Anatomy of Neck Triangles: Structures and Boundaries

Anatomy of neck triangles is an important topic for NEET PG preparation. This blog post will help you understand what are Neck Triangles, the Anatomy of Neck Triangles, their structures and boundaries in detail. So, let’s get started! 

Q. All of the following are in the anterior triangle of neck EXCEPT

1. Digastric
2. Subclavian
3. Muscular 
4. Submental 

• Subclavian is in the posterior triangle  

Explanation

• Posterior triangle has two parts
  • Occipital 
  • Subclavian 
• Anterior triangle has 3 parts
  • Digastric
  • Muscular 
  • Submental

• Posterior triangle 
• Anterior triangle
• Boundary - Sternocleidomastoid
  • Origin from sternum and clavicle
  • Inserted into mastoid

Posterior triangle

• Boundaries
  • Anteriorly - sternocleidomastoid
  • Posteriorly - trapezius muscle
  • Base - middle 1/ 3 of clavicle
• Omohyoid has superior or anterior belly and inferior or posterior belly
• Inferior belly of omohyoid divides posterior triangle into
  • Larger occipital triangle 
  • Smaller subclavian triangle
• Occipital triangle - near occipital bone
• Subclavian triangle
  • Block can be given in posterior triangle of neck - subclavian triangle
    • Third part of subclavian artery can be blocked here 
    • Cervical plexus block
    • Brachial plexus block 

Anterior triangles

1. Carotid triangle - bounded by superior belly of omohyoid

• Common carotid artery bifurcation
• All 3 arteries - common carotid, external and internal carotid arteries are contents

2. Digastric triangle 

• bounded by anterior and posterior belly of digastric
• Base - mandible

3. Muscular triangle- strap muscles - anterior neck muscles 

• Thyroid gland covered by strap muscles
  • Sternohyoid
  • Sternothyroid
  • Thyrohyoid
  • Omohyoid 

4. Submental triangle - one in midline (Others are one on right and one on the left side)

• Under the mentum of mandible 
• Bounded by anterior belly of digastric on either side
• Base - hyoid bone
• Submental lymph nodes- drain tip of tongue 
  • They then drain into submandibular or directly into lower deep cervical lymph nodes
• Base of Mandible bone
• Hyoid bone in the midline
• Mastoid process of temporal bone
• Occipital bone 
• Sternum articulates with clavicle on either side
• Scapula
• Sternocleidomastoid 
  • Called so because it originates from sternum and clavicle
  • Inserted into mastoid process
  • Boundary of posterior triangle
• Posterior triangle bounded by
  • Anterior : Sternomastoid
  • Posterior : Trapezius 
  • Base : clavicle
• 2 triangles in posterior triangle
  • Omoclavicular triangle - subclavian triangle 
  • Occipital triangle 
• Anterior triangle
  • Digastric triangle
  • Between two anterior bellies and base - hyoid - submental triangle
  • Carotid - sternomastoid, posterior belly of digastric, Superior belly of omohyoid
  • Muscular triangle in midline 

Triangles - front view

• In midline
  • Mandible
  • Hyoid bone
  • Sternum 
• Sternocleidomastoid - divides neck into posterior triangle and anterior triangle
• Posterior triangle - boundaries 
  • Sternocleidomastoid
  • Trapezius - muscle of upper back
    • From occipital bone to scapula and clavicle 
  • Base - Middle 1/ 3 of clavicle 
• Inferior belly of omohyoid - from scapula to hyoid bone
  • Divides posterior triangle 
  • Subclavian Triangle - boundaries
    • Inferior belly of omohyoid
    • Sternomastoid muscle
    • Clavicle 
  • Occipital triangle
    • Inferior belly of omohyoid
    • Sternomastoid
    • Trapezius
• Anterior triangle 
  • Digastric triangle
    • Anterior belly of digastric - to hyoid bone and mastoid process
    • Posterior belly - hyoid and mastoid process
    • Base - mandible
    • Bounded by two bellies of digastric muscle - so called digastric triangle
  • Midline triangle - submental triangle 
    • Base - hyoid 
    • Anterior belly on both sides 
    • One triangle in midline
    • Below the Mentum of mandible 
  • Muscular triangle 
    • Thyroid gland - one half on each muscular triangle - one lobe in each
    • Thyroid - covered by strap muscles
• Sternohyoid - ribbon like muscle - sternum to hyoid 
• Carotid triangle
  • Common carotid artery bifurcation into external and internal carotid artery -In the middle of triangle
  • Boundaries 
    • Sternomastoid
    • Superior belly of omohyoid
    • Posterior belly digastric 

