Sternalis Muscle: A Cadaveric Case Report of a Rare Variant With Multiple Branching

The sternalis muscle is a rare anatomical variant located in the anterior thoracic wall. Understanding variations in the sternalis muscle anatomy is essential for clinicians, especially radiologists and surgeons to prevent misdiagnosis and avoid complications during surgical procedures in the anterior thoracic region. We present a unique case of bilateral branched sternalis muscles. On either side, the sternalis muscle lies deep to the breast and superficial fascia and superficial to pectoralis major muscle and pectoral fascia. Each sternalis muscle is branched into medial and lateral slips with the medial slip larger than the lateral slip. The medial slip of the right sternalis was larger than the medial slip of the left sternalis. The lateral slip of the left sternalis was larger than the lateral slip of the right sternalis. The lateral slip of the left sternalis muscle has a curved course with superior-lateral convexity and inferomedial concavity. Near its middle, the left sternalis lateral slip is branched into two smaller slips separated by a narrow cleft. The left sternalis muscle in our report is a new variant with multiple branching, which cannot be matched to any type of the previously described classifications. In this case report, we discussed the need of modification of the currently available sternalis classification system to accommodate all types of the previously reported sternalis muscles including the branching pattern of this muscle.