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Scalene muscles

The scalene muscles are a trio of deep neck muscles—comprising the anterior, middle, and posterior scalenes, along with an inconstant scalenus minimus—that originate from the transverse processes of the cervical vertebrae (C2–C7) and insert onto the first and second ribs, facilitating respiration and neck movement.^[1] Located laterally to the cervical spine and deep to the sternocleidomastoid muscle, these muscles form key boundaries of the interscalene triangle, through which the brachial plexus and subclavian artery pass.^[1] Their primary functions include elevating the first and second ribs during inspiration to aid breathing, as well as contributing to ipsilateral lateral flexion and ipsilateral rotation of the neck.^[2] Innervated by anterior rami of spinal nerves C3–C8 via the cervical and brachial plexuses, the scalenes receive blood supply from branches of the subclavian artery and thyrocervical trunk.^[1] Clinically, they are implicated in thoracic outlet syndrome due to potential compression of neurovascular structures, and anatomical variations, such as the presence of scalenus minimus in 30–71% of individuals or atypical insertions, can influence surgical approaches in the neck.^[1]

Anatomy

Location and attachments

The scalene muscles consist of three strap-like muscles—anterior, middle, and posterior—situated in the lateral aspect of the neck, where they form the floor of the posterior triangle. This triangle is bounded superiorly by the inferior belly of the omohyoid muscle, posteriorly by the trapezius, and anteriorly by the posterior border of the sternocleidomastoid muscle. The scalenes lie deep to the sternocleidomastoid and platysma muscles, lateral to the cervical spine, and are enveloped within the prevertebral fascia, separating them from adjacent structures like the brachial plexus and subclavian vessels.^[1]^[3] These muscles originate collectively from the transverse processes of the cervical vertebrae, spanning levels C3 to C7, with fibers arising from the anterior, posterior, or intermediate tubercles depending on the specific muscle. The anterior scalene originates from the anterior tubercles of C3–C6, the middle scalene from the posterior tubercles of C2–C7 (between anterior and posterior tubercles), and the posterior scalene from the posterior tubercles of C5–C7. They course inferolaterally in a parallel fashion, typically measuring 5–10 cm in length and 1–2 cm in width, tapering as they descend toward the upper thorax.^[1]^[3]^[4] The insertions of the scalene muscles occur on the upper ribs, anchoring them to the thoracic cage. The anterior and middle scalenes insert on the first rib—the anterior at the scalene tubercle on its superior surface, and the middle on the superior surface posterior to the subclavian artery groove—while the posterior scalene inserts on the anterior surface of the second rib. This arrangement positions the muscles to bridge the cervicothoracic junction, with the anterior and middle scalenes partially enclosing the subclavian artery and brachial plexus in the interscalene triangle.^[1]^[3]

Anterior scalene

The anterior scalene muscle is the most superficial member of the scalene group, positioned in the lateral neck deep to the sternocleidomastoid muscle. It originates from the anterior tubercles of the transverse processes of the third through sixth cervical vertebrae (C3-C6).^[1] The muscle's fibers course obliquely downward and laterally, reflecting their role in bridging the cervical spine to the upper thorax.^[5] This orientation contributes to its superficial accessibility compared to the deeper middle and posterior scalenes. The anterior scalene inserts primarily at the scalene tubercle on the superior surface of the first rib, with some fibers potentially extending to contact the dome of the pleura.^[1] Measuring approximately 7-8 cm in length, it is thickest near the level of the cricoid cartilage and tapers toward its origins and insertion points.^[1] Anatomical variations may include origins extending to C2 or omitting C6, or insertions reaching the second or third rib, though such anomalies occur infrequently.^[1] A key anatomical relation of the anterior scalene is its formation of the anterior (lateral) boundary of the interscalene triangle, alongside the middle scalene posteriorly and the first rib inferiorly; this space transmits the roots of the brachial plexus and the subclavian artery.^[6] The phrenic nerve (arising from C3-C5) courses obliquely along the muscle's anterior surface, often sharing a fascial sheath and passing anterior to the subclavian artery within the triangle.^[6]^[1]

