The Anatomy of the Deltoid Muscle

The main muscle of the shoulder

In This Article

The deltoid muscle is the main muscle of the shoulder. It consists of three muscle heads: the anterior deltoid, lateral deltoid, and posterior deltoid. All assist with arm elevation during a process called glenohumeral elevation and play a large role in the movement and overall stability of the shoulder joint and upper arm. The entire deltoid muscle spans from the spine of the scapula to the lateral portion of the clavicle.

Anatomy

Each head of the deltoid has slightly different insertion points, allowing for more control and a full range of motion at the shoulder joint. The anterior head of the deltoid works closely with pectoralis major, which allows for full stabilization near its more superior insertion on the clavicle.

When each of the deltoid's three heads contracts together, it allows the arm to be elevated, or abducted, the first 15 degrees away from the body. This then triggers the remainder of the glenohumeral joint to assist with stabilizing the shoulder and contracting additional muscles to provide the remainder of the arc of motion.

The anterior deltoid rotates the shoulder joint medially by drawing the arm inward. This flexion and medial rotation allow the arm to move forward, sometimes referred to as forward flexion. This action can be seen in a variety of functional tasks and is quite integral to upper extremity movement. The forward-flexed motion moves the arm toward the insertion point of the anterior deltoid at the clavicle.

The lateral deltoid rotates the shoulder joint laterally moving the arm outward, referred to as abduction. This is important while walking, reaching, and completing many other tasks in a lateral plane. This abduction moves the shoulder joint downward to accommodate for the movement of the entire arm outward. The contraction of the lateral deltoid pushes the arm toward the insertion point of the lateral deltoid, which is on the proximal humerus.

The posterior deltoid rotates the joint laterally which moves the arm backward and outward. This moves the entire arm toward the spine, which is where this head of the deltoid inserts. This motion is seen often when dressing, reaching backward, or throwing, among other functional tasks.

Structure

Embryonic development of all aspects of the deltoid muscle originates from the mesodermic layer of cells during pregnancy. Myoblasts, which are early muscle cells and later evolve into muscle fibers, often develop into upper extremities and lower extremities during early development. The deltoid muscle, in particular, develops from ventral muscle cells, which are toward the front of the body.

The anterior deltoid is composed of clavicular muscle fibers in accordance with its insertion on the clavicle. The lateral deltoid is composed of acromial muscle fibers to reflect its insertion on the acromion process of the humerus. The posterior deltoid is composed of spinal fibers due to their insertion on spinal processes of the vertebrae.

Anatomical Variations

A common anatomical variation of the deltoid includes the presence of separate fascial sheaths and muscle fibers on the posterior deltoid. This may cause medical professionals to mistake the separate muscle fibers of the posterior deltoid for an adjoining muscle, teres minor.

In some cases, a difference in the insertion point of the lateral deltoid into the medial epicondyle of the humerus is seen. This is different from its typical and traditional insertion point on the acromion process of the humerus. This variation can complicate blood supply and innervation, making it important for medical professionals to be mindful of this during internal procedures such as surgery.

An additional variation may be present involving blood supply. The thoracoacromial artery plays a large role in providing each of the deltoid heads with blood supply and oxygen. This artery typically runs through a groove between the deltoid and pectoralis muscles.

However, a variation may be noted in some individuals where this artery tunnels through the deltoid rather than around it. If any of the deltoid muscles are sprained or injured in any way, this variation can cause the pain, spasming, decreased or altered sensation, or sometimes a lack of blood supply.

Function

As mentioned earlier, the deltoid muscle plays a large role in the gross movements of the arm. Each head of the deltoid muscle also plays a role in the stabilization of the glenohumeral joint which serves to improve the smoothness and overall quality of arm movement. The glenohumeral joint, consisting of the scapula and humerus, relies on upper arm musculature for stabilization and overall maintenance of the joint integrity.

The deltoid is a muscle, meaning motor function is its sole and primary job.

Motor function which the deltoid is responsible for includes shoulder abduction, shoulder flexion, and shoulder extension (which allows the shoulder to stay at its resting position while also giving the option of backward movement).

Each of these motions plays a large role in the overall use of the arm in daily tasks.

