Anterior Triangle of neck

INTRODUCTION

– The anterior triangle is a triangular region on either side of the anterior median line of the neck.

Boundaries

• Anteriorly: It is bounded by the anterior median line extending from the symphysis menti to the suprasternal notch.
• Posteriorly: It is bounded by the anterior border of the sternocleidomastoid muscle.
• Base: It is directed upward and formed by the lower border of the mandible and a line joining its angle to the mastoid process.
• Apex: It is directed downward, where the anterior border of sternocleidomastoid meets the anterior median line.
• Roof: It is formed by the skin, superficial fascia containing platysma, and investing layer of deep cervical fascia.

Structures in the anterior median region of the neck

– The anterior median region is a vertical strip, approximately 2–3 cm wide, extending from the symphysis menti to the suprasternal notch.

Superficial Structures

• Skin: It forms the superficial covering of the region.
• Platysma: Its medial fibres may cross the midline approximately 1–2 cm below the chin.
• Anterior jugular vein: It begins in the submental region and descends near the midline. Above the sternum, it pierces the investing fascia and passes laterally deep to sternocleidomastoid.
• Submental lymph nodes: They lie beneath the chin, near the symphysis menti.
• Transverse cervical nerve: Its branches supply the skin of the anterior and lateral neck.
• Investing layer of deep cervical fascia: It encloses the submandibular gland superiorly and forms the suprasternal space inferiorly.

Deep Structures Above the Hyoid Bone

• Mylohyoid muscle and its median raphe.
• Anterior belly of digastric muscle.
• Superficial part of the submandibular gland, situated lateral to the midline.
• Nerve and vessels supplying mylohyoid.
• Submental branch of the facial artery.

Deep Structures Below the Hyoid Bone

• Pretracheal fascia: It forms the false capsule of the thyroid gland and contributes to the ligament of Berry.
• Infrahyoid muscles: They include sternohyoid, sternothyroid, thyrohyoid, and omohyoid.
• Thyrohyoid membrane: It is pierced laterally by the internal laryngeal nerve and superior laryngeal vessels.
• Other structures include the thyroid cartilage, cricothyroid membrane, cricoid cartilage, cricothyroid muscle, thyroid gland, and trachea.

SURFACE LANDMARKS OF ANTERIOR TRIANGLE

• Mandible: Its horseshoe-shaped lower border forms the upper boundary of the anterior triangle.
• Chin: It is the median prominence of the lower face, above the inferior border of the mandible.
• Angle of mandible: It is palpable at the junction of the body and ramus of the mandible.
• Hyoid bone: It is a U-shaped bone palpable in the anterior neck at the level of the C3 vertebra.
• Thyroid cartilage: Its anterior projection forms the laryngeal prominence (Adam’s apple), which is more prominent in males.
• Cricoid cartilage: It lies immediately below the thyroid cartilage at the level of the C6 vertebra.
• Trachea: It begins below the cricoid cartilage. Its second to fourth rings are usually covered anteriorly by the thyroid isthmus.
• Suprasternal notch: It lies between the medial ends of the clavicles, above the manubrium, approximately at the level of the lower border of T2 vertebra.
• Jugular venous arch: It connects the two anterior jugular veins within the suprasternal space.

Structures at the Level of C6 Vertebra

• The cricoid cartilage lies at this level.
• The larynx continues inferiorly as the trachea.
• The pharynx continues inferiorly as the oesophagus.
• The common carotid artery can be compressed against the carotid tubercle of C6.

Additional Anatomical Facts

• Thyroidea ima artery: It is an occasional artery arising commonly from the brachiocephalic trunk or occasionally from the arch of aorta. It ascends anterior to the trachea to supply the thyroid gland, particularly its isthmus.
• Median thyrohyoid ligament: It is the median thickening of the thyrohyoid membrane.
• Subhyoid bursa: It separates the posterior surface of the hyoid bone from the thyrohyoid membrane.
• Vertebral levels: The hyoid bone lies at C3, the upper border of thyroid cartilage at approximately C4, and the cricoid cartilage at C6.

SUBDIVISION OF ANTERIOR TRIANGLE

– The anterior triangle is subdivided by the anterior belly of digastric and superior belly of omohyoid into four triangles.

1. Submental triangle (half on each side).
2. Digastric (submandibular) triangle.
3. Carotid triangle.
4. Muscular triangle.

