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Subacromial space

The subacromial space (also called the supraspinatus outlet) is the narrow interval between the humeral head and the acromion process of the scapula. It serves as a critical passageway for the supraspinatus tendon, subacromial-subdeltoid bursa, and other soft-tissue structures of the rotator cuff.

This space allows smooth gliding of the rotator cuff tendons beneath the acromion during shoulder elevation. Reduction in this space from inflammation, bone spurs, or anatomical variants can lead to impingement syndromes and rotator cuff tears.

Synonyms

  • Supraspinatus outlet

  • Subacromial interval

  • Subdeltoid space

Location and Boundaries

  • Superiorly: Acromion, acromioclavicular joint, and coracoacromial ligament

  • Inferiorly: Head of the humerus and the supraspinatus tendon

  • Anteriorly: Coracoacromial ligament and anterior acromion

  • Posteriorly: Posterior aspect of the acromion and deltoid muscle

  • Medially: Base of the coracoid process and supraspinatus muscle

  • Laterally: Deltoid muscle overlying the humeral greater tuberosity

Contents

  • Supraspinatus tendon (passing from supraspinous fossa to greater tuberosity)

  • Subacromial-subdeltoid bursa (largest bursa in the shoulder, facilitating tendon gliding)

  • Joint capsule of the shoulder (superior aspect)

  • Portions of the infraspinatus and long head of biceps tendons (in proximity)

  • Coracoacromial ligament and arch forming the roof of the space

Relations

  • Superior: Deltoid muscle and acromion

  • Inferior: Humeral head and supraspinatus tendon

  • Anterior: Coracoacromial ligament and clavicle

  • Posterior: Infraspinatus tendon and posterior deltoid fibers

Function

  • Facilitates tendon movement: Allows frictionless gliding of rotator cuff tendons beneath the acromion.

  • Distributes load: Absorbs and disperses stress during abduction and elevation.

  • Protects tendons: The bursa cushions the supraspinatus tendon from compressive forces.

  • Maintains shoulder mechanics: Ensures smooth articulation between humeral head and scapula.

Clinical Significance

  • Subacromial impingement syndrome: Most common pathology; due to narrowing of the space compressing the supraspinatus tendon or bursa.

  • Rotator cuff tears: Chronic impingement can cause partial or full-thickness tears, particularly of the supraspinatus tendon.

  • Bursitis: Inflammation of the subacromial-subdeltoid bursa causing pain and restricted motion.

  • Osteophyte formation: At the acromion or acromioclavicular joint reduces space height.

  • Acromial morphology: Hooked or curved acromions predispose to impingement.

  • Postoperative changes: Scar tissue or postoperative fibrosis can cause secondary impingement.

MRI Appearance

  • T1-weighted images:

    • Deltoid and supraspinatus muscles: Intermediate signal intensity

    • Tendons: Low signal (dark), continuous fibers

    • Bursa: Normally collapsed and not visualized; effusion appears intermediate-to-bright

    • Marrow of acromion and humeral head: Bright fatty signal

    • Pathology: Effusion or bursitis shows intermediate-to-bright signal; tendinopathy shows thickened low-signal tendon

  • T2-weighted images:

    • Bursa: Appears as a thin potential space; effusion or inflammation shows bright hyperintensity

    • Tendons: Low signal unless torn or degenerated

    • Marrow: Bright, slightly less than on T1

    • Pathology: Rotator cuff tears appear as fluid-bright defects within tendon; impingement causes bursal fluid and tendon thickening

  • STIR:

    • Normal tissue: Intermediate-to-dark signal

    • Bursal fluid, edema, or tendon inflammation: Bright hyperintense

    • Excellent for detecting bursitis, tendinopathy, and soft-tissue impingement

  • Proton Density Fat-Saturated (PD FS):

    • Normal space: Collapsed bursa and low-signal tendons

    • Inflammation or fluid: Bright hyperintense signal separating acromion from supraspinatus

    • Tendinopathy: Focal or diffuse bright intratendinous signal without fiber disruption

    • Partial or full-thickness tear: Bright signal extending through tendon to joint or bursal surface

  • T1 Fat-Sat Post-Contrast:

    • Normal bursa: Minimal enhancement

    • Bursitis or synovitis: Diffuse bursal enhancement

    • Postoperative scarring: Irregular enhancing tissue between acromion and rotator cuff

    • Infective or inflammatory bursitis: Enhancing walls with surrounding soft-tissue edema

CT Appearance

Non-Contrast CT:

  • Acromion and humeral head: High-attenuation bony margins forming space roof and floor

  • Bursa and tendons: Soft-tissue density, often indistinguishable unless fluid present

  • Pathology:

    • Detects acromial spurs, acromioclavicular osteophytes, and calcific tendinitis

    • Measures acromiohumeral distance (normally 7–14 mm; reduced in impingement)

    • Identifies cortical irregularities or erosions from chronic friction

MRI image

Subacromial space shoulder axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Subacromial space shoulder axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Subacromial space shouldercoronal cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image

Subacromial space ct axial

CT image

Subacromial space ct coronal