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Anatomy, Head and Neck: Sternocleidomastoid Muscle
Introduction
The sternocleidomastoid (SCM) is a prominent superficial neck muscle that divides the neck into anterior and posterior triangles (see Image. Superficial Neck Anatomy). Originating from the sternal manubrium and medial clavicle, the muscle inserts onto the mastoid process and superior nuchal line. The SCM facilitates lateral neck flexion, contralateral head rotation, and bilateral cervical flexion or extension, depending on cervical spine rigidity, and contributes to inspiration.
The muscle's blood supply emanates from branches of the external carotid artery. Innervation is primarily via the accessory nerve (cranial nerve XI), with proprioceptive fibers coming from the cervical plexus. Anatomical variants include differences in origins, insertions, number of muscle heads, and occasional aberrant innervation. The SCM also participates in posture regulation, temporomandibular function, and complex cervicofacial muscle coordination.
The SCM serves as a key landmark in the assessment of neck and head pathologies and the execution of surgical procedures such as carotid endarterectomy. Dysfunction or injury of this muscle can contribute to postural abnormalities, torticollis, impaired cervical mobility, and altered temporomandibular function. Electrophysiological studies indicate that SCM activity reflects broader cervicofacial muscular coordination, making its evaluation useful in diagnosing neuromuscular disorders. SCM palpation and functional testing can aid clinicians in identifying muscle hypotrophy, asymmetry, or altered activation patterns associated with chronic neck pain, congenital muscular abnormalities, or secondary musculoskeletal adaptations.
Structure and Function
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Structure and Function
Anatomy
The SCM divides the neck into anterior and posterior triangles. The anterior triangle is bounded by the posterior border of the SCM, the inferior border of the mandible, and the medial line of the neck. Suprahyoid and infrahyoid muscles lie within the anterior triangle. The posterior triangle is bounded by the SCM anteriorly, the clavicle inferiorly, and the trapezius posteriorly. Scalene muscles reside within the posterior triangle. The SCM is a large, easily identifiable, and palpable muscle (see Image. Cross-Section of the Human Neck).
The SCM originates from the upper edge of the sternal manubrium and the medial quarter of the clavicle, with the 2 heads merging into a single muscle belly directed upward and laterally. Insertions occur at the mastoid process of the temporal bone and the anterior portion of the superior nuchal line. Fibers are arranged in parallel. The SCM is not pennate.
The SCM may be subdivided into 4 portions based on origin and insertion: sternomastoid, sterno-occipital, cleidomastoid, and cleido-occipital (see Image. Anterior Neck Muscles and Related Structures). The sternomastoid portion generates the greatest contractile force, whereas the cleido-occipital portion produces the least.[1]
Function
The unilateral contraction of the SCM produces a triple movement: rotation of the head to the side opposite the contracting muscle, lateral inclination toward the contracting side, and extension. The effects of simultaneous contraction of both SCM muscles depend on the state of contraction of other cervical spine muscles. If the cervical spine is mobile, bilateral contraction induces cervical hyperlordosis with head extension and dorsal bending of the cervical spine. If the cervical spine is rigid and rectilinear due to paravertebral muscle contraction, simultaneous SCM contraction produces cervical flexion along the dorsal spine and forward flexion of the head. The SCM also contributes to inspiration by anchoring to the temporal bone and elevating the sternum and clavicles.[2]
The SCM contributes significantly to cervical and overall body posture. Vestibular stimulation electrically activates the SCM, demonstrating a close connection between the vestibular system and SCM motoneurons.[3] Lateral inclination is the movement in which the SCM exhibits maximal speed and force.[4]
The SCM also supports proper temporomandibular joint function. During mastication, a trigeminal-cervical reflex activates the SCM, and its innervation is essential for optimal temporomandibular joint occlusion. Mandibular occlusal alterations disrupt SCM function, leading to muscular incoordination and abnormal neck inclinations. Correction of occlusal defects or dental treatment has resolved torticollis in some cases. During unilateral mastication, SCM activity synchronizes with the masseter, whereas bilateral chewing elicits anticipatory SCM activation, likely to stabilize the neck.[5]
Embryology
The SCM derives from paraxial (preoptic) mesoderm and occipital (postoptic) somites and partially from neural crests.[6][7] The SCM appears on the 14th day of gestation in animal models. Recent studies indicate that progenitor cells forming the neck muscles occupy the same region as cardiac progenitor cells within the cardiopharyngeal mesoderm.[8]
Blood Supply and Lymphatics
The arterial supply of the SCM derives from branches of the external carotid artery, including the occipital artery and the superior thyroid artery, which are palpable in the medial-anterior portion of the muscle. Blood flow to the respiratory muscles, including the SCM, during intense physical activity increases at the expense of limb muscles.[9] The external jugular vein courses inferiorly and posteriorly to the SCM, draining venous blood via anterior and posterior branches. The lymphatic drainage of the SCM follows the vertical chain, comprising the anterior superficial lymph nodes and the posterior triangle lymph nodes inferiorly.
