Magnetic resonance imaging of the shoulder in abduction.

To simulate the painful arm position in subacromial impingement syndrome, magnetic resonance imaging of the shoulder was done with the arms at the sides and at 80 degrees abduction in two groups of patients. Group A had 20 shoulders in 20 patients with subacromial impingement syndrome and Group B had 19 shoulders in 19 patients without impingement syndrome. When arm position was changed from adduction to abduction, an increase in signal intensity of the rotator cuff tendon was seen more often in Group A (45%) than in Group B (26%). Group A shoulders showed encroachment of the acromion or the acromioclavicular joint to the rotator cuff in 25% of the shoulders with the arms at the sides, and in 75% with the arms in abduction. Similar findings were observed in only 14% and 21% of Group B shoulders in the respective arm positions. Abduction of the arms seemed to cause encroachment of the overlying structure, especially of the acromioclavicular joint, to the rotator cuff in patients with subacromial impingement syndrome, even if there were no such findings in the images with the arms at the sides.