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Lumbar triangle

The lumbar triangle, also called the triangle of Petit, is an anatomical space in the posterolateral abdominal wall. It is a weakened area of the abdominal wall that is clinically important as a potential site for lumbar hernias.

The lumbar triangle is bounded by the latissimus dorsi muscle medially, the external oblique muscle laterally, and the iliac crest inferiorly. Its floor is formed by the internal oblique muscle, and it is covered superficially by skin and fascia.

The triangle provides a natural weak spot where herniation of abdominal contents (retroperitoneal fat, bowel, or kidney) may occur, leading to lumbar hernia, a rare but clinically significant condition. This area is also used surgically as an access point for certain retroperitoneal or renal procedures.

Synonyms

  • Triangle of Petit

  • Inferior lumbar triangle

Function

  • Serves as an anatomical landmark in posterior abdominal wall

  • Functions as a potential site of herniation (lumbar hernia)

  • Provides a surgical window to retroperitoneal structures

MRI Appearance

T1-weighted images:

  • Muscular boundaries (latissimus dorsi, external oblique, internal oblique) appear as intermediate signal

  • Fat within the triangle appears hyperintense, aiding in visualization of the space

T2-weighted images:

  • Muscles show intermediate-to-low signal, fat remains bright

  • Herniated bowel loops or fluid collections appear hyperintense

STIR:

  • Suppresses fat, enhancing visualization of muscular boundaries and pathologic edema

  • Useful for detecting inflammatory changes, hernia sacs, or fluid collections

T1 Fat-Saturated (Pre-contrast):

  • Fat suppression accentuates the soft tissue structures and hernia contents

  • Herniated viscera are easier to distinguish from surrounding fat

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Muscles enhance mildly and homogeneously

  • Abnormal findings such as hernia sac inflammation, infection, or neoplasms enhance more strongly

  • Defines extension of retroperitoneal pathology into the lumbar triangle

MRI 3D Reconstructions:

  • Useful for surgical planning, allowing clear visualization of the triangle’s spatial relationships and any hernia defect

CT Appearance

CT Pre-Contrast:

  • Shows the fat-filled space of the lumbar triangle bounded by muscle and bone

  • Detects herniated bowel loops or fat protrusion

CT Post-Contrast:

  • Enhances vascular and soft tissue structures, improving delineation of hernia contents

  • Essential for detecting lumbar hernias, abscesses, or tumors in the lumbar triangle

  • 3D CT reconstructions clearly demonstrate the triangle boundaries and hernia defects

CT image

Lumbar triangle CT sagittal anatomy image-img-00000-00000

CT image

Lumbar triangle CT axial anatomy image-img-00000-00000

MRI image

Lumbar triangle MRI coronal anatomy image-img-00000-00000