Deltoid muscle

The deltoid is a large, thick, triangular-shaped shoulder muscle (it is the largest muscle of the group). Its name derives from the Greek letter delta (Δ), as the shape of the muscle resembles an inverted delta. Being a superficial shoulder muscle, the deltoid is easily palpated. Additionally, it serves as a site for intramuscular injections.

The deltoid surrounds the shoulder, spreading anteroposteriorly and giving the shoulder its rounded contour. Because of its wide origin, it envelops the shoulder (glenohumeral) joint from all sides. It arises from two bones - the scapula and the clavicle - and extends inferolaterally to insert into the deltoid tuberosity of the humerus.

According to the site of origin, the deltoid can be subdivided into three parts - clavicular, acromial, and scapular spinal (or simply the spinal part). Each part has a different origin site, and all parts are distinct portions both anatomically and functionally.

The clavicular part arises from the anterior border and superior surface of the lateral third of the clavicle. It provides the internal (medial) rotation and flexion of the arm at the glenohumeral (shoulder) joint. The acromial part originates from the lateral aspect of the acromion of the scapula. It is the strongest portion and is responsible for the abduction of the arm at the shoulder joint beyond the initial 15 degrees. The first 15 degrees of the abduction are initiated by the supraspinatus. The clavicular and scapular spinal fibers stabilize and guide the arm through this movement.

The scapular spinal part originates from the lateral two-thirds of the inferior aspect of the posterior border of the scapular spine. It provides external (lateral) rotation and extension of the arm at the shoulder joint. Overall, all parts of the deltoid stabilize the glenohumeral joint and prevent its inferior displacement.

Origin: clavicular part - anterosuperior aspect of the lateral third of the clavicle (acromial end of the clavicle); acromial part - lateral aspect of the acromion of the scapula; scapular spinal part - lateral two-thirds of the inferior aspect of the posterior border of the spine of the scapula.

Insertion: deltoid tuberosity of the humerus.

Action: clavicular part - internal rotation and flexion of the arm at the glenohumeral (shoulder) joint, stabilization of the the arm during the action of the acromial part; acromial part - abduction of the arm at the glenohumeral joint beyond the initial 15 degrees; scapular spinal part - external rotation and extension of the arm at the glenohumeral joint, stabilization of the arm during the action of the acromial part. All parts of the deltoid stabilize the glenohumeral joint and prevent its inferior displacement.

Innervation: axillary nerve (C5 - C6).

Blood supply: deltoid and acromial branches of the thoracoacromial artery, anterior and posterior circumflex humeral arteries (all arise from the axillary artery), and the deltoid branch of the deep brachial artery (a branch of the brachial artery).