Submental Triangle

– The submental triangle is a median triangular area situated between the chin and hyoid bone. Each anterior triangle contributes one half.

Boundaries

• Apex: It is formed by the symphysis menti.
• Base: It is formed by the body of the hyoid bone.
• Lateral boundaries: They are formed by the anterior bellies of the digastric muscles.
• Floor: It is formed by the mylohyoid muscles and their median raphe.
• Roof: It consists of the skin, superficial fascia, and investing layer of deep cervical fascia.

Contents

• Submental lymph nodes are present within the triangle.
• Submental veins unite to contribute to the formation of the anterior jugular veins.

Digastric Triangle

– The digastric triangle, also called the submandibular triangle, lies below the lower border of the mandible.

Boundaries

• Anteroinferiorly: It is bounded by the anterior belly of digastric.
• Posteroinferiorly: It is bounded by the posterior belly of digastric and stylohyoid.
• Superiorly: It is bounded by the lower border of the mandible and a line extending from its angle to the mastoid process.
• Apex: It is formed by the intermediate tendon of digastric.
• Floor: It is formed by the mylohyoid, hyoglossus, and middle constrictor muscles, arranged from anterior to posterior.
• Roof: It consists of the skin, superficial fascia containing platysma, and investing layer of deep cervical fascia. The cervical branch of facial nerve and branches of transverse cervical nerve traverse the superficial tissues.

Contents

– The stylomandibular ligament separates the anterior and posterior parts of the region.

Anterior part

– It contains the superficial part of submandibular gland, facial artery and vein, submandibular lymph nodes, submental artery, nerve and vessels to mylohyoid, and hypoglossal nerve.

Posterior part

• It contains the lower part of parotid gland and external carotid artery.
• Its deeper structures include the styloid process, styloglossus, stylopharyngeus, and glossopharyngeal nerve.
• The carotid sheath and its contents lie still deeper.

Carotid Triangle

– The carotid triangle contains the terminal part of the common carotid artery and proximal parts of the external and internal carotid arteries.

Boundaries

• Anterosuperiorly: It is bounded by the posterior belly of digastric and stylohyoid.
• Anteroinferiorly: It is bounded by the superior belly of omohyoid.
• Posteriorly: It is bounded by the anterior border of sternocleidomastoid.
• Floor: It is formed by the thyrohyoid, hyoglossus, middle constrictor, and inferior constrictor muscles.
• Roof: It consists of the skin, superficial fascia containing platysma, and investing layer of deep cervical fascia.

Contents

• Arteries: The common, external, and internal carotid arteries are present with the carotid sinus and carotid body.
• Veins: The internal jugular vein receives the common facial, lingual, and pharyngeal veins.
• Cranial nerves: The vagus, accessory, and hypoglossal nerves are related to the triangle.
• Other nerves: These include the superior laryngeal nerve with its internal and external branches, superior root of ansa cervicalis (descendens hypoglossi), and cervical sympathetic trunk.
• Carotid sheath: It encloses the common and internal carotid arteries, internal jugular vein, and vagus nerve.
• Lymph nodes: The deep cervical lymph nodes include the jugulodigastric and jugulo-omohyoid nodes.

Muscular Triangle

– The muscular triangle occupies the lower part of the anterior triangle, close to the midline.

Boundaries

• Anteriorly: It is bounded by the anterior median line extending from the hyoid bone to the suprasternal notch.
• Posterosuperiorly: It is bounded by the superior belly of omohyoid.
• Posteroinferiorly: It is bounded by the anterior border of sternocleidomastoid.
• Floor: It is formed by the pretracheal fascia and underlying thyroid and cricoid cartilages and trachea.
• Roof: It consists of the skin, superficial fascia containing platysma, and investing layer of deep cervical fascia.

Contents

• It contains the infrahyoid muscles, particularly sternohyoid, sternothyroid, and thyrohyoid. The superior belly of omohyoid forms its superolateral boundary.
• Deeper structures include the thyroid gland, larynx, and trachea.

Note: Some authors describe the infrahyoid muscles as forming the floor of the muscular triangle.

