Impingement is one of the most common causes of pain in the adult shoulder. It results from pressure on the rotator cuff from part of the shoulder blade (scapula) as the arm is lifted.

The rotator cuff is a tendon linking four muscles: the supraspinatus, the infraspinatus, the subscapularis, and the teres minor. These muscles cover the "ball" of the shoulder (head of the humerus). The muscles work together to lift and rotate the shoulder.

The acromion is the front edge of the shoulder blade. It sits over and in front of the humeral head. As the arm is lifted, the acromion rubs, or "impinges" on, the surface of the rotator cuff. This causes pain and limits movement.

The pain may be due to a "bursitis," or inflammation, of the bursa overlying the rotator cuff or a "tendonitis" of the cuff itself. In some circumstances, a partial tear of the rotator cuff may cause impingement pain.

Impingement is common in both young athletes and middle-aged people. Young athletes who use their arms overhead for swimming, baseball, and tennis are particularly vulnerable. Those who do repetitive lifting or overhead activities using the arm, such as paper hanging, construction, or painting are also susceptible. Pain may also develop as the result of minor trauma or spontaneously with no apparent cause.

Beginning symptoms may be mild. Patients frequently do not seek treatment at an early stage.

• Minor pain that is present both with activity and at rest
• Pain radiating from the front of the shoulder to the side of the arm
• Sudden pain with lifting and reaching movements
• Athletes in overhead sports may have pain when throwing or serving a tennis ball

Impingement commonly causes local swelling and tenderness in the front of the shoulder. There may be pain and stiffness when the arm is lifted. There may also be pain when the arm is lowered from an elevated position.

As the problem progresses, there may be pain at night. Strength and motion may be lost. It may be difficult to do activities that place the arm behind the back, such as buttoning or zippering.

In advanced cases, loss of motion may progress to a "frozen shoulder." In acute bursitis, the shoulder may be severely tender. All movement may be limited and painful.

To diagnose shoulder impingement, an orthopaedic surgeon reviews the symptoms and physically examines the shoulder.

He or she may take X-rays. A special X-ray view, called an "outlet view," sometimes will show a small bone spur on the front edge of the acromion (see xray). The doctor may request further imaging studies, such as an MRI (magnetic resonance imaging). These can show fluid or inflammation in the bursa and rotator cuff. In some cases, partial tearing of the rotator cuff will be seen.

An impingement test, which involves injecting a local anesthetic into the bursa, can help to confirm the diagnosis.

After surgery, the arm may be placed in a sling for a short period of time. This allows for early healing. As soon as comfort allows, the sling may be removed to begin exercise and use of the arm. The surgeon will provide a rehabilitation program based on the patient's needs and the findings at surgery. This will include exercises to regain range of motion of the shoulder and strength of the arm. It typically takes two to four months to achieve complete relief of pain, and may take up to a year.