EXCURSION OF THE ROTATOR CUFF UNDER THE ACROMION - PATTERNS OF SUBACROMIAL CONTACT

Nine fresh-frozen, human cadaveric shoulders were elevated in the scap ular plane in two different humeral rotations by applying forces along action lines of rotator cuff and deltoid muscles. Stereophotogrammetr y 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-degree of elevation; it shifts medially with arm elevat ion. On the humeral surface, contact shifts from proximal to distal on the supraspinatus tendon with arm elevation. When external rotation i s 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 mor e pronounced for Type III acromions. Mean acromiohumeral interval was 11.1 mm at 0-degree of elevation and decreased to 5.7 mm at 90-degrees , when greater tuberosity was closest to the acromion. Radiographs sho w bone-to-bone relationship; stereophotogrammetry assesses contact on soft tissues of the subacromial space. Contact centers on the supraspi natus insertion, suggesting altered excursion of the greater tuberosit y may initially damage this rotator cuff region. Conditions limiting e xternal rotation or elevation may also increase rotator cuff compressi on. Marked increase in contact with Type III acromions supports the ro le of anterior acromioplast when clinically indicated, usually in olde r patients with primary impingement.