MUSCLES OF ANTERIOR TRIANGLE OF NECK

– The muscles of the anterior triangle are classified into two groups:

A. Suprahyoid muscles (four pairs)

1. Digastric
2. Stylohyoid
3. Mylohyoid
4. Geniohyoid

B. Infrahyoid muscles (four pairs)

1. Sternohyoid
2. Sternothyroid
3. Thyrohyoid
4. Omohyoid

Infrahyoid Muscles

• The infrahyoid muscles are thin, ribbon-shaped muscles, commonly called strap muscles.
• They are arranged in two layers:
  • Superficial layer: Sternohyoid and omohyoid.
  • Deep layer: Sternothyroid and thyrohyoid.
• They are supplied by fibres from the anterior rami of C1–C3 spinal nerves.
• All receive their nerve supply through the ansa cervicalis, except thyrohyoid, which receives C1 fibres through the hypoglossal nerve.
• They mainly depress or stabilise the hyoid bone and larynx during swallowing and speech.

Sternohyoid

• Description: It is a long, narrow strap muscle extending upward and medially.
• Origin: It arises from the posterior surface of the manubrium and adjoining medial end of the clavicle.
• Insertion: It attaches to the inferior border of the body of the hyoid bone.
• Nerve supply: Ansa cervicalis (C1–C3).
• Action: It depresses the hyoid bone after its elevation during swallowing.

Sternothyroid

• Description: It is a flat strap muscle, shorter and broader than sternohyoid.
• Origin: It arises from the posterior surface of the manubrium and adjoining first costal cartilage.
• Insertion: It attaches to the oblique line of the thyroid cartilage.
• Nerve supply: Ansa cervicalis (C1–C3).
• Action: It depresses the larynx after its elevation during swallowing.

Thyrohyoid

• Description: It is a short, quadrilateral muscle situated deep to sternohyoid.
• Origin: It arises from the oblique line of the thyroid cartilage.
• Insertion: It attaches to the inferior border of the body and greater horn of the hyoid bone.
• Nerve supply: C1 fibres carried by the hypoglossal nerve.
• Actions:
  • It depresses the hyoid bone when the larynx is fixed.
  • It elevates the larynx when the hyoid bone is fixed.

Omohyoid

• Description: It consists of superior and inferior bellies connected by an intermediate tendon, which is anchored to the clavicle by a fascial sling.
• Origin: The inferior belly arises from the superior border of the scapula near the suprascapular notch.
• Insertion: The superior belly attaches to the inferior border of the hyoid bone, lateral to sternohyoid.
• Nerve supply: Ansa cervicalis (C1–C3).
• Actions:
  • It depresses and retracts the hyoid bone.
  • It tenses the cervical fascia through its intermediate tendon.

Ansa cervicalis

– Definition: Ansa cervicalis, also called ansa hypoglossi, is a loop of cervical spinal nerve fibres situated on or within the anterior wall of the carotid sheath.

Formation

It is formed by the union of two roots:

• Superior root (descendens hypoglossi): It contains C1 fibres that travel briefly with the hypoglossal nerve before descending over the internal and common carotid arteries.
• Inferior root (descendens cervicalis): It contains fibres from C2 and C3 spinal nerves. It passes around the internal jugular vein and joins the superior root, usually in front of the common carotid artery, to form a loop.

Distribution

• Superior root: It supplies the superior belly of omohyoid.
• Ansa cervicalis loop: It supplies sternohyoid and sternothyroid.
• Inferior root or loop: It supplies the inferior belly of omohyoid.

Note: C1 fibres travelling with the hypoglossal nerve supply thyrohyoid and geniohyoid directly, without passing through the ansa cervicalis.

Carotid sinus and carotid body

Carotid Sinus

• Location: It is a fusiform dilatation at the origin of the internal carotid artery, near the bifurcation of the common carotid artery.
• Structure: Its wall contains baroreceptors sensitive to changes in arterial pressure.
• Innervation: Its sensory fibres travel mainly through the carotid sinus nerve (Hering’s nerve), a branch of the glossopharyngeal nerve. Additional innervation comes from the vagus and sympathetic nerves.
• Function: Increased arterial pressure stimulates baroreceptors, producing a reflex reduction in heart rate and blood pressure.

Carotid Body

• Location: It is a small, oval, reddish-brown structure situated near the posterior aspect of the common carotid bifurcation.
• Structure: It contains peripheral chemoreceptors that monitor changes in blood gases and pH.
• Innervation: Its sensory fibres travel mainly through the glossopharyngeal nerve, with additional contributions from the vagus and sympathetic nerves.
• Function: Reduced arterial oxygen tension, increased carbon dioxide, or decreased pH stimulates chemoreceptors, producing a reflex increase in the rate and depth of respiration.

