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Muscular triangle

The muscular triangle is one of the four subdivisions of the anterior triangle of the neck, a superficial anatomical region located in the anterior compartment inferior to the hyoid bone and superior to the clavicle. It is primarily defined by its role in housing the infrahyoid strap muscles that facilitate head and neck movements, as well as supporting vital structures involved in respiration, swallowing, and endocrine function.^[1] This triangle is clinically significant due to its proximity to the thyroid gland and major neurovascular elements, making it a key area in surgical interventions such as thyroidectomy and tracheostomy.^[2] The boundaries of the muscular triangle are precisely delineated to distinguish it from adjacent regions like the carotid and submandibular triangles. Medially, it is bordered by the midline of the neck extending from the hyoid bone to the sternum; laterally and posteriorly by the anterior margin of the sternocleidomastoid muscle; and superiorly by the superior belly of the omohyoid muscle, with the inferior border of the hyoid bone contributing to its upper limit.^[2] ^[1] These boundaries are enclosed by layers of the deep cervical fascia, including the investing layer superficially and the pretracheal layer deeply, which help compartmentalize contents and provide surgical landmarks.^[1] Key contents of the muscular triangle include the four infrahyoid strap muscles—sternohyoid, sternothyroid, thyrohyoid, and omohyoid (superior belly)—which depress the hyoid bone and larynx during swallowing and phonation, innervated primarily by the ansa cervicalis nerve loop formed from cervical plexus branches (C1–C3).^[2] Deep to these muscles lie the thyroid gland, larynx, trachea, and esophagus, along with associated vessels such as the superior thyroid artery, anterior jugular vein, and inferior thyroid veins, as well as lymph nodes in the anterior cervical, prelaryngeal, pretracheal, and paratracheal groups.^[2] ^[1] Nerves traversing or adjacent to the region include branches of the vagus nerve, such as the superior laryngeal nerve, which is vulnerable during procedures in this area.^[1] Clinically, the muscular triangle is notable for its involvement in endocrine and airway surgeries, where precise dissection of the strap muscles is required to access the thyroid and parathyroid glands without damaging the recurrent laryngeal nerve, which could lead to vocal cord paralysis.^[2] Injuries or pathologies here, such as thyroid goiters or infections, can impair swallowing or cause airway obstruction, underscoring the region's functional importance in daily physiology and its relevance in head and neck oncology.^[1]

Anatomy

Location

The muscular triangle, also known as the inferior carotid triangle or omotracheal triangle, is defined as the inferior subdivision of the anterior triangle of the neck.^[2] It occupies the lower portion of this anterior region, which is itself delineated by the midline of the neck medially, the anterior border of the sternocleidomastoid muscle laterally, and the inferior border of the mandible superiorly.^[3] This positioning situates the muscular triangle centrally in the anterior aspect of the neck, anterior to the sternocleidomastoid muscle and inferior to the more superior subdivisions of the anterior triangle.^[4] The apex of the muscular triangle is located superiorly at the inferior border of the hyoid bone.^[3] Its base extends inferiorly to the sternum along the midline, forming an irregular triangular shape that widens toward the base.^[2] As a paired structure, the muscular triangle exists bilaterally, with one on each side of the neck midline, mirroring the symmetrical anatomy of the cervical region.^[3] This bilateral configuration distinguishes it from the posterior triangle, which lies posterior to the sternocleidomastoid muscle.^[4]

Boundaries

The muscular triangle, a subdivision of the anterior triangle of the neck, is defined by three primary boundaries that enclose its region inferior to the hyoid bone. The superior boundary is formed by the superior belly of the omohyoid muscle, which extends obliquely from the hyoid bone to its intermediate tendon.^[3] The medial boundary follows the midline of the neck, extending from the hyoid bone inferiorly to the jugular notch of the sternum.^[5] The lateral boundary is delineated by the anterior border of the sternocleidomastoid muscle, specifically its superior portion, which runs from the intersection with the omohyoid muscle down toward the sternum.^[4] This configuration results in an apex at the hyoid bone, formed by the inferior border between the midline and the origin of the superior belly of the omohyoid muscle, and a base at the jugular notch of the sternum formed by the medial boundary.^[3] Due to the oblique course of the superior belly of the omohyoid muscle, the muscular triangle often appears irregular or quadrilateral in certain anatomical views, leading some descriptions to characterize it as a "slightly dubious" triangle with effectively four sides: the hyoid bone serving as a distinct superior limit separate from the omohyoid's trajectory.^[3]^[1]

