Biomechanics of an Accessory Clavicular Head to the Sternocleidomastoid Muscle: Cadaveric Case and 3D Modeling

& Objective The sternocleidomastoid (SCM) is a muscle of the head and neck. Its sternal (SH) and clavicular (CH) heads ascend the neck, merge, and insert on the skull. An accessory clavicular head (ACH) can develop from a remnant slip of the trapezius when the SCM and trapezius separate from the same embryological muscle mass. Variations in gross presentation of accessory SCM heads have been well documented, but their mechanics are less described and the literature could benefit from 3D modeling of a SCM ACH . The objective of this study is to examine a case of bilateral SCM ACH on a human cadaver and calculate its biomechanical capacity through direct measurements and 3D modeling. Materials & Methods Bilateral SCMs with ACHs were dissected, eviscerated, and photographed (iPad Pro, Apple, Inc.). Image color contrast was utilized to aide structure identification (Adobe Photoshop 20.0.9). Muscles, fascicle bundles, attachments, and pennation angles were measured (ImageJ, 1.52q) before removing and weighing each muscle belly (Ohaus Scale, Model V22PWE1501T). Each head's physiological cross‐sectional area (PCSA) was calculated and multiplied by an accepted specific tension value for fast‐acting skeletal muscle (22.5 N cm ‐2 ) to determine their theoretical maximal isometric forces (MIF) of contraction. MIF ACH was compared to MIF SH,CH and considered for its mechanical effect on clavicle and skull movements. All calculations were derived from mean values between the bilateral SCM heads. 3D modeling is in process and will be used to determine angles and force vectors created by SCM ACH on the clavicle and skull. Additionally, electron microscopy imaging is in process to evaluate SCM ACH sarcomere state. Results The SCM ACH had a mass of 3.41 grams and measured 16.8 cm in total length. Its muscle fibers were parallel and composed 15.8 cm (94.0%) of the muscle's total length. Its 3.0 cm wide origin attached to the clavicle 5.6 cm lateral to the sternoclavicular joint, and it inserted with the SCM SH,CH on the mastoid process and superior nuchal line. The PCSA of the SCM SH , SCM CH , and SCM ACH was 1.02 cm 2 , 0.54 cm 2 , and 0.20 cm 2 , generating MIFs of 23.03 N, 12.04 N, and 4.59 N, respectively. In this regard, the MIF ACH would have increased the force of the normal SCM SH,CH on the skull by 13.1%. Compared to the MIF CH alone, the MIF ACH would exert 38.1% more force on the clavicle at a more distal location from the clavicle's fulcrum than SCM CH . Conclusions & Significance Presence of a SCM ACH can generate considerably more force on the distal clavicle and posteroinferior skull than normal. This additional capacity to the normal SCM could assist in shoulder elevation, ipsilateral neck flexion, contralateral head rotation as well as help maintain static support and position of the pectoral girdle and neck/head. Alternatively, it could decrease access to neurovascular structures of the neck, complicate physical examination and radiologic imaging, and impact fractures of the clavicle. The force vectors acting on the pectoral girdle are dimensionally complex, so 3D modeling and biomechanical analysis is necessary for a complete investigation. The gross imaging, 3D modeling, and biomechanics of the SCM ACH presented in this study may help clinicians with diagnoses, procedures, and treatments pertaining to the head and neck.