Deltoid (posterior head)

Anatomy

Origin:
Anterior fibres from lateral third of the clavicle. Lateral/middle fibres from the acromion. Posterior fibres from the spine of the scapula.

Insertion:
Deltoid tuberosity of the humerus.

Key Relations:
Its origin mirrors the insertion of trapezius.

Functions

-Powerful abductor of the arm after 15°.*
-Anterior fibres flexes and medially rotates the arm at the shoulder joint.
-Posterior fibres extends and laterally rotates the arm at the shoulder joint.
-Together these actions allow for swinging of the arms as when you are walking.

*Supraspinatus is responsible for abduction of the forearm for the first 15°.

Supply

Nerve Supply:
Axillary nerve (C5, C6).

Blood Supply:
-Posterior circumflex humeral artery
-Deltoid branch of thoracoacromial artery.

Clinical

Deltoid is the site for many intramuscular injections and vaccinations.

Paralysis of the axillary nerve, for example, in shoulder dislocation when the downward displacement of the humerus may crush it results in loss of function and wasting of the deltoid muscle.

Deltoid is tested clinically by passive and then active (against resistance) abduction of the forearm above 15°, i.e. ask the patient to raise their arm out to the side and to maintain this while the clinician tries to push the arm back towards the torso.

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