Excursion of the rotator cuff under the acromion. Patterns of subacromial contact.

Nine fresh-frozen, human cadaveric shoulders were elevated in the scapular plane in two different humeral rotations by applying forces along action lines of rotator cuff and deltoid muscles. Stereophotogrammetry determined possible regions of subacromial contact using a proximity criterion; radiographs measured acromiohumeral interval and position of greater tuberosity. Contact starts at the anterolateral edge of the acromion at 0 degrees of elevation; it shifts medially with arm elevation. On the humeral surface, contact shifts from proximal to distal on the supraspinatus tendon with arm elevation. When external rotation is decreased, distal and posterior shift in contact is noted. Acromial undersurface and rotator cuff tendons are in closest proximity between 60 degrees and 120 degrees of elevation; contact was consistently more pronounced for Type III acromions. Mean acromiohumeral interval was 11.1 mm at 0 degrees of elevation and decreased to 5.7 mm at 90 degrees, when greater tuberosity was closest to the acromion. Radiographs show bone-to-bone relationship; stereophotogrammetry assesses contact on soft tissues of the subacromial space. Contact centers on the supraspinatus insertion, suggesting altered excursion of the greater tuberosity may initially damage this rotator cuff region. Conditions limiting external rotation or elevation may also increase rotator cuff compression. Marked increase in contact with Type III acromions supports the role of anterior acromioplasty when clinically indicated, usually in older patients with primary impingement.