Triangles Of The Neck
While the primary focus of this article is to review anatomical landmarks of the triangles of the neck, it is important to first discuss the anatomy of the neck. There are several important functions of the neck:
- The neck works with the shoulders to provide support for the head and facilitates rotation of the head about its axis.
- It acts as a conduit for neurovascular structures (including the spinal cord) travelling to and from the head.
- It is the major passageway from the upper digestive and respiratory tract to the lower regions of the same.
- It can be used as an emergency route to ventilate a patient when the oral and nasal routes are compromised or use of those pathways is contraindicated.
The neck is the bridge between the thorax and the head. It is of variable length and width depending on the individual’s gender, age and body habitus. While those parameters may vary from person to person, other features remain constant. The neck extends from the bases of the skull and mandible (superiorly) to the level of the thoracic inlet. It can be divided into symmetrical halves by an imaginary line known as the median line of the neck through the mandibular symphysis (symphysis menti). It consists of cutaneous, fascial, muscular, fatty and bony layers.
This article will look at the gross anatomy of the neck in order to lay the foundation for understanding the triangles of the neck.
Gross Anatomy of the Neck: Fascial Layers
There are two fascial layers in the neck – there is a superficial and a deep cervical fascia. The latter is further subdivided into investing, pretracheal and prevertebral layers.
Superficial cervical fascia
A thin lamina mixed with adipose tissue is found immediately deep to the skin; it is known as the superficial cervical fascia. This fascial layer, which lies over the platysma muscle, merges with the aponeurosis of the same muscle inferiorly. The merged tissue may either form skin ligaments or merge with the deltopectoral fascia (covering the deltoid and pectoralis major muscles). The fascia is continuous circumferentially around the entire neck.
Deep cervical fascia
The deep cervical fascia was previously believed to be a single layer of fascia. However, subsequent studies supported the notion that the deep cervical fascia is further subdivided into three other layers. The investing (superficial) layer of the deep cervical fascia also circumferentially wraps around the neck by meeting with the analogous fascia of the contralateral side, while simultaneously covering the sternocleidomastoid and trapezius muscles. The investing layer has periosteal attachments at along the base of the mandible, the mastoid process and the superior nuchal line of the occipital bone superiorly.
In the mandibulomastoid region, the fascia travels behind the parotid gland and attaches to the arch of the zygomatic bone. There are also inferior periosteal attachments between the investing layer and the manubrium sterni, acromion and clavicle. Just above the attachment to the manubrium sterni, the investing layer of deep cervical fascia divides into superficial and deep layers after integrating with the aponeurosis of the platysma . The superior layer attaches to the anterior border of the manubrium, while the deep layer attaches to the clavicular ligament and posterior surface of the manubrium. The two layers create a space known as the suprasternal space that houses the jugular venous arch and the caudal part of the anterior jugular veins, the sternal heads of sternocleidomastoid and areolar tissue.
The pretracheal layer of the deep cervical fascia is deep to the investing layer of the same. Cranially, it attaches to the hyoid bone and caudally it extends into the superior mediastinum adjacent to the great vessels before merging with the pericardium. Laterally, it is continuous with the investing layer of deep cervical fascia as well as the carotid sheath. The fascia also encircles the oesophagus, infrahyoid strap muscles, thyroid and parathyroid glands, as well as the trachea, larynx and pharynx.
The last layer of the deep cervical fascia is the prevertebral layer. As the name suggest, this fascial layer is superficial the anterior group of vertebral muscles. Loose areolar tissue occupies the retropharyngeal space, which lies posterior to the buccopharyngeal fascia (covering of the pharynx) and anterior the prevertebral fascia. Inferiorly, the fascia extends into the superior mediastinum (anterior to longus colli) before blending with the anterior longitudinal ligament. Superiorly it attaches to the base of the cranium. It continues bilaterally to cover the scalenus anterior and medius muscles, as well as the levator scapulae muscle. As it becomes thin and loose areolar tissue laterally, it merges with the fascia of the sternocleidomastoid and the carotid sheath. It is drawn inferolaterally by the emerging subclavian artery and brachial plexus as the axillary sheath, in the retroclavicular space.
The carotid sheath is a fascial layer that encases the internal jugular vein, vagus nerve (CN X), parts of the ansa cervicalis (C1, C2, C3) and the common and internal carotid arteries. It is a product of consolidating parts of the deep cervical fascia.
