It is generally recognized that the prevalence of sternalis varies greatly in different races and populations [3, 5, 10]. The sternalis muscle is more often unilateral than bilateral, and it is more frequently on the right side than left side [1, 4, 10]. According to Snosek et al [10], sternalis muscle was classified into simple type, mixed type and other based on each hemithorax separately. Our case reported here fits into “other” category, which is absent from previous classifications.
It has been shown that the innervation of sternalis muscle is most likely to be external or internal thoracic nerves or the intercostal nerves with blood supplies from internal thoracic artery [9, 10]. In our case, the sternalis muscle located in the right thoracic region was innervated by the anterior cutaneous branches of the right intercostal nerve, while the fasciculus ascended with the left sternocleidomastoid muscle was innervated by the branches of left accessory nerve entering the left sternocleidomastoid muscle. The blood supply for thoracic part of sternalis came from internal thoracic artery (figure not shown).
Previously, it has been reported that sternalis may have a function of skin tension or chest wall movement in respiration [3, 8]. The sternalis with variation of sternocleidomastoid muscle found in our case may have extra function similar to sternocleidomastoid. Normally, the sternocleidomastoid muscle ascends obliquely across the side of the neck and attaches inferiorly consists of two heads, one from the sternum and one from the clavicle. In our case, SCMs have four heads of origin (three from the sternum and one from the clavicle). It is shown that the sternocleidomastoid muscle can have multiple attachments, and its variations become functionally, clinically, or surgically important only when the lower attachment or upper attachment shows variations [6]. During the dissection, the variation of cephalic vein and brachial plexus has also been found (figure not shown). Whether the variation of sternalis can be accompanied by other adjacent variations or is just partial of a larger group of variations is unknown. Microdissection technology is recommended for meticulous dissection and further exploration [8].
Because of the incomplete description of the sternalis in most anatomy textbooks, the absence of sternalis training or the deficiencies of sternalis detection technologies, many physicians, surgeons and radiologists have insufficient understanding or even no idea about the sternalis [1, 2, 3, 7, 10]. The existing of sternalis can affect clinical diagnosis (eg. abnormal ECG and misdiagnosis of imaging), clinical treatments (eg. radiation and surgery) [8, 9, 10]. In addition, for reconstruction surgery of the head, neck, anterior chest wall and breast, sternalis can also be used as muscle flap [4, 5, 7].