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Piriformis muscle

The piriformis is a thick, pear-shaped muscle located in the gluteal region. It originates from the sacrum and passes laterally through the greater sciatic foramen to attach to the femur. As one of the short external rotators of the hip, the piriformis plays a vital role in stabilizing the femoral head in the acetabulum. Clinically, it is well known for its association with piriformis syndrome, where compression of the sciatic nerve by the muscle leads to buttock and leg pain.

Synonyms

  • Pear-shaped muscle of the hip

  • Piriformis of the gluteal region

  • External rotator of the hip (piriformis)

Origin, Course, and Insertion

  • Origin:

    • Arises from the anterior surface of the sacrum, between the second to fourth sacral foramina

    • May also have small fibers arising from the sacrotuberous ligament and ilium near the greater sciatic notch

  • Course:

    • The muscle passes laterally and inferiorly, filling most of the greater sciatic foramen

    • It emerges into the gluteal region above the superior border of the ischial spine

    • Divides the greater sciatic foramen into a suprapiriform and infrapiriform compartment for neurovascular passage

  • Insertion:

    • Inserts via a rounded tendon into the superior border of the greater trochanter of the femur

Relations

  • Anteriorly: Pelvic sacral plexus, rectum

  • Posteriorly: Gluteus maximus

  • Superiorly: Superior gluteal vessels and nerve (suprapiriform foramen)

  • Inferiorly: Sciatic nerve, inferior gluteal vessels and nerve, pudendal nerve (infrapiriform foramen)

  • Medially: Sacrum, sacral foramina

Nerve Supply

  • Nerve to piriformis (direct branch from sacral plexus, S1–S2)

Arterial Supply

  • Superior gluteal artery

  • Inferior gluteal artery

  • Lateral sacral arteries

Venous Drainage

  • Venous blood drains into the superior and inferior gluteal veins, ultimately into the internal iliac vein

Function

  • External rotation of the thigh at the hip (when extended)

  • Abduction of the thigh (when flexed)

  • Stabilizes the femoral head in the acetabulum

  • Acts as a landmark in the gluteal region for division of neurovascular structures

Clinical Significance

  • Piriformis syndrome: entrapment of the sciatic nerve leading to buttock pain radiating down the leg

  • May complicate hip surgeries due to close relation to neurovascular bundles

  • Can be injured in posterior hip dislocations or pelvic fractures

  • Key landmark for identifying gluteal vessels and nerves in surgical approaches

MRI Appearance

T1-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Chronic fatty infiltration may appear bright

T2-weighted images:

  • Low-to-intermediate baseline signal intensity

  • Acute edema, strain, or inflammation appears as bright signal

STIR (Short Tau Inversion Recovery):

  • Normal muscle has low-to-intermediate signal

  • Pathology (strain, myositis, inflammation) appears bright

Proton Density Fat-Sat (PD FS):

  • Muscle demonstrates low-to-intermediate signal intensity

  • Injuries or tendinopathy appear as bright focal or diffuse signal

T1 Fat-Sat Post-Contrast:

  • Normal muscle enhances mildly and homogeneously

  • Pathological muscle shows heterogeneous or rim enhancement depending on etiology

CT Appearance

Non-Contrast CT:

  • Appears as homogeneous soft tissue density within the gluteal region

  • Hematomas appear as hyperdense regions

  • Fatty degeneration shows as low attenuation

Post-Contrast CT:

  • Muscle enhances mildly and evenly under normal conditions

  • Inflammatory or neoplastic lesions show stronger or irregular enhancement

  • Abscess appears as central low density with rim enhancement

MRI image

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MRI image

Piriformis muscle mri axial image image anatomy image-img-00000-00000_00001

MRI image

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CT image

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CT image

Piriformis muscle ct coronal image image anatomy image-img-00000-00000

MRI image

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