Muscles

The muscular triangle of the neck, also known as the inferior carotid triangle, is primarily occupied by the infrahyoid strap muscles, which form its characteristic muscular content and contribute to its nomenclature. These muscles lie anterior to the carotid sheath and are arranged in superficial and deep layers, facilitating movements of the hyoid bone and larynx essential for deglutition and phonation.^[6]^[4] The sternohyoid muscle is the most superficial of the infrahyoid group, originating from the medial end of the clavicle and the manubrium of the sternum, and inserting into the medial aspect of the body of the hyoid bone. It functions to depress the hyoid bone and larynx, playing a key role in lowering the hyoid during swallowing.^[7]^[8] Lying deep to the sternohyoid is the sternothyroid muscle, which originates from the posterior surface of the manubrium of the sternum and inserts onto the oblique line of the thyroid cartilage. This muscle depresses the larynx independently of the hyoid bone, aiding in the stabilization required for vocalization and swallowing.^[7]^[8] The thyrohyoid muscle represents a superior continuation of the sternothyroid, originating from the oblique line of the thyroid cartilage and inserting into the greater cornu and body of the hyoid bone. It elevates the thyroid cartilage to approximate the hyoid or, conversely, depresses the hyoid when the larynx is fixed, thereby supporting laryngeal elevation during swallowing and speech.^[7]^[8] The superior belly of the omohyoid muscle forms the superolateral boundary of the muscular triangle, originating from the intermediate tendon (which is anchored to the clavicle) and inserting into the hyoid bone, while its tendon crosses through the triangle. Although primarily a boundary structure, it contributes to depressing the hyoid bone, enhancing the overall stability of the region.^[7]^[4] Collectively, these strap muscles stabilize the hyoid bone, enabling coordinated laryngeal movements critical for swallowing and speech production. The sternohyoid, sternothyroid, and omohyoid muscles are innervated by branches of the ansa cervicalis, while the thyrohyoid muscle receives innervation from C1 fibers via the hypoglossal nerve, ensuring precise control during these functions.^[8]^[7]

Glands and viscera

The thyroid gland, a key endocrine organ regulating metabolism, is situated in the anterior neck within the muscular triangle, spanning the vertebral levels from C5 to T1. It consists of two lateral lobes positioned on either side of the trachea, connected by a midline isthmus that overlies the anterior surface of the 2nd to 4th tracheal rings, forming a characteristic butterfly shape. The posterior surface of the thyroid relates closely to the recurrent laryngeal nerves, which course in the tracheoesophageal groove adjacent to its lobes.^[9]^[10] The parathyroid glands, four small endocrine structures responsible for calcium homeostasis, are embedded on the posterior aspect of the thyroid gland within the muscular triangle. The superior pair lies along the mid-posterior border of each thyroid lobe, near the level of the cricothyroid junction. The inferior pair is typically located near the inferior poles of the thyroid lobes, though their position can vary slightly among individuals.^[11]^[12] The lower portion of the larynx resides in the superior aspect of the muscular triangle, encompassing the thyroid and cricoid cartilages. The thyroid cartilage, the largest laryngeal cartilage, protrudes anteriorly to form the laryngeal prominence, while the signet ring-shaped cricoid cartilage lies inferiorly, providing attachment for the trachea and marking the transition to the airway's subglottic region.^[13] The cervical trachea occupies the midline of the muscular triangle, originating from the inferior border of the cricoid cartilage at the level of C6 and extending approximately 5 cm inferiorly before bifurcating in the thorax. Posterior to the trachea lies the esophagus, a muscular tube that begins at the same cricoid level and runs parallel in the midline, facilitating the passage of food into the superior mediastinum.^[14]^[15]

