Posterior Triangle

Introduction

Introduction to the Posterior Triangle of the Neck

The posterior triangle of the neck is a critical anatomical region bounded by the sternocleidomastoid muscle anteriorly, the trapezius muscle posteriorly, and the clavicle inferiorly. It serves as a conduit for neurovascular structures, including the brachial plexus, cervical plexus, and subclavian vessels, making it essential for both surgical and diagnostic considerations. Understanding its boundaries, contents, and fascial layers is foundational for procedures such as nerve blocks, vascular access, and lymph node biopsies.

Clinical and Anatomical Significance

The posterior triangle is subdivided into the occipital and subclavian (omoclavicular) triangles by the inferior belly of the omohyoid muscle. Its superficial location and rich neurovascular supply render it vulnerable to trauma, compression syndromes, and iatrogenic injury. Mastery of this region is vital for clinicians managing neck pain, thoracic outlet syndrome, and metastatic spread from head and neck malignancies.

Study

Boundaries and Subdivisions

The posterior triangle is defined by three key boundaries: the posterior border of the sternocleidomastoid muscle, the anterior border of the trapezius muscle, and the middle third of the clavicle. The inferior belly of the omohyoid muscle divides the triangle into two smaller regions: the larger occipital triangle superiorly and the smaller subclavian triangle inferiorly. The occipital triangle contains the accessory nerve and cervical lymph nodes, while the subclavian triangle houses the subclavian artery and brachial plexus roots.

Fascial Layers and Compartments

The posterior triangle is enveloped by the investing layer of deep cervical fascia, which splits to surround the sternocleidomastoid and trapezius muscles. Deep to this lies the prevertebral fascia, covering the scalene muscles and forming the floor of the triangle. The carotid sheath, though primarily associated with the anterior triangle, may extend into the posterior triangle near its medial border. These fascial planes guide the spread of infections and influence the localization of pathological processes such as abscesses or tumors.

Neurovascular Contents

The posterior triangle contains several critical neurovascular structures. The accessory nerve (CN XI) courses superficially across the triangle, innervating the sternocleidomastoid and trapezius muscles, and is vulnerable to injury during surgical procedures. The cervical plexus, formed by the ventral rami of C1-C4, gives rise to sensory branches such as the lesser occipital, great auricular, transverse cervical, and supraclavicular nerves. The brachial plexus roots (C5-T1) emerge between the anterior and middle scalene muscles, while the subclavian artery and vein traverse the inferior aspect of the triangle.

Muscular Floor and Related Structures

The floor of the posterior triangle is formed by the prevertebral muscles, including the splenius capitis, levator scapulae, and the scalene muscles (anterior, middle, and posterior). The anterior scalene muscle separates the subclavian artery from the subclavian vein and is a key landmark for identifying the brachial plexus. The phrenic nerve (C3-C5) descends along the anterior surface of the anterior scalene, providing motor innervation to the diaphragm. The omohyoid muscle, with its inferior belly crossing the triangle, further subdivides the region and serves as a surgical landmark.

Lymphatic Drainage and Clinical Correlates

The posterior triangle contains lymph nodes that drain the scalp, ear, and posterior neck, as well as the upper limb via the axillary nodes. Enlargement of these nodes may indicate metastatic spread from head and neck cancers, particularly squamous cell carcinoma or lymphoma. The accessory nerve’s superficial course makes it susceptible to injury during lymph node biopsies, potentially leading to trapezius muscle paralysis and shoulder dysfunction. Thoracic outlet syndrome, characterized by compression of the brachial plexus or subclavian vessels, often manifests with symptoms referable to the posterior triangle.

Summary

Key Takeaways

The posterior triangle of the neck is bounded by the sternocleidomastoid, trapezius, and clavicle, and is subdivided into occipital and subclavian triangles by the omohyoid muscle. It contains critical neurovascular structures, including the accessory nerve, cervical plexus, brachial plexus roots, and subclavian vessels, all of which are vulnerable to injury or compression. Fascial layers within the triangle influence the spread of infections and guide surgical approaches.

Clinical Correlate

Injury to the accessory nerve in the posterior triangle can result in trapezius muscle paralysis, leading to shoulder droop and impaired abduction. Thoracic outlet syndrome may present with neurovascular compression symptoms, such as paresthesia or vascular insufficiency in the upper limb. Lymphadenopathy in this region often warrants evaluation for metastatic disease or systemic conditions like lymphoma.

Surgical and Diagnostic Relevance

The posterior triangle is a common site for nerve blocks, such as the cervical plexus block for neck procedures, and vascular access via the subclavian vein. Understanding the anatomical relationships in this region is essential for avoiding iatrogenic injury during lymph node biopsies, central line placement, or neck dissections. Imaging studies, such as ultrasound or MRI, are frequently employed to assess pathology in this anatomically complex area.