The Sternalis Muscle: A Case Report

Background: The sternalis muscle is considered to be a rare anatomical variation found in about 5-6% of cases. First discovered in the early 1600s, the sternalis muscle has been a controversial topic due to variations that exist in its morphology, innervation and vascular supply. Method: During a routine dissection of a 73-year-old (Male) cadaver, a single, large sternalis muscle was discovered on the left thoracic wall. Nine other donors were dissected (n=10) to look for the presence of other sternalis muscle variants. Results: Out of ten cadavers, one sternalis muscle was identified. The sternalis muscle was located on the left thoracic wall. The proximal tendon was a continuation of the sternocleidomastoid, with the medial and lateral borders of the muscle attaching to the pectoral fascia. At the superior border of the 4th rib, the medial aspect of the muscle has a firm attachment to the gladiolus. Laterally the muscle attaches firmly to the 5th rib. Both aspects of the muscle attach inferiorly to the 6th rib via a diffuse tendon and blend in with surrounding deep fascia. Muscle fibers are vertical in orientation. Conclusion: The sternalis muscle is clinically relevant and should be considered by radiologists and surgeons when dealing with the thoracic wall. The sternalis muscle can be mistaken for a breast nodule on mammography, which can lead to unnecessary testing if clinicians are unaware of its existence.