Cadaveric case report and biomechanical analysis of an accessory clavicular head to the sternocleidomastoid

Introduction: The bilateral sternocleidomastoid (SCM) muscle is a contralateral rotator and ipsilateral flexor of the head and neck that typically consists of one sternal head (SH) and one clavicular (CH) head. Variations in SCM muscle topography, attachments, existence, and number of muscle bellies have been reported with clinical considerations but accompanying mechanical analyses and three-dimensional modeling are scarce. The objective of this study was to investigate a case of bilateral SCM accessory clavicular heads (ACH) discovered in an adult human cadaver and determine the biomechanical implications with three-dimensional modeling. Materials and methods: Left and right SCMs were dissected, photographed, and eviscerated. Fascicle bundle lengths, attachment site locations, pennation angles, and muscle belly masses were recorded for each head. Sarcomere lengths were measured from transmission electron microscopy (TEM) imaging to normalize fascicle lengths from current post-mortem state to that of optimal human skeletal muscle sarcomeres. Physiological cross-sectional area (PCSA) was calculated to determine each head's maximal isometric force (F_max) generating capacity. SCM specifications were modeled virtually in three dimensions for broader visualization of ACH in anatomical position and calculation of ACH and CH torques the clavicle. Results are discussed as averages of the left and right heads. Results: SH, CH, and ACH fusiform muscle bellies weighed 18.03 g, 9.30 g, and 3.41 g with PCSAs of 1.24 cm², 0.65 cm², and 0.25 cm², respectively. TEM examination of the SCM tissue revealed a mean sarcomere length of 3.19 μm, 0.55 μm more than the optimal human sarcomere length. Fascicle bundle lengths for SH, CH, and ACH were normalized and determined to be able to generate 27.84 N, 14.56 N, and 5.57 N, respectively. Despite only being able to generate 38.3% the F_max of CH, the ACH effectively doubles the SCM torque on the distal clavicle (31.01 N⋅cm plus 32.32 N⋅cm) due to its further distal attachment from sternoclavicular joint (5.6 cm vs. 2.3 cm) and more perpendicular angle of attachment (83.8° vs. 74.8°, respectively). Conclusions: Presence of a smaller but more laterally located ACH can generate considerably more force from the distal clavicle on posteroinferior skull than a typical SCM. Gross imaging, biomechanical analysis, and three-dimensional modeling of the SCM with ACH as presented in this study may serve as a new and important resource for healthcare providers, massage therapists, myofascial workers, and medical educators when considering musculoskeletal anomalies of the head and neck.