Nerves
The cutaneous branches of the cervical plexus emerge from the posterior edge of the SCM and contribute to the muscle’s proprioceptive functions. The accessory nerve passes through the posterior triangle to innervate the trapezius and the SCM.[10]
Muscles
The muscles of the neck are components of the myofascial system, which establishes both an anatomical and functional continuum (see Image. Muscles of the Head, Face, and Neck).[11] Dysfunction of a single muscle or muscle segment produces functional alterations throughout the neck musculature. For example, ocular pathology modifies the electromyographic activity of the masseter and neck muscles, including the SCM.[12][13]
Superficial and deep neck muscles are activated by the cortical system via the reticulospinal pathway, with synchronous activation regardless of muscle layer depth.[14] Therefore, consideration of the entire neck muscle complex is essential when evaluating pathology that appears to involve a single muscle.
In healthy subjects, the SCM contains mostly white, anaerobic fibers (~65%) and fewer red, aerobic fibers (~35%).[15] The muscle generates substantial force rapidly with minimal resistance over prolonged periods. The relative proportion of white and red fibers in the SCM changes with age, with red fibers increasing to approximately 44% at the expense of white fibers.[16] The SCM demonstrates adaptive responses to the surrounding environment and age-related physiological changes.
Physiologic Variants
Agenesis of the SCM, which may be accompanied by the absence of the trapezius, is a rare anomaly that often does not produce clinical or functional deficits. This outcome likely reflects compensatory adaptations by other neck muscles.[17]
Other anatomical variations of the SCM involve its origins, which can influence surgical approaches in the region. The clavicular attachment may be narrow or wide, measuring up to 7 to 8 cm. Multiple clavicular attachments may also be observed. Variations in origin can affect the acromioclavicular joint or produce additional muscular bellies in the SCM. Insertions at the sternoclavicular joint have been documented, altering neck anatomy.[18]
An increased number of SCM heads is relatively common. For example, one side may exhibit 2 sternomastoid heads, a cleido-occipital head, and a cleidomastoid occipital head, whereas the contralateral side may present a single sternomastoid, a cleido-occipital, and 2 cleidomastoid heads, totaling 4 heads.[19]
Rarely, the margin of the SCM contacts the trapezius, likely due to embryological malformations. Known insertion variants include cleido-epistrophic, cleido-cervical, and cleido-atlantic types, each with 1 or more heads.[20]
Innervation of the SCM exhibits variability. A study reported that the lower portion of the SCM receives fibers from a branch of C1 via the ansa cervicalis (descendens hypoglossi). Similar innervation may occur in the upper portion.[21] An aberrant branch of the facial nerve has been documented to innervate the deep portion of the upper 1/3 of the SCM.[22]
The SCM is also recognized by alternative names, including "nutator capitis," "mastoideus colli," "sternocleidomastoid muscle of Kopfnicker," and "sternomastoid muscle." Awareness of the muscle's anatomical variations is essential when performing surgical interventions in the region.
Surgical Considerations
The SCM can serve as an autograft for repairing surgical defects. This application allows reconstruction of soft tissue or bony areas in the head and neck.
