The purpose of this study was to examine the anatomy of the infraspinatus including the orientation of muscle fibers and the insertion to the greater tuberosity. Ninety-three shoulders from 52 cadavers were minutely dissected. After resection of the acromion and removal of the coracohumeral ligament, the infraspinatus muscle was carefully investigated macroscopically. After the orientation of muscle fibers was confirmed, the muscle was peeled from the proximal part to the distal part and the insertion of the infraspinatus tendon was examined. In 4 shoulders, muscle fibers were completely removed in water and the direction and insertion of the tendon were examined. The infraspinatus muscle originated both from the inferior surface of the spine of the scapula and the infraspinatus fossa, and inserted to the greater tuberosity. The muscle fibers originated from the spine were running dorsally and horizontally to the greater tuberosity. On the other hand, the fibers from the fossa were running ventrally and diagonally to the greater tuberosity. These fibers were merged at the insertion. The infraspinatus tendon had vast insertion to the greater tuberosity, and the most anterior part of the tendon was inserted to the most anterior portion of the greater tuberosity, bordering on the most anterior part of the supraspinatus tendon. The supraspinatus tendon is regarded as the most affected tendon in rotator cuff tears. However, the results of this study suggested that the infraspinatus tendon could be involved in the majority of rotator cuff tears. The infraspinatus may act not only in external rotation but also in abduction, because the infraspinatus tendon was inserted to the most anterior part of the greater tuberosity.
