Expansion
SCM in front, trapezius behind, clavicle's the floor you'll find
Mnemonic
Three borders, easily recalled as “a muscle in front, a muscle behind, a bone below”:
- Anterior - posterior border of sternocleidomastoid
- Posterior - anterior border of trapezius
- Inferior - the middle third of the clavicle
The roof is investing fascia, and the floor is formed by prevertebral fascia over splenius capitis, levator scapulae and the scalenes.
The structure at risk is the accessory nerve (XI), which crosses the triangle superficially, roughly at the junction of the upper and middle thirds of sternocleidomastoid. It is the commonest nerve injured in lymph node biopsy, causing trapezius weakness and lateral winging of the scapula.
Expansion
- Anterior boundary - posterior border of sternocleidomastoid
- Posterior boundary - anterior border of trapezius
- Inferior boundary - middle third of the clavicle