Repetitive impingement of the bursa, rotator cuff, and biceps tendon produces pathologic changes in these structures that progress in a predictable pattern. Early on, repetitive motion produces mechanical inflammation of the subacromial bursa and underlying rotator cuff. As activities that cause impingement continue, inflammation of the rotator cuff tendons worsens. Chronic inflammation in time leads to degeneration and eventual tearing of the rotator cuff. Degeneration of the rotator cuff exposes the biceps tendon, making it susceptible to degeneration and rupture. As the soft tissue restraints of the shoulder wear out, degenerative disease sets in and is typical of the advanced stages of the impingement syndrome.
Most of the pathologic changes in the rotator cuff due to impingement occur near the humeral insertion of the tendon. This area is referred to as the critical zone and has been identified as relatively avascular. Repetitive compression causes relative ischemia in this area. Over time this area degenerates and ultimately fails. The critical zone is the most common site of all rotator cuff abnormalities. The supraspinatus, due to its location in the coracoacromial arch, is the most commonly affected muscle of the rotator cuff.
Three stages of impingement are identifiable. Stage 1 is characterized by local inflammation, edema, and hemorrhage and is most commonly associated with subacromial bursitis and early rotator cuff tendinitis. These changes are considered reversible. Stage 2 is characterized by inflammation, thickening, and fibrosis of the rotator cuff tendons. Stage 3 is characterized by degeneration and rupture of the rotator cuff tendon. Degenerative changes in the bony structures of the shoulder usually accompany stage 3. Stages 2 and 3 are considered to be irreversible.
Specific maneuvers on physical examination test for signs of impingement by compressing the rotator cuff and bursa between the humeral head and coracoacromial arch. Neer's impingement test requires the examiner to move the patient's straightened arm smoothly but forcibly to full abduction. This compresses the cuff and bursa against the undersurface of the acromion. A second test, Hawkins' impingement test, requires the examiner to position the patient's arm in 90° of abduction and 90° of elbow flexion. Rotation of the arm inwardly across the front of the patient's body compresses the cuff and bursa between the humeral head and coracoacromial ligament. These tests are considered positive if they reproduce pain.
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