The sternalis muscle

Do you have a sternalis muscle? Or maybe even two? Frequently bilateral, the sternalis muscle is slender band of fibers running parallel to the sternum. It lies superficial to the pectoral fascia, arises from the sheath of the rectus abdominis muscle or adjacent structures and terminates upon the pectoral fascia, upper sternum, costal cartilages, or the sternal or medial heads of the sternocleidomastoid muscle (Huntington et al., 1905; Bergman et al., 1992). The innervation is generally from the pectoral nerves (Kida and Kudoh, 1991; Kida et al., 2000), although isolated cases of innervation from the intercostal nerves have been documented (O’Neill and Folan-Curran, 1999). Anatomists tell us that if you are a woman, you are slightly more apt to have one or two sternalis muscles than if you are male (8.7% of females, compared with 6.4% of males) (Bergman et al., 1992). Although the incidence varies with ethnicity, we might expect to find this slender muscle in anywhere from 4–11% of our patients. In some populations, for example the Taiwanese, the incidence may be as low as 1% (Shen et al., 1992; Jeng and Su, 1998). When the sternalis accompanies partial or complete congenital absence of the pectoralis major muscle, it may achieve significant size (Kitamura et al., 1985). Yet clinicians are surprisingly unaware of this common variation. When one of the authors of this vignette (ASA-J) encountered this muscle during performance of a modified radical mastectomy (Fig. 1), it was the first such example either of us had observed in many years of surgery in this anatomic region. We are not alone in our ignorance; Bailey et al. (1999) surveyed 65 physicians and medical students and documented “near universal lack of familiarity” with the sternalis. They further commented that the paucity of clinical encounters appeared incongruous with the reported incidence derived from cadaver studies. Certainly it is easy to overlook this slender muscle (Fig. 2), which varies from a few slips of muscle fibers or a simple aponeurotic structure to a well-developed band that may be visualized on mammography or other radiologic studies. It has been hypothesized that the sternalis is a vestige of the panniculus carnosus. It appears to have little functional significance, except when it enlarges to compensate for the absence of pectoralis major. And what difference does it make, anyhow? As radiologic techniques have become more precise, the sternalis is occasionally imaged on mammography, magnetic resonance imaging, or computerized tomographic scans. Four cases were discovered among approximately 32,000 mammograms, stimulated by an index case who underwent open surgical biopsy for a presumed breast mass, actually a sternalis muscle (Bradley et al., 1996). Current mammographic practice emphasizes patient positioning to maximize radiographic access to all breast tissue, including the medial portion of the breast, formerly considered a blind area. With these techniques the normal pectoralis major muscle is seen on craniocaudal views in at least 30% of cases and it is anticipated that the sternalis muscle will be noted more commonly than in years past (Bradley et al., 1996; Murphy and Nokes, 1996). Follow-up with magnetic resonance imaging or computerized tomographic scans may be necessary to differentiate this normal variant from a breast tumor. Thus it may be that it is as a shadow upon an X-ray film this slender and underappreciated muscle is most