Variation of Sternalis Muscle: A Case Report

Purpose: The sternalis muscle was first documented in the 19th century and since has been shown to have variations in its origins, insertions, direction, extra muscle slips, and nerve innervations. This case report describes anomalous features not previously reported in the medical literature. Methods: During routine gross anatomy dissection of the anterior chest wall of an 100-year old woman who died of “natural causes” at UNTHSC, obvious anomalies in the bilateral appearance of the sternalis muscle were observed and documented. Results: The left sternalis originated from the clavicular origin of the left sternocleidomastoid (SCM) muscle, while the right emerged from the sternal origin of the right SCM muscle. Both the left and right original tendons of the sternalis’ were separated but wrapped in a tight connective tissue band. The fleshy muscle bellies of each sternalis travelled down inferolaterally to insert on the aponeurosis of the left and right external oblique muscles, respectively. The innervation to the muscle could be traced to the 4th, 5th, and 6th anterior intercostal nerves. Conclusions: Several anomalies found here have not been previously reported in the literature including (1) the sternalis muscles originating from the clavicular head of the SCM muscle, (2) the tendinous band suspended over and slightly left of the sternal angle with only a loose connection to the underlying sternal angle, and (3) the inferolateral and nearly symmetrical 30 degree angle traveled by the sternalis to the sternum. Awareness of the location of the sternalis will help radiologists and reconstructive surgeons avoid misdiagnosis during mammography or misjudgment during breast surgery. Because of its superior attachment to the sternocleidomastoid muscle, therapists may need to be aware that a person with such an anomaly may have an automatic accessory inspiration with head rotation.