Clinical Integration

ARTERIES OF THE ANTERIOR TRIANGLE OF NECK

Common Carotid Artery (CCA)

• The common carotid artery is a major source of blood supply to the head and neck, including the brain.
• Origin:
  • Right CCA: It arises from the brachiocephalic trunk behind the right sternoclavicular joint.
  • Left CCA: It arises directly from the arch of the aorta.
• Course: It ascends within the carotid sheath towards the upper border of the thyroid cartilage.
• Termination: It divides into the external and internal carotid arteries at approximately the level of the upper border of the thyroid cartilage (C3–C4 level).
• Branches: It usually gives no branches in the neck.

Internal Carotid Artery

• The internal carotid artery begins at the bifurcation of the common carotid artery, initially posterolateral to the external carotid artery.
• It ascends within the carotid sheath and enters the cranial cavity through the carotid canal.
• Branches: It gives no branches in the neck.

Structures between Internal and External Carotid Arteries

– The upper parts of these arteries are separated by the following structures:

1. Part of the parotid gland.
2. Styloid process of temporal bone.
3. Styloglossus muscle.
4. Stylopharyngeus muscle.
5. Glossopharyngeal nerve.
6. Pharyngeal branch of vagus nerve.

External Carotid Artery

• The external carotid artery supplies most extracranial structures of the head and neck.
• Origin: It arises from the common carotid artery at the upper border of the thyroid cartilage, approximately opposite the C3–C4 level.
• Course: It ascends through the carotid and digastric triangles before entering the parotid gland.
• Termination: It divides into the superficial temporal and maxillary arteries within the parotid gland, behind the neck of the mandible.

Relations

In the carotid triangle

• Superficial relations: These include sternocleidomastoid, cervical branch of facial nerve, hypoglossal nerve, and facial, lingual, and superior thyroid veins.
• Deep relations: These include the pharyngeal wall, superior laryngeal nerve, ascending pharyngeal artery, and internal carotid artery.

In the digastric triangle

– The artery passes deep to the posterior belly of digastric and stylohyoid before entering the parotid gland.

Within the parotid gland

• Superficially: It is related to the retromandibular vein and facial nerve with its branches.
• Deeply: It is related to the deep part of parotid gland.
• Other deeper relations include the internal carotid artery, styloid process, styloglossus, stylopharyngeus, glossopharyngeal nerve, and pharyngeal branch of vagus.

Branches of External Carotid Artery

– The external carotid artery gives eight branches, arranged as follows:

Medial branch

1. Ascending pharyngeal artery.

Anterior branches

• Superior thyroid artery.
• Lingual artery.
• Facial artery.

Posterior branches

• Occipital artery.
• Posterior auricular artery.

Terminal branches

• Superficial temporal artery.
• Maxillary artery.

Note: Five branches commonly arise within the anterior triangle: ascending pharyngeal, superior thyroid, lingual, facial, and occipital arteries.

Ascending Pharyngeal Artery

• It is usually the smallest branch of the external carotid artery and arises from its medial aspect.
• Course: It ascends vertically along the lateral wall of the pharynx towards the skull base.
• Branches:
  • Pharyngeal branches: They supply the pharynx, palatine tonsil, soft palate, and pharyngeal opening of the auditory tube.
  • Inferior tympanic artery: It enters the tympanic cavity through the inferior tympanic canaliculus and supplies the middle ear.
  • Meningeal branches: They enter the cranial cavity through openings including the jugular foramen and hypoglossal canal.

Superior Thyroid Artery

• It is usually the first anterior branch of the external carotid artery, arising near the greater horn of the hyoid bone.
• Course: It descends forwards towards the superior pole of the thyroid gland, passing deep to the infrahyoid muscles.
• Branches:
  • Infrahyoid branch: It runs along the inferior border of the hyoid bone.
  • Superior laryngeal artery: It pierces the thyrohyoid membrane with the internal laryngeal nerve and supplies the larynx.
  • Cricothyroid branch: It supplies the cricothyroid region and anastomoses with its opposite counterpart.
  • Sternocleidomastoid branch: It supplies the sternocleidomastoid muscle.
  • Terminal branches: The anterior and posterior glandular branches supply the thyroid gland.

