Teres Intermedius: An Accessory Rotator Cuff Muscle Discovered During Routine Cadaveric Dissection

Abstract

The teres minor is a unipennate intrinsic shoulder muscle that arises on the lateral border of the scapula and runs obliquely to insert on the greater tubercle of the humerus. It stabilizes and contributes to external rotation of the glenohumeral joint. Teres major, while resembling the teres minor in origin and fiber direction, ultimately diverges from the teres minor as it passes anterior to the humerus and acts predominantly as a medial rotator and adductor of the shoulder. Described variants in the teres musculature are rare. We found a unilateral accessory muscle in the teres region of the right shoulder in an 81‐year‐old Caucasian male cadaver in a traditional anatomy lab. The accessory muscle arises from the lateral aspect of the scapula and travels parallel and posterior to the teres minor to insert onto the posterior head of the deltoid. The variant muscle is of similar size to the teres minor (also present), but its fibers are encapsulated in their own distinct fascial sheath, entirely separate from that of the anatomically normal teres muscles. We provide a detailed description of the accessory muscle and propose that similar variants may be clinically important due to their proximity to the anatomical quadrangular space, as compression of the structures traversing this area may result in pain, tenderness, paresthesia, and cyanosis in the shoulder region.