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Posterior superior iliac spine

The posterior superior iliac spine (PSIS) is a prominent bony projection located at the posterior end of the iliac crest of the ilium. It lies at the level of the second sacral vertebra (S2) and marks the posterior limit of the iliac crest. It is typically visible and palpable as two dimples on the lower back, known clinically as the "dimples of Venus."

The PSIS serves as an important landmark for clinical examination, imaging, and surgery. It forms part of the sacroiliac joint region and serves as an attachment point for key ligaments and muscles.

Attachments

  • Ligaments: Posterior sacroiliac ligaments, interosseous sacroiliac ligament, and sacrotuberous ligament

  • Muscles: Multifidus, erector spinae fascia, and gluteus maximus (via aponeurotic connections)

Synonyms

  • PSIS

  • Iliac spine posterior superior

  • Spina iliaca posterior superior

Function

  • Provides ligamentous attachment for posterior sacroiliac ligaments

  • Serves as a muscle attachment for parts of gluteus maximus and multifidus

  • Acts as a palpable surface landmark for identifying sacroiliac joint and S2 vertebral level

  • Guides clinical procedures and pelvic alignment assessments

Nerve Supply

  • Sensory innervation of skin over PSIS via dorsal rami of lumbar and sacral spinal nerves

  • Muscular attachments (multifidus, gluteus maximus) supplied by corresponding segmental nerves

Arterial Supply

  • Iliolumbar artery (branch of posterior division of internal iliac artery)

  • Superior gluteal artery (branch of posterior division of internal iliac artery)

Venous Drainage

  • Drains via venous plexuses into internal iliac veins

  • Small tributaries accompany arterial supply

MRI Appearance

T1-weighted images:

  • PSIS cortical bone appears as low signal intensity

  • Bone marrow within ilium shows intermediate signal

T2-weighted images:

  • Cortical bone: low signal

  • Marrow: variable intermediate signal

  • Joint-related edema or pathology appears bright hyperintense

STIR:

  • Suppresses fat, making bone marrow edema, sacroiliitis, or trauma hyperintense

  • Excellent for detecting early inflammatory changes

T1 Fat-Saturated (Pre-contrast):

  • Bone marrow appears intermediate signal against suppressed fat

  • Improves visualization of subtle marrow changes

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Enhances in cases of active inflammation, infection, or neoplasm

  • Helps differentiate chronic vs acute sacroiliitis at sacroiliac joint near PSIS

MRI 3D Reconstructions:

  • Provide clear depiction of PSIS surface anatomy and relation to sacroiliac joint

  • Used in preoperative surgical planning

CT Appearance

CT Pre-Contrast:

  • Cortical outline of PSIS appears as a dense, high-attenuation structure

  • Marrow seen as lower-density trabecular bone

  • Excellent for evaluating fractures, bony variations, or sacroiliac alignment

CT Post-Contrast:

  • Typically no direct enhancement of bone

  • Useful for detecting adjacent soft tissue pathology, inflammatory extension, or tumor involvement

  • 3D CT reconstructions show precise PSIS anatomy for orthopedic planning

CT image

Posterior superior iliac spine ct axial image

CT VRT 3D image

Posterior superior iliac spine ct 3d image

MRI image

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