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Internal oblique muscle

The internal oblique muscle is the middle of the three flat muscles of the anterolateral abdominal wall, lying deep to the external oblique and superficial to the transversus abdominis. It arises from the thoracolumbar fascia, the anterior two-thirds of the iliac crest, and the lateral two-thirds of the inguinal ligament. From here, its fibers run superomedially (at right angles to the external oblique) and insert into the inferior borders of the lower ribs (10–12), the linea alba, and the conjoint tendon (formed with the transversus abdominis at the pubic crest).

It plays a major role in trunk flexion and rotation, abdominal wall stability, and increasing intra-abdominal pressure for functions like forced expiration, defecation, and childbirth. Clinically, the internal oblique is important in inguinal canal anatomy, as its fibers contribute to the formation of the conjoint tendon, a key structure in hernia surgery.

Synonyms

  • Obliquus internus abdominis

  • Middle abdominal wall muscle

Function

  • Bilateral contraction: flexes the trunk and compresses abdominal contents

  • Unilateral contraction: rotates trunk ipsilaterally (to the same side)

  • Supports abdominal viscera and contributes to forced expiration

  • Provides dynamic stability to the lumbar spine and pelvis

  • Contributes fibers to conjoint tendon, reinforcing the posterior wall of inguinal canal

Nerve Supply

  • Lower intercostal nerves (T7–T11)

  • Subcostal nerve (T12)

  • Iliohypogastric and ilioinguinal nerves (L1)

Arterial Supply

  • Lower posterior intercostal arteries

  • Subcostal artery

  • Deep circumflex iliac artery

  • Iliolumbar artery

Venous Drainage

  • Follows arterial supply via intercostal, subcostal, and iliolumbar veins → azygos and iliac systems

MRI Appearance

T1-weighted images:

  • Normal muscle shows intermediate signal intensity with fascicular architecture

  • Surrounded by hyperintense subcutaneous and retroperitoneal fat

T2-weighted images:

  • Normal muscle appears intermediate-to-low signal

  • Injury, edema, or strain shows hyperintense signal changes

STIR:

  • Suppresses fat, highlighting muscle edema, inflammation, or hematoma as bright signal

  • Sensitive for trauma and infection

T1 Fat-Saturated (Pre-contrast):

  • Muscle fibers appear intermediate signal, distinguishable from suppressed fat background

  • Helps detect subtle muscle volume loss or asymmetry

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal muscle enhances mildly and uniformly

  • Pathologies (abscess, myositis, tumor) show heterogeneous or intense enhancement

MRI Non-Contrast 3D Imaging:

  • Demonstrates full thickness and orientation of internal oblique fibers

  • Useful in preoperative abdominal wall reconstruction planning

CT Appearance

CT Pre-Contrast:

  • Internal oblique appears as a thin, soft tissue muscle layer between external oblique and transversus abdominis

  • Can detect muscle atrophy, hernia, or hematoma

CT Post-Contrast:

  • Muscle enhances mildly and homogeneously

  • Useful for evaluating abdominal wall pathology, tumors, infections, or surgical planning

  • 3D reconstructions assist in hernia repair planning and flap reconstructions

CT images

Internal Oblique Muscle axial ct image 2

CT images

Internal Oblique Muscle axial ct image

CT images

Internal Oblique Muscle coronal ct image

MRI images

Internal Oblique Muscle MRI axial image anatomy image -img-00000-00000

MRI images

Internal Oblique Muscle MRI coronal image anatomy image -img-00000-00000

CT image

Internal oblique muscle CT anatomy Location and Imaging Appearance -img-00000-00000