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Acromial part of deltoid muscle

The acromial part of the deltoid muscle (also known as the middle or lateral part) forms the thick, lateral aspect of the deltoid, giving the shoulder its characteristic rounded contour. It arises from the lateral border and superior surface of the acromion process of the scapula and inserts into the deltoid tuberosity of the humerus.

This part of the muscle is primarily responsible for abduction of the shoulder joint, acting as the chief abductor of the arm beyond the initial 15° initiated by the supraspinatus. It is particularly important in dynamic shoulder motion, overhead activities, and maintaining glenohumeral joint stability.

Synonyms

  • Middle deltoid

  • Lateral deltoid

  • Acromial deltoid

Origin, Course, and Insertion

  • Origin: Lateral border and superior surface of the acromion process of the scapula

  • Course: Fibers run vertically downward over the lateral aspect of the shoulder

  • Insertion: Deltoid tuberosity on the lateral surface of the humerus, via a thick tendinous attachment

Relations

  • Superficial: Subcutaneous tissue and skin of the shoulder region

  • Deep: Subdeltoid (subacromial) bursa and supraspinatus tendon

  • Anteriorly: Clavicular part of the deltoid

  • Posteriorly: Spinal part of the deltoid

  • Inferiorly: Continuity with deep fascia of the arm overlying the brachialis and triceps

Nerve Supply

  • Axillary nerve (C5, C6) — branch of the posterior cord of the brachial plexus

Arterial Supply

  • Posterior circumflex humeral artery (branch of the axillary artery)

  • Deltoid branch of thoracoacromial artery

Venous Drainage

  • Accompanying veins of the posterior circumflex humeral artery, draining into the axillary vein

Function

  • Abduction: Chief abductor of the shoulder joint from 15° to approximately 90°

  • Stabilization: Maintains humeral head alignment within the glenoid cavity during motion

  • Assistance in rotation: Coordinates with anterior and posterior deltoid fibers to balance shoulder movement

  • Dynamic control: Plays a vital role in arm elevation, lifting, and overhead mechanics

Clinical Significance

  • Tears or strains: Result from repetitive overhead activity or trauma

  • Deltoid atrophy: Seen in axillary nerve injury, post shoulder dislocation, or deltoid detachment

  • Injection site: Common site for intramuscular injections due to its bulk and accessibility

  • Impingement or bursitis: Subacromial bursitis can cause pain beneath the acromial deltoid

  • Imaging importance: MRI useful for assessing tears, fatty atrophy, and denervation changes

MRI Appearance

  • T1-weighted images:

    • Muscle belly: Intermediate signal intensity with visible fascicular pattern

    • Tendon: Low signal (dark linear band) attaching to humerus

    • Marrow of humerus and acromion: Bright fatty signal

    • Atrophy: Appears as loss of muscle bulk with increased intramuscular fat signal (bright)

  • T2-weighted images:

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

    • Tendon: Low signal, sharply defined

    • Pathology: Strain or tear appears as bright hyperintense area within muscle or tendon

    • Bursitis: Subacromial fluid shows bright signal between deltoid and supraspinatus

  • STIR:

    • Normal muscle: Intermediate-to-dark signal

    • Pathology: Bright hyperintense in edema, muscle strain, or acute denervation

    • Highlights perimuscular and subacromial bursal fluid collections

  • Proton Density Fat-Saturated (PD FS):

    • Normal: Uniform intermediate-to-dark muscle signal

    • Tear or inflammation: Bright hyperintensity in affected fibers or at insertion

    • Excellent for detecting low-grade muscle strain, tendinitis, and peritendinous fluid

  • T1 Fat-Sat Post-Contrast:

    • Normal muscle: Mild uniform enhancement

    • Inflammatory or postoperative changes: Focal enhancement along fibers or tendon insertion

    • Chronic denervation or scarring: Peripheral rim enhancement with central low-signal fibrosis

CT Appearance

Non-Contrast CT:

  • Muscle: Homogeneous soft-tissue density lateral to the shoulder joint

  • Tendon: Linear density inserting at humeral deltoid tuberosity

  • Acromion: Cortical margins clearly defined beneath the deltoid origin

  • Pathology: Detects calcifications, chronic scarring, or soft-tissue atrophy; fracture fragments at deltoid origin may be seen

Post-Contrast CT (standard):

  • Muscle: Homogeneous enhancement

  • Inflamed or torn fibers: Show subtle increased enhancement or localized irregularity

  • Bursal inflammation: Enhancing fluid or thickening in subacromial region

  • Useful for: Postoperative assessment, calcific tendinitis, and soft-tissue lesion evaluation

MRI image

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MRI image

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CT image

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