Bilateral subclavius posticus innervated by the lateral pectoral nerve: a cadaveric and ultrasound case report

Purpose: The subclavius posticus is a well-known variation of the subclavius muscle. This study aims to describe the morphology, innervation, and anatomical relationships of a bilateral subclavius posticus identified in a cadaver, to demonstrate its detection by high-resolution ultrasonography, and to discuss its anatomical and clinical implications. Methods: Routine dissection of a soft-embalmed adult cadaver, a 54-year-old donor who died from rupture of a cerebral aneurysm, was performed. The infraclavicular and thoracic outlet regions were examined bilaterally. Morphometric data, muscle attachments, and neurovascular relationships were recorded. High-resolution ultrasonography was used to evaluate the contralateral side prior to dissection. Innervation and venous drainage patterns were identified through careful tracing of peripheral nerves and associated vessels. Results: An accessory muscle consistent with the subclavius posticus was identified bilaterally. On both sides, the muscle originated near the junction of the first rib and costal cartilage and inserted onto the medial aspect of the coracoid process. Ultrasound successfully detected the left-sided muscle prior to dissection. Notably, both muscles were innervated by the lateral pectoral nerve, a finding not previously reported in the literature. Venous drainage was observed through a tributary to the cephalic vein. Conclusion: This report documents the first bilateral subclavius posticus innervated by the lateral pectoral nerve. These findings broaden the recognized spectrum of subclavius-related variants and highlight the utility of ultrasound for identifying accessory musculature in the thoracic outlet region.