Superficial structures of the neck: Posterior triangle

Definitions & Key takeaways

The posterior neck triangle is an anatomical region in the posterolateral aspect of the neck. It is anteriorly bordered by the posterior border of the sternocleidomastoid, posteriorly by the anterior border of the trapezius muscle, and inferiorly by the middle third of the clavicle.

It has a roof formed by the investing layer of deep cervical fascia and a floor formed by the muscles splenius capitis, levator scapulae, middle scalene, posterior scalene, and occasionally a portion of anterior scalene, which are covered by the prevertebral layer of deep cervical fascia.

The posterior triangle is further divided by the inferior belly of the omohyoid into two other triangles: the occipital triangle located superiorly and the omoclavicular triangle located inferiorly. The posterior triangle includes arteries such as the suprascapular artery, cervicodorsal trunk, the third part of the subclavian artery and part of the occipital artery, veins such as the external jugular vein and nerves such as spinal accessory nerve, brachial plexus, and cervical plexus.

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The neck is the anatomical region between the base of the cranium superiorly and the clavicles inferiorly, joining the head to the trunk and limbs, and serving as a major conduit for structures passing between them.
In the neck, there are superficial structures, located in the anterior and posterior triangles, and deep structures, including the cervical viscera and prevertebral muscles.
Now, the neck is divided into the anterior and posterior triangles mainly by the borders of the sternocleidomastoid, or SCM, and trapezius muscles.
Now, the SCM is a broad, strap-like muscle which has a sternal and a clavicular head. The sternal head is rounded and its inferior portion originates from the manubrium of the sternum, while the clavicular head is thick and its inferior portion originates from the superior surface of the medial third of the clavicle.
The two heads of the SCM are separated inferiorly, forming a superficially visible space called the lesser supraclavicular fossa, which looks like a small triangular depression.
Superiorly, the two heads of SCM join as they go toward the cranium and insert on the mastoid process of the temporal bone and the superior nuchal line of the occipital bone.
The posterior border of this muscle forms the anterior boundary of the posterior triangle. When the SCM contracts, it produces movement at either the craniovertebral joints, or the cervical intervertebral joints, or at both.
So, unilateral contraction of SCM leads to lateral flexion of the neck to the same side, and also rotates the head so your face is turning superiorly towards the opposite side.
Bilateral contraction of SCM can lead to three different movements: extension of the neck at the atlanto-occipital joint, which makes the chin rise, flexion of the neck which makes the chin approach the manubrium, and extension of the superior cervical vertebrae combined with flexion of the inferior vertebrae which makes the chin thrust forward with head kept level.
Additionally, bilateral contraction of SCM with the head and neck fixed can lead to elevation of the clavicles and manubrium and thus the elevation of the anterior ribs.
This action makes the SCMs act as accessory muscles of respiration as they assist in lifting the sternum anterosuperiorly, which is referred to as the pump-handle movement of the thoracic wall.
Finally, the SCM is innervated by the spinal accessory nerve or cranial nerve 11, as well as fibers from C2 and C3. And then there’s the trapezius, which is a large triangular muscle that covers the posterolateral aspect of the neck and thorax.
It originates from the medial third of superior nuchal line, external occipital protuberance, nuchal ligament and the spinous processes of the C7 toT12 vertebrae.
It inserts onto the lateral third of the clavicle, acromion and spine of the scapula. The anterior border of the trapezius forms the posterior boundary of the posterior triangle.
Now, the trapezius also causes many different actions. Its superior fibers elevate the scapula, its middle fibers retract the scapula, and its inferior fibers depress the scapula.
The superior and inferior fibers can also work together to produce upward rotation of the scapula, causing the glenoid cavity to tilt superiorly.
And finally, when the shoulders are fixed, bilateral contraction of the superior fibers extends the neck, while unilateral contraction causes lateral flexion to the same side.Just like the SCM, the trapezius is innervated by the spinal accessory nerve and also by spinal nerves C2 and C3.Now, the posterior triangle, like any respectable triangle, has three sides, called boundaries.
The anterior boundary is formed by the posterior border of the SCM, the posterior boundary is formed by the anterior border of the trapezius and the inferior boundary is formed by the middle third of the clavicle between the trapezius and the SCM.
The posterior triangle has an apex where the SCM and trapezius meet on the superior nuchal line of the occipital bone, a deep boundary - or floor - formed by several prevertebral muscles covered by the prevertebral layer of deep cervical fascia, and a superficial boundary - or roof - formed by the investing layer of deep cervical fascia.The floor of the posterior triangle is formed primarily by four muscles: the splenius capitis, levator scapulae, middle scalene and posterior scalene.
Ok, so, the splenius capitis originates on the inferior half of the nuchal ligament and the spinous processes of C7 to T4.
It inserts onto the mastoid process of the temporal bone and the lateral third of the superior nuchal line. Levator scapulae originates from the posterior tubercles of the transverse processes of the C1 to C4 vertebrae and inserts onto the superior part of the medial border of the scapula.
The middle scalene is the largest and longest of the three scalene muscles and it originates from the posterior tubercles of the transverse processes of C3 to C7 and inserts onto the superior surface of the first rib, posterior to the subclavian groove.
The posterior scalene muscle is the smallest and most deeply seated of the scalene muscles. It originates from the posterior tubercles of the transverse processes of C5 to C7 and inserts onto the external border of the second rib.
There’s also the anterior scalene muscle, which lies deep to the sternocleidomastoid muscle. This means it’s usually hidden, but sometimes it can be visualized in the inferomedial portion of the posterior triangle.
