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Infraspinous fossa

The infraspinous fossa is a large, triangular depression occupying the posterior surface of the scapula below the spine. It forms a major portion of the scapular body and serves as the origin site for the infraspinatus muscle, one of the four rotator cuff muscles.

The fossa is slightly concave, smooth near its center, and bordered by ridges and lines that give attachment to muscle fibers and septa. It plays a crucial role in shoulder stabilization, humeral rotation, and force transmission from the upper limb to the trunk.

Synonyms

  • Fossa infraspinata scapulae

  • Subspinous fossa

  • Posterior scapular depression

Location and Structure

  • The infraspinous fossa lies on the posterior aspect of the scapular body, below the spine of the scapula.

  • Shape: Triangular and shallow in its center, thickened near its borders.

  • Boundaries:

    • Superiorly: Spine of scapula

    • Inferiorly: Inferior angle of scapula

    • Medially: Vertebral (medial) border

    • Laterally: Axillary (lateral) border and glenoid cavity

  • Surface features:

    • Smooth central depression for infraspinatus origin

    • Slightly roughened ridges and crests near borders for fascial and tendinous attachments

  • Septa: Bony ridges divide the fossa into smaller compartments, anchoring muscle fibers of the infraspinatus.

Relations

  • Anteriorly: Subscapular fossa (separated by scapular body) and posterior thoracic wall

  • Posteriorly: Skin and superficial fascia

  • Superiorly: Spine of scapula and supraspinous fossa (via spinoglenoid notch)

  • Laterally: Glenoid cavity and capsule of shoulder joint

  • Medially: Trapezius and rhomboid major muscles overlie its medial border

Attachments

  • Infraspinatus muscle: Occupies nearly the entire fossa, arising from its medial two-thirds.

  • Infraspinous fascia: Thick fascia covering the muscle, forming a strong fibrous sheath that attaches to the margins of the fossa and the spine.

  • Septa: Provide fibrous partitions within the muscle belly.

  • Occasional vascular foramina: Allow entry of suprascapular vessels and nerves.

Nerve Supply

  • Suprascapular nerve (C5–C6) — passes through the spinoglenoid notch to innervate the infraspinatus muscle and supply periosteum of the fossa.

Function

  • Muscular origin: Serves as the attachment site for the infraspinatus muscle, essential for external rotation of the humerus.

  • Shoulder stability: Contributes to the rotator cuff complex, preventing anterior humeral head translation.

  • Force transmission: Distributes mechanical stress from muscle contraction to the scapular body.

  • Protection: Shields posterior thoracic wall and underlying neurovascular structures.

Clinical Significance

  • Fractures: May occur with direct trauma or scapular body fractures; often associated with rib or clavicular injuries.

  • Atrophy: Infraspinatus muscle wasting secondary to suprascapular nerve entrapment at the spinoglenoid notch.

  • Tendinopathy or tear: Chronic stress or rotator cuff pathology may cause irregular cortical erosion.

  • Osteolytic lesions or metastasis: Occasionally involve the fossa, requiring MRI or CT for evaluation.

  • Surgical relevance: Landmark for posterior shoulder approaches and scapular bone grafts.

MRI Appearance

  • T1-weighted images:

    • Cortex: Low signal (dark).

    • Marrow: Bright, reflecting fatty composition.

    • Infraspinatus muscle: Intermediate signal intensity; well defined.

    • Fascia: Low-signal thin band covering the muscle.

    • Pathology: Fracture or lesion—linear low-signal line with surrounding marrow changes.

  • T2-weighted images:

    • Cortex: Low signal.

    • Marrow: Bright, depending on fat content; hyperintense in marrow edema.

    • Infraspinatus muscle: Intermediate-to-low signal, slightly darker than T1.

    • Pathology: Fracture, contusion, or tumor shows hyperintense marrow replacement.

  • STIR:

    • Normal marrow: Intermediate-to-dark signal.

    • Abnormal marrow: Bright hyperintensity in edema, contusion, or infection.

    • Soft tissue edema: Clearly visible around the muscle or fascia.

  • Proton Density Fat-Saturated (PD FS):

    • Normal fossa and muscle: Intermediate-to-dark homogeneous signal.

    • Abnormal: Bright areas indicating bone marrow edema, stress reaction, or muscle strain.

    • Sensitive for detecting subtle cortical disruption and periosseous fluid.

  • T1 Fat-Sat Post-Contrast:

    • Normal bone and muscle: Mild, uniform enhancement.

    • Inflammation or tumor: Irregular or nodular enhancement.

    • Post-traumatic reaction: Peripheral enhancement with central low-signal hematoma or fibrosis.

CT Appearance

Non-Contrast CT:

  • Cortex: High attenuation with clearly defined borders.

  • Trabecular bone: Fine honeycomb pattern.

  • Surface: Smooth concavity of the fossa with visible ridges.

  • Pathology: Detects fractures, erosions, sclerosis, and bony irregularities with high resolution.

  • Useful for: Trauma evaluation, surgical planning, and assessing neoplastic involvement.

Post-Contrast CT (standard):

  • Bone: Minimal enhancement.

  • Soft tissue: Enhancing inflammatory or neoplastic changes in the infraspinatus muscle or fascia.

  • Applications: Defines the extent of bone lesions, infection, or soft-tissue mass near the spinoglenoid region.

MRI images

Infraspinous fossa of scapula axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI images

Infraspinous fossa of scapula sag cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT images

Infraspinous fossa ct axial

CT images

Infraspinous fossa sacpuls sag image

CT VRT 3D image

Infraspinous fossa ct vrt image