The deltoid muscle serves no sensory function, though nerves and arteries which run through it allow for its oxygenation and movement. Therefore, severe injury to the deltoid muscle may indirectly cause injury to the underlying nerves and arteries which run through or are adjacent to the deltoid.

Associated Conditions

Conditions associated with the deltoid often are related to injury of the deltoid muscle or adjacent muscles with similar functions. Muscles in the upper arm region include the supraspinatus, infraspinatus, teres minor, and subscapularis (together known as the rotator cuff). The most common injury to this group of muscles as a whole is a rotator cuff tear.

While the deltoid is not one of the rotator cuff muscles, it may be impacted as a result of poor motor function and/or surgery to this set of muscles. This may place undue mechanical stress on the deltoid muscle, which can cause a muscle sprain if it continues over a long period of time. Compensatory patterns causing this undue stress can be addressed through proper rehabilitation of the entire upper arm after such an injury or surgery.

More specific to the deltoid muscle is its integration within the entire arm, as this plays a role in surgical approaches.

Due to the frontal placement of the anterior head of the deltoid, this muscle is an important consideration when surgeons choose an approach which involves access through the front of the arm.

Surgeries such as open capsular reconstructions for frontal shoulder instability, shoulder replacement surgeries, biceps tendon repairs, and rotator cuff repair surgeries all may utilize the deltopectoral approach. The deltopectoral approach uses fibers and anatomical landmarks from the deltoid and pectoralis major muscles to guide surgical incisions.

Both frontal and side approaches to surgery include the splitting of deltoid muscle fibers followed by fibers being sewn back together.

Any of these approaches which include the splitting of the deltoid fibers can result in injury to the deltoid muscle. This may require some rehabilitation to the deltoid muscle along with the typical rehabilitation course specific to the surgical procedure.

Due to the placement of the axillary nerve immediately below the deltoid muscle, the nerve supply to the deltoid and other upper arm muscles may also be impacted through surgery or traumatic injuries. This would require more extensive rehabilitation efforts, along with potentially more procedures to attempt nerve regeneration and repair.

This nerve loss can result in a partial or complete loss of motor function to the deltoid muscle, along with the other muscles the axillary nerve supplies. Motor loss would also be accompanied by loss of sensation to the deltoid muscle, depending on the severity of the nerve loss.

The cephalic vein runs adjacent to the deltoid muscle and assists with circulation and fluid management. An injury of any kind to the cephalic vein can result in fluid buildup to the upper arm. If not addressed immediately and properly, fluid buildup can lead to a host of other complications, including skin changes, loss of blood flow, nerve damage, loss of muscle strength, and more.

Rehabilitation

Rehabilitation of the deltoid muscle looks much the same as rehabilitation of most large muscles of the arm.

If someone has surgery on the deltoid muscle or even an injury which requires extensive rehabilitation, treatment of the deltoid will typically follow a specific protocol. This protocol will vary slightly based on the injury which occurred and whether surgery was performed.

Most protocols require an individual to wear a brace which immobilizes the arm for two to three weeks. This stability gives the muscle adequate time to heal from a repair without causing further or repeat injury.

While the arm is immobilized, an occupational or physical therapist will provide either active or passive motion to the joints below the upper arm. This includes the elbow, wrist, and fingers to ensure movement is maintained throughout the entire arm. Lack of motion to these joints can cause muscle weakness, resulting in an overall lengthened rehabilitation period.

More aggressive exercises and range of motion activities are completed starting about six weeks after surgery or injury. These exercises can then be graduated to functional tasks including dressing, writing, driving, throwing, and more. Progression to the next step of this protocol is dependent on good tolerance to previous steps with no complications or excessive pain limiting participation.

This protocol may be different if an injury or surgery has resulted in nerve loss. In this case, pure strengthening is not the focus of the rehabilitative professional. Re-education of nerves in conjunction with muscles will be immediately indicated in order to increase the chance of regaining the neurological connection between the brain and the injured muscle.

In any case of injury or surgery, it is important to maintain these precautions and closely follow the instructions of an occupational or physical therapist. They maintain close contact with your doctor and will update each professional involved.

Early treatment in instances such as these will help ensure motion and nerve function, if this is included in treatment, have a high probability of being effectively restored.

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