A bilateral cervical accessory biceps muscle: anatomical description and clinical implications

PURPOSE: Accessory muscles between the levator scapulae and serratus posterior superior are rarely reported and may be interpreted as variants of either adjacent muscle. This study describes bilateral accessory muscles in the posterior cervical and upper thoracic region and discusses their anatomical classification and clinical relevance. METHODS: A routine anatomical dissection was performed in a formalin-fixed adult male cadaver. The posterior cervical and upper thoracic regions were dissected to define the topography, attachments, morphology, distal interface, and innervation of the accessory muscles. Morphometric measurements were obtained bilaterally using a caliper. RESULTS: Bilateral elongated muscular structures were identified between the levator scapulae and serratus posterior superior muscles. Each muscle had an individual proximal attachment at the transverse process of the second cervical vertebra, followed an inferomedial course approximately parallel to the splenius capitis, and remained distinct from the main belly of the levator scapulae. Distally, each muscle was separated from the serratus posterior superior by a bursal interface, without direct muscular or aponeurotic continuity or independent scapular insertion. Both muscles were bicipital and fusiform and were innervated by posterior rami that also supplied the splenius capitis. CONCLUSION: These findings support the interpretation of the structures as paired individualized accessory muscles rather than accessory slips of adjacent muscles, expanding the known spectrum of posterior cervical and upper thoracic anatomical variations.