Journal of the Anatomical Society of India

CASE REPORT
Year
: 2021  |  Volume : 70  |  Issue : 2  |  Page : 113--115

Sternalis and transversus thoracis muscles: An anatomical variation and its clinical implicationss


Monica Adriana Vaida1, Nawwaf Sebastian Damen2, Adelina Maria Jianu1, Laura Grigorita1,  
1 Department of Anatomy and Embryology, “Victor Babes” University of Medicine and Pharmacy, Timisoara, Romania
2 Department of Pediatric Surgery, “Victor Babes” University of Medicine and Pharmacy, Timisoara, Romania

Correspondence Address:
Dr. Adelina Maria Jianu
Department of Anatomy and Embryology, “Victor Babes” University of Medicine and Pharmacy, 2 Eftimie Murgu Square, 300041 Timisoara
Romania

Abstract

An unusual muscle was discovered during the dissection of the presternal and pectoral regions, which, according to Jelev classification, can be considered a sternalis muscle. The identified right sternalis muscle has a common origin with the sternal head of the right sternocleidomastoid muscle and then splits in two bellies, the right one, much longer, which inserts on the right 2nd–5th costal cartilages, and the left one which inserts on the 2nd–3th left sternocostal joints. The sternalis muscle was associated with a very poor developed right transversus thoracis muscle. The study is important for the anatomists and more important for the clinicians, as this muscle's presence may cause diagnostic errors in the pectoral region.



How to cite this article:
Vaida MA, Damen NS, Jianu AM, Grigorita L. Sternalis and transversus thoracis muscles: An anatomical variation and its clinical implicationss.J Anat Soc India 2021;70:113-115


How to cite this URL:
Vaida MA, Damen NS, Jianu AM, Grigorita L. Sternalis and transversus thoracis muscles: An anatomical variation and its clinical implicationss. J Anat Soc India [serial online] 2021 [cited 2021 Sep 19 ];70:113-115
Available from: https://www.jasi.org.in/text.asp?2021/70/2/113/320283


Full Text



 Introduction



In literature, the sternalis muscle was reported as a uni- or bilateral parasternal muscle, located between the superficial fascia of the anterior thoracic wall and the pectoral fascia. The origin is on the manubrium of the sternum, sternocleidomastoid fascia, pectoralis major, the upper ribs, and their costal cartilages and insertion on the lower ribs and their costal cartilages, the rectus abdominis sheath, and the aponeurosis of external oblique muscle.[1] Transversus thoracis also called sternocostalis or triangularis sternae,[2] was classically described as four to five slips, which spreads on the internal surface of the anterior thoracic wall. Its origin is on the xiphoid process, inferior part of the sternum, and the adjacent costal cartilages and insertion on the 2nd–6th costal cartilages.[3]

According to Jelev et al., in 44.2% of cases, the transversus thoracis muscle was asymmetrical.[3] Sternalis muscle may occur as an isolated supernumerary muscle or associated with other anterior thoracic wall unusual muscles, such as the chondroepitrochlearis muscle, pectoralis quartus, and pectoralis minimus. To the best of our knowledge, the association between sternalis and the transversus thoracis muscle anomaly have not been reported before.

Most of the studies regarding sternalis muscle were anatomical studies performed by cadaveric dissection; that's why such an accessory muscle's clinical significance is controversial. The sternalis muscle was frequently discovered on magnetic resonance or computed tomography imaging, showing its importance among surgeons because it may cause diagnostic errors in the pectoral region[4] and may be used in breast reconstruction.[5]

 Case Report



The purpose of the present study is to describe a rare anatomical variant of the sternalis muscle, associate with a variation of the transversus thoracis muscle, significant for anatomists, but much more important for clinicians.

