Logan Steele
The sternalis muscle is an uncommon anatomical variant of the chest wall musculature. It is an accessory muscle that can originate from the upper sternum and the infraclavicular region, and it may be a variation of the pectoralis major or of the rectus abdominis. The sternalis muscle often runs along the anterior aspect of the body of the sternum and can display varying insertions such as the pectoral fascia, lower ribs, and costal cartilages. Its presence is estimated at around 7.8% in the general population, with a higher incidence of unilateral presentation and a slightly higher prevalence in females. The sternalis muscle is important in medicine as it can be mistaken for pathological lesions or growths, leading to clinical misdiagnoses.
| Characteristics | Values |
|---|---|
| Location | Anterior chest wall, in front of the sternal end of the pectoralis major |
| Embryological Origin | Unknown, possibly a derivative of the hypaxial myotomes/dermomyotomes or adjacent muscles such as the pectoralis major, sternocleidomastoid, or rectus abdominis |
| Prevalence | Approximately 7.8% in the general population, with higher prevalence in females and individuals of African or Asian descent |
| Presentation | Usually unilateral, with rare instances of bilateral presentation |
| Innervation | External or internal thoracic nerves (55% of cases), intercostal nerves (43% of cases), or both (remaining cases) |
| Function | Unknown, possibly a proprioceptive sensor for thoracic wall movements or involved in shoulder joint movement or chest wall elevation |
| Clinical Significance | Often mistaken for a benign or malignant growth, can alter electrocardiogram readings and confuse mammography |
| Morphology | Varies, can be classified as Type I, II, or III, or as triple-bellied/double-headed |
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What You'll Learn
Sternalis muscle origin and embryonic development
The sternalis muscle is an uncommon anatomic variant of the chest wall musculature. It often originates from the upper part of the sternum and can insert into the pectoral fascia, lower ribs, costal cartilages, rectus sheath, or aponeurosis of the abdominal external oblique muscle. It may be present unilaterally or bilaterally.
There is significant debate and uncertainty surrounding the embryonic origin of the sternalis muscle. Its embryological origin is unclear, and there are multiple theories attempting to explain it. One theory suggests that it is a derivative of the hypaxial myotomes or dermomyotomes, which are responsible for the development of the ventral and lateral body wall muscles of the thorax and abdomen.
Other theories propose that the sternalis muscle originates from adjacent muscles or their blastemas, such as the sternocleidomastoid, rectus abdominis, or panniculus carnosus muscle sheet. Some authors argue that it develops from the rectus abdominis sheath or the pectoralis major due to a defect in muscle patterning.
The varying innervation of the sternalis muscle is likely due to its individual topographical location, allowing it to attract fibres from different adjacent nerves during development. This muscle's innervation has been reported to be from the external or internal thoracic nerves in most cases, with some supplied by the intercostal nerves and a minority by both nerve types.
The sternalis muscle has been documented in anatomical literature since 1604, and its presence can impact medical imaging and surgical procedures. Its existence varies between different populations, with higher prevalence reported in individuals of African and Asian descent compared to Caucasian individuals.
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Sternalis muscle innervation
The sternalis muscle is an uncommon anatomical variant of the chest wall musculature. It is an accessory muscle that can originate from the upper sternum and the infraclavicular region. Its origin and insertion are variable, and it may be present unilaterally or bilaterally.
The sternalis muscle was first reported in 1604, but its exact embryological origin and developmental sequence remain unknown and highly debated. There are many theories about its origin, including that it is a derivative of the hypaxial myotomes/dermomyotomes from which the ventral and lateral body wall muscles of the thorax and abdomen are developed. It may also originate from adjacent muscles or their blastemas, such as the sternocleidomastoid, rectus abdominis, or panniculus carnosus muscle sheet.
The innervation of the sternalis muscle is also enigmatic. O'Neil and Folan-Curran reported that in most cases, the muscle was innervated by the external or internal thoracic nerves (55%) or the intercostal nerves (43%), with the remaining cases supplied by both nerves. This indicates that the sternalis muscle does not always derive from the same embryonic origin.
The presence of the sternalis muscle is asymptomatic, but it can cause aesthetic concerns due to chest asymmetry or deviation of the nipple-areola complex. It may also alter electrocardiogram readings and confuse routine mammography, leading to clinical misdiagnoses. Thus, it is essential for healthcare professionals to be aware of this unique muscle to ensure accurate diagnoses and minimise patient distress.
Cadaveric studies have shown that the sternalis muscle has a mean prevalence of around 7.8% in the population, with a higher incidence in females and individuals of African or Asian descent. The prevalence among subpopulations ranges from 0.5% to 23.5%, possibly due to variations in the definition of the sternalis muscle.
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Sternalis muscle prevalence
The sternalis muscle is an unusual variant muscle in the anterior chest wall, located superficially in the direction of the sternum. Its presence has been a topic of interest and discussion among anatomists and clinicians due to its variable prevalence and unique characteristics.
The prevalence of the sternalis muscle has been reported to vary significantly across different populations and ethnic groups. Several studies have been conducted to investigate the occurrence of this muscle, with varying results. One study examined 100 cadavers of both sexes and found that the sternalis muscle was present in 8% of the cases, with a higher prevalence in males (10%) compared to females (6%). Another study involving 400 cadavers reported a slightly higher prevalence, with the sternalis muscle observed in 11.5% of the specimens, again with a higher incidence in males (14.5%) than in females (8.5%).
