17 - Identification of a Rare Pectoral Muscle Variation in a Cadaveric Specimen and Its Potential Clinical Relevance

Poster Board Number: 17
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Abstract Body : The pectoral region is a common site of muscular, nervous, and arterial anatomical variations. One muscular variation, the Thoracobrachialis muscle, has been observed infrequently with an incidence rate of 0.5-2% since its initial discovery by Duvernoy in 1955. There exist several variations in the anatomy of the muscle itself, leading to variable nomenclature including Thoracobrachialis and Chondroepitrochlearis. Descriptions of this muscle and its tendonous attachment have previously arisen from cadaveric and clinical case reports. In this report, we describe an incidence of the Thoracobrachialis muscle, its tendonous attachment, and implications for median and ulnar nerve entrapment in the brachial region.

Routine dissection of the brachial region of an 86-year-old female cadaver was performed. Upon discovery of the anomalous Thoracobrachialis muscle, all cadavers in the dissection laboratory (n=38) were inspected for the presence of the muscle.

The unilateral Thoracobrachialis muscle and its tendonous attachment were discovered in the left pectoral and brachial regions of the 86-year-old female cadaver. No other cadavers in the laboratory possessed the muscle. The muscle originated from the Pectoralis major muscle and emerged as an independent muscular slip that inserted into a tendon running from the lateral lip of the intertubercular groove of the humerus to the medial epicondyle of the humerus. Additional fibers from the tendon blended into the medial intermuscular septum at the medial epicondyle. The tendon was located deep to the brachial fascia and crossed diagonally and anterior to the Biceps brachii muscle and median nerve. At its attachment to the medial epicondyle, the tendon closely approximated the ulnar nerve at the cubital tunnel. The Thoracobrachialis was innervated by the lateral pectoral nerve and appeared to receive blood supply from the lateral thoracic artery. Upon further inspection of the pectoral region, no additional muscular, arterial, or nervous anomalies were observed.

This report summarizes the anatomy of an anomalous muscular slip in the pectoral region and its associated tendonous attachment. The appearance of only one unilateral Thoracobrachialis muscle in a sample size of 38 cadavers supports previous reports of an extremely low incidence rate.

Several case reports have described clinical outcomes in which the tendon of Thoracobrachialis led to median nerve entrapment. Due to the proximity of the tendon to the ulnar nerve in this case, it is also important to consider the potential for ulnar nerve entrapment in the cubital tunnel. While there have been several reports of the Thoracobrachialis muscle over the years, the muscle is highly variable, and thus, descriptions of these variations are necessary for clinical and surgical reference. Additionally, the extraneous tendon of Thoracobrachialis may be used as a tendon graft for procedures such as ulnar collateral ligament or rotator cuff tendon repair.

The authors would like to thank the donor whose generous offering made this discovery possible. Additionally, we would like to thank the Pennsylvania State University College of Medicine for providing the laboratory and tools needed for cadaveric dissection. The physician assistant students who performed the initial dissection are Ethan Lee, Rachel Hayes, and Ella Gourley. Identification efforts were furthered aided by Dr. Kimberly Harbaugh, Chief of Peripheral Neurosurgery at Penn State Milton S. Hershey Medical Center.