Typical and atypical shoulder impingement syndrome: diagnosis, treatment, and pitfalls.

The cause of shoulder impingement syndrome usually is considered to be compression of the rotator cuff and subacromial bursa against the anterolateral aspect of the acromion. The typical symptom is anterolateral shoulder pain that worsens at night and with overhead activity. However, the pain may be caused by factors other than a hooked acromion. Atypical impingement syndrome most commonly results from an os acromiale, a subcoracoid disorder, acromioclavicular joint undersurface hypertrophy, a deconditioned rotator cuff, or scapular dyskinesis. The correct diagnosis is made through the patient history and physical examination, with appropriate diagnostic imaging. Nonsurgical treatment is successful for most types of impingement syndrome; if it is not successful, all structural causes of mechanical impingement must be corrected.