Vessels and nerves

The muscular triangle of the anterior neck contains several critical vascular and neural structures that supply and innervate the infrahyoid muscles, thyroid gland, and adjacent larynx. These elements include arteries originating from the external carotid and thyrocervical trunk, superficial and deep veins draining midline and glandular tissues, nerves from the cervical plexus and vagus, and lymph nodes facilitating drainage from endocrine and aerodigestive structures. While the carotid sheath and its contents lie more posteriorly in the adjacent carotid triangle, select branches and pathways traverse the muscular triangle to reach midline targets. The arterial supply to the muscular triangle is dominated by the superior thyroid artery and inferior thyroid artery. The superior thyroid artery arises directly from the anterior aspect of the external carotid artery just below the level of the greater cornu of the hyoid bone, then descends across the triangle to enter the superior pole of the thyroid gland, providing branches that supply the gland, sternothyroid and thyrohyoid muscles, and the larynx via the superior laryngeal artery. The inferior thyroid artery originates from the thyrocervical trunk of the subclavian artery in the root of the neck, courses superiorly and medially behind the carotid sheath to enter the muscular triangle posteriorly, supplying the inferior and posterior aspects of the thyroid gland, parathyroid glands, and esophagus while anastomosing with the superior thyroid artery; it also gives off ascending branches that reinforce thyroid vascularization. These arteries collectively ensure robust perfusion to the glandular and muscular contents, with the inferior thyroid artery notably vulnerable to compression during thyroid enlargement due to its posterior entry path. Venous drainage parallels the arterial pattern but includes more superficial components. The anterior jugular veins, arising from submental venous plexuses, course superficially within the muscular triangle along the midline anterior to the strap muscles, receiving tributaries from the anterior neck skin and platysma before uniting inferiorly to form a common trunk that drains into the subclavian vein or external jugular vein. Deeper, the superior thyroid vein emerges from the superior thyroid gland pole, accompanying the artery to drain directly into the internal jugular vein, while the middle thyroid vein arises from the lateral thyroid surface, draining into the internal jugular vein independently without a corresponding artery. These veins handle the bulk of glandular effluent, with variability in the middle thyroid vein's presence noted in up to 20% of cases where drainage merges with the inferior thyroid vein. Neural elements in the muscular triangle primarily provide motor and sensory innervation to the infrahyoid strap muscles and larynx. The ansa cervicalis, a neural loop from the cervical plexus (C1–C3 roots), descends superficially on the carotid sheath before entering the triangle to innervate the omohyoid, sternohyoid, and sternothyroid muscles, facilitating their roles in head stabilization and hyoid depression; its superior root (from hypoglossal nerve) and inferior root (from C2–C3) form the loop around the vessels. The external branch of the superior laryngeal nerve, originating from the vagus nerve (CN X) at the nodose ganglion, pierces the thyrohyoid membrane to enter the triangle and supplies motor innervation to the cricothyroid muscle, modulating vocal pitch. The recurrent laryngeal nerve, also from the vagus, ascends obliquely in the tracheoesophageal groove after looping inferiorly around the right subclavian artery or left aortic arch, traversing the muscular triangle to enter the larynx posterior to the cricothyroid joint and innervating all intrinsic laryngeal muscles except cricothyroid; its right-sided loop is shorter and higher than the left. Lymphatic structures within the muscular triangle include the prelaryngeal (delphian), pretracheal, and paratracheal nodes, which collect lymph from the thyroid gland, larynx, trachea, and anterior cervical skin before draining into the deep cervical chain along the internal jugular vein. The prelaryngeal node lies anterior to the cricothyroid ligament and is a sentinel for thyroid pathology, while pretracheal and paratracheal nodes flank the trachea and esophagus, handling efferent vessels from the thyroid isthmus and lobes; these nodes are integral to metastatic spread assessment in thyroid cancers due to their direct glandular drainage. Although primarily posterior in the carotid sheath, the vagus nerve and cervical sympathetic trunk influence the muscular triangle via branches, with the vagus contributing the laryngeal nerves described above and the sympathetic trunk providing vasomotor fibers to regional vessels without direct traversal of the triangle. Major carotid artery branches, such as the common or internal carotid, remain confined to the adjacent carotid triangle and do not enter the muscular space.

Clinical significance

Surgical relevance

The muscular triangle provides critical surgical access to midline structures in the anterior neck, facilitating procedures that require exposure of the trachea, thyroid gland, and adjacent vasculature while minimizing disruption to surrounding tissues. Surgeons often retract the sternocleidomastoid muscle posteriorly and divide the omohyoid muscle to expose the common carotid artery within or adjacent to this region, enabling ligation below the level of the omohyoid for control in cases of hemorrhage or trauma. In thyroidectomy, a low transverse collar incision is made approximately 1-2 cm above the sternal notch to align with natural skin creases, followed by elevation of superior and inferior subplatysmal flaps to reflect the platysma muscle. The strap muscles (sternohyoid and sternothyroid) are then separated or divided vertically along the midline raphe and retracted laterally, providing direct visualization of the thyroid lobes within the muscular triangle's boundaries. Careful identification and preservation of the recurrent laryngeal nerve is essential to prevent vocal cord paralysis.^[16] For tracheostomy, the optimal site lies in the midline between the cricoid cartilage and the sternal notch, typically targeting the second to third tracheal rings to avoid the thyroid isthmus, which may require ligation or division if it overlies the trachea. After incising through the platysma and separating the strap muscles laterally, the pretracheal fascia is opened to access the trachea, ensuring rapid airway establishment while preserving vascular integrity.^[17] Early anatomical dissections in the 19th century, such as those informing carotid artery ligations for penetrating neck trauma, underscored the muscular triangle's utility in achieving vascular control by delineating safe pathways through the strap muscles and midline structures.^[18]

