The deltoid is the most powerful muscle in the shoulder. It is made up of three muscle bundles which participate in the various movements of the shoulder (abduction, flexion, rotation, extension). The functioning of the deltoid can be altered by pathologies or abnormalities that can affect the muscle, the tendons that serve as an attachment to the skeleton or the circumflex nerve that invades it. These pathologies are diagnosed using X-rays, MRIs, ultrasounds, blood tests, biopsies or electromyographies.

The deltoids are two muscles that cover the outer sides of the shoulders. These large and thick muscles are shaped like cones with a downward apex and form the link between the shoulder girdle (collarbone, sternum and scapula) and the top of the humerus. They are innervated by the axillary nerve, also called the circumflex nerve.

The deltoids are made up of three muscle bundles, the fibers of which converge downwards and meet at the level of an area called the deltoid “V” located on the upper first third of the humerus:

• The anterior (or clavicular) bundle that fits over the upper anterior border of the clavicle
• The middle (or acromial) bundle which is inserted on the upper part of the acromion (small bony protrusion of the scapula which articulates with the clavicle)
• The posterior (or spinal) bundle which is inserted at the level of the spine of the scapula

The deltoids are separated from the humerus by a synovial bursa, a flattened pocket filled with synovia (a viscous colorless fluid that lubricates the joint surfaces).

In static

The deltoid is a suspensory muscle of the humerus. It also acts as a contractile cushion, especially during a fall on the shoulder stump.

In dynamics

The deltoid is the most powerful muscle in the shoulder. As a whole, it participates in the abduction movement of the arm (lateral elevation of the arm which moves away from the median plane of the body) in synergy with the supraspinatus (muscle stretched between the scapula and the top of the humerus)

Other shoulder movements specifically involve one or more deltoid bundles:

• Shoulder flexion (raising the arm forward) involves the anterior and middle bundles;
• Medial shoulder rotation (counterclockwise, inward) involves the anterior bundle;
• Shoulder extension (raising the arm back) involves the posterior bundle;
• Lateral shoulder rotation (clockwise, outward) involves the posterior bundle.

Inserts

In rare cases, the deltoids can undergo a disinsertion (rupture of the point of attachment between the bone and the muscle) which can have several origins:

• Lesions associated with a massive rupture of the rotator cuff (a group of four tendons);
• Isolated lesions;
• A postoperative complication after shoulder surgery, especially after acromioplasty (surgical planing of the acromion);
• A traumatic rupture during road accidents, trauma caused by the seat belt or during weight training exercises.

Contracture

Deltoid contracture is a long-lasting, involuntary contraction of the muscle. It can be congenital, post-traumatic or idiopathic. Also, the deltoid is a common site for intramuscular injection of drugs. With repeated injections, the muscle can also contracture.

Atrophy

The deltoid can be atrophied:

• As a result of traumatic injury to the circumflex nerve during a fracture of the humerus or a dislocation of the shoulder;
• In Parsonage and Turner syndrome, inflammation of the nerves controlling the muscles of the shoulders and arms;
• In quadrilateral space syndrome (anatomical orifice of the shoulder where the axillary nerve travels), chronic pain of the circumflex nerve.

Paralysis of the circumflex nerve

Isolated paralysis of the axillary nerve is usually the consequence of trauma to the shoulder (dislocation, fracture, contusion). It can have consequences on the motor skills of the various muscles innervated by the circumflex nerve, in particular the deltoid.

Myosites

Myositis are rare autoimmune muscle diseases, in other words diseases where the immune system goes out of order and attacks the muscles.

Tendinitis

The attachment of the deltoid tendons to the acromion can be achieved in the context of spondyloarthritis, inflammatory and chronic rheumatism which mainly affects the lumbar region and the pelvis.

Anomalies and pathologies of the deltoids are managed by orthopedists and orthopedic surgeons.

Disinsertions of the deltoid

Their treatment depends on the patient, the impairment of muscle functions and the aetiology. It can be:

• Medical treatment to relieve pain (analgesics, anti-inflammatory drugs, physiotherapy);
• In case of intolerance, the tendon and muscle can be reinserted on the acromion;
• The placement of a reverse total prosthesis if the patient retains at least one functional deltoid bundle.

Contractures

In the case of deltoid contracture, the patient is exposed to various complications (omarthosis, acromial hyperplasia, flattening of the humeral head). Treatment for this condition is usually surgical.

Atrophies

The treatment of quadrilateral space syndrome is mainly conservative with, in a few cases, surgery with neurolysis (surgical release of a nerve stuck in tissue). In Parsonage and Turner syndrome, treatment is symptomatic (analgesics) and rehabilitation is important.

Paralysis of the circumflex nerve

The first treatment considered is rehabilitation. In case of failure, an exploratory intervention can be performed.

Myosites

The treatment of myositis is mainly based on corticosteroids (prednisone). At the onset of the disease, corticosteroid therapy is often combined with:

• Intravenous administration of immunoglobulins (plasma proteins with immune properties);
• Plasma exchanges: in the hospital, a medical device makes it possible to replace the patient’s plasma which contains antibodies liable to participate in the attack on the muscles by a similar pharmacological product;
• Cyclophosphamide, an immunosuppressant administered in a hospital environment reserved for severe forms of the disease.

The different anomalies and pathologies of the deltoid can be diagnosed thanks to different examinations that it is often necessary to combine to obtain a complete diagnosis.

• The clinical examination consists in testing the muscle strength of the deltoid, in particular by maintaining specific positions;
• The radiography allows to have a good evaluation of the bone structures, and it makes it possible to visualize indirect signs of damage to the rotator cuff. It is often performed as a first-line treatment;
• Ultrasound examines soft tissue. It can detect tendon damage or muscle atrophy;
• MRI is also used to examine soft tissue, it analyzes muscles and provides information on their size. In the event of muscle damage, particularly in the presence of inflammatory muscle lesions (myositis), the latter will appear abnormally white in the images;
• Electromyography is a test that records the electrical activity of a muscle or nerve. It is used in particular in the diagnosis of paralysis of the circumflex nerve or of an atrophied deltoid;
• Blood tests are used to measure certain muscle enzymes, such as creatine kinase. If they present an abnormally high level, this may indicate muscle damage;
• A muscle biopsy is an examination that is done under local anesthesia. Small fragments of the deltoid are taken for analysis.