Subacromial space width changes during abduction and rotation - a 3-D MR imaging study

The objectives of this study were to determine systematic changes of the no rmal subacromial space width during abduction and rotation, and to analyze the spatial relationship of the supraspinatus muscle with the acromion and clavicle. 12 healthy volunteers were imaged by an open MR scanner in 5 diff erent positions of abduction and in 3 positions of rotation. After three di mensional (3D) reconstruction and 3D Euclidian distance transformation, the minimal spatial distances between the humerus and the acromion and the hum erus and clavicle were computed. The minimal acromio-humeral distance decre ased significantly from 30 degrees of abduction (mean 7.0 mm +/- 1.6 mm) to 120 degrees (mean 3.9 mm; +/- 1.8 mm; p<0.0001). At 30 degrees, the minima l distance penetrated the supraspinatus, whereas at 120 degrees it was alwa ys located lateral to the supraspinatus tendon. At 90 degrees with internal rotation (7.6 mm, +/- 2.3 mm) the minimal acromio-humeral distance was lar ger than in neutral rotation (5.4 mm, +/- 2.3 mm) or external rotation (4.4 mm, +/- 2.2 mm; p<0.05), but it penetrated the supraspinatus tendon at its most vulnerable part, reaching the acromion at its anterior inferior borde r. We conclude that the subacromial space width changes during abduction an d rotation and that the supraspinatus is in closest contact to the anterior inferior border of the acromion in 90 degrees of abduction with 45 degrees internal rotation. These values obtained in volunteers can be used as a ba sis for further investigations in patients with the impingement syndrome.