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Serratus posterior inferior muscle

The serratus posterior inferior muscle is a thin, quadrilateral muscle of the intermediate back muscles, located in the lower thoracic and upper lumbar region. It originates from the spinous processes of T11–L2 vertebrae and the supraspinous ligaments, and inserts into the inferior borders of ribs 9–12 near their angles.

This muscle lies deep to the latissimus dorsi and superficial to the erector spinae group. Its fibers run obliquely upward and laterally from spine to ribs. Although once thought to assist in respiration, its role in ventilation is minor. More importantly, it contributes to stabilization of lower ribs during forced expiration and trunk movements.

Clinically, the serratus posterior inferior may be a source of myofascial pain in the thoracolumbar region. It is also encountered during posterior thoracic and spinal surgical approaches.

Synonyms

  • Musculus serratus posterior inferior

  • Posteroinferior serratus muscle

Function

  • Assists in depression of ribs 9–12 during forced expiration

  • Stabilizes lower ribs during diaphragmatic contraction

  • Contributes to trunk stabilization and thoracolumbar mechanics

  • May play a minor role in respiratory dynamics

Nerve Supply

  • Anterior rami of intercostal nerves T9–T12

Arterial Supply

  • Posterior intercostal arteries (9th–12th)

  • Small contributions from lumbar arteries

Venous Drainage

  • Posterior intercostal veins → azygos and hemiazygos systems

  • Lumbar veins → ascending lumbar and azygos system

MRI Appearance

T1-weighted images:

  • Normal serratus posterior inferior appears as intermediate signal intensity with clear fascicular pattern

  • Surrounded by fat planes, which appear hyperintense

T2-weighted images:

  • Normal muscle: intermediate-to-low signal intensity

  • Muscle edema (strain, trauma, myositis) appears bright hyperintense

STIR:

  • Suppresses fat, highlighting muscle edema, inflammation, or hematomas

  • Very sensitive for acute muscle pathology

T1 Post-Gadolinium (Gd-enhanced MRI):

  • Normal muscle enhances mildly and uniformly

  • Myositis, abscess, or tumor shows abnormal heterogeneous enhancement

  • Useful for assessing paraspinal infections or infiltrating tumors

MRI Non-Contrast 3D Imaging:

  • Provides 3D reconstruction of thoracolumbar musculature

  • Useful in surgical planning for spinal and posterior thoracic approaches

CT Appearance

Non-contrast CT:

  • Muscle appears as a thin, soft-tissue density sheet posterior to ribs 9–12

  • Helpful in identifying calcifications, atrophy, or asymmetry

CT Post-Contrast:

  • Enhances visualization of soft tissue pathology, infection, or neoplasms

  • Delineates relation to lower ribs, spine, and thoracolumbar fascia

  • Assists in planning thoracic and lumbar surgical approaches

MRI images

Serratus posterior inferior muscle mri axial image

MRI images

Serratus posterior inferior muscle mri coronal image

MRI IMAGE

Serratus posterior inferior muscle mri axial anatomy image-img-00000-00000

MRI IMAGE

Serratus posterior inferior muscle mri axial anatomy image-img-00000-00000_00001