During the routine dissection studies for medical students in the presternal and pectoral regions', a unilateral variant of the sternalis muscle was found located on the right side in a 60-year-old male formalin-fixed cadaver. This muscle takes origin on the sternum's manubrium's anterior surface by a common tendon with the sternal head of the sternocleidomastoid muscle. It then splits into two flat bundles, right and left, their direction being perpendicular to the major pectoralis muscle. Pectoralis major muscles were normally placed. The right bundle measured 8 cm in length, and 1.4 cm at its broadest part, runs craniocaudally, parasternal, and inserts on the costal cartilages at the 2nd–5th rib level. The left flatband, measured 2 cm in length, had a craniocaudal trajectory on the sternum's anterior surface, across the midline, becoming parasternal and inserted on the 2nd–3th left sternocostal joints [Figure 1].{Figure 1}

At the same time, the dissection revealed a very poorly developed transversus thoracis muscle on the right side and a very well-developed transversus thoracis muscle on the left side [Figure 2].{Figure 2}

A drawing was made to reveal the present case [Figure 3].{Figure 3}

 Discussion



Cabrolius first reported the sternalis muscle in 1604,[6] when he found a longitudinal band-like muscle situated superficial to the sternum. Because of its relations, the muscle was called “parasternalis” or “rectus sterni muscle,” “sternalis brutorum” or “episternalis,” “rectus thoracicus superficialis,” “rectus sternalis,” or “mystery” muscle.[1],[7],[8]

Many authors investigated the sternalis muscle in different populations and appeared to be present in 8% of the cases.[9] Due to its peculiar morphological position, it has produced a literature of its own.[10] Its nerve supply is variable, by the pectoral nerves or by the intercostal nerves.[1] Although its function is still unknown, most of the authors agreed that it might participate in the chest and shoulder joint movements, such as elevating the lower part of the chest and acting as an accessory inspiratory muscle.[1] The transversus thoracis represents the primary muscle of breathing in dogs, but in humans is an accessory muscle for respiration, supporting the active expiration by drawing down the costal cartilages.[2] The intercostals nerves innervate it.

According to Huntington,[10] the sternalis is always innervated by the anterior thoracic nerves, and hence neurologically, it belongs to the pectoralis group muscles. He reported three hypotheses regarding the derivation of the sternalis muscle: Turner's hypotheses describing the sternalis as a reversional persistent remnant of the thoracic cuticular muscle; Cunnigham's hypotheses describing the sternalis as a displaced and rotated segment of the pectoralis major muscle; Eisler's hypotheses describing the direct derivation of the sternalis from the pectoralis major due to an atypical widening of the ventral parts of intercostal spaces. Huntington[10] classified the sternalis in three types: well-developed sternalis coexisting with pectoralis major anomalies; a slender form of the sternalis muscle with normal pectoralis major; sternalis muscle which presents a direct connection with pectoralis major. According to Sadler,[11] the myoblasts from thoracic and abdominal hypomeres split into three layers and a ventral longitudinal column. The thorax's three layers consist of external intercostal, internal intercostal, and transversus thoracis muscle, and in the abdomen of internal oblique, external oblique, and transversus abdominis muscles. The longitudinal column is represented by the rectus abdominis muscle in the abdominal region, infrahyoid muscles in the cervical region, and disappears in the thoracic region or it may be found as the sternalis muscle. The embryological origin of the transversus thoracis muscle and the sternalis muscle associated with a very poorly developed transversus thoracis muscle may explain sternalis muscle presence like a “compensatory” muscle for the transversus thoracis.

According to the Jelev et al. classification,[1] the muscle that we found is located between the superficial fascia and pectoral fascia of the anterior thoracic region, originates on the sternum and inserts on the costal cartilages. Therefore, it can be considered sternalis muscle, Type II2 (one muscle and two simple asymmetric bellies). A well-developed muscular belly, like the right belly, in this case, associated with an anomaly of the right transversus thoracis muscle, can be regarded as a “symptomatic” sternalis muscle.

 Conclusions



We report a variant of sternalis muscle, associate with a variation of the transversus thoracis muscle, important for anatomists, but much more significant for clinicians. Its presence may interfere with the proper positioning of the breast implants, cause changes in the electrocardiogram and pain in the pectoral region, or misdiagnosed as a tumor or a lesion in the anterior chest wall.

Acknowledgment

The authors would like to thank Mihaela Damen who made a drawings of the present case.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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