Acromial structure and tears of the rotator cuff.

Rotator cuff lesions have been related to the structure of the acromion. We report a clinical review of 56 shoulders and the analysis of their acromial structure as seen on the radiographic arch (outlet) view and magnetic resonance imaging. The shoulders were classified as acromial type I (flat), type II (curved), or type III (hooked). On plain radiographs 89% of type III acromions had tearing of the rotator cuff (p < 0.001). The association between acromial type as determined on magnetic resonance imaging and the presence of rotator cuff tearing was less significant. Magnetic resonance imaging offered no additional benefit over plain radiographs for determining acromial type. In addition, a method of quantitating acromial structure (the "acromial angle") was devised. This angle showed a significant association with acromial types (p < 0.0001) seen on plain radiographs and had good interobserver reproducibility (coefficient of variation, 0.1). With this measurement system a type I acromion had an acromial angle of 0 degrees to 12 degrees; a type II acromion, 13 degrees to 27 degrees; and a type III acromion, greater than 27 degrees.