An unusual bilateral sternalis muscle

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An unusual bilateral sternalis muscle

K. Natsis, K. Vlasis, T. Totlis, G. Paraskevas, P. Tsikaras
Cazuri clinice, no. 2, 2008
* Department of Anatomy, Medical School, Aristotle University of Thessaloniki, Greece
* Department of Anatomy

The sternalis is an accessory muscle which appears with variable frequency among the different ethnic groups and ranges from less than 0.5% in the Taiwanese population to 22% in the Chinese population (1). In the literature, there is a controversy about the innervation, origin and incidence of this variation. In the current study, we present a bilateral sternalis muscle with unusual right-sided presentation and we discuss the controversial characteristics of this variation as well as its clinical relevance.

Case report
The bilateral sternalis muscle was found during routine dissection of a 64 years-old Caucasian male cadaver (fig. 1). At the right side, there were three almost parallel strap-like muscle slips at the place of sternalis muscle. Each slip arised with a separate aponeurosis from the sheath of the rectus abdominis muscle, along the 6th rib. After an arched course superiorly and medially, the medial slip inserted aponeurotic into the 3rd costal cartilage and the adjacent part of the body of sternum, the middle slip inserted aponeurotic into the 2nd costal cartilage and the lateral slip inserted aponeurotic into the pectoral fascia at the level of the 3rd rib’s inferior border. A small branch of the 3rd intercostal nerve’s anterior cutaneous branch was found to innervate the medial slip of the muscle. We did not find the nerve branches for the two other slips. At the left side, the sternalis muscle consisted of a single but broad muscle band, which arised aponeurotic from the sheath of the rectus abdominis muscle, along the 7th rib, and it inserted with a trigonal tendon into the 3rd costal cartilage and the adjacent part of the body of sternum. As a result, the left sternalis muscle and the medial band of the right sternalis muscle formed a musculotendinous arch. A small branch of the 4th intercostal nerve’s anterior cutaneous branch was found to innervate the left sternalis muscle.
Figure 1
In the literature the sternalis muscle is described as a fleshy band of longitudinal fibers, of varying length and width (2). The sternalis muscle that we observed at the right side of the cadaver had an unusual morphology, namely it consisted of three almost parallel, arched and strap-like muscle slips.
Sternalis muscle’s innervation and origin are under controversy. O’Neil and Folan-Curran (1998) made an examination of the innervation patterns and concluded that the sternalis muscle is either pectoralis major derived with an innervation from the pectoral nerves or rectus abdominis derived with an innervation from the intercostal nerves (3). In our case, branches of the intercostal nerves innervated the sternalis muscle bilaterally and thus it was derived from the rectus abdominis muscle.
The sternalis muscle seems to have little functional significance, except when it enlarges to compensate for the absence of pectoralis major muscle (4). However, this muscle is occasionally imaged on craniocaudal projection of mammography as a pseudotumor, which may be confused with a breast tumor (5, 6). Awareness of the sternalis muscle and its early identification is imperative in order to proceed in an appropriate plane during mastectomy (7). A subcutaneous, craniocaudally oriented muscle slip is very confusing if the surgeon is not aware of its identify (8). Additionally, the sternalis muscle has been used in reconstructive surgery (9).
The three muscle slips which we found at the right side of the cadaver, in the place of sternalis muscle, prove that sternalis morphology may vary a lot. This knowledge is important not only to anatomists, but also to radiologists and surgeons, during interpretation of mammographies and breast surgery, respectively.

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