A flap of the SCM may be employed during parotid gland resection for tumor removal. The muscle facilitates adequate flap length and rotation over the incision site, reduces depression of the parotidectomy area, and decreases the risk of necrosis due to the SCM’s rich vascularization. Complete prevention of Frey syndrome (auriculotemporal nerve injury) cannot be guaranteed.[23]
The SCM is utilized in a variety of procedures requiring reconstruction of orofacial and pharyngeal structures. The selection of muscular flaps or flaps incorporating bony segments depends on the specific surgical objective.[24] Examples of structures that may be reconstructed using SCM flaps include the following:
- Tongue and buccal floor
- Oral cavity, oropharynx, and laryngotracheal complex
- Regions of the head and neck
- Jawbone and mastoid defects
- Esophagopharyngeal complex
- Cheek
SCM muscle flaps are employed in the surgical repair of congenital muscular torticollis. In muscular torticollis, shortening and fibrosis of the SCM alter head and shoulder alignment, producing ipsilateral lateral flexion and contralateral rotation of the face. Treatment options include rehabilitation or surgical intervention. Delayed diagnosis without therapy may result in persistent shortening of the SCM and formation of a rigid muscle band. Severe cases can cause lasting craniofacial deformities.[25]
Favorable outcomes are achievable within the first 5 years of life, with earlier intervention providing optimal results.[26] Surgical release of the rigid SCM band can improve facial and cervical deformities in adults with untreated congenital stiff neck, although results are never equivalent to early childhood intervention. The standard surgical approach in both children and adults involves partial resection of the SCM.[27]
The SCM also serves as a critical anatomical landmark for surgical exposure during carotid endarterectomy, guiding incision placement and access to the carotid sheath. Identification and retraction of the SCM facilitate safe dissection while helping preserve adjacent neurovascular structures (see Image. Preoperative Markings for Carotid Endarterectomy).
Clinical Significance
Sternocleidomastoid Muscle Function Evaluation
Assessment begins with the patient in a seated position to observe hypotrophy of the SCM and postural abnormalities of the neck, head, shoulder, scapula, clavicle, and sternal manubrium. Voluntary neck movements are performed to evaluate motor or pain limitations. Forced inhalation and simulated mastication are used to assess SCM function.
Reflexes are tested using a small tendon hammer at the clavicular insertion of the SCM. Muscle strength is evaluated by having the patient move the head through flexion, rotation, and lateral inclination against minimal resistance applied by the examiner. Lesions involving the SCM may affect the accessory nerve, although such occurrences are infrequent.[28] Spinal accessory nerve injury results in absent tendon reflexes, atrophy of the SCM and trapezius, shoulder depression, and the sign of Sicard, characterized by increased depth of the supraclavicular fossa. Paralysis of the SCM can produce torticollis.
Types of Torticollis
Torticollis can be classified into several types. Paralytic torticollis results from injury to the spinal accessory nerve.[29] Congenital torticollis is frequently associated with intrauterine packaging disorders, including metatarsus adductus, developmental dysplasia of the hip, acetabular dysplasia, and congenital hip dislocations.[30][31] Metatarsus adductus occurs in approximately 15% of congenital torticollis cases.[32] Spasmodic torticollis is a form of segmental dystonia. Ocular torticollis arises when diplopia affects SCM posture.[33]
Symptomatic torticollis has variable etiologies, such as pain, inflammation, infection, or abnormal cervical vertebral positioning.[34] “Psychic pillow” torticollis is observed in severe neurological conditions, including Parkinson disease and catatonic disorders, in which patients maintain the head bent forward as if resting on a pillow, even in the supine position. Psychogenic torticollis is characterized by fear of correct neck movements due to pain or vertigo. Accurate diagnosis of these disorders typically requires electromyography and imaging studies, including magnetic resonance imaging, computed tomography, or ultrasonography.
Other Issues
Manual Approach: Physiotherapy
All superficial and deep muscle layers should be assessed when managing SCM dysfunction. In congenital torticollis, which accounts for approximately 1/3 of congenital muscular abnormalities, physiotherapy plays a central role in resolving dysfunction or accelerating recovery following surgical intervention. Recommended conservative therapy includes stretching exercises, guided voluntary movements to improve posture in children of appropriate age, and parental modifications of the child’s posture. Conservative management resolves the condition in many cases.[35] Therapeutic approaches to SCM dysfunction vary according to the therapist’s assessment and specific medical indications.
Certain pathologies necessitate primary surgical intervention. These conditions include intramuscular hemangioma, pseudosarcomatous proliferative myositis, pseudotumor of infancy (fibromatosis colli), and rupture of the SCM.