Middle scalene

The middle scalene muscle, also known as scalenus medius, originates from the posterior tubercles of the transverse processes of the second through seventh cervical vertebrae (C2-C7).^[1]^[4] This attachment spans six vertebrae, making it the most extensive origin among the scalene muscles, in contrast to the anterior scalene (C3-C6) and posterior scalene (C5-C7).^[1]^[4] It inserts onto the superior surface of the first rib, specifically on the inner aspect posterior to the groove (sulcus) for the subclavian artery.^[1]^[4] This positioning places the muscle fibers running obliquely downward and laterally from their cervical origins to the rib insertion. As the largest and longest of the three scalene muscles, it occupies a central location between the anterior and posterior scalenes in the lateral aspect of the neck.^[4]^[7] The middle scalene forms the posterior boundary of the interscalene triangle, a key anatomical space bounded anteriorly by the anterior scalene muscle and inferiorly by the first rib.^[1] The roots of the brachial plexus and the subclavian artery traverse this triangle, passing between the anterior and middle scalene muscles immediately anterior to the latter.^[1] In some cases, branches such as the C7 or C8 nerve roots or the upper long thoracic nerve may pierce the middle scalene itself.^[1]

Posterior scalene

The posterior scalene muscle, also known as scalenus posterior, is the smallest and most deeply situated of the three scalene muscles in the lateral neck. It originates from the posterior tubercles of the transverse processes of the C5 to C7 vertebrae.^[3]^[4] Its fibers descend posterolaterally with a steep downward slope to insert on the external surface of the second rib, posterior to the attachment of the serratus anterior muscle.^[7] This muscle is positioned deepest relative to the skin surface. Unlike the anterior and middle scalenes, the posterior scalene does not have major nerves or blood vessels piercing its belly directly, distinguishing its isolated position in the deep neck. It forms part of the floor of the posterior triangle of the neck, alongside the middle scalene, levator scapulae, and splenius capitis muscles.^[1]^[8]

Anatomical variations

The scalene muscles exhibit several anatomical variations, including fusions between individual muscles and the presence or absence of accessory components. A common variation involves the fusion of the middle and posterior scalene muscles, where fibers from the posterior scalene merge with the middle scalene, often extending the origin from C3 to C7 vertebrae; this occurs in a notable proportion of cases, with cadaveric studies reporting such fusions.^[1] Another frequent alteration is the partial or complete absence of the anterior scalene muscle, which is rare and typically unilateral, based on surgical and cadaveric observations.^[9] Accessory scalene muscles, such as the scalenus minimus, represent another key variation, arising between the anterior and middle scalenes from the transverse process of C7 and inserting onto the first rib or Sibson's fascia; this muscle is present in 7.8% to 71.7% of individuals according to anatomical dissections, with the wide range attributed to differences in study methodologies rather than solely racial factors.^[10]^[1] These accessory structures may contribute to a narrower interscalene space, though their clinical relevance varies. Developmentally, these variations stem from the embryological origins of the scalene muscles in the hypaxial portion of the cervical myotomes (C3-C8), where myoblasts migrate and differentiate during the seventh week of gestation under the influence of surrounding connective tissues; incomplete separation of these myotomal components can lead to fusions or accessory muscles.^[1]^[11]

Innervation and blood supply

Nerve supply

The scalene muscles receive their primary motor innervation from the anterior rami of the cervical spinal nerves C3 through C8.^[1] This segmental contribution arises from the ventral roots of the spinal cord, forming part of both the cervical and brachial plexuses, which supply the neck and upper limb musculature.^[1] The anterior scalene muscle is primarily innervated by branches from the anterior rami of C4 to C6, derived from the cervical plexus.^[7]^[12] In contrast, the middle scalene receives a broader supply from C3 to C8, with branches often perforating the muscle belly, particularly from C7 and C8 roots of the brachial plexus.^[1]^[7] The posterior scalene is supplied by anterior rami of C6 to C8, typically via direct branches from the brachial plexus roots that may traverse or pierce the muscle.^[1]^[7] These pathways reflect the muscles' position in the interscalene triangle, where the brachial plexus trunks pass between the anterior and middle scalenes, allowing for close anatomical integration of neural supply.^[1]

Vascular supply

The scalene muscles derive their arterial supply primarily from the ascending cervical artery, a branch of the thyrocervical trunk that arises from the first part of the subclavian artery.^[1] This artery provides blood to the anterior, middle, and posterior scalene muscles through its ascending branches along the cervical vertebrae.^[7] The anterior scalene receives additional direct supply from the inferior thyroid artery, another branch of the thyrocervical trunk.^[1] The posterior scalene may also be supplied by the superficial cervical artery in some cases.^[13] Venous drainage from the scalene muscles follows accompanying veins that empty into the subclavian vein, ultimately joining the internal jugular vein to form the brachiocephalic vein.^[1] The middle scalene's drainage pathway typically routes through the subclavian vein directly into the brachiocephalic trunk.^[1] The arterial network supporting the scalene muscles includes anastomoses between the ascending cervical artery and the deep cervical artery, a branch of the costocervical trunk from the second part of the subclavian artery; these connections provide collateral flow to the deep cervical musculature.^[14] Anatomical variations in the arterial supply to the scalene muscles are rare, but the ascending cervical artery may occasionally originate from the thyrocervical trunk, suprascapular artery, transverse cervical artery, superficial cervical artery, or rarely directly from the subclavian artery, rather than the more common origin from the inferior thyroid artery, as observed in vascular imaging and cadaveric studies.^[14]

