Deltoid

The deltoid muscle is the primary abductor of the shoulder joint, lifting the arm away from the body in the coronal plane. Abduction beyond 15 degrees at the glenohumeral joint is almost entirely dependent on an intact deltoid. The muscle's three portions—anterior, middle, and posterior—also contribute to shoulder flexion, extension, and rotation.

The anterior deltoid fibers flex and medially rotate the arm, making them active in forward-reaching movements. The middle (lateral) fibers are the primary abductors. The posterior fibers extend and laterally rotate the arm, contributing to backward arm movement and rowing-type motions. All three portions converge on the deltoid tuberosity of the humerus.

The deltoid is innervated by the axillary nerve (C5, C6), which wraps around the surgical neck of the humerus. Injury to the axillary nerve—from a shoulder dislocation, proximal humerus fracture, or improper injection—results in inability to abduct the arm beyond about 15 degrees and loss of sensation over the lateral shoulder (regimental badge area).

The deltoid is preferred for intramuscular injections because it is readily accessible, has adequate muscle mass in most adults, and has few major underlying structures compared to other injection sites. The injection is typically given into the middle third of the muscle, two to three finger-breadths below the acromion, to avoid the axillary nerve and vessels located inferiorly.

The deltoid muscle is responsible for the rounded, cap-like contour of the shoulder. When the deltoid is paralyzed or significantly atrophied, the shoulder takes on a squared or flattened appearance because the underlying bony prominences of the humeral head and acromion become more visible.