Posterior Triangle — MRCS Part B Anatomy

The posterior triangle of the neck, with the accessory nerve as the structure the examiner is leading towards.

Boundaries

In front, the posterior border of sternocleidomastoid; behind, the anterior border of trapezius; below, the middle third of the clavicle; the apex where the two muscles meet on the superior nuchal line. The roof is the investing layer of deep cervical fascia with platysma over it, and the floor is the prevertebral fascia over splenius capitis, levator scapulae and the three scalene muscles. The inferior belly of omohyoid crosses the lower part and divides it into an occipital triangle above and a supraclavicular (subclavian) triangle below.

Contents

Nerves: the accessory nerve crossing the triangle on the levator scapulae, the cutaneous branches of the cervical plexus (lesser occipital, great auricular, transverse cervical and supraclavicular) emerging at the nerve point halfway down the posterior border of sternocleidomastoid, the trunks of the brachial plexus between scalenus anterior and medius, and the phrenic nerve on the front of scalenus anterior. Vessels: the third part of the subclavian artery, the transverse cervical and suprascapular arteries, the external jugular vein ending in the subclavian vein, and the subclavian vein behind the clavicle. Lymph nodes along the accessory nerve and in the supraclavicular fossa, including Virchow's node on the left.

The accessory nerve

Its spinal root arises from the upper five cervical segments, climbs through the foramen magnum, joins the cranial root briefly and leaves the skull through the jugular foramen. It runs down and back across the internal jugular vein, enters the deep surface of sternocleidomastoid and supplies it, emerges from the posterior border about a third of the way down, crosses the posterior triangle on levator scapulae, and passes under the anterior border of trapezius to supply it. It is motor to those two muscles only; the cervical plexus branches that run with it carry sensation.

Why it matters

It lies just under the fascia in the triangle, so it is the nerve cut in a lymph node biopsy there, and it is at risk in neck dissections. Loss of trapezius drops the shoulder, tilts the scapula, and stops the patient raising the arm above the horizontal because trapezius rotates the scapula for the final part of abduction. Loss of sternocleidomastoid weakens turning the head to the opposite side.

Testing it

Ask the patient to shrug both shoulders against resistance and compare the two sides, then to turn the head against your hand and feel sternocleidomastoid contract on the opposite side. Look for winging of the scapula on abducting the arm and for a drooped shoulder at rest.

The external jugular vein

Formed behind the angle of the mandible, it crosses sternocleidomastoid under platysma, pierces the roof of the triangle above the clavicle and ends in the subclavian vein. It is used to estimate venous pressure when the internal jugular cannot be seen, and it is easily cut in a lacerated neck; the danger is air embolism when the open vein is held tented by the fascia.