Rotator cuff disorders are caused by a combination of factors. The rotator cuff moves within a confined space called the subacromial space. Also within that space is a small sac of fluid (subacromial bursa), which serves as a cushion between the tendons and bone. When the subacromial space becomes smaller due to inflammation, bone spurs, or fluid buildup, the rotator cuff tendons may be squeezed and rub against bone (impingement). As a result, the tendons may become damaged and irritated, causing bleeding and inflammation of the bursa or tendons. With continuing damage, the tendons develop scarring and become stringy (fibrous), which makes them weaker and less flexible. Eventually, this may lead to partial or even complete tears.

A rotator cuff is more easily damaged or torn as age-related degeneration develops. The factors below often occur together or overlap.

• Bones that are irregularly shaped can affect how the rotator cuff moves in the subacromial space. You may be born with these irregularities, or they may occur after some type of injury, such as a broken bone or dislocation of the shoulder joint. If the acromion is hooked or curved, impingement or degeneration of rotator cuff tendons may be more likely.
• As you age, everyday activities and normal wear and tear lead to some changes in the rotator cuff, such as:
  • General degeneration of the tendon, which includes thinning, fraying, and tearing.
  • Reduced blood supply to the tendons.
  • Arthritis of the acromioclavicular (AC) joint, which can cause bony growths that can damage the rotator cuff.
• Joint looseness and muscle imbalance in the shoulder can cause damage, including the formation of scar tissue and changes in the tissue itself.
• Repetitive activities, especially forceful overhead motions, can damage the rotator cuff. These motions are common in certain sports or occupations, including throwing a baseball, the overhead swing in tennis, swimming, lifting, or painting.
  • Repetitive activity can cause tendons to rub or scrape against the acromion, which can irritate the rotator cuff.
  • Repeated overhead motions can damage stabilizing ligaments and result in an imbalance of opposing shoulder muscles, which may cause tendons to rub against the bones (impingement).
• Overuse may occur with or be closely related to repetitive activities. Normal motions made frequently over a long period can stress or injure rotator cuff tissues. Athletes, including young people, may develop tendinitis from overuse in throwing, swimming, and racquet sports.
  • Overuse can cause the shoulder joint to become unstable and allow the ball of your upper arm bone (humeral head) to move upward, narrowing the subacromial space where the rotator cuff moves. In this narrowed space, the rotator cuff is squeezed, forcing the tendon to rub against bone (impingement).