  • Marrow signal appears intermediate, standing out against suppressed fat planes

  • Enhances detection of subtle marrow lesions

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Highlights active inflammation, infection, or neoplastic infiltration

  • Sacroiliac joint inflammation near PIIS enhances heterogeneously

MRI 3D Reconstructions:

  • Demonstrate PIIS surface anatomy, greater sciatic notch, and relation to sacroiliac joint

  • Helpful for preoperative surgical planning

CT Appearance

CT Pre-Contrast:

  • PIIS appears as a dense cortical projection forming part of the posterior ilium and upper sciatic notch

  • Excellent for identifying fractures, alignment, and sacroiliac pathology

CT Post-Contrast:

  • Bone itself does not enhance, but adjacent soft tissue inflammation, vascular structures, or tumors may show enhancement

  • 3D CT reconstructions provide clear visualization of PIIS and pelvic borders for trauma and surgery

CT image

Posterior inferior iliac spine ct axial image

CT VRT 3D image

Posterior inferior iliac spine ct 3d image

MRI image

Posterior inferior iliac spine MRI axial anatomy image -img-00000-00000