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Infraspinatus muscle

The infraspinatus muscle is one of the four rotator cuff muscles of the shoulder. It originates from the infraspinous fossa of the scapula and inserts onto the middle facet of the greater tubercle of the humerus.

It lies inferior to the supraspinatus and superior to the teres minor, forming a large triangular muscle that covers most of the posterior scapula. The infraspinatus works in close coordination with the teres minor to provide external rotation of the humerus. Importantly, it also acts as a dynamic stabilizer of the glenohumeral joint, keeping the humeral head centered within the glenoid fossa during arm movements.

This muscle is especially vulnerable in rotator cuff tears (second only to supraspinatus) and may show selective atrophy in suprascapular nerve entrapment at the spinoglenoid notch.

Synonyms

  • Musculus infraspinatus

  • Scapular external rotator

  • Posterior rotator cuff muscle

Function

  • External rotation of the humerus

  • Stabilization of the shoulder joint by compressing the humeral head into the glenoid fossa

  • Assists in horizontal abduction and extension of the arm

  • Provides posterior shoulder stability during overhead activity

Nerve Supply

  • Suprascapular nerve (C5–C6), branch of the upper trunk of the brachial plexus

Arterial Supply

  • Suprascapular artery (branch of thyrocervical trunk)

  • Circumflex scapular artery (branch of subscapular artery)

Venous Drainage

  • Suprascapular vein → external jugular vein → subclavian vein

  • Circumflex scapular vein → subscapular vein → axillary vein

MRI Appearance

T1-weighted images:

  • Appears as intermediate signal intensity muscle with fascicular pattern

  • Fatty degeneration or chronic tear: high signal intensity due to fatty infiltration

T2-weighted images:

  • Normal muscle: intermediate-to-low signal

  • Edema or acute tear: bright hyperintense areas within the muscle belly or tendon

STIR:

  • Suppresses fat, making muscle edema, strain, or inflammation conspicuous

  • Useful for identifying early tendon tears, denervation edema, or bursitis

T1 Post-Gadolinium (Gd-enhanced MRI):

  • Normal infraspinatus: mild homogeneous enhancement

  • Pathological states (myositis, tumors, infections): intense heterogeneous enhancement

  • Enhances evaluation of rotator cuff tendon tears and capsular involvement

MRI Non-Contrast 3D Imaging:

  • Provides detailed 3D reconstructions of the infraspinatus tendon, footprint, and relation to glenohumeral joint

  • Crucial for surgical planning in rotator cuff repair

CT Appearance

Non-contrast CT:

  • Muscle visible as a soft tissue density below the scapular spine

  • Detects atrophy, calcifications, or chronic fatty replacement

CT Post-Contrast:

  • Highlights inflammatory, infectious, or neoplastic involvement of infraspinatus

  • Defines relationship with adjacent bone and soft tissues

  • 3D reconstructions help in orthopedic preoperative planning

MRI image

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

CT image

Infraspinatus muscle anatomy MRI axial image -img-00000-00000_00001

CT images

Infraspinatus muscle ct axial

MRI image

Infraspinatus muscle sag cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000