Definition

By: Gregory R. Waryasz, MD

The deltoid muscle of the musculoskeletal system is characterized by its appearance as an inverted Greek letter delta (Δ).

It is part of the scapulohumeral muscles.  It consists of skeletal muscle fibers.

Its unique structural features include its three separate parts.  The anterior and posterior parts are unipennate. The middle part is multipennate.  The entire muscle is thick and coarsely textured.  The deltoid muscle gives the shoulder its rounded appearance.  The anterior part of the deltoid marks the lateral border for the deltopectoral interval which is where the cephalic vein is located. This is a common surgical approach for access to the glenohumeral joint.

The origin is the lateral third of the clavicle, the acromion, and the spine of the scapula.

The insertion is the deltoid tuberosity of the humerus.

The blood supply is from the posterior circumflex humeral artery and venous drainage is from the accompanying veins.

The innervation is from the axillary nerve.

The deltoid muscle as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in the embryo.

The function of the deltoid is to abduct the arm when all three parts contract together.  The anterior and posterior deltoid parts steady the abducted arm. These parts of the deltoid are used to swing the arms during walking.  The deltoid works with the supraspinatus during the first 15 degrees of abduction.  The anterior division abducts, horizontally flexes, and medially rotates the humerus at the shoulder by working with the rest of the deltoid, supraspinatus, and pectoralis major clavicular division.  The medial division abducts the humerus by working with the rest of the deltoid, and the supraspinatus.  The posterior division abducts, horizontally extends, and laterally rotates the humerus at the shoulder by working with the rest of the deltoid and the suprasinpatus.

Common diseases include muscle strain, deltoid contracture, and injury to the axillary nerve.

Deltoid contracture can occur after multiple intra-muscular injections. It causes shoulder pain, difficultly with abduction, and a palpable fibrous band.

Injury to the axillary nerve can result in inability to use the deltoid muscle and a patch of sensory deficit over the lateral shoulder.  The deltoid may atrophy after injury.

Commonly used diagnostic procedures include clinical history, physical exam, and MRI.

It is usually treated with physical therapy and NSAIDs for muscle strain.  Deltoid contracture may require a distal deltoid release surgically.

References

Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.

Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.

Wheeless’ Textbook of Orthopaedics: Deltoid Muscle (http://www.wheelessonline.com/ortho/deltoid_muscle)