In conclusion, internal impingement apparently occurs in nearly all patients and is demonstrable on MR imaging. Pathologic changes associated with internal impingement seem to develop with repetitive placement of the arm into a position of extreme external rotation and abduction. Findings may include lesions of the posterior superior labrum, undersurface irritation, or tearing of the supraspinatus-infraspinatus junction near the attachment site and cystic changes of the posterior superior glenoid and posterior lateral greater tuberosity. There is no evidence for a particular sequence of pathologic changes. Instability may be associated with but does not appear to be a prerequisite for the development of the pathologic lesions of internal impingement.