Digastric triangle

• Floor of digastric triangle - floor of mouth - Mylohyoid muscle
  • Mandible to hyoid
  • Depressor of mandible - open mouth - along with anterior belly of digastric
• Submandibular lymph nodes
  • Lateral side of tongue drained by submandibular lymph nodes

Submental triangle

• Under the mentum of mandible 
• Submental lymph nodes
  • Drain tip of tongue 

Anterior muscular triangle

• Thyroid gland covered by strap muscles
• Sternohyoid - sternum to hyoid 

Carotid triangle

• Common Carotid artery bifurcation into 2 branches
• Followed by Internal jugular vein outside 
• Carotid sheath cover internal jugular vein and common carotid artery 
• Contents of carotid triangle
  • Carotid sheath
  • Common carotid artery - medially
  • External carotid artery
  • Internal carotid artery 
  • Internal jugular vein -laterally
• Boundaries
  • Superior belly of omohyoid
  • Posterior belly of digastric
  • sternomastoid

Subclavian artery

• Second part covered by scalenus anterior
• Proximal to the muscle - first part
• Distal to muscle - third part
• Third part of subclavian artery - in subclavian triangle - posterior triangle 
• Lateral to scalenus anterior
  • Subclavian artery - 3rd part can be blocked in subclavian triangle 
  • Brachial plexus can be blocked subclavian triangle - in case of multiple fracture in upper limb

Q. All is true about digastric triangle EXCEPT:

1. On either side is anterior belly of digastric
2. Floor is formed by mylohyoid muscle
3. Floor is formed by hyoglossus muscle
4. Contains mylohyoid nerve and vessels

Explanation

• Digastric triangle is bounded by anterior belly and posterior belly of digastric
• The statement is true for submental triangle. On either side is anterior bellies of digastric for submental triangle 
• Floor of digastric triangle

• Mylohyoid
• Hyoglossus 
• Mylohyoid nerve and vessels

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Digastric triangle

• Anterior belly of digastric
  • Origin - mandible
  • Insertion - hyoid 
  • Depressor of mandible to open mouth 
• Posterior Belly of digastric
  • Mastoid and hyoid
• Base - mandible
• Floor of digastric triangle 
  • Mylohyoid - mandible to hyoid 
  • Hyoglossus - hyoid bone and tongue
    • Depressor of tongue
    • Supplied by 12th nerve 
  • 12th nerve → hypoglossal canal → enter carotid triangle → goes to digastric triangle → supply tongue muscles  
  • Mylohyoid nerve and artery

Carotid triangle 

• Common carotid bifurcates into external and internal carotid arteries
• Internal jugular vein
• Both internal jugular vein (laterally) and common carotid artery (medially) is covered by carotid sheath
• External carotid artery comes out of the triangle, later only internal carotid is a content
• Hypoglossal nerve
• Vagus nerve - between the internal jugular vein and common carotid artery and behind them - inside the carotid sheath 
• Ansa cervicalis - embedded anteriorly to the carotid sheath
  • Supply anterior neck muscles
    • Sternohyoid muscle
    • Omohyoid muscle
• Behind the carotid sheath - Sympathetic chain 
  • Stellate ganglion block given here in case of Reynaud’s phenomenon 
    • To reduce sympathetic vasoconstriction 
    • Reynaud’s phenomenon - Impending gangrene and Hyperhidrosis due to -Overactivity of sympathetic system

Digastric and Carotid triangles

Content of both digastric and carotid triangle - hypoglossal nerve To supply hyoglossus at the floor of digastric triangle  Floor of both digastric and submental triangles - mylohyoid Lymph nodes in digastric - submandibular lymph node - lateral side of tongue In submental triangle - submental lymph node- tip of tongue  Carotid sheath - enclose Common carotid artery (later, internal carotid artery) Internal jugular vein Vagus between and behind common carotid artery and internal jugular vein Ansa cervicalis - embedded anteriorly on the carotid sheath Supply sternohyoid Omohyoid  Behind carotid sheath - Sympathetic trunk  Stellate ganglion block can be given here

Stellate ganglion block

• Carotid sheath moved laterally 
• Using ultrasound, the sympathetic trunk is marked
• Block is given
• Done in case of Reynaud’s phenomenon
• Sympathetic overactivity
  • Vasoconstriction is more → ischemia and impending gangrene 
  • Hyperhidrosis - sweat gland is under sympathetic cholinergic system
• Using ultrasound probe → mark sympathetic trunk in front of sternomastoid, in anterior triangle 
• Behind carotid sheath → sympathetic trunk, stellate ganglion blocked
• In Reynaud’s phenomenon → the periphery becomes pale → severe ischemia → gangrene 