The anterior scalene originates from the anterior tubercles of the transverse processes of the C3 to C6 vertebrae and inserts onto the scalene tubercle on the first rib.
Let’s take a quick break and try to identify the muscles that form the boundaries and floor of the posterior triangle. Now, the posterior triangle is further divided by the inferior belly of the omohyoid into two other triangles: the occipital triangle, located superiorly, and the much smaller omoclavicular triangle, located inferiorly.
The occipital triangle contains the occipital artery at its apex and also the spinal accessory nerve. The omoclavicular triangle, also called the subclavian triangle, can be seen on the surface of the neck as the supraclavicular fossa, and it’s bounded by the inferior belly of the omohyoid posterosuperiorly, the posterior border of SCM anteriorly, and the middle third of the clavicle inferiorly.
The inferior part of the external jugular vein crosses the triangle superficially, and deep to it is the investing layer of deep cervical fascia.Now, let’s talk about the subclavian artery, which arches medial to the clavicle and passes between the anterior and middle scalene muscles.
In the inferior part of the omoclavicular triangle, the third and final part of the subclavian artery can be visualized.
It is the longest and most superficial part of the subclavian artery, beginning at the lateral border of the anterior scalene muscle and ending at the lateral border of the first rib where it continues as the axillary artery.
It lies deep in the inferior part of the omoclavicular triangle, where it is in contact with the first rib and is situated posterosuperior to the subclavian vein.
This third part of the artery lies directly anterior to the inferior trunk of the brachial plexus. Its pulsations can be felt by applying deep pressure in the omoclavicular triangle.
The posterior triangle also contains the lateral branches of the thyrocervical trunk and part of the occipital artery. The thyrocervical trunk is a branch of the subclavian artery and its lateral branches are the suprascapular artery and the cervicodorsal trunk.
The suprascapular artery travels inferolaterally across the anterior scalene muscle and phrenic nerve, running inferior to the cervicodorsal trunk.
It crosses the third part of the subclavian artery and the cords of the brachial plexus and then passes posterior to the clavicle to supply muscles on the posterior aspect of the scapula.The cervicodorsal trunk, also known as the transverse cervical artery runs superficially and laterally across the phrenic nerve and anterior scalene muscle, and splits in two other branches: the superficial cervical artery and dorsal scapular artery.
These branches either cross or pass through the trunks of the brachial plexus and give branches to their vasa nervorum, which are blood vessels that supply nerves.
The superficial cervical artery then runs deep to the trapezius, and the dorsal scapular artery runs deep to the levator scapulae and rhomboids, supplying both.
Finally, the occipital artery is a branch of the external carotid artery, and it enters the posterior triangle at its apex and climbs over the head to supply the posterior half of the scalp.
Now, the veins in the posterior triangle include the external jugular vein, which begins near the angle of the mandible by the union between the posterior division of the retromandibular vein and the posterior auricular vein.
It crosses the SCM obliquely, deep to the platysma, and enters the anteroinferior part of the posterior triangle. Then, it pierces the investing layer of deep cervical fascia at the posterior border of SCM.
Then, the external jugular vein descends to the inferior part of the omoclavicular triangle and drains into the subclavian vein.
Superior to the clavicle, it receives the cervicodorsal, suprascapular and anterior jugular veins and through all its tributaries it drains most of the scalp and side of the face.Finally, the nerves in the posterior triangle are the spinal accessory nerve, or cranial nerve 11 , as well as the brachial plexus and the cervical plexus.
The spinal accessory nerve runs deep to the SCM and innervates it before entering the posterior triangle at or inferior to the junction of the superior and middle thirds of the posterior border of SCM.
Then it passes posteroinferiorly, within or deep to the investing layer of deep cervical fascia and runs on the levator scapulae from which it is separated by the prevertebral layer of deep cervical fascia.
Finally, it disappears deep to the anterior border of the trapezius and innervates it.Next, the brachial plexus is formed by the anterior rami of the spinal nerves C5, C6, C7, C8 and T1, and it emerges between the anterior and middle scalene muscles, to descend inferolaterally through the posterior triangle.
The plexus then passes between the first rib, clavicle and superior border of the scapula to enter the axilla where it provides innervation for most of the upper limb.
From the superior trunk of the brachial plexus arises the suprascapular nerve which runs laterally across the posterior triangle to supply the supraspinatus and infraspinatus muscles on the posterior aspect of the scapula.
Finally, the cervical plexus can also be found in the posterior triangle, including its superficial branches which are cutaneous nerves that emerge at the posterior border of the SCM.
The roots of the cervical plexus arise from the anterior rami of the spinal nerves C1, C2, C3 and C4. All right, as a quick recap.
The posterior triangle is a major region of the neck which is bounded anteriorly by the posterior border of the SCM, posteriorly by the anterior border of the trapezius, and inferiorly by the middle third of the clavicle between the trapezius and the SCM.
It has a roof formed by the investing layer of deep cervical fascia and a floor formed by the muscles splenius capitis, levator scapulae, middle scalene, posterior scalene, and occasionally a portion of anterior scalene, which are covered by the prevertebral layer of deep cervical fascia.
The posterior triangle is further divided by the inferior belly of omohyoid into two other triangles: the occipital triangle located superiorly and the omoclavicular triangle located inferiorly.
The posterior triangle include arteries such as the suprascapular artery, cervicodorsal trunk, the third part of the subclavian artery and part of the occipital artery, veins such as the external jugular vein and nerves such as spinal accessory nerve,
Superficial structures of the neck: Posterior triangle: Video | Osmosis