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Iliacus muscle

The iliacus muscle is a flat, triangular muscle that lines the iliac fossa of the pelvis. Together with the psoas major, it forms the iliopsoas muscle, the primary flexor of the hip joint. It arises from the upper two-thirds of the iliac fossa, inner lip of the iliac crest, anterior sacroiliac ligaments, and ala of the sacrum. The fibers converge and join the tendon of the psoas major to insert onto the lesser trochanter of the femur.

Anatomically, the iliacus lies posterior to the abdominal wall and anterior to the hip joint capsule. It is covered by the iliac fascia and is in close relation to the femoral nerve and vessels. Clinically, the iliacus is important in hip flexion mechanics, iliopsoas abscesses, femoral nerve compression, and musculoskeletal pain syndromes.

Synonyms

  • Musculus iliacus

  • Iliac muscle

  • Iliopsoas component

Function

  • Primary flexor of the hip joint, especially when trunk is fixed

  • Assists in external rotation of the femur

  • Helps stabilize the pelvis and hip joint during locomotion and posture

  • With psoas major, forms iliopsoas, a key muscle in walking, running, and climbing

Nerve Supply

  • Femoral nerve (L2–L4)

  • Small contributions from the lumbar plexus via branches of L2 and L3

Arterial Supply

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

  • Contributions from medial circumflex femoral artery

  • Contributions from obturator artery

Venous Drainage

  • Parallels arterial supply

  • Iliolumbar vein and branches drain into the common iliac vein

  • Connections to femoral and obturator venous networks

MRI Appearance

T1-weighted images:

  • Iliacus muscle shows intermediate signal intensity with clear fascicular pattern

  • Surrounded by hyperintense fat, aiding delineation

T2-weighted images:

  • Normal muscle: intermediate to low signal

  • Muscle edema, strain, or abscess: hyperintense signal changes

STIR:

  • Suppresses fat, highlighting muscle edema, hematoma, or infection as bright hyperintensity

  • Sensitive for iliopsoas abscess or inflammatory myositis

T1 Fat-Saturated (Pre-contrast):

  • Muscle appears as intermediate signal against suppressed fat background

  • Enhances recognition of muscle borders and subtle pathology

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal muscle enhances mildly and homogeneously

  • Pathology (abscess, tumor, myositis) shows heterogeneous or rim enhancement

  • Useful for mapping iliopsoas abscesses and post-surgical changes

MRI Non-Contrast 3D Imaging:

  • Provides 3D reconstruction of iliacus and iliopsoas complex

  • Helpful in preoperative surgical planning and complex pelvic evaluations

CT Appearance

CT Pre-Contrast:

  • Iliacus appears as a homogeneous soft tissue density in iliac fossa

  • Useful for detecting calcification, hematoma, or abscess gas

CT Post-Contrast:

  • Muscle enhances homogeneously

  • Abscesses show rim enhancement with central low density

  • Defines relationship with femoral nerve, iliac vessels, and hip joint capsule

MRI images

Iliacus muscle MRI axial image anatomy image -img-00000-00000

MRI images

Iliacus muscle MRI coronal image anatomy image -img-00000-00000

CT images

Iliacus muscle CT coronal image

CT images

Iliacus muscle CT axial image