The Deltoid Muscle is a muscle that surrounds the shoulder and plays a key role in arm movements. It takes its name from the Greek word "delta" (Δ) due to its triangular shape. It is also referred to as the deltoideus in some anatomical references. It functions as the main abductor muscle (elevator muscle) of the arm and contributes to the stabilization of the shoulder joint.

Anatomy

Morphology

The Deltoid Muscle is a large, triangular-shaped muscle consisting of three distinct anatomical divisions around the shoulder. These divisions are called the anterior (clavicular) head, the middle (acromial) head, and the posterior (spinal) head. The anterior head originates from the lateral third of the clavicle and primarily contributes to flexion (forward elevation) and internal rotation of the arm. The middle head originates from the acromion process and serves as the primary muscle for abduction (lateral extension) of the arm. The posterior head originates from the spinous process of the scapula and is active in extension (backward extension) and external rotation of the arm. All fibers of the deltoid muscle terminate by inserting on the deltoid tuberosity, located lateral to the humerus. This structural integrity allows the muscle to generate multi-directional force during shoulder movements.

Embryological Origin and Classification

The Deltoid Muscle derives from somitic mesenchyme derived from paraxial mesoderm and differentiates from the dorsal muscle mass that develops within the limb bud. Classified among the scapulohumeral muscles, this muscle is one of the important muscles of the upper extremity due to both its superficial location and multiple functions.

Nerve Innervation and Vascular Supply

The Deltoid Muscle is innervated by the axillary nerve (C5–C6). This nerve arises from the posterior cord of the brachial plexus and sends branches to the deep surface of the muscle. The axillary nerve also innervates the teres minor muscle and, as it passes around the humeral neck, is vulnerable to trauma. The muscle's vascular supply is primarily provided by the posterior circumflex humeral artery. This artery, a branch of the axillary artery, branches deep to the deltoid muscle, ensuring adequate perfusion.

Function and Clinical Relationships

Thanks to its multi-headed structure, the deltoid muscle contributes to movements in almost every direction of the shoulder joint. Its most prominent function is abduction of the arm; particularly in movements beyond 15 degrees, the deltoid takes over from the supraspinatus muscle and elevates the arm laterally. Its anterior fibers play an active role in flexion and internal rotation, while its posterior fibers play an active role in extension and external rotation. The muscle also contributes to passive stabilization of the shoulder joint by holding the humeral head in the glenoid cavity. Clinically, the deltoid muscle is a frequently preferred site for intramuscular injections; it is particularly well-vascularized and easily accessible in adults. However, it can be injured by overuse, direct trauma, or repetitive microtrauma, leading to shoulder pain. Injury to the axillary nerve can severely impact the deltoid muscle, resulting in muscle atrophy and significant weakness in shoulder abduction. Nerve damage can often develop after humeral head fractures, shoulder dislocations, or surgical interventions.