Function

Respiratory role

The scalene muscles serve as accessory inspiratory muscles, primarily elevating the first and second ribs to expand the thoracic cavity during forced inspiration, thereby increasing lung volume and facilitating greater airflow.^[1] This action is achieved through their attachments to the upper ribs, with the anterior and middle scalenes attaching to the first rib and the posterior scalene to the second rib, making the anterior and middle scalenes particularly active in respiratory mechanics.^[1] In quiet, tidal breathing, the scalene muscles exhibit low-level electrical activity but play a minor role compared to the diaphragm, acting as obligatory inspiratory muscles with recruitment from the onset of inspiration.^[15] Their significance increases markedly during labored or forced respiration, such as in conditions like chronic obstructive pulmonary disease (COPD), where heightened neural drive leads to strong inspiratory contractions to compensate for increased mechanical loads and airflow limitation.^[16]^[17] Electromyography (EMG) studies confirm this pattern, showing progressive recruitment and elevated activity in the scalenes as inspiratory demand rises, with anterior and middle scalenes demonstrating the most consistent involvement.^[18] Biomechanically, scalene contraction elevates and stabilizes the upper ribs, enhancing thoracic expansion and supporting diaphragmatic descent by counteracting inward rib displacement during inspiration.^[1] This rib fixation aids overall ventilatory efficiency, particularly under load. The scalenes interact synergistically with other inspiratory muscles, including the sternocleidomastoid for additional neck stabilization and the intercostal muscles for coordinated rib cage expansion, ensuring integrated chest wall movement during elevated respiratory efforts.^[19]^[20]

Role in neck movement

The scalene muscles contribute to various movements of the head and neck, acting primarily on the cervical spine through their attachments to the vertebrae and ribs. All three scalene muscles—anterior, middle, and posterior—produce ipsilateral lateral flexion of the neck upon unilateral contraction, drawing the cervical vertebrae toward the same side as the active muscle.^[7]^[1] This action is essential for tilting the head sideways and is supported by the oblique orientation of the muscle fibers relative to the cervical transverse processes.^[7] In addition to lateral flexion, the anterior and middle scalene muscles facilitate contralateral rotation of the neck when contracting unilaterally, rotating the head away from the side of activation due to their line of pull crossing the midline of the cervical spine.^[7] The anterior scalene specifically contributes to slight forward flexion of the neck during bilateral contraction, assisting in nodding motions, while the middle scalene emphasizes rotational torque, and the posterior scalene focuses more on lateral flexion with minimal rotational influence.^[1] These muscle-specific actions arise from differences in their origins (C3–C6 for anterior, C2–C7 for middle, C5–C7 for posterior) and insertions on the upper ribs.^[7] Beyond primary movements, the scalene muscles serve synergistic roles in stabilizing the cervical spine during dynamic activities, such as shoulder elevation, where they fix the vertebral attachments to prevent unwanted head tilt.^[1] They also maintain postural stability in the upright position by counteracting gravitational forces on the head through sustained tonic activity, ensuring balanced support between the neck and shoulder girdle.^[1] This stabilization is modulated by their innervation from the cervical plexus (C3–C8), enabling coordinated motor control.^[7]

