Shoulder and arm

The shoulders and arms belong to the upper extremity or upper limb region, which starts from the shoulder joints and ends at the hands(1).

Both the shoulders and arms are vital for upper limb mobility. These body parts are used for performing everyday activities, such as lifting an object and pushing a door open.

Anatomy of the Shoulder

Shoulder joints have the most extensive range of movement among all other joints in the human body(2).

While advantageous for daily living and sports, the shoulders’ wide flexibility may lead to problems, like instability and greater injury risk.

Bones of the Shoulder

Three types of bones make up the shoulder joint(3):

• The scapula or the shoulder blade
• The clavicle or the collarbone
• The humerus or the upper arm bone

The shoulder joint, or the glenohumeral joint, is where the entire upper limb is connected to the trunk(4). The glenohumeral joint is often described as a ball-and-socket joint(5).

The description is based on how the humerus bone fits into the scapula’s glenoid cavity (6).

However, the glenohumeral joint may be described more accurately as a ball-and-saucer joint(7).

Unlike the hip and upper leg, another ball-and-socket joint in the body, the glenoid has a shallower curve, akin to a saucer. The shallow curve permits a wide range of movement, though the joint and its movements may be compromised or less stable(8).

Cartilage and Membrane of the Shoulder

Cartilage and soft tissues surround and support the shoulder joint, ensuring that the shallow socket can hold all the components together. Inside the shoulder socket or glenoid sits a soft fibrous ring of cartilage called the labrum(9).

This cartilage ring surrounds the end of the humerus bone and makes a deeper socket for the bone to fit(10).

The labrum deepens the socket by almost 50%, making a stronger fit for the arm bone(11). The deeper socket helps stabilize the joint without restricting arm movement(12).

A thin membrane called the synovium also lines the shoulder joint. The synovial fluid produced by the synovium serves as a lubricant and additional cushion for the joint(13).

Muscles of the Shoulder

Two groups of muscles reinforce the shoulder joint and keep the humerus centered in the glenoid or shoulder socket(14):

• Superficial muscles: deltoid and trapezius
• Deep layer muscles: supraspinatus, infraspinatus, teres minor, and subscapularis

The deltoid and trapezius muscles give the shoulder its shape or contour(15).

The deltoid muscle is also known as the strongest shoulder muscle. It provides the power needed for lifting the upper limb and heavy weights(16).

The deep layer muscles are also known as rotator cuff muscles(17).

These muscles cover the end of the humerus, ensure it stays in place, and enable rotational movement of the arm(18).

Anatomy of the Arm

In medical or anatomical terms, the arm pertains to the part of the upper extremity stretching between the shoulder and the elbow joint(19).

The part of the upper limb starting below the elbow and before the hand is considered the forearm.

Bones of the Arm

The humerus is the only bone of the arm. It serves as an anchor or pillar for all the soft tissue structures or muscles of the arm. The upper end of the arm bone connects to the scapula or shoulder blade. Meanwhile, the lower end (also known as distal humerus) attaches to the elbow joint(20).

Muscles of the Arm

The five muscles of the arm are divided into two compartments(21):

• Anterior (near the front) compartment: biceps brachii, brachialis, and coracobrachialis
• Posterior (near the back compartment: triceps brachii and anconeus muscle

The anterior compartment muscles are flexor muscles, meaning their primary function is flexion or bending of the arm(22).

Meanwhile, the posterior compartment muscles are extensor muscles that function primarily to extend or straighten the arm(23).

The triceps brachii is attached to the scapula and acts as a weak extensor of the arm from the shoulder joint. However, this muscle serves as the main extensor of the forearm from the elbow joint(24).

Common Shoulder and Arm Injuries

Shoulder problems may be divided into four major categories: tendon inflammation or tears, instability, arthritis, and fractures or broken bones. Shoulder dislocations, which fall under the instability category, are common in the US(25).

A study showed that the incidence rate of shoulder dislocations presented at emergency departments in the US is estimated at 23.9 per 100,000 people(26).

The findings further note that 71.8% of the dislocations occurred among males. Moreover, the shoulders had the second-highest injury incidence rate (16.8%) among other upper limb injuries(27).

