Supernumerary insertions of the sternocleidomastoid muscle: a rare anatomical variation with clinical significance

BACKGROUND: The sternocleidomastoid (SCM) muscle is a prominent feature of the neck related to significant neurovascular structures with major clinical relevance in head and neck surgery. Variations of its anatomy and course have been widely documented; however, anatomical variants of its insertion are considered a rare occurrence. The aim of this study is to present a considerably uncommon anatomical variation of the insertion of the sternocleidomastoid muscle, discuss the clinical implications of its presence and suggest a possible role in the treatment of facial palsy. CASE REPORT: We present a rare case of bilateral supernumerary insertions of the sternocleidomastoid muscle that we came across during routine dissection on a male cadaver of a Caucasian donor to the Department of Anatomy. Specifically, the right sternocleidomastoid ascended as a single muscle belly that gave rise to six distinct tendons which inserted into the superior nuchal line of the occiput, while the left sternocleidomastoid muscle formed four distinct tendons prior to insertion. CONCLUSIONS: Morphological variations of the sternocleidomastoid muscle are relevant for clinical, surgical and radiological approaches of the neck. To our knowledge, the present case is the second reported insertional variation of the sternocleidomastoid muscle with supernumerary tendons. Its potential role in the treatment of facial palsy as a split muscle flap is also discussed and confirmation in the setting of a clinical scenario is encouraged.