Anatomical relations

The scalene muscles are situated in the lateral aspect of the neck, deep to the sternocleidomastoid muscle and lateral to the cervical vertebrae, with their origins on the transverse processes of C3 to C7 and insertions on the first and second ribs.^[1] They collectively contribute to bounding key anatomical spaces, including the interscalene triangle, which is delimited anteriorly by the anterior scalene muscle, posteriorly by the middle scalene muscle, and inferiorly by the first rib; through this triangle pass the roots of the brachial plexus and the subclavian artery.^[6] The costoclavicular space, positioned between the clavicle superiorly and the first rib inferiorly, is influenced by the insertions of the anterior and middle scalene muscles on the first rib, facilitating the passage of the subclavian artery, vein, and brachial plexus.^[21] Specific relational proximities include the phrenic nerve, which courses along the anterior surface of the anterior scalene muscle, often sharing its fascial covering.^[1] The middle scalene muscle lies posterior to the brachial plexus and subclavian artery, which traverse the interscalene triangle anterior to it.^[6] The posterior scalene muscle is positioned deep to superficial structures in the posterior cervical triangle, including the external jugular vein, which descends superficially across the region.^[22] In terms of lymphatic relations, the scalene muscles are in close proximity to the supraclavicular (scalene) lymph nodes, located posteriorly by the scalene muscles and anteriorly by the sternocleidomastoid, facilitating drainage from the neck and upper limb structures into the jugular lymphatic trunks.^[23] The muscles are enclosed within the prevertebral layer of the deep cervical fascia, which surrounds the scalenes anterior to the vertebrae and allows for gliding of adjacent pharyngeal and esophageal structures during neck movements.^[24] On imaging, the scalene muscles are well-visualized on MRI and CT scans relative to the cervical vertebrae and ribs, appearing as distinct muscular bands originating from transverse processes and inserting on the rib surfaces; for instance, T1-weighted MRI can delineate accessory scalene branches or compressions within the scalene triangle.^[25]^[26]

Clinical significance

Thoracic outlet syndrome

Thoracic outlet syndrome (TOS) is a condition characterized by compression of the neurovascular structures in the thoracic outlet, particularly involving the scalene muscles, leading to symptoms in the upper extremity.^[27] The syndrome is classified into three main types: neurogenic TOS, which accounts for approximately 95% of cases and results from compression of the brachial plexus often due to scalene muscle hypertrophy or anomalies; arterial TOS, involving the subclavian artery (about 1-2% of cases); and venous TOS, affecting the subclavian vein (3-5% of cases).^[28] In pathophysiology, the middle scalene muscle frequently contributes to impingement within the interscalene triangle, where it, along with the anterior scalene and first rib, narrows the space for the brachial plexus trunks or subclavian artery, exacerbated by repetitive overhead activities or anatomical variations.^[29] This compression leads to clinical manifestations including pain radiating from the neck to the arm, paresthesia, and muscle weakness, particularly in the hand and fingers.^[28] Diagnosis relies on a combination of clinical provocation tests and imaging. The Adson test involves neck extension and rotation to assess radial pulse diminution or symptom reproduction indicative of vascular or neurogenic compression, while the Roos test (elevated arm stress test) provokes symptoms by sustaining arm abduction and external rotation for three minutes.^[30]^[31] Confirmatory imaging includes MRI to visualize scalene muscle hypertrophy or brachial plexus impingement and angiography for vascular involvement.^[28] The incidence of TOS is estimated at 1-3 cases per 100,000 population annually.^[32] Treatment begins with conservative measures, such as physical therapy focused on scalene stretches, postural correction, and strengthening exercises, which improve symptoms in a majority of neurogenic cases within 6-12 months.^[29] For refractory cases, surgical interventions include scalene muscle release (scalenectomy) to decompress the interscalene triangle or first rib resection to expand the thoracic outlet, often via supraclavicular or transaxillary approaches.^[28] Surgical outcomes demonstrate significant improvement, with approximately 90% of interventions resulting in symptom improvement post-procedure.^[33]

Other injuries and conditions

Trauma to the scalene muscles commonly occurs in whiplash injuries sustained during motor vehicle collisions (MVCs), where rapid hyperextension and flexion of the neck lead to strain or spasm in these muscles.^[34] Symptoms typically include acute neck pain radiating to the shoulder and arm, often accompanied by discomfort or tingling in the upper extremity due to myofascial irritation or secondary nerve involvement.^[35] Approximately 20-40% of individuals experience persistent symptoms beyond one year following whiplash-associated disorders, with scalene involvement contributing to chronic neck stiffness and referred arm pain in affected cases.^[36] Myofascial pain syndrome in the scalene muscles arises from active trigger points, which generate referred pain patterns to the head, temple, shoulder, and upper arm, mimicking tension headaches or cervicogenic discomfort.^[37] These trigger points, often resulting from overuse or postural strain, can cause localized tenderness and restricted neck mobility.^[38] Treatment frequently involves dry needling, a technique that targets the trigger points to reduce pain intensity and improve muscle function, with evidence showing short-term relief in inspiratory capacity and discomfort levels after a single session.^[39] The scalene muscles may contribute secondarily to cervical radiculopathy, where tightness or spasm exacerbates nerve root compression, leading to radiating arm pain, paresthesia, or weakness that simulates disc prolapse symptoms.^[40] Post-surgical complications following neck dissection, such as in head and neck cancer treatment, can involve scalene dysfunction due to inadvertent trauma or scarring near the brachial plexus, resulting in persistent shoulder impairment or neuropathic pain.^[41] In these cases, preservation of the scalene muscles during surgery is critical to minimize functional deficits.^[42] Rare pathologies affecting the scalene muscles include abscesses from infections like tuberculosis, presenting as localized swelling and pain requiring drainage and antimicrobial therapy, and benign tumors such as intramuscular hemangiomas, which manifest as noncompressible masses with potential for compression symptoms.^[43] ^[44] Electromyography (EMG) serves as a key diagnostic tool for detecting denervation in the scalene muscles, revealing reduced activity or fibrillations indicative of nerve injury, which aids in differentiating these conditions from other neuropathies.^[45]

