Sternalis muscle: an underestimated anterior chest wall anatomical variant

The sternalis muscle is an anatomical variant of the anterior thoracic region musculature well documented and familiar to anatomists but quite unknown among clinicians and radiologists [1]. It lies superficially and perpendicular to the pectoralis major muscle and parallel to the sternum. Many more terms have been used in the literature to describe sternalis muscle such as "parasternalis" and "rectus sterni" muscle [2,3]. The muscle usually arises from the upper sternum and the infraclavicular region and can display variable insertion points such as the pectoral fascia, lower ribs, costal cartilages, rectus abdominis muscle sheath or the abdominal external oblique muscle aponeurosis [4,5]. However, there is a great variation in height (4.8±1.97 cm), width (15.1±6.84 mm), and thickness (3±0.91 mm) [6]. Its presence ranges from a few short fibers to a well-formed muscle, found unilaterally or bilaterally. Variation also exists in the reported incidence among different populations ranging from 1% in Taiwanese to 18.2% in North Chinese [7,8]. According to Scott-Corner et al, it is more usual in females (8.7%) than in males (6.4%) [9].The early detection of its presence is critical in regular mammogram screening in order to avoid possible differential diagnostic dilemma. Additionally, there are potential surgical benefits, as it can be used as a flap in reconstruction surgery of the head and neck, anterior chest wall, and breast. We describe a rare observation of a bilateral sternalis muscle in a female cadaver, and we discuss about the muscle's possible embryological origin, innervation, and clinical significance.During an educational thoracoabdominal dissection of a female formalin-fixed cadaver, a long well defined vertical muscle was encountered on each hemithorax. Both muscles were strap-like, flattened, located parallel to the sternum in a paramedian position, and lying superficial to the pectoralis major muscle and the pectoral fascia (Figure 1). Specifically, on