Two Cases of Rectus Sternalis Muscle

Abstract

A unilateral rectus sternalis muscle was observed in a 80-year-old male cadaver. Located on the right side, the muscle took origin by a small fleshy belly from the lower part of pectoral fascia, at the level of 7 th costal cartilage. The fusiform muscle belly tapered into a thin delicate tendon at the level of 4 th costal cartilage, continued, for a short distance along right sternal margin, before crossing the sternum from right to left over the sternal angle and the left clavicle to merge with the sternal head of left sternocleidomastoid muscle [Table/ Fig-1 The sternalis muscle, an uncommon anatomical variant of the chest wall musculature, though perhaps well known to anatomists, is quite unfamiliar to clinicians and radiologists despite attempts to highlight its clinical importance in recent years. During routine dissection for undergraduate medical teaching, in the department of anatomy, we came across two cases of sternalis muscle. The first was a unique case of unilateral right sternalis with contralateral insertion on the left sternocleidomastoid, and the second case where bilateral presence of the muscle was noted with ipsilateral insertion. The former was supplied by medial pectoral nerve and the latter by lower intercostal nerves. Usually present between the pectoral and superficial fasciae, wide ranging prevalence and morphology have been attributed to this muscular variant. Opinions differ on its development and nerve supply. Its presence can be misdiagnosed as a wide range of benign and malignant anterior chest wall lesions and tumours, but it is also of great use as a pedicle flap or flap microvascular anastomosis in reconstructive surgeries of anterior chest wall, head and neck and breast. In this paper, two cases of sternalis muscle which presented very differently from each other are discussed. Case 2Bilateral sternalis muscle was observed in a 70-year-old male cadaver. Muscles on both sides, roughly fusiform in shape, had a fleshy origin from external oblique aponeurosis at the level of seventh costal cartilage. The muscles became tendinous near the sternal angle, continued upwards and crossed the clavicle, to insert into the sternal head of ipsilateral sternocleidomastoid. Both muscles were innervated by lower intercostal nerves [Table/ Fig-2].

A Right Sternalis Muscle: Clinical and Surgical Significance

“…The sternalis muscle found conforms to the morphological characteristics described by Jelev [2] for this muscle; it corresponds to the I1 type of their description (simple unilateral belly). Its format is similar to that reported by other authors in the literature: unilateral [3,[10][11][12][13][14], fusiform [14], in the right hemithorax, in a parasternal position and with the fibres parallel to the sternum [15] and superficial to the pectoralis major and its fascia, its fibres not being in continuity with fibres of any other adjacent muscle [13]. The bilateral sternalis muscle is more uncommon [8,11,16] and, in case it appears, it is usually asymmetric, with the right side better developed than the left one [17].…”

“…The most uncommon variation is a digastric sternalis muscle [18]. The presence of the muscle has been revealed during cadaveric dissection in medicine schools [10][11][12][13][14]16,[19][20][21][22][23] that are in the literature as case reports. Its presence is usually asymptomatic, although one symptomatic case with long-term pain in the parasternal region has been described [24].…”

“…For others, it has its origin in the rectus abdominis if the innervation is supplied by the intercostal nerves [2,3]. In any case, it seems clear that the innervation derives from the intercostal nerves, the pectoral nerves or both [13,14,19,26,31]. For Jelev [2], vascularisation was supplied by perforating branches of the internal thoracic artery; however, in our case it was not tried to prove vascularisation or innervation, probably damaged when separating the muscle on its lateral margins.…”

Exploring Anatomic Variants to Enhance Anatomy Teaching: Musculus Sternalis

“…Although it is often described as a “rare” variant [ 6 , 7 ], the appearance of M sternalis was documented as early as the 17th century [ 14 ], and its identification as a known anatomic variant is common through the early 20th century [ 10 , 13 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ], after which it tends to appear only chiefly in case reports of unusual anatomic features. This may be because M sternalis appears to be of little clinical significance except for those who might encounter it in medical imaging or surgical professions [ 11 , 12 , 13 , 28 , 29 ].…”

“…To complicate things more, Pillay et al report the innervation of a unilateral M sternalis via the medial pectoral nerve, but that of a bilateral M sternalis by intercostal nerves [ 27 ]. And Hung et al, also report finding innervation from both the intercostal and the medial pectoral nerves [ 22 ].…”