Influence of distinct anatomic subregions of the supraspinatus on humeral rotation.

The supraspinatus, having distinct anterior and posterior subregions, is most commonly considered an abductor of the humerus, but it has also been shown to induce humeral rotation. The objective of this study was to quantify the magnitude and direction of humeral rotation that results from loading the distinct anterior and posterior subregions of the supraspinatus. Fourteen cadaver specimens were tested under four loading conditions based on physiological cross section area of the supraspinatus: (1) anterior only; (2) posterior only; (3) physiologic (each subregion loaded simultaneously); and (4) nonphysiologic (the tendon loaded as a whole). Each specimen was tested at 0, 15, 30, 45, and 60 degrees of glenohumeral abduction in the scapular plane and from 60 degrees of internal to 45 degrees of external rotation in 15 degrees increments. The humeral rotation that occurred with loading from the initial starting rotation position was measured using a rotary variable inductance transducer. In the scapular plane, the anterior subregion of the supraspinatus acts as both an internal and external rotator depending on the initial position of the humerus. The posterior subregion either acted as an external rotator or did not induce rotation. This study demonstrated a distinct functional difference between the anatomic subregions of the supraspinatus. This understanding will help to improve testing methods and the development of repair strategies of the supraspinatus.