Lingual Artery

• It arises from the anterior aspect of the external carotid artery near the greater horn of the hyoid bone.
• Course: It follows a tortuous course and is divided into three parts by its relationship to the hyoglossus muscle.

First part

• It lies in the carotid triangle and forms an upward convex loop, allowing movements of the hyoid bone and tongue.
• The hypoglossal nerve crosses superficial to this part.

Second part

• It passes deep to the hyoglossus muscle, near the greater horn of the hyoid bone.
• The hypoglossal nerve runs superficial to hyoglossus.

Third part

• It continues as the deep lingual artery, running forwards towards the tip of the tongue.
• It lies deep to the mucosa on either side of the frenulum linguae.

Branches

1. Suprahyoid branch: It runs along the superior border of the hyoid bone.
2. Dorsal lingual branches: These supply the posterior tongue, palatine tonsil, and soft palate.
3. Sublingual artery: It supplies the sublingual gland and floor of the mouth.
4. Deep lingual artery: It is the terminal continuation supplying the anterior tongue.

Clinical Integration

– During extensive tongue surgery, the lingual artery may be ligated proximally to control bleeding.

Facial Artery

• The facial artery arises from the anterior aspect of the external carotid artery, usually just above the lingual artery.
• It is divided into cervical and facial parts.

Cervical part

• It ascends deep to the posterior belly of digastric and stylohyoid.
• It grooves the submandibular gland before curving around the inferior border of the mandible to enter the face.

Branches of the cervical part

1. Ascending palatine artery: It ascends between styloglossus and stylopharyngeus and supplies the soft palate, pharynx, palatine tonsil, and auditory tube.
2. Tonsillar artery: It pierces the superior constrictor and provides a major blood supply to the palatine tonsil.
3. Glandular branches: They supply the submandibular gland.
4. Submental artery: It accompanies the nerve to mylohyoid and supplies the submental region.

Branches of the facial part

1. Inferior labial artery.
2. Superior labial artery.
3. Lateral nasal artery.

Occipital Artery

• It arises from the posterior aspect of the external carotid artery, usually opposite the origin of the facial artery.
• Course: It passes upwards and backwards, deep to the posterior belly of digastric, sternocleidomastoid, and splenius capitis.
• It occupies a groove medial to the mastoid process and reaches the posterior scalp by piercing the muscles and fascia of the occipital region.

Branches

1. Sternocleidomastoid branches: The upper branch accompanies the accessory nerve, while the lower branch may loop around the hypoglossal nerve.
2. Mastoid branch: It enters the mastoid foramen and supplies the mastoid region and meninges.
3. Meningeal branches: They enter the skull through the jugular foramen and condylar canal.
4. Muscular branches: They supply adjacent neck muscles.
5. Descending branch: Its superficial division anastomoses with the transverse cervical artery, while its deep division joins the deep cervical artery.
6. Occipital branches: They supply the posterior scalp.

Posterior Auricular Artery

• It arises from the posterior aspect of the external carotid artery.
• Course: It passes upwards and backwards near the superior border of the posterior belly of digastric, between the parotid gland and styloid process.
• It divides into auricular and occipital branches between the mastoid process and auricle.

Branches

1. Stylomastoid artery: It enters the stylomastoid foramen and supplies the facial nerve, middle ear, and mastoid air cells.
2. Auricular branch: It supplies the auricle.
3. Occipital branch: It supplies the scalp behind the auricle.

Superficial Temporal Artery

• It is a terminal branch of the external carotid artery arising within the parotid gland behind the neck of the mandible.
• Course: It ascends across the root of the zygomatic arch and divides into frontal and parietal branches.

Branches

1. Transverse facial artery: It crosses the masseter and supplies the parotid gland, temporomandibular joint, and face.
2. Anterior auricular branches: They supply the auricle and external acoustic meatus.
3. Zygomatico-orbital branch: It supplies the lateral facial and orbital regions.
4. Middle temporal artery: It pierces the temporal fascia and supplies temporalis.
5. Frontal and parietal branches: They supply the temporal and parietal regions of the scalp.

Maxillary Artery

• It is the larger terminal branch of the external carotid artery.
• Origin: It begins within the parotid gland behind the neck of the mandible.
• Course: It passes forwards into the infratemporal fossa, supplying deep structures of the face and adjoining regions.