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Coccygeus muscle

The coccygeus muscle, also known as the ischiococcygeus, is a triangular muscle of the pelvic floor. It lies posterior to the levator ani, forming part of the pelvic diaphragm. It is closely related to the sacrospinous ligament, which it blends with, and it plays a supportive role in maintaining pelvic organ position. Though relatively small, it is clinically relevant in cases of pelvic floor dysfunction, trauma, and coccygeal pain.

Synonyms

  • Ischiococcygeus

  • Small pelvic floor muscle

  • Posterior pelvic diaphragm component

Origin, Course, and Insertion

  • Origin: Arises from the ischial spine

  • Course: Fibers pass medially and inferiorly, running parallel and anterior to the sacrospinous ligament

  • Insertion: Attaches to the lateral borders of the coccyx and lower sacrum (S5 segment)

Nerve Supply

  • Anterior rami of S4 and S5 spinal nerves

Arterial Supply

  • Inferior gluteal artery

  • Internal pudendal artery branches

  • Lateral sacral artery contributions

Venous Drainage

  • Drains into the internal iliac venous system through veins accompanying its arterial supply

Function

  • Supports the coccyx and sacrum, reinforcing the pelvic floor

  • Assists levator ani in supporting pelvic viscera

  • Stabilizes the sacroiliac and sacrococcygeal joints

  • Helps flex the coccyx slightly after defecation or childbirth

MRI Appearance

T1-weighted images:

  • Coccygeus muscle shows low-to-intermediate signal intensity

  • Fat planes surrounding the muscle appear bright, outlining its triangular shape

T2-weighted images:

  • Normal muscle shows low-to-intermediate signal

  • Edema, strain, or inflammation appear bright

STIR (Short Tau Inversion Recovery):

  • Baseline low-to-intermediate signal

  • Pathological involvement (strain, trauma, inflammation) appears as bright hyperintensity

Proton Density Fat-Sat (PD FS):

  • Normal muscle appears low-to-intermediate signal

  • Acute injury or pathology appears bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal muscle enhances mildly and homogeneously

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

CT Appearance

Non-Contrast CT:

  • Appears as a small triangular soft tissue density along the pelvic wall, anterior to the sacrospinous ligament

  • Hemorrhage appears as dense focus within the muscle

  • Chronic fatty replacement may reduce its density

Post-Contrast CT:

  • Normal muscle demonstrates mild uniform enhancement

  • Inflammatory or neoplastic processes show irregular or increased enhancement

  • Abscess or necrosis presents as low-density center with rim enhancement

MRI image

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

CT image

coccygeus muscle ct axial image