Regional differences in the prevalence of the sternalis muscle have also been noted. A study conducted in the Indian population revealed a relatively higher prevalence, with the muscle present in 22.5% of the individuals examined. In contrast, a study in a Turkish population reported a lower prevalence, with the sternalis muscle found in only 3.7% of the cases. These variations could be attributed to genetic factors, environmental influences, or a combination of both.
The sternalis muscle has also been observed with varying prevalence rates across different ethnic groups. A study among African Americans, Caucasians, and Hispanics found the muscle in 11%, 14%, and 19% of the respective populations. These differences suggest that genetic factors may play a role in the development and expression of the sternalis muscle. Additionally, familial occurrence of the sternalis muscle has been reported, providing further evidence for a potential genetic influence.
While the sternalis muscle is typically considered a variant anatomy, its presence can have clinical implications. In some cases, the muscle may cause diagnostic confusion, mimicking other pathologies or altering the presentation of underlying structures. For example, the presence of a well-developed sternalis muscle can simulate a sternotomy scar, leading to potential misinterpretation during clinical examinations or imaging assessments.
In conclusion, the sternalis muscle prevalence varies significantly across different populations, ethnic groups, and regions. Its occurrence ranges from single-digit percentages in some studies to nearly a quarter of individuals in others. These variations highlight the importance of anatomical research and awareness among healthcare professionals. Understanding the prevalence and characteristics of the sternalis muscle can aid in accurate diagnosis, surgical planning, and the interpretation of imaging studies, ensuring optimal patient care.
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Sternalis muscle identification and imaging
The sternalis muscle is an uncommon anatomical variant of the chest wall musculature. It is often mistaken for a benign or malignant growth on the anterior chest wall, which can lead to clinical misdiagnoses. Therefore, it is important that healthcare professionals are aware of this unique muscle to ensure accurate diagnoses and patient care.
The sternalis muscle is typically discovered during cadaveric dissection or diagnostic imaging. It is found in around 7.8% of the population, with a higher incidence in females. The muscle is usually unilateral, found on the right side of the body, and is rarely bilateral.
The muscle can be identified by its location on top of the pectoralis major, with its origin and insertion variable. It often originates from the upper sternum and can insert onto the pectoral fascia, lower ribs, costal cartilages, rectus sheath, or aponeurosis of the abdominal external oblique muscle.
When it comes to imaging, the sternalis muscle is typically visible in the medial aspect of the breast on a craniocaudal mammogram, appearing as a small soft tissue mass. Its margins and shape are variable and it usually measures around 1-2 cm. Ultrasound, CT, or MRI imaging may be used for confirmation when the diagnosis is uncertain.
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Sternalis muscle function
The sternalis muscle is an uncommon anatomical variant of the chest wall musculature. It is present in around 7.8% of the population, with a higher incidence in females. The muscle is often discovered during cadaveric dissection or diagnostic imaging and typically presents as a unilateral band of muscular fibres on the right side of the body, with rare instances of left-side or bilateral muscle fibres. The embryological origin and function of the sternalis muscle are still debated, with many theories proposed.
Origin and Insertion
The origin and insertion of the sternalis muscle vary. It often originates from the upper part of the sternum and can insert onto various structures such as the pectoral fascia, lower ribs, costal cartilages, rectus sheath, and the aponeurosis of the abdominal external oblique muscle. On the right side, the sternalis muscle runs in a convex course and terminates at the right sternocostal arch. On the left side, it splits into two bellies and is inserted onto the left 10th-12th costal cartilages and costochondral junctions, terminating at the left sternocostal arch.
Embryological Origin
There are several theories regarding the embryological origin of the sternalis muscle. Some suggest that it arises as a derivative of adjacent muscles such as the pectoralis major, sternocleidomastoid, or rectus abdominis. Others propose that it may be similar to the panniculus carnosus, a ventral thoracic wall muscle found in lower vertebrates. It is also claimed to originate from the abdominal external oblique muscle or the ventrolateral part of the diaphragm. The disagreement about its embryological origin highlights the complex nature of this muscle.
Function
The exact function of the sternalis muscle is unknown, and it is considered an accessory or vestigial muscle without a known functional significance to the human body. However, several theories have been proposed regarding its potential function. One theory suggests that it may function as a proprioceptive sensor for thoracic wall movements. Another theory proposes that it may be involved in the movement of the shoulder joint or the elevation of the chest wall. While the muscle's presence is asymptomatic, it can cause aesthetic concerns, such as chest asymmetry or deviation of the nipple-areola complex. Additionally, the presence of the sternalis muscle can alter electrocardiogram readings and confuse routine mammography, impacting clinical diagnoses and patient care.
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Frequently asked questions
The sternalis muscle is an uncommon anatomical variant of the chest wall musculature. It is also known as the rectus sternalis or parasternalis.
There is no apparent physiological function of the sternalis muscle. However, there are several theories for its function, including that it may function as a proprioceptive sensor for thoracic wall movements or that it may take part in the movement of the shoulder joint.
The embryological origin of the sternalis muscle is still debated. It is theorised to be a derivative of the hypaxial myotomes/dermomyotomes from which the ventral and lateral body wall muscles of the thorax and abdomen are developed. It may also originate from adjacent muscles such as the sternocleidomastoid, rectus abdominis, or pectoralis major.
Cadaveric studies have shown that the sternalis muscle has a mean prevalence of around 7.8% in the general population, with a higher incidence in females. The unilateral sternalis muscle has been reported in 4.5% of subjects, while the bilateral manifestation is found in less than 1.7%.
