Shoulder pain is the result of an injury, degenerative disease, or wear and tear from overuse or aging that damages the soft tissues – including muscles, tendons and ligaments – or bones of the shoulder joint or the area around it. These tissues must remain healthy and strong if they are to keep the shoulder joint anchored, stabilized and working properly. If these tissues become damaged, shoulder pain can result. 

Each year, around 4 million people in the United States seek medical care for pain related to shoulder sprains, strains, dislocations or other problems, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases. In addition, orthopaedic surgeons – physicians who treat disorders of the bones, muscles and related structures – receive 1.5 million annual visits related to conditions affecting the shoulder.

The shoulder is the most movable joint in the body. It includes four bones – the humerus (arm bone), the clavicle (collarbone), the scapula (shoulder blade) and the sternum (breastbone). The primary shoulder joint, technically known as the glenohumeral joint, it is a modified ball and socket in which a “ball” in the upper humerus fits against a shallow, disc-shaped socket (glenoid fossa) in the scapula. A soft-tissue envelope known as a capsule encircles the glenohumeral joint and is lined by a thin, smooth synovial membrane.

Additional joints in both shoulders include:

• Acromioclavicular (AC) joint. Several ligaments and a cartilaginous disc connect the acromion (the highest point of the shoulder at the top of the scapula) and the clavicle. The AC joint is often injured in collision sports such as football and bicycling.
  
  
• Sternoclavicular joint. Several strong ligaments connect the clavicle to the sternum near the body’s midline. This synovial joint is the most stable and least injured of the shoulder joints.  

Muscles, tendons (including four rotator cuff tendons) and ligaments keep the bones of the shoulder in place. Ligaments attach the shoulder bones to each other, and the tendons attach the bones to the shoulder muscles. These soft tissues help the muscles move the shoulder.

Other important structures of the shoulder include the rotator cuff, which is a combination of tendons and muscles that holds the “ball” of the humerus against the shallow socket of the glenohumeral joint. This helps provide great mobility and strength to the shoulder joint.

In addition, two filmy, sac-like structures called bursae help bone, muscle and tendon to work together smoothly while minimizing friction. The bursae protect the rotator cuff form the bony arch of the acromion.

The shoulder’s anatomy allows a wide range of motion. However, this flexibility also makes the joint unstable and prone to injury.