A novel cadaveric study of the morphometry of the serratus anterior muscle: one part, two parts, three parts, four?

The serratus anterior is portrayed as a homogeneous muscle in textbooks and during functional activities and rehabilitation exercises. It is unclear whether the serratus anterior is composed of subdivisions with distinctive morphology and functions. The purpose of this study was to determine whether the serratus anterior could be subdivided into different structural parts on the basis of its segmental architectural parameters. Eight formalin-embalmed serratus anterior muscles were dissected and the attachments of each fascicle documented. Orientation and size of each fascicle were measured and the physiological cross-sectional area (PCSA) calculated. Three subdivisions of the serratus anterior were identified. A new finding was the discovery of two distinctive fascicles attached to the superior and inferior aspects of rib 2. The rib 2 inferior fascicle had the largest PCSA (mean 1.6 cm²) and attached, with the rib 3 fascicle, along the medial border of the scapula to form the middle division. The rib 2 superior and rib 1 fascicles attached to the superior angle of the scapula (upper division). Fascicles from ribs 4–8/9 attached to the inferior angle of the scapula (lower division). Mean fascicle angle relative to a vertical midline reference and PCSA for each division were 29° and 1.3 cm² (upper), 90° and 2.2 cm² (middle) and 59° and 3.0 cm² (lower). This novel study demonstrated the presence of morphologically distinct serratus anterior subdivisions. The results of this study will inform the development of optimal techniques for the assessment, treatment and rehabilitation of this architecturally complex muscle in shoulder and neck pain.