Learning Objectives

By the end of this section, the learner should be able to:

Define the anatomical boundaries of the posterior triangle of the neck
Identify its muscular, vascular, and neural contents
Describe its subdivisions
Apply anatomical knowledge to relevant clinical correlations

Overview

The posterior triangle of the neck is an anatomical region located on the posterolateral aspect of the neck. It serves as an important conduit for major nerves and vessels supplying the upper limb and neck.

In this section, we will examine the borders, contents, subdivisions, and clinical relevance of the posterior triangle.

Borders

The posterior triangle is bounded by three muscular and bony landmarks:

Anterior border: Posterior border of the sternocleidomastoid (SCM) muscle
Posterior border: Anterior border of the trapezius muscle
Inferior border: Middle one-third of the clavicle

Additional boundaries include:

Roof: Investing layer of the deep cervical fascia
Floor: Prevertebral fascia covering the underlying vertebral muscles

Muscles

Several muscles contribute to the boundaries and floor of the posterior triangle.

A key muscle in this region is the omohyoid muscle, which has superior and inferior bellies connected by an intermediate tendon.

The inferior belly traverses the posterior triangle in a superomedial direction
It divides the triangle into two distinct subdivisions
The muscle then passes deep to the SCM to enter the anterior triangle

The floor of the posterior triangle is formed by vertebral muscles covered by prevertebral fascia, including:

Splenius capitis
Levator scapulae
Anterior, middle, and posterior scalene muscles

Activity

Vasculature

The external jugular vein is a major superficial vein of the neck. It is formed by the union of the retromandibular and posterior auricular veins and crosses superficially over the SCM before entering the posterior triangle.

Within the posterior triangle:

The external jugular vein pierces the investing fascia
It drains into the subclavian vein

The subclavian vein is clinically important as a common site for central venous access.

Other vascular structures present include:

Transverse cervical vein
Suprascapular vein

These veins are accompanied by their corresponding arteries.

The subclavian artery can be identified as it emerges between the anterior and middle scalene muscles. After crossing the first rib, it continues as the axillary artery, supplying the upper limb.

Nerves

The spinal accessory nerve (CN XI) enters the posterior triangle after innervating the sternocleidomastoid muscle. It crosses the triangle obliquely within the investing fascia and lies relatively superficially, making it susceptible to injury.

The cervical plexus forms within the muscles of the floor of the triangle. Its branches include:

Phrenic nerve (C3–C5): Descends within the prevertebral fascia to innervate the diaphragm
Muscular branches to the vertebral muscles
Cutaneous branches supplying the neck and scalp

The trunks of the brachial plexus also pass through the floor of the posterior triangle.

Figure 2 demonstrates the neural structures within the posterior triangle.

Subdivisions

The inferior belly of the omohyoid muscle divides the posterior triangle into two regions:

Occipital triangle:
- Located superior to the omohyoid
- Contains accessory nerve and cervical plexus branches
Subclavian (supraclavicular) triangle:
- Located inferior to the omohyoid
- Contains the distal portion of the subclavian artery

Activity

Clinical Relevance

External Jugular Vein Injury

Due to its superficial course, the external jugular vein is vulnerable to trauma. When severed:

The lumen remains open because of the surrounding investing fascia
Air may be drawn into the venous system, leading to air embolism
This can impair right atrial blood flow and cause cyanosis

Management: Immediate application of pressure to prevent further air entry and bleeding.

Cervical Plexus Nerve Block

A cervical plexus block may be used for regional anaesthesia of the neck.

Local anaesthetic is injected along the posterior border of the SCM
At the junction of its superior and middle thirds
This location corresponds to the nerve point of the neck, where cutaneous branches of the cervical plexus emerge