Magnetic resonance imaging analysis of the subacromial space in the impingement sign positions.

External shoulder impingement is commonly diagnosed by passively moving the shoulder into various positions of elevation and internal rotation in order to perform impingement sign maneuvers. There is a lack of agreement among clinicians regarding the positions of the anatomic structures in the subacromial space when these maneuvers are performed. The purpose of this study was to use magnetic resonance imaging to identify and measure the changes in anatomic structures in the subacromial space as the arm was moved from complete rest to 160 degrees of forward flexion during the Neer and Hawkins impingement sign maneuvers. Ten subjects with normal shoulders (mean age, 32 years) were studied. The acromiohumeral interval was smallest with the arm at the side (mean, 6.4 mm) and progressively increased as the arm was elevated from 90 degrees to 160 degrees (mean, 7.7-14.2 mm). In no instance was the rotator cuff found to be in contact with the anterior acromion. The rotator cuff insertion appeared to be in closest proximity to the anteroinferior acromion, not at full elevation (Neer sign position), but at 90 degrees of flexion (Hawkins sign position). Our data suggest that a clinically positive Hawkins sign is consistent with external shoulder impingement. Clinical suspicion that mechanisms other than impingement may be involved, particularly if the Neer impingement sign is also positive, is nonetheless advised.