Posterior triangle of the neck

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Posterior triangle of the neck

The posterior triangle, or lateral neck region, is bordered as follows:
- Apex: where the sternocleidomastoid (SCM) and trapezius meet near the superior nuchal line of the occipital bone
- Anterior boundary: the posterior edge of the SCM
- Posterior boundary: the anterior edge of the trapezius
- Inferior boundary: the middle third of the clavicle
- Roof: investing layer of the deep cervical fascia
- Floor (from top to bottom): semispinalis capitis, splenius capitis, levator scapulae, scaleneus posterior, and scaleneus medius

The inferior belly of the omohyoid muscle crosses this area about 2.5 cm above the clavicle, dividing it into two smaller triangles:
- Nerves and plexuses
- Vessels
- Lymph nodes
- Muscles

Key points
- The accessory nerve (CN XI) runs through this triangle and is at risk during lymph node biopsy; damage can make it hard to shrug the shoulders or raise the arm because the trapezius is affected.
- The external jugular vein lies close to the surface here and can be injured.