Biomechanical analysis and clinical implications of an interscalene muscle slip involving the brachial plexus

Introduction: The brachial plexus courses above the first rib and between the anterior and middle scalene muscles. Morphological variations of the scalene muscles can be clinically significant, but biomechanical analyses of such variations are scarcely reported. The objective of this study is to perform an anatomical and biomechanical investigation of a unique interscalene muscle variation involving the brachial plexus. Methods: A unilateral (left) interscalene muscle slip was discovered during routine dissection of an elderly male human cadaver. The anatomy was cleanly dissected and photographed in situ with scale. Mass, fascicle length, and mean fixed sarcomere length were measured for the anterior scalene, interscalene, and middle scalene muscles, and the normalized maximal isometric force (Fmax) for each muscle was calculated. Results: The 5.20 cm interscalene muscle slip was bicipital; its medial and lateral heads originated from the proximal aspect of the middle scalene muscle posterior to the C5 ventral ramus, converged to a common belly between the C6 and C7 ventral rami, and inserted on the distal aspect of the anterior scalene muscle anterior to the C7 ventral ramus. The Fmax of the anterior scalene, interscalene, and middle scalene muscles were 13.58 N, 0.46 N, and 13.82 N, respectively. Conclusion: The interscalene muscle variation may jeopardize the C5 and C6 ventral rami and complicate medical procedures involving the scalene triangle (e.g., interscalene nerve block). Rarity of the variation may cause its oversight as a differential diagnosis and misinform the treatment and therapeutic progress for other conditions. This report may benefit clinicians, anatomists, and medical students as they deliberate complicated differential diagnoses and medical procedures involving the scalene triangle region.