Gross Anatomy of the Neck: Musculature
There are several muscular layers in the neck. They can be divided into superficial and deep muscles, as well as suprahyoid and infrahyoid muscles (i.e. above and below the hyoid bone). Owing to the fact that there are numerous muscles in the neck (most of which have been covered in previous articles), this piece will focus on those neck muscles that form the borders of the triangles of the neck.
The platysma is a superficial neck muscle that primarily acts as a muscle of facial expression. It originates from the cranial portion of the fascia of pectoralis major and the fascia of the deltoid muscle. Its fibers have attachments to the lower border of the mandible, the integument of the lower face, and the lower lip. This muscle receives arterial supply from the submental branch of the facial artery and from the thyrocervical trunk via the suprascapular artery. Innervation to the muscle is derived from CN VII (facial nerve).
The sternocleidomastoid muscle is a prominent structure located on the lateral aspect of the neck. It has two heads and therefore two origins. Medially, the sternal head arises from the anterior surface of manubrium sterni; laterally, the clavicular head arises from the upper surface of medial third of the clavicle. The former travels in a posterosuperior manner while the latter in an almost vertical direction until they meet midway along the neck and begin to blend. Fibers of the clavicular head insert into the lateral surface of mastoid process and those of the sternal head insert into the lateral half of superior nuchal line of occipital bone . The superior thyroid and occipital arteries give direct branches to sternocleidomastoid. It is also perfused by the occipital branch of posterior auricular and muscular branch of suprascapular arteries. Ventral rami of C2, C3 and C4 and the spinal part of CN XI (spinal accessory nerve) give motor innervation to the muscle.
The digastric muscle is a paired, suprahyoid structure that has two bellies. The anterior belly originates in the digastric fossa near the midline of the base of the mandible, while the posterior belly originates in the mastoid notch of the temporal bone. The muscles slope posteroinferiorly and anteroinferiorly (respectively), to insert in the intermediate tendon attached to the greater cornu (horn) of the hyoid bone. The two bellies of the digastric muscle have different blood supplies and innervations. The anterior belly is supplied by branches of the facial artery and the mylohyoid part of the inferior alveolar nerve, while the posterior belly receives the occipital and posterior auricular arteries and CN VII.
Like the digastric muscle, the omohyoid muscle has two bellies that meet at an intermediate tendon. The inferior belly of the omohyoid muscle originates from the superior scapular border, adjacent to the scapular notch. The narrow, flat muscle travels superomedially, deep to sternocleidomastoid and inserts in the intermediate tendon. From the intermediate tendon, the superior belly continues superiorly to insert in the inferior border of the body of the hyoid bone. Its blood supply is derived from the external carotid artery via the superior thyroid and lingual branches. The nerve supply to the inferior belly is derived from the ansa cervicalis (C1, C2, and C3), while that to the superior belly is from the superior ramus of the ansa cervicalis (C1).
While the trapezius muscle is primarily considered a superficial back muscle, its superior fibers also provide support to the neck. The muscle originates from the superior nuchal line and the external occipital protuberance of the occipital bone, in addition to the nuchal ligament and the spinous processes of C7 to T12 vertebrae. The acromion, scapular spine and lateral third of the clavicle serve as insertion points for the muscle. Blood supply to trapezius is from the transverse cervical artery as well as contributing dorsal perforating branches of the posterior intercostal arteries. Innervation to the muscle is via CN XI.
Gross Anatomy of the Neck: Other Structures
In addition to the muscles and fascia discussed above, the neck also contains numerous large vessels that carry blood to the head and back to the heart. The common, internal and external carotids and the jugular venous system are responsible for supplying the head and its contents. Branches of the subclavian arteries and veins perform the same function to structures in the neck and head as well.
Viscera and bones
Furthermore, the neck also houses visceral such as the thyroid and parathyroid glands, oesophagus, larynx and numerous lymph nodes. At the core of the neck is the proximal segment of the vertebral column (from C1 to T1) and its constituents. The hyoid bone (mentioned previously) is located in the anterior part of the neck.
Anatomical Triangles of the Neck
As was mentioned earlier, the median line of the neck divides the neck into symmetrical halves. The sternocleidomastoid muscle, in its oblique (posterosuperior) course, further divides the neck into anterior and posterior triangles. The anterior triangle of the neck is further subdivided into four smaller triangles, while the posterior triangle is broken up into two smaller triangles.
