The sternalis is an anomalous superficial chest wall muscle running vertically on the anterior chest, parallel to the lateral sternum and overlying the pectoralis major. Present in approximately 4-8% of individuals (bilaterally in half), it is one of the most frequently encountered anatomical muscle variants. It varies widely in size, shape, and attachment.
| Origin | Variable: manubrium sterni, clavicular head of sternocleidomastoid, or pectoralis major fascia |
|---|---|
| Insertion | Rectus abdominis aponeurosis or costal cartilages of ribs 5-7; runs vertically along the lateral sternal margin |
| Nerve Supply | Intercostal nerves (T2-T6); anterior thoracic nerve branches |
| Blood Supply | Internal thoracic artery perforators |
| Actions | Uncertain; too thin and variable to have significant mechanical function; likely a vestigial muscle with no specific action in most individuals |
|---|
The sternalis muscle is a diagnostic pitfall on mammography and breast MRI, where its variable appearance can simulate a breast mass, asymmetric tissue, or anterior chest wall tumour. Recognition of its characteristic vertical orientation parallel to the sternum and its muscle signal on MRI prevents unnecessary biopsy. It is also encountered as an unexpected finding during breast reconstruction, mediastinal surgery, and cardiothoracic procedures. The sternalis is clinically silent in the vast majority of individuals in whom it is present.
A unilateral sternalis muscle overlying the medial breast on mammography produces a soft tissue opacity in the anterior chest wall that may be mistaken for an asymmetric density or superficial mass; correlation with MRI demonstrating a vertical muscle belly parallel to the sternum with normal muscle signal confirms the anatomical variant and prevents unnecessary intervention.