History and etymology

Historical anatomy

The scalene muscles have been recognized as part of the lateral neck musculature since ancient times, with early accounts based on animal dissections that did not clearly distinguish them from other cervical structures. In the 16th century, Andreas Vesalius advanced the understanding of neck anatomy through direct human dissections in his 1543 work De Humani Corporis Fabrica, providing more precise depictions of cervical attachments and correcting several inaccuracies in prior descriptions. The 18th century saw further delineation of the scalene muscles' anatomical relations, with the space between the anterior and middle scalene muscles and the first rib recognized as a key landmark for neurovascular passages in the neck, now known as the interscalene triangle. The 19th century saw practical applications emerge with the recognition of the scalene muscles' role in compressive pathologies; in 1861, Holmes Coote performed the first surgery for thoracic outlet syndrome by removing a cervical rib to alleviate vascular compression, marking an early intervention in the scalene region.^[46] Advancements continued into the 20th century, with clinical studies in the 1940s, such as those by Greene and Burch, confirming the scalene muscles' active involvement in respiration.^[47] Imaging techniques evolved from plain X-rays in the early 1900s, which primarily visualized bony variations like cervical ribs affecting the scalenes, to computed tomography (CT) and magnetic resonance imaging (MRI) by the late 20th century, enabling detailed assessment of muscular variations and soft-tissue compressions in the scalene region.

Etymology

The term "scalene" derives from the Ancient Greek skalēnós (σκαληνός), meaning "uneven" or "unequal," a reference to the differing spans of origin among the three muscles, such as the anterior scalene attaching to the transverse processes of vertebrae C3–C6 and the middle scalene to C2–C7, evoking the unequal sides of a geometric scalene triangle.^[48]^[49] The name highlights the asymmetrical attachments that distinguish these muscles from more uniform neck structures.^[50] In Latin anatomical nomenclature, the muscles were termed musculi scaleni, with the anterior scalene specifically designated scalenus anticus to denote its forward position.^[7] This terminology appeared in Renaissance-era texts, reflecting the adoption of Greek roots into Latin for precise description of anatomical variations.^[51] Related terms include "interscalene," which describes the interval between the anterior and middle scalene muscles, forming part of the interscalene triangle through which the brachial plexus passes.^[6] The "scalene tubercle," or Lisfranc tubercle, refers to the bony prominence on the inner border of the first rib serving as the primary insertion site for the anterior scalene muscle. The nomenclature transitioned into English anatomical literature during the 18th century, appearing in translations and original works that standardized classical terms for broader medical education.^[4]

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[50]
Etymology of Neck Terms - Dartmouth Web Hosting
Scalene – Comes from the Greek word scalenos meaning uneven. If you remember your geometry, you will recall that a scalene triangle is one with three unequal ...
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Scalene - Etymology, Origin & Meaning
Originating from Greek skalēnos meaning "unequal" and Late Latin scalenus, this word means "having unequal sides," especially in math describing triangles.Missing: muscles | Show results with:muscles
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Scalene Muscles, Its Attachments and Actions - Yoganatomy
Nov 7, 2017 · They function unilaterally, to assist in lateral flexion of the neck to the same side. The scalenes as a group also assist as secondary ...<|control11|><|separator|>
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scalene - Wiktionary, the free dictionary
Etymology. Borrowed from Late Latin scalēnus (“of unequal sides”), from ... (anatomy) Ellipsis of scalene muscle. (trigonometry) Ellipsis of scalene ...<|control11|><|separator|>
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Scalene tubercle - Wikipedia
The scalene tubercle is a small projection that runs along the medial border of the first rib between two grooves, which travel anteriorly for the subclavian ...Missing: etymology | Show results with:etymology