Shoulder injuries are common and generally occur as a result of trauma (a sudden, forceful injury). However, the most common cause of injury to the shoulder joint is repetitive overuse of the muscles and tendons that support and move the joint. Athletes and workers who repetitively perform certain movements, such as throwing a baseball or using a hammer, have a higher risk of developing overuse injuries.

Cause of injury
As a result of repetitive muscle contractions, the tendons that attach muscle to bone can sustain slight tears, or microtears, in the tissue. Fortunately, the natural healing process of your body usually heals these tears. However, when there is an accumulation of microtears from overuse, your body's healing response cannot keep up, leading to serious tissue and joint damage.

Some characteristics or risk factors have been identified as predisposing a person to an overuse injury in the shoulder. These include age over 30 years, a job or sport that requires a high activity level or a demanding technique, being out of shape, and participating in an event that overexerts your shoulder three or more times per week for 30 minutes or more per session.

How can overuse affect my shoulder?
The part of the shoulder that is most commonly affected by overuse is the rotator cuff. The rotator cuff is a group of muscles and tendons that originate from the scapula (shoulder blade) and attach to the humerus (upper arm bone) (see Fig above). Localized injury to the tendons can occur from the wear and tear of repetitive motions, resulting in irritation and swelling of the tendons. This in turn will weaken your rotator cuff muscles, causing the tendons between the humeral head and overlying bony prominence (acromion) in your shoulder to compress, or impinge. This is known as primary impingement syndrome. Bone spurs (Fig. 2) can also form, causing fraying and tearing of the tendon, particularly during chest-level or overhead activities, with pain occurring more frequently at night. Shoulder weakness is also common.

When diagnosed early, the injury to the rotator cuff can be treated with nonsurgical methods. However, in more advanced stages, rotator cuff impingement may require surgery.

Overuse can also cause ailments such as tendinitis and tendinosis. And although they sound alike, they are completely different. Tendinitis refers to an inflammation of the tendons and is caused mainly by overuse activity. Tendinitis is generally treated with conservative measures, including rest, ice, and nonsteroidal anti-inflammatory drugs (such as ibuprofen).

Tendinosis is not inflammatory in nature, but rather degenerative. This is commonly seen more in older patients, where tissue deteriorates over a period of time from either a previous injury or traumatic event. Certain factors are usually responsible for the tendon damage, including repetitive strain, muscle fatigue and weakness, and an age-related decrease in blood supply to the tendon. Your ability to naturally heal becomes hindered due to the decreased blood supply, and the degenerative process advances. This process is generally chronic (long lasting) and irreversible and may require surgery to repair any partial or complete tendon ruptures (Fig. 3).

How can I prevent this from happening to me?
Unfortunately, no single injury can be totally prevented, but there are some measures that can be taken to lower your risk of injury. Stretching and strengthening your muscles can help lower your risk of injury. Just remember to see your physician to rule out any other possible problems before undergoing a stretching and strengthening program.

George M. McCluskey, III, M.D.
Columbus, Georgia