Deltoid

The deltoid is a superficial muscle overlying the glenohumeral joint and is one of the six intrinsic muscles of the shoulder. The other five intrinsic muscles of the shoulder include the teres major and the four rotator cuff muscles (i.e., supraspinatus, infraspinatus, teres minor, and subscapularis muscles), which are powerful rotators of the glenohumeral joint. The deltoid muscle receives its blood supply from various branches of the axillary artery, including the thoracoacromial artery, the circumflex humeral arteries, and the deep brachial artery. It’s innervated by the axillary nerve, which runs around the neck of the humerus once exiting the axilla.   

Anatomically, the deltoid muscle can be divided into three parts: the anterior, or clavicular; lateral, or acromial; and posterior, or spinal. Proximally, each of these parts attach to a different region of the shoulder, known as the “origin” of the muscle. The anterior part attaches to the lateral third of the clavicle, the lateral part attaches to the acromion process, and the posterior part attaches to the spine of the scapula. The deltoid muscle fibers run laterally and inferiorly towards the humerus and converge into a narrow base that attaches to the deltoid tuberosity of the humerus. This is the “insertion” of the deltoid muscle.  

The deltoid has a significant role in shoulder abduction as well as smaller roles in flexion and extension movements of the arm. When the three parts of the deltoid contract simultaneously, it results in abduction of the shoulder when the arm is already held at 15 degrees of abduction. The deltoid cannot initiate abduction, so the initial 15 degrees of abduction are produced by the supraspinatus muscle.  

In addition, each part of the deltoid muscle can contract independently, producing other arm movements. When the anterior deltoid contracts alone, it produces extension and lateral rotation of the arm. When the lateral part of the deltoid contracts independently, it causes the arm to abduct. Contraction of the posterior deltoid causes the arm to flex and medially rotate. On top of that, the anterior and posterior deltoid muscle fibers are used to swing the arms when walking.  

Lastly, the deltoid plays an important role in stabilizing the glenohumeral joint, especially when carrying weights with the arm completely adducted (e.g., when carrying grocery bags) where it prevents inferior displacement of the glenohumeral joint.  

The deltoid muscle can become injured by repeated overhead activities; sports injuries such as from water polo, swimming, or baseball; and motor vehicle crashes, all of which can lead to tears or strains. Deltoid muscle tears are uncommon and typically related to massive rotator cuff tears or traumatic shoulder dislocations.   

In addition, injuries to the axillary nerve, including those caused by humeral fractures, dislocation of the shoulder, or mechanical stress (e.g., incorrect use of crutches) may affect the functioning of the nerve and, consequently, the deltoid muscle. Symptoms of axillary nerve palsy (i.e., paralysis) or injury can lead to atrophy of the deltoid muscle, causing loss of muscle tone and a flat-appearing shoulder. There may also be muscle weakness and loss of sensation to the skin overlying the deltoid muscle.   

Finally, injury to the deltoid muscle may occur due to repeated intramuscular injections of vaccines, antibiotics, and other medications as well as during an anterior surgical approach to the shoulder joint. 

Treatment of deltoid injuries depends on the degree and type of injury. In general, deltoid strains and partial tears can be treated with rest, ice, and medications to help alleviate the pain, such as acetaminophen, or paracetamol, and ibuprofen. Once movement becomes pain free, individuals may benefit from physical therapy and gentle stretching to expedite healing and prevent future strains. Minor strains may take 1-2 weeks to resolve, whereas deltoid tears may take a few months.