The anterior triangle is formed by the anterior border of sternocleidomastoid posteriorly, the median line of the neck anteriorly and by the base of the mandible together with a horizontal line extending to the mastoid process superiorly. The apex of the anterior triangle extends towards the manubrium sterni. These triangles can also be described as infrahyoid (muscular) and suprahyoid (submental and digastric) triangles; the carotid triangle crosses the hyoid bone.
The muscular triangle also shares one margin with the anterior triangle – the median line of the neck. However, the muscular triangle begins at the inferior border of the body of the hyoid bone. It has two posterior borders – the proximal part of the anterior border of sternocleidomastoid inferiorly and the anterior part of the superior belly of omohyoid superiorly. This puts the apex of the muscular triangle at the intersection of sternocleidomastoid and omohyoid. The muscular triangle contains:
- Superior thyroid artery
- Inferior thyroid artery
- Anterior jugular veins
- Thyroid gland
- Parathyroid glands
Similar to the muscular triangle, the carotid triangle has the omohyoid and sternocleidomastoid muscles as parts of its borders. However, it is the posterior margin of the superior omohyoid muscle that limits the triangle anteriorly and the anterior margin of the sternocleidomastoid posteriorly. Superiorly, the posterior belly of the digastric muscle and stylohyoid close the triangle. It is floored by the inferior and middle pharyngeal constrictors, hyoglossus and parts of thyrohyoid. Its roof is formed by deep and superficial fascia, platysma and skin. This triangle contains:
- Common carotid artery
- External carotid artery (and branches except maxillary, superficial temporal and posterior auricular)
- Internal carotid artery (and sinus)
- Internal jugular vein
- Common facial vein
- Lingual vein
- Superior thyroid vein
- Middle thyroid vein
Like the anterior triangle, the digastric (submandibular) triangle is limited superiorly by the same structures. Its inferior boundaries are formed by the posterior belly of the digastric and stylohyoid muscles posteriorly, and the anterior belly of the digastric muscle anteriorly. The apex of the triangle rests at the intermediate tendon of the digastric muscle. Its floor is formed by the mylohyoid and hyoglossus, while it is roofed by skin, fascia and platysma. The digastric triangle houses:
- Submandibular gland and lymph nodes (anteriorly)
- Caudal part of the parotid gland (posteriorly)
- Facial artery and vein
- Submental artery and vein
- Lingual arteries and veins
The submental triangle is located between the anterior bellies of the left and right digastric muscles. The base of the triangle is formed by the body of the hyoid bone and its apex extends towards the symphysis menti. This triangle, like the submandibular triangle, is floored by the mylohyoid muscles and roofed by the platysma, fascia and skin. Small venous tributaries to the anterior jugular vein, and the submental lymph nodes also occupy this space.
The posterior border of sternocleidomastoid and the anterior border of trapezius form the anterior and posterior borders of the posterior triangle of the neck, respectively. The base of the posterior triangle is formed by the middle third of the clavicle. The investing layer of deep cervical fascia and integument forms the roof of the space, while the floor is covered with the prevertebral fascia along with levator scapulae, splenius capitis and the scalene muscles . The inferior belly of omohyoid subdivides the posterior triangle into a small supraclavicular, and a large occipital, triangle.
The anterior and posterior margins of the occipital triangle are the same as those of the posterior triangle. However, its base is now formed by the superior margin of the inferior omohyoid muscle. The semispinalis capitis (occasionally), splenius capitis, levator scapulae and scaleni medius and posterior muscles line the floor of the occipital triangle in that craniocaudal order. The roof of the triangle is (from superficial to deep) skin, superficial and deep fascia.
Finally, the supraclavicular triangle (greater supraclavicular fossa) is the smaller of the two posterior triangles. It shares anterior and inferior margins with the posterior triangle. However, it is limited superiorly by the inferior border of omohyoid. Scalenus medius, the first digitation of serratus anterior and the first rib are in the floor of this triangle. The roof is formed from the skin, fascia and platysma.
The subdivisions of the posterior triangle are occupied by the following:
- The third part of the subclavian artery
- Suprascapular and transverse cervical branches of the thyrocervical trunk
- External jugular vein
- The trunks of the brachial plexus
- Fibers of the cervical plexus