Unilateral Ectopic Insertion of the Pectoralis Minor: Clinical and Functional Significance

During routine dissection of a 70-year old female we observed a unilateral ectopic insertion of the left pectoralis minor muscle. The tendon was cord-like and passed through a tendon sheath superior to the coracoid process to insert on the greater tubercle of the humerus. Additionally, an aponeurosis extended from the distal aspect of the muscle's tendon and passed medially to insert near the base of the coracoid process. This is the first report of an additional aponeurosis extending from the tendon of the pectoralis minor and attaching to the coracoid process. We also observed that the pectoralis minor tendon caused an unusually smooth deep indentation on the superior aspect of the coracoid process; considering its insertion on the humerus we hypothesize that the muscle acted as an internal rotator as well as an abductor of the shoulder along with the supraspinatus. Similar to the action of the supraspinatus, this aberrant muscle may have also helped prevent the downward dislocation of the humerus. Pectoralis minor variations have been described since 1897, however few studies have demonstrated functional or clinical significance. The redundancy of the actions of this muscle along with its long tendon suggest a potential source for autograft.