Boundaries and subdivisions
The posterior triangle is bounded in front by the posterior border of sternocleidomastoid, behind by the anterior border of trapezius and below by the middle third of the clavicle. Its apex lies where the two muscles meet on the superior nuchal line of the occipital bone.
- Roof: the investing layer of deep cervical fascia, with platysma over its lower part and skin.
- Floor: the prevertebral fascia over, from above down, semispinalis capitis, splenius capitis, levator scapulae, scalenus posterior and scalenus medius, with scalenus anterior at the lower anterior corner behind sternocleidomastoid.
The inferior belly of omohyoid crosses the lower part and divides it into a large occipital triangle above and a small supraclavicular (subclavian) triangle below.
Contents
Most important contents of the posterior triangle lie either just under the roof or on the floor, so depth matters as much as position.
| Group | Structures |
|---|---|
| Nerves | Accessory nerve; lesser occipital, great auricular, transverse cervical and supraclavicular nerves; trunks of the brachial plexus; dorsal scapular and suprascapular nerves; nerve to subclavius |
| Arteries | Third part of the subclavian artery; transverse cervical and suprascapular arteries; occipital artery at the apex |
| Veins | External jugular vein ending in the subclavian vein; transverse cervical and suprascapular veins |
| Lymph nodes | Nodes along the accessory nerve; supraclavicular nodes |
The phrenic nerve runs down on the front of scalenus anterior deep to the prevertebral fascia. It lies mostly behind sternocleidomastoid rather than in the triangle proper. The brachial plexus trunks and subclavian artery emerge between scalenus anterior and scalenus medius into the supraclavicular triangle.
Accessory nerve and cervical plexus branches
The spinal accessory nerve (CN XI) is the key structure in the posterior triangle because it lies superficially and is easily cut. Its fibres arise from the upper cervical segments of the spinal cord (about C1 to C5), ascend through the foramen magnum and leave the skull through the jugular foramen.
- It passes deep to sternocleidomastoid and supplies it.
- It emerges from the posterior border of sternocleidomastoid near the junction of its upper and middle thirds, a little above the great auricular nerve.
- It runs down and back across the triangle on levator scapulae, within or just under the investing fascia.
- It passes under the anterior border of trapezius, above the clavicle, and supplies it.
The cutaneous branches of the cervical plexus emerge together near the midpoint of the posterior border of sternocleidomastoid, a spot called the nerve point of the neck.
| Nerve | Roots | Course and territory |
|---|---|---|
| Lesser occipital | C2 | Hooks round the accessory nerve and runs up along sternocleidomastoid to the scalp behind the ear |
| Great auricular | C2, C3 | Ascends across sternocleidomastoid to the skin over the parotid, angle of mandible and ear lobe |
| Transverse cervical | C2, C3 | Crosses sternocleidomastoid horizontally to the front of the neck |
| Supraclavicular | C3, C4 | Descends over the clavicle to the skin of the shoulder and upper chest |
Clinical relevance
Surgery in the posterior triangle is dominated by the risk to the accessory nerve.
- Accessory nerve palsy: after a lymph node biopsy or neck dissection, trapezius is paralysed. The shoulder droops, the scapula wings laterally, shrugging is weak and abduction above the horizontal is difficult because trapezius helps rotate the scapula.
- Testing the nerve: shrug against resistance for trapezius, and turn the head against resistance for sternocleidomastoid of the opposite side.
- Supraclavicular nodes: an enlarged left supraclavicular node (Virchow's node, Troisier's sign) suggests abdominal malignancy spreading via the thoracic duct.
- Brachial plexus block: interscalene and supraclavicular blocks target the trunks between the scalenes and above the clavicle.
- External jugular vein: visible for venous pressure and cannulation; a cut vein held open by fascia risks air embolism.
On the specimen
This lateral neck dissection extends beyond the triangle, so several labelled structures belong to the anterior triangle or the face.
- Splenius capitis or levator scapulae: splenius capitis runs up and laterally to the mastoid process and superior nuchal line; levator scapulae runs down from the upper cervical transverse processes to the superior angle of the scapula.
- Scalenus medius or scalenus posterior: medius is the larger and inserts on the first rib behind the subclavian groove; posterior is smaller, lies behind it and inserts on the second rib.
- Trapezius: the posterior boundary, its anterior edge forming the back of the triangle.
- Great auricular nerve: climbs over sternocleidomastoid towards the ear lobe, parallel to and behind the external jugular vein.
- Anterior neck structures: the parotid lies above, the submandibular gland with a submandibular node below the mandible, and the posterior belly of digastric below the parotid. The hypoglossal nerve loops forward across the external carotid artery, and the facial artery arches over the submandibular gland.