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Psoas major muscle

The psoas major is a long, thick, fusiform muscle located in the retroperitoneum of the posterior abdominal wall. It originates from the transverse processes, lateral surfaces of vertebral bodies, and intervertebral discs of T12 to L5. The fibers descend inferolaterally, passing anterior to the hip joint, and insert on the lesser trochanter of the femur, where they join with fibers of the iliacus muscle to form the iliopsoas tendon.

The psoas major is a powerful hip flexor and also contributes to trunk flexion, lateral bending, and stabilization of the lumbar spine. It forms part of the posterior abdominal wall and lies adjacent to important retroperitoneal structures, including the kidneys, ureters, sympathetic trunk, lumbar plexus, and major vessels (aorta, IVC).

Clinically, the psoas major is involved in psoas abscesses (tuberculous or pyogenic), hematomas, retroperitoneal tumors, and lumbar plexus entrapment syndromes. It is also a key landmark in retroperitoneal surgery and radiology.

Synonyms

  • Iliopsoas (when combined with iliacus)

  • Musculus psoas major

  • Retroperitoneal hip flexor muscle

Function

  • Primary flexor of the hip joint

  • Assists in external rotation of the thigh

  • Stabilizes the lumbar spine during standing and walking

  • Contributes to trunk flexion when lower limb is fixed

  • Important in maintaining upright posture and gait mechanics

Nerve Supply

  • Direct branches of the lumbar plexus (ventral rami of L1–L3, sometimes L4)

Arterial Supply

  • Lumbar arteries (branches of abdominal aorta)

  • Contributions from iliolumbar artery (branch of internal iliac artery)

  • Small branches from external iliac and femoral arteries near insertion

Venous Drainage

  • Psoas veins drain into the lumbar veins → IVC

  • Iliolumbar and femoral veins contribute near insertion

MRI Appearance

T1-weighted images:

  • Normal psoas major shows intermediate signal intensity with fascicular pattern

  • Surrounded by retroperitoneal fat (high signal), aiding delineation

T2-weighted images:

  • Muscle appears intermediate-to-low signal

  • Edema, abscess, or hematoma shows hyperintense signal within or around the muscle

STIR:

  • Suppresses fat and highlights muscle edema, infection, or trauma as bright hyperintensity

  • Sensitive for early detection of psoas abscess or hematoma

T1 Fat-Saturated (Pre-contrast):

  • Psoas muscle fibers appear intermediate signal, standing out against suppressed fat planes

  • Useful for assessing atrophy and asymmetry

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal muscle shows mild homogeneous enhancement

  • Abscesses show rim enhancement with central non-enhancing fluid

  • Tumors show heterogeneous or intense enhancement

MRI Non-Contrast 3D Imaging:

  • Provides multiplanar 3D evaluation of psoas morphology, size, and relation to vertebrae, nerves, and vessels

  • Useful for preoperative planning and retroperitoneal pathology mapping

CT Appearance

CT Pre-Contrast:

  • Psoas muscle appears as a soft-tissue density structure along the posterior abdominal wall

  • Detects calcifications, hemorrhage, and asymmetry

CT Post-Contrast:

  • Normal psoas enhances mildly and homogeneously

  • Abscess: low-density fluid with peripheral rim enhancement

  • Hematoma: hyperdense in acute phase, with variable evolution

  • Tumor infiltration: irregular, heterogeneous enhancement

  • 3D reconstructions delineate iliopsoas tendon insertion and surgical relations

MRI images

Psoas major muscle MRI axial image anatomy image -img-00000-00000

MRI images

Psoas major muscle MRI axial image anatomy image -img-00000-00000_00001

MRI images

Psoas major muscle MRI coronal image anatomy image -img-00000-00000

CT images

Psoas major muscle ct axial

CT images

Psoas major muscle ct coronal

CT image

Psoas major muscle ct sfov axial

MRI image

Psoas major muscle MRI axial anatomy image-img-00000-00000

MRI images

Psoas major MRI sagittal image-img-00000-00000