26 - Presence of Bilateral Rectus Sternalis Muscles in an 88 Year-Old White Female Donor

Poster Board Number: 26
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Abstract Body : INTRODUCTION & OBJECTIVE
The rectus sternalis or sternalis muscle is a vestigial muscle which lies just deep to the fascia of the anterior thoracic wall, superficial to the pectoralis major muscle. The sternalis muscle is generally described as originating proximally and inserting distally along the sternum, with a great deal of variability in the specific sites of both origin and insertion. It was first described by French surgeon Barthelemy Cabrol in the 16th century, with multiple anatomical variations reported over the past 600 years. Cadaveric studies estimate the prevalence of the rectus sternalis muscle as 7.8% in the general population, with unilateral presentation being more common. As imaging modalities and thoracic procedures have advanced, the prevalence and variations of the sternalis muscle have become more relevant.

MATERIALS AND METHODS
This case report involves bilateral sternalis muscles in an 88-year-old White female donor dissected during the fall of 2022 at the Uniformed Services University of the Health Sciences. In the course of the anterior chest dissection, we identified bilateral accessory muscles deep to the superficial fascia layer, but superficial to the pectoralis major muscle. Initial dissection was performed by medical students assigned to the donor, with anatomical instructor assistance after the variant was recognized.

RESULTS
Rectus sternalis muscle bellies were present bilaterally, with the right-sided sternalis muscle presenting significantly larger than the left (2.7 cm wide versus 1.3 cm). We hypothesized that the left sternalis muscle had been damaged during an antemortem sternotomy. Both muscle bellies arose as a continuation of the sternal head of each sternocleidomastoid muscle. The right-sided muscle belly inserted on the surface of ribs four through ten, as well as the fascia of the rectus abdominis muscle. Its fibers were oriented laterally. Conversely, the left-sided muscle belly inserted on the inferior portion of the sternum with vertically oriented fibers. No contributions from the pectoral nerves or pectoral branches of the thoracoacromial artery were noted on either side.

CONCLUSIONS
Given the location of these muscle bellies between the superficial fascia layer and the pectoralis major muscle, the origin and insertion sites, and the fiber orientation, it is reasonable to conclude that these findings represent a bilateral rectus sternalis or sternalis muscle.

SIGNIFICANCE
Although the rectus sternalis muscle has been well-known to anatomists for hundreds of years, further description and characterization is needed to bring awareness to clinicians. Additionally, improved understanding of the prevalence and epidemiology of this anatomical variant is needed to expand clinical applications. The rectus sternalis muscle has been known to interfere with mammography screening, contributing to false positives and leading to unnecessary patient stress and procedures, including biopsies. While the presence of the sternalis muscle variant may lead to confusion and hindrance when encountered unexpectedly, its presence may be used for flap reconstruction procedures and contribute to improved sling formation in breast reconstruction surgeries. Improving understanding of this anatomic variant, its innervation and blood supply can help to reduce confusion during imaging interpretation and allow surgeons to consider the rectus sternalis muscle for procedural applications.