sternalis

The sternalis, or sternalis muscle, is an anatomical variant of the anterior chest wall found in a minority of humans. It typically runs parallel to the sternum, usually lying superficial to the pectoralis major, and may appear as a vertical or oblique muscular slip along the midsagittal chest.

Origin and insertion of the sternalis are highly variable. It may arise from the fascia over the pectoralis major, from upper costal cartilages, or from the rectus sheath, and it commonly inserts into the sternum near the manubrium or xiphoid process, or into the proximal portion of the pectoralis major. Its course and attachments can differ between individuals and sides.

Prevalence and distribution reports vary, but estimates typically place the sternalis in about 8 to 15 percent of people, with unilateral forms more frequent than bilateral ones; left-sided variants are commonly described in the literature.

Developmentally, the sternalis is regarded as a vestigial or atavistic muscle, arising from the ventral thoracic musculature or the panniculus carnosus. In most individuals this muscle regresses, but it persists in a minority, leading to its presentation as a distinct muscular slip on imaging or examination.

Clinical significance is generally limited, as the sternalis is usually asymptomatic. Nonetheless, its presence can complicate imaging interpretation by mimicking a soft-tissue mass on mammography, CT, or MRI, and awareness of this variant is important for radiologists and surgeons performing procedures in the chest wall or breast region.