Attachment, relations, nerve supply and action of the deltoid muscle

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Deltoid Muscle

The deltoid is a large, triangular muscle that caps the shoulder. Its base is attached superiorly along the clavicle and scapula, and its apex points inferiorly to the humerus - forming a shape that is essentially a mirror image of the trapezius insertion above it.

Attachments

Origin (proximal / base of triangle)

The deltoid originates along a continuous U-shaped line from three parts:

Part	Origin
Clavicular (anterior)	Lateral third of the clavicle
Acromial (middle)	Lateral border of the acromion
Spinal (posterior)	Spine of the scapula (entire lower lip)

Insertion (distal / apex of triangle)

Deltoid tuberosity on the lateral surface of the shaft of the humerus (approximately at its midpoint)

Attachments and neurovascular supply of the deltoid - posterior view

Fig. 7.36 - Attachments and neurovascular supply of the deltoid and trapezius. Gray's Anatomy for Students

Nerve Supply

Axillary nerve (C5, C6) - a branch of the posterior cord of the brachial plexus
The axillary nerve, together with the posterior circumflex humeral artery and vein, enters the deep surface of the deltoid by passing posteriorly around the surgical neck of the humerus (through the quadrangular space)

Clinical note: Fractures of the surgical neck of the humerus or anterior shoulder dislocations can injure the axillary nerve, resulting in weakness of abduction and loss of sensation over the "regimental badge" area of the lateral arm.

Actions

The deltoid's actions vary by part, and depend on the angle of arm abduction:

Cross-section showing three parts of deltoid around shoulder joint

Cross-section through the right shoulder joint showing the three parts. THIEME Atlas of Anatomy

Part	Primary Actions
Clavicular (anterior)	Flexion, internal rotation, adduction of the arm
Acromial (middle)	Abduction (the prime mover, 0-90°)
Spinal (posterior)	Extension, external rotation, adduction of the arm

Variable actions with degree of abduction

An important concept is that the clavicular and spinal parts change their role depending on arm position:

Below 60° of abduction: The clavicular and spinal parts act as antagonists to the acromial part - they are adductors and resist abduction.
Above 60° of abduction: As the arm passes the abduction/adduction axis, the clavicular and spinal parts flip their role and become synergistic abductors, assisting the acromial part.

Three parts of deltoid and abduction/adduction axis, anterior view

Variable actions of the three deltoid components relative to the abduction/adduction axis. THIEME Atlas of Anatomy

Summary Table

Feature	Detail
Origin	Lateral 1/3 clavicle, acromion, spine of scapula
Insertion	Deltoid tuberosity of humerus
Nerve	Axillary nerve (C5, C6) - posterior cord of brachial plexus
Blood supply	Posterior circumflex humeral artery
Main action	Abduction of the arm (acromial part)
Other actions	Flexion & medial rotation (clavicular part); Extension & lateral rotation (spinal part)

Sources: Gray's Anatomy for Students (9780323934237); THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System (9781626237186)

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Here are the full relations of the deltoid muscle, organized systematically:

Relations of the Deltoid Muscle

Deep surface of deltoid showing axillary nerve distribution and borders

Deep surface of left deltoid showing the axillary nerve coursing transversely ~5 cm distal to the muscle's origin. Campbell's Operative Orthopaedics

Superficial (Outer) Surface

Covered by skin and superficial fascia
The platysma extends onto the upper part of the muscle
The supraclavicular nerves (C3, C4) supply the skin over the upper part of the deltoid

Deep Surface

The deep surface is separated from underlying structures by the subdeltoid (subacromial) bursa, which lies between the deltoid/acromion above and the rotator cuff tendons/joint capsule below. The deep surface overlies:

Structure	Location
Glenohumeral joint capsule	Central deep surface
Rotator cuff muscles (supraspinatus, infraspinatus, teres minor)	Directly deep
Subacromial/subdeltoid bursa	Intervening between deltoid and rotator cuff
Biceps brachii (long head tendon)	Deep, anteriorly
Coracobrachialis	Deep, anteromedially
Axillary nerve + posterior circumflex humeral vessels	Course transversely ~5 cm below the origin, on the deep surface, entering from the quadrangular space posteriorly

Anterior Border

Separated from pectoralis major by the deltopectoral groove (interval)
In this groove runs the cephalic vein (ascending to drain into the axillary vein at the infraclavicular fossa after piercing the clavipectoral fascia) and the deltoid branch of the thoracoacromial artery
This interval is the key surgical approach to the shoulder (deltopectoral approach)

Posterior Border

Overlaps the long head of triceps brachii posteriorly
Related to teres major and teres minor posteroinferiorly

Superior / Bony Relations

Overlies the acromion and acromioclavicular joint superiorly
The trapezius inserts on the same bony line (clavicle, acromion, spine of scapula) immediately medial to the deltoid origin - the two muscles mirror each other

Neurovascular Content Within / On Deep Surface

Axillary nerve - enters deep surface posterolaterally after passing through the quadrangular space; courses transversely approximately 5 cm distal to the acromial origin before branching into the muscle
Posterior circumflex humeral artery and vein - accompany the axillary nerve

Clinical note: When splitting the deltoid for a lateral surgical approach, incisions must not extend more than 5 cm below the acromion to avoid damaging the axillary nerve branches on the deep surface.

3D superior and posterosuperior views showing deltoid relations with pectoralis major, teres major, and triceps

Superior and posterosuperior views of the deltoid (purple) showing its relations to pectoralis major (yellow-green), teres major, triceps brachii, clavicle, acromion, and scapula. Imaging Anatomy Atlas Vol. 3

Sources: Campbell's Operative Orthopaedics 15th Ed (9780443117633); Imaging Anatomy Atlas Vol. 3 (9781626239845); Gray's Anatomy for Students (9780323934237); THIEME Atlas of Anatomy (9781626237186)

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Relations of the Deltoid Muscle

Superficial (Outside)

Skin and superficial fascia
Supraclavicular nerves over the skin

Deep Surface (Underneath)

Subdeltoid bursa (a cushion separating it from below)
Rotator cuff (supraspinatus, infraspinatus, teres minor)
Shoulder joint capsule
Axillary nerve + posterior circumflex humeral vessels — run across the deep surface about 5 cm below the acromion

Anterior Border

Meets pectoralis major at the deltopectoral groove, which contains:
- Cephalic vein (drains upward into axillary vein)
- Deltoid branch of thoracoacromial artery

Posterior Border

Overlaps long head of triceps
Related to teres major and teres minor

Above (Superior)

Overlies the acromion and acromioclavicular joint
Trapezius inserts just medial to it — the two muscles mirror each other

⚠️ Key clinical point: Never split the deltoid more than 5 cm below the acromion — the axillary nerve runs there and will be damaged.

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