Bilateral sternalis muscle variation with an asymmetric X-shaped pattern: a case report

The sternalis muscle is a rare anatomical variant of the anterior thoracic wall musculature with clinical implications for diagnosis and surgery. However, bilateral sternalis variations with an asymmetric X-shaped crossing have rarely been documented. This study aimed to supplement anatomical data and provide a clinical reference by reporting such a unique case.

During routine anatomical dissection for medical education, a bilateral sternalis muscle variation was identified in a 70-year-old female cadaver. The muscle's origin, insertion, morphology, and anatomical relationships were observed, photographed, and subjected to precise morphometric measurements.

The bilateral sternalis muscles exhibited asymmetric morphology and formed an X-shaped crossing superiorly with the sternocleidomastoid muscles. The right muscle originated from the 6th-7th costal arches, inserted near the sternoclavicular joint, and measured 8.56 cm (lateral margin) and 6.27 cm (medial margin) in length. The left muscle arose from the 7th rib and inserted lateral to the 5th sternocostal junction, with a lateral margin length of 4.43 cm and a medial margin length of 4.14 cm. The right muscle was significantly larger than the left.

This bilateral asymmetric X-shaped sternalis variation partially conforms to the Jelev II2 classification but has distinct inferior attachment patterns. Clinically, it may mimic breast masses, interfere with electrocardiographic (ECG) readings, or complicate thoracic and breast surgery. Enhanced recognition of this variant through preoperative imaging is crucial to avoid misdiagnosis and optimize clinical outcomes.