Injuries to the upper arm had the lowest incidence rate (3.7%). Researchers also noted that fractures are the most common injury to the upper limbs(28).

Radiology and diagnostic imaging techniques allow doctors to correctly diagnose shoulder and arm problems and keep track of the healing progress. Some diagnostic imaging modalities used to examine the shoulders and arms are X-ray exams, magnetic resonance (MR) imaging, and computed tomography (CT) scans.

1. Vasković, J. (2020, Oct. 29). Upper extremity anatomy. Kenhub. Retrieved from https://www.kenhub.com/en/library/anatomy/upper-extremity-anatomy
2. Terry, G. C., & Chopp, T. M. (2000). Functional anatomy of the shoulder. Journal of athletic training, 35(3), 248–255. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1323385/pdf/jathtrain00003-0018.pdf
3. Athwal, G. & Widmer, B. (2018, March). Shoulder Pain and Common Shoulder Problems. American Academy of Orthopaedic Surgeons. Retrieved from https://orthoinfo.aaos.org/en/diseases—conditions/shoulder-pain-and-common-shoulder-problems/
4. Vasković, J. (2020, Oct. 29). Upper extremity anatomy. Op. cit.
5. Forro, S.D., Munjalm, A., & Lowe, J.B. (2020, Jan.) Anatomy, Shoulder and Upper Limb, Arm Structure and Function. (Updated 2020 Aug 10). StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK507841/
6. Athwal, G. & Widmer, B. (2018, March). Op. cit.
7. Forro, S.D., Munjalm, A., & Lowe, J.B. (2020, Jan.) Op. cit.
8. Ibid.
9. Arthritis Foundation. Shoulder Anatomy. Retrieved from https://www.arthritis.org/health-wellness/about-arthritis/where-it-hurts/shoulder-anatomy
10. Ibid.
11. Athwal, G. (2017, Oct.). Shoulder Joint Tear (Glenoid Labrum Tear). American Academy of Orthopaedic Surgeons. Retrieved from https://orthoinfo.aaos.org/en/diseases—conditions/shoulder-joint-tear-glenoid-labrum-tear/
12. International Cartilage Regeneration & Joint Preservation Society. The Shoulder. Retrieved from https://cartilage.org/patient/about-cartilage/welcome-to-our-joint/the-shoulder/
13. Arthritis Foundation. Shoulder Anatomy. Op. cit.
14. Vasković, J. (2020, Oct. 29). Upper extremity anatomy. Op. cit.
15. Mytilinaios, D. (2020, Oct. 29). Arm and shoulder anatomy. Kenhub. Retrieved from https://www.kenhub.com/en/library/anatomy/arm-and-shoulder-anatomy
16. Arthritis Foundation. Shoulder Anatomy. Op. cit.
17. Mytilinaios, D. (2020, Oct. 29). Op. cit.
18. Arthritis Foundation. Shoulder Anatomy. Op. cit.
19. Vasković, J. (2020, Oct. 29). Upper extremity anatomy. Op. cit.
20. Della Rocca, G. J. (2016, Dec.). Distal Humerus Fractures of the Elbow. American Academy of Orthopaedic Surgeons. Retrieved from https://orthoinfo.aaos.org/en/diseases—conditions/distal-humerus-fractures-of-the-elbow/
21. Pirie, E. (2020, Dec. 22). Arm muscles. Kenhub. Retrieved from https://www.kenhub.com/en/library/anatomy/arm-muscles
22. Ibid.
23. Ibid.
24. Ibid.
25. Zacchilli, M. A., & Owens, B. D. (2010). Epidemiology of shoulder dislocations presenting to emergency departments in the United States. The Journal of bone and joint surgery. American volume, 92(3), 542–549. https://doi.org/10.2106/JBJS.I.00450
26. Ibid.
27. Ootes, D., Lambers, K. T., & Ring, D. C. (2012). The epidemiology of upper extremity injuries presenting to the emergency department in the United States. Hand (New York, N.Y.), 7(1), 18–22. https://doi.org/10.1007/s11552-011-9383-z
28. Ibid.