A new classification system for the infrahyoid muscles according to their morphology and innervation

Background: The aim is to reveal the basic morphology of the infrahyoid muscles by proposing a new classification system. Materials and methods: Data were analyzed from 67 sides from 34 Japanese cadavers. We propose a new classification system for the infrahyoid muscles, dividing them into 12 areas: superficial (S) and deep (D), cranial (1, 3, 5) and caudal (2, 4, 6), lateral (1, 2), intermediate (3, 4) and medial (5, 6). This system integrates morphology and innervation pattern of these muscles more accurately. In the superficial layer, top of the intermediate tendon of the omohyoid muscle serves as the boundary between the cranial and caudal parts. In the deep layer, the thyroid cartilage represents the boundary. Additionally, innervation by the cranial and caudal branches of the ansa cervicalis is a required criterion for classification. Results: The incidences for six areas — S1, S2, S5, S6, D3, and D4 — exceeded 99%, with this pattern observed in 71.6% of cases, suggesting that these may represent standard anatomical patterns. In contrast, areas such as S3, S4, D1, D2, D5, and D6 showed lower incidences. Notably, three novel anatomical variations were observed and described in detail. Conclusions: The proposed classification system allows for a more comprehensive understanding of the basic morphology of the infrahyoid muscle variation. Clinically, this is relevant for surgical procedures involving the infrahyoid area, such as flap reconstruction, where unexpected variations may occur surgical risks. Therefore, the proposed classification system offers anatomical and surgical framework of the infrahyoid muscles.