Anatomy Atlas

Infraspinatus

Musculus infraspinatus
MuscleHigh-yielddraftMSK

Rotator cuff muscle in the infraspinous fossa and the main lateral (external) rotator of the arm.

Core anatomy

Location

Infraspinous fossa of the scapula, below the spine, covered partly by deltoid and trapezius.

Group

Rotator cuff (SITS); scapulohumeral muscles.

Origin

Infraspinous fossa of the scapula and the deep fascia covering it.

Insertion

Middle facet of the greater tubercle of the humerus.

Innervation

Suprascapular nerve (C5, C6)

Blood supply

Suprascapular and circumflex scapular arteries verify

Action
  • Laterally (externally) rotates the arm
  • With the other rotator cuff muscles, holds the humeral head in the glenoid cavity
Relations

The suprascapular nerve reaches infraspinatus by passing around the lateral border of the scapular spine through the spinoglenoid notch after supplying supraspinatus.

Mentioned in

Generated automatically from links in other entries.

Rotator cuff (SITS)Lateral view of the right glenoid fossa with the humerus removed, anterior to the right. Supraspinatus passes over the top of the joint beneath the acromion, infraspinatus and teres minor lie behind it, and subscapularis lies in front. The long head of biceps arises from the supraglenoid tubercle and the long head of triceps from the infraglenoid tubercle. The inferior part of the capsule has no cuff support. The table lists each muscle: supraspinatus to the superior facet of the greater tubercle, suprascapular nerve, initiates abduction, empty-can test; infraspinatus to the middle facet, suprascapular nerve, lateral rotation; teres minor to the inferior facet, axillary nerve, lateral rotation; subscapularis to the lesser tubercle, upper and lower subscapular nerves, medial rotation, lift-off and belly-press tests. All are supplied mainly by C5–C6.SupraspinatusInfraspinatusTeres minorSubscapularisAcromionCoracoid processLong head of biceps(supraglenoid tubercle)Rotator intervalInferior capsule: no cuff(dislocation is usuallyanterior–inferior)Long head of triceps(infraglenoid tubercle)Glenoid fossaand labrumRight glenoid, lateral view, humerus removedposterior ← · → anterior · not to scaleMuscleHumerusNerveActionExam testSupraspinatusGreater tubercle (superior facet)SuprascapularInitiates abductionEmpty canInfraspinatusGreater tubercle (middle facet)SuprascapularLateral rotationResisted ERTeres minorGreater tubercle (inferior facet)AxillaryLateral rotationResisted ER, hornblowerSubscapularisLesser tubercleUpper + lower subscapularMedial rotationLift-off, belly pressAll four are supplied mainly by C5–C6. Supraspinatus initiates abduction (about the first 15°);deltoid takes over after that. Supraspinatus is the tendon most often torn or impinged under theacromion. Supraspinatus, infraspinatus and teres minor insert on the greater tubercle; subscapularis on the lesser.

Swipe sideways to see the whole diagram →

Rotator cuff (SITS). Right glenoid seen from the side with the humerus removed. The cuff covers the top, back and front of the joint but not its inferior part, which is why most dislocations are anterior–inferior. The table summarizes attachments, nerves, actions and bedside tests.
Original schematic, created for this atlas.
Posterior view of supraspinatus, infraspinatus, teres minor, teres major and triceps brachii
Posterior scapular muscles and triceps brachii.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 412. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Suprascapular nerve entrapment

Mechanism / lesion

Compression at the suprascapular notch (affects both supraspinatus and infraspinatus) or at the spinoglenoid notch (affects infraspinatus only). Causes include a paralabral ganglion cyst (often with a labral tear), traction in overhead athletes (volleyball, baseball) and fractures.

Classic presentation

Deep, poorly localized posterior shoulder pain, weak external rotation, and visible wasting of the infraspinous fossa. Isolated infraspinatus atrophy points to the spinoglenoid notch.

Posterosuperior rotator cuff tear PANCE

Mechanism / lesion

Extension of a supraspinatus tear posteriorly, or an isolated tear, usually degenerative.

Classic presentation

Weak external rotation with a positive external rotation lag sign, and difficulty with overhead and reaching activities.

High-yield PANCE pearls

  • Infraspinatus = main external rotator; tested by resisted external rotation with the elbow at the side.
  • Suprascapular nerve supplies both supraspinatus and infraspinatus; spinoglenoid notch lesions affect infraspinatus only.
  • Infraspinous fossa wasting → think suprascapular neuropathy or a chronic posterior cuff tear.

Imaging anatomy

X-ray

Usually normal. Shoulder radiographs exclude fractures of the scapula or glenoid.

MRI

Assesses tendon tears, muscle atrophy and fatty infiltration (sagittal oblique). A paralabral cyst in the spinoglenoid notch with denervation edema of infraspinatus alone suggests suprascapular nerve compression at that level.

Ultrasound

The posterior approach shows the infraspinatus tendon inserting on the greater tuberosity and posterior joint recess. Dynamic external/internal rotation helps.

Key views & landmarks
  • MRI sagittal oblique (muscle bulk) and axial (spinoglenoid notch)

Physical exam

Surface landmarks

Infraspinous fossa below the palpable spine of the scapula. Atrophy shows as hollowing.

Strength / function testing

Resisted external rotation with the arm adducted and the elbow flexed at 90°.

External rotation lag sign

Technique

With the elbow flexed at 90° and the arm at the side (slight abduction), the examiner passively rotates the shoulder into near-maximal external rotation and asks the patient to hold the position after release.

Positive test

The forearm drifts back toward internal rotation (a lag).

Significance

Suggests a tear of infraspinatus (± supraspinatus).

Self-quiz

Q1.

A 26-year-old volleyball player has vague posterior shoulder pain and weak external rotation. There is visible wasting of the infraspinous fossa, but the supraspinous fossa is normal and abduction is strong. MRI shows a ganglion cyst. Where is the cyst most likely located?

Show answer

B

At the spinoglenoid notch the suprascapular nerve has already supplied supraspinatus, so only infraspinatus is affected. A lesion at the suprascapular notch would weaken both muscles. The quadrangular space contains the axillary nerve (teres minor and deltoid).

Q2.

Which pair of muscles laterally rotates the arm?

Show answer

C

Infraspinatus (suprascapular nerve) and teres minor (axillary nerve) are the primary lateral rotators, with help from the posterior deltoid. The other pairs are mainly medial rotators, abductors or flexors.

Q3.

The infraspinatus tendon inserts on which part of the humerus?

Show answer

C

From superior to inferior on the greater tubercle the insertions are supraspinatus (superior facet), infraspinatus (middle facet) and teres minor (inferior facet). Subscapularis inserts on the lesser tubercle.

Q4.

Which nerve supplies infraspinatus?

Show answer

A

The suprascapular nerve (C5, C6) supplies supraspinatus, then passes through the spinoglenoid notch to infraspinatus. Teres minor, the other external rotator, is supplied by the axillary nerve.

Q5.

Which bedside test best isolates infraspinatus?

Show answer

C

Infraspinatus is the main external rotator and is tested by resisted external rotation with the elbow at the side; a positive external rotation lag sign suggests a posterosuperior cuff tear. Empty-can tests supraspinatus, lift-off tests subscapularis, Speed tests the biceps tendon, and Hawkins–Kennedy is an impingement sign.

Flashcards

See also