Associated pathologies

The muscular triangle of the neck encompasses structures vulnerable to several pathologies, particularly those involving the thyroid and parathyroid glands, larynx, trachea, vasculature, and lymphatic system. Thyroid disorders commonly manifest within this region due to the gland's location. Goiter, an enlargement of the thyroid gland, can cause compressive symptoms such as dysphagia and dyspnea when the mass expands sufficiently to impinge on adjacent structures like the esophagus and trachea.^[19] Thyroiditis, exemplified by Hashimoto's disease, involves autoimmune-mediated chronic inflammation of the thyroid, leading to glandular swelling and potential hypothyroidism through progressive destruction of thyroid tissue.^[20] Thyroid cancer, primarily papillary and follicular types, originates within the gland and may present as a neck mass with hoarseness or swallowing difficulties if it invades local tissues; papillary carcinoma is the most prevalent subtype, often with an indolent course, while follicular variants tend to affect older individuals and carry a risk of distant metastasis.^[21] Parathyroid pathologies also impact the muscular triangle, as these glands lie posterior to the thyroid. Primary hyperparathyroidism frequently results from a benign adenoma in one parathyroid gland, causing excessive parathyroid hormone secretion and resultant hypercalcemia, which manifests with fatigue, kidney stones, and bone pain.^[22] Hypoparathyroidism, conversely, often arises post-surgically from inadvertent removal or damage to the glands during thyroidectomy, leading to hypocalcemia with symptoms including muscle cramps, tetany, and paresthesias; this complication underscores the need for careful intraoperative preservation of parathyroid blood supply.^[23] Laryngeal and tracheal conditions within the triangle can compromise airway patency. Laryngitis, an inflammation of the larynx typically due to viral infection or overuse, presents with hoarseness, voice weakness, and throat discomfort, potentially narrowing the airway and causing stridor in severe cases.^[24] Tracheal stenosis, often post-intubation or from inflammatory scarring, narrows the tracheal lumen and impacts airflow in the triangle, resulting in dyspnea, wheezing, and recurrent infections, with diagnosis relying on bronchoscopy to assess the degree of obstruction.^[25] Vascular anomalies in the region include aneurysms of the thyroid arteries and thrombosis of the anterior jugular veins. Superior or inferior thyroid artery aneurysms, though rare, can rupture spontaneously, leading to hematoma formation, neck swelling, and acute airway compromise requiring emergent intervention.^[26] Anterior jugular vein thrombosis may occur secondary to local trauma or hypercoagulability, presenting with neck pain, swelling, and a palpable cord-like structure, with risks of propagation to deeper veins if untreated.^[27] Lymphatic involvement often stems from metastatic spread, as lymph nodes in the muscular triangle drain head and neck regions. Metastatic nodes from head and neck squamous cell carcinomas, such as those originating in the oral cavity or pharynx, commonly enlarge in this area, causing painless masses that signal advanced disease and necessitate imaging and biopsy for staging.^[28]

References

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[1] The muscular triangle is outlined by the anterior aspect of the sternocleidomastoid and superior belly of the omohyoid, the midline of the neck, and the ...
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The anterior triangle is a region located at the front of the neck. In this article, we shall look at the anatomy of the anterior triangle of the neck – its ...
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The infrahyoid muscles are a group of four muscles that are located inferiorly to the hyoid bone in the neck. They can be divided into two groups.
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Hashimoto's disease is an autoimmune disorder. The immune system creates antibodies that attack thyroid cells as if they were bacteria, viruses or some other ...Overview · Symptoms · When to see a doctor · Causes
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Sep 19, 2025 · A growth on a parathyroid gland that isn't cancer. This is called an adenoma. It's the most common cause of primary hyperparathyroidism. · Two or ...Diagnosis & treatment · Parathyroidectomy · Treatment of parathyroid...
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