The most common causes are bursitis, tendonitis, rotator cuff tears, dislocations (which produce "instability"), "frozen shoulder" and fractures. Other less common causes of shoulder pain are shoulder separation, degenerative arthritis, SLAP lesions and biceps tendonitis.

Some uncommon causes are pain referred from a neck problem, cysts around the shoulder joint, nerve injuries and lung tumors (Pancoast tumor).

The Causes of Shoulder Pain Defined
Bursitis is inflammation of a bursa. A bursa is a tiny fluid-filled sac that functions as a gliding surface to reduce friction between tissues of the body. There are 160 bursae in the body. The major bursae are located adjacent to the tendons near the large joints, such as the shoulders, elbows, hips, and knees.

The shoulder is the most mobile joint in the body. When a person reaches overhead, the tendons that move the shoulder must glide almost two inches past a bone surface. The acromion is the large bursa in the shoulder that protects and cushions the tendons as they glide beneath the shoulder blade. With repetitive activities above shoulder level, overuse, aging or falls, this bursa may become inflamed, producing noises when the shoulder is moved. Pain coming from shoulder bursitis is frequently felt halfway down the upper arm toward the elbow.

Tendonitis can be especially difficult to deal with when your daily job or athletic activity puts a strain on the very tendons that are irritated regularly. The tendons are a densely packed area of soft tissue that connects muscle to bone. Because the tendons are so dense, they do not stretch as well and can become easily irritated with repetitive body movements that place a continuous pull on them. The resulting tendon irritation and inflammation is known as tendonitis. The most common areas of tendonitis are the elbow, wrist, biceps, shoulder (including rotator cuff attachments), leg, knee (patellar), ankle, hip, ankle and achilles.

Tendonitis involving the shoulder’s rotator cuff is the most common cause of shoulder pain in individuals more than 40 years old. Pain and weakness with lifting the arm are frequent symptoms of tendonitis. While tendonitis may be thought of as a fraying or irritation of the tendon, it may progress to a partial or complete rotator cuff tear.

Rotator cuff tears are damage to a tendon in the rotator cuff, a group of four tendons that stabilize the shoulder joint and move the shoulder in various directions. This can be due to trauma (from falling and injuring the shoulder or from overuse in sports, particularly involving repetitive overhead motions), inflammation (from tendinitis, bursitis, or arthritis of the shoulder) or degeneration (from aging).

Rotator cuff tears that involve the full thickness of the tendon are called “complete tears.” Once a tendon has this degree of separation, it does not heal back to the bone on its own. Shoulders with torn rotator cuffs frequently lose strength and often cause pain at night. Over time, the tears can enlarge, like a seam splitting, resulting in greater weakness.

SLAP lesions -- The socket of the shoulder joint is like a saucer with a rim around the edge that helps keep the ball in place. The socket is called the glenoid, and the rim is called the labrum, which means “lip.” An important tendon from the biceps muscle also attaches in this part of the shoulder. When the rim of the saucer tears from the rest of the saucer, it is called a SLAP (Superior Labrum Anterior to Posterior) lesion.

SLAP lesions may produce pain, clicking, and weakness when the arm is in front of the body. Although physical examination and MRI tests may suggest a SLAP lesion, the diagnosis is frequently only confirmed and treated during arthroscopic examination.

Dislocations involve the separation of the ball of the shoulder joint (the top rounded portion of the upper arm bone, or humerus) from the socket of the joint (the glenoid fossa of the shoulder blade or scapula). This is the most frequently dislocated major joint of the body, reflecting the fact that it sacrifices stability for mobility. Most of these dislocations are due to trauma. Individuals less than 25 years old who dislocate their shoulder have an 85 percent chance of dislocating again because the ligaments that hold the shoulder in the socket are usually torn.

"Frozen shoulder" is severe limitation of the range of motion of the shoulder due to scarring around the  joint (adhesive capsulitis). A minor trauma may cause the shoulder to produce excessive amounts of scar tissue around the ball and socket, gradually reducing the “slack” in the joint. Women, around age 50, and people with diabetes are at higher risk for this condition. While shoulder motion may be lost gradually, symptoms may be suddenly apparent when he shoulder is abruptly moved to its limit, and becomes suddenly painful. Night pain is common.

Fractures -- The most common include:

Clavicle Fractures -- The collarbone is frequently fractured with falls or trauma.
 See our special page on the A/C joint.
 

Greater tuberosity fractures -- The largest bump on the ball at the top of the upper arm bone is called the greater tuberosity. Three of the four rotator cuff muscles attach to it. If this bone breaks away from the rest of the ball, it may heal in a position that produces bursitis, pain and limited shoulder motion.

Shoulder separation -- Direct blows to the shoulder during sports may cause the ligaments between the collar bone (clavicle) and shoulder blade (scapula) to tear. This may produce tenderness over the joint where these two bones come together on the top of the shoulder. More severe sprains can cause the collarbone to remain elevated relative to the shoulder blade. This produces a bump, which is permanent, but tenderness in the area disappears in 2–3 months.

Degenerative arthritis is caused by the breakdown and eventual loss of the cartilage of one or more joints. Cartilage is a substance made of protein and water that serves as a "cushion" between the bones of the joints, like a living “Teflon” coating. Also known as osteoarthritis, degenerative arthritis is mostly related to aging. With aging, the water content of the cartilage increases and the protein makeup of cartilage degenerates. Repetitive use of the joints over the years irritates and inflames the cartilage, causing pain and swelling. Eventually, cartilage begins to degenerate by flaking or forming tiny crevasses.

In the shoulder, degenerative arthritis is less common than arthritis in the knee or hip and it may involve the large ball-and-socket joint and produce loss of motion. Replacing the worn shoulder joint with an artificial one can help reduce pain, and often improves motion.
Read Shoulder Replacement Surgery in Brief.

Degenerative arthritis of the joint between the collarbone and the shoulder blade (acromioclavicular, or AC joint) often accompanies rotator cuff disease. The tip of the collarbone is tender, and movements of the arm across the front of the shoulder produce pain. Painful degeneration of the AC joint may be treated with arthroscopic removal of the end of the clavicle so that the two bones no longer bang into each other, and so that bone spurs from the AC joint no longer protrude down into the rotator cuff, which runs just beneath.

Biceps tendonitis -- The biceps muscle is the powerful, prominent muscle centered on the front of the upper arm. The long tendon from this muscle passes up the front of the shoulder, through a groove between the two bumps (tuberosities), and attaches to the top of the rim of the shoulder socket. This tendon is prone to wearing out in middle age. Like a cable, it may fray, and torn portions of the tendon may catch and become irritated as the shoulder moves. Biceps tendonitis produces tenderness in the front of the shoulder where the tendon passes through the groove.

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