Anterior triangle - carotid triangle In front of sternomastoid Contents  Common carotid artery Internal jugular vein CN X Ansa cervicalis Sympathetic trunk (stellate ganglion block ) CN XII

Posterior triangle  Occipital triangle and Subclavian triangle Contents Subclavian artery  External jugular vein Cervical plexus (nerve block) Brachial plexus trunk Phrenic nerve CN XI

Three blocks can be given in posterior triangle

1. Brachial plexus can be blocked - multiple fracture of upper limb 
   • Lateral to scalenus anterior in subclavian triangle 
2. Cervical plexus block 
3. Subclavian artery block

Cervical plexus block 

• Posterior border of sternomastoid
• In the midpoint of sternomastoid 
• Gives 4 branches
  • Greater auricular nerve
  • Lesser occipital nerve
  • Transverse cervical nerve
  • Supraclavicular nerve 

Spinal accessory nerve - supply sternomastoid and trapezius 

• While draining an abscess in the posterior triangle - be careful 
• Surface mark the spinal accessory nerve
  •  To avoid iatrogenic injury to 11th nerve 
• This paralyzes trapezius → difficulty in shrugging the shoulder 
• The Sternomastoid is spared
• Because in nerve injury → only distal part is affected 
• Sternomastoid is already supplied
• Phrenic nerve should never be blocked - difficulty breathing respiratory muscle paralysis 

Subclavian artery block

• Superior surface of first rib 
• Third part of subclavian artery blocked using thumb
• Lateral to sternomastoid and scalenus anterior
• Subclavian artery third part can be blocked in neck region
• Subclavian artery continue as axillary artery in axilla 
• Posterior border of sternomastoid - Midpoint - cervical plexus block

Brachial plexus block

• Scalenus anterior inserted into inner border of first rib - carotid tubercle 
• Scalenus medius is also attached to first rib - superior surface 
• Interscalene triangle - between scalenus anterior and scalenus medius
• Both blocks are given here
  • Subclavian artery block and brachial plexus block
• Third part distal to scalenus anterior
• Artery is compressed on superior surface of first rib
• Trunks of brachial plexus in neck
• Cords are in axilla

Scalenus anterior and medius inserted to first rib In between the scalene muscles - interscalene triangle Brachial plexus and subclavian artery can be blocked in interscalene triangle Axillary sheath covers brachial plexus and axillary artery Axillary vein is outside the sheath  Subclavian artery crosses first rib to become axillary artery  Similarly, Axillary vein becomes subclavian vein Subclavian artery and vein sandwich scalenus anterior muscle  Subclavian artery becomes axillary artery Axillary vein - outside axillary sheath Becomes subclavian vein in neck Brachial plexus pass behind clavicle Trunk of brachial plexus in interscalene triangle Covered by prevertebral fascia Prevertebral fascia  At the floor of posterior triangle  Covers scalenus medius  Extends into axilla and form axillary sheath As Brachial plexus pull the fascia into axilla  Axillary sheath - continuation of prevertebral fascia in the axilla Contents - axillary vein and brachial plexus Axillary vein is outside

• Cervical plexus blocked in occipital triangle 
• Posterior border of sternomastoid
• Midpoint of sternomastoid

Phrenic nerve

• Branch of cervical plexus
• Anterior surface of scalenus anterior muscle
• Motor supply to diaphragm
• Sensory to
  • central diaphragm 
  • Mediastinum - pleura, pericardium, Peritoneum 

Cervical plexus nerve block

• Behind sternomastoid 
• Block given during surgery around the region
• Posterior border of sternomastoid- midpoint of sternomastoid 
• Phrenic nerve cannot be blocked → respiratory paralysis

Three things blocked in posterior triangle 

• 3rd part of subclavian triangle - interscalene triangle - subclavian triangle
• Cervical plexus - occipital triangle- posterior border, midpoint of sternomastoid
• Trunk of brachial plexus -subclavian triangle 

11th nerve - spinal accessory nerve 

• Supply sternomastoid and trapezius
• Could be damaged iatrogenically during drainage in posterior triangle 
• Spares sternomastoid 
• Trapezius paralyzed - cannot shrug shoulder

Q. If there is superficial cut in the region of middle part of posterior triangle of neck, patient will experience problem in

1. Adduction of arm
2. Protraction of scapula
3. Shrugging of shoulder
4. Overhead abduction of arm

Explanation 

• Trapezius help in shrugging of shoulder - which is paralyzed so cannot shrug the shoulder 
• Though trapezius help in overhead abduction, serratus anterior is required along with trapezius for overhead abduction
• Sternomastoid is spared during the injury 

We hope you have understood Neck Triangles in detail through this detailed article. For more informative articles like this, keep following our blog and download the PrepLadder app to access high-yield content.