Recent studies demonstrate increased electrical activity of the SCM in individuals with chronic neck pain compared to subjects without pain. Chronic cervical pain is also associated with greater fat infiltration within the SCM.[36] Incorporating stretching and massage into conventional physiotherapy improves clinical outcomes in this context.[37]
Alterations in the electromyographic spectrum of the SCM correlate with the presence of temporomandibular disorders. Electromyographic evaluation of the SCM provides a tool to detect mandibular dysfunctions.[38]
Osteopathy and Manual Therapy
Osteopathic treatment aimed at SCM recovery after surgery can also positively influence scar formation. Gentle, noninvasive osteopathic manipulation addresses all myofascial layers of the neck and the intervertebral spaces of the cervical spine.[39][40][41]
Obstructive sleep apnea syndrome (OSAS) induces nonphysiological adaptation of the SCM, reducing the muscle's viscoelasticity. Greater muscle stiffness correlates with increased OSAS severity. Assessment of the SCM provides clinical insight for OSAS diagnosis. Improving muscle elasticity may offer therapeutic benefit.[42]
Swallowing difficulties frequently occur in patients with advanced head and neck cancer following radiotherapy. SCM stiffness strongly correlates with reduced laryngeal elevation. Enhancing SCM mobility may improve laryngeal elevation and mitigate swallowing impairments.[43]
Abnormal cervical lordosis is associated with altered neuromuscular activity of the SCM. This adaptation increases fatigue of the neck muscles and produces pain during movement. Optimizing SCM biomechanics may reduce these symptoms.[44]
Significant traumatic injury to the SCM is uncommon. Ultrasound or contrast-enhanced computed tomography can detect hematomas if injury is suspected. Conservative management with immobilization is sufficient when adjacent vital structures remain uncompromised.[45]
Parkinson disease is associated with instability in daily activities, including standing, sitting, and walking. Increased myoelectric activity of the SCM contributes to dynamic postural alterations in Parkinson disease. Further investigation is required.[46]
Chronic myofascial pain of the SCM occurs in approximately 55% of patients with head and neck cancer after treatment. This pain is linked to depression. Gentle therapies aimed at improving muscle tone may provide clinical benefit.[47]
Media
(Click Image to Enlarge)
Superficial Neck Anatomy. This left lateral-view illustration shows the anterior and posterior triangles. The anterior triangle is further divided into the submental, submandibular, carotid, and muscular triangles. The muscles in this illustration include the mylohyoideus, digastricus, omohyoideus (venter superior and inferior), sternocleidomastoideus, scalenus medius and anterior, and trapezius. Bony structures include the mandible, mastoid process, left hyoid, and clavicle.
Illustrated by B Palmer
(Click Image to Enlarge)
Preoperative Markings for Carotid Endarterectomy. This clinical photograph demonstrates the surgical markings on the lateral neck prior to a carotid endarterectomy. The red arrow indicates the planned incision line along the anterior border of the sternocleidomastoid, which allows for optimal arterial exposure. The blue arrow identifies the inferior border of the mandible, which ensures the incision is positioned correctly for distal access.
Contributed by S Dulebohn, MD
(Click Image to Enlarge)
Muscles of the Head, Face, and Neck. The epicranius, galea aponeurotica, frontalis, temporal fascia, auricularis superior, auricularis anterior, auricularis posterior, occipitalis, sternocleidomastoid, platysma, trapezius, orbicularis oculi, corrugator, procerus, nasalis, dilator naris anterior, dilator naris posterior, depressor septi, mentalis, orbicularis oris, masseter, zygomaticus, and risorius muscles are shown in the image.
Henry Vandyke Carter, Public Domain, via Wikimedia Commons
(Click Image to Enlarge)
Cross-Section of the Human Neck. This diagram illustrates the anatomical relationships between the cervical fascia, major muscle groups, and visceral organs at the C6 level. Key neurovascular structures, including the carotid artery and vagus nerve, are shown within their respective compartments.
Henry Vandyke Carter, Public Domain, via Wikimedia Commons
(Click Image to Enlarge)
Anterior Neck Muscles and Related Structures. This illustration shows the suprahyoid, infrahyoid, styloglossus, hyoglossus, geniohyoideus, mylohyoideus, digastricus, stylohyoideus, omohyoideus, sternothyroideus, sternohyoideus, omohyoideus, sternocleidomastoideus, trapezius, and omohyoideus muscles. The mandibular symphysis, thyroid cartilage, thyroid gland, hyoid bone, clavicles, scapula, and sternum are also shown.
Henry Vandyke Carter, Public Domain, via Wikimedia Commons
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