Trapezius

Musculus trapezius

Also known as: traps, trapezoid muscle

A large, flat, triangular muscle of the upper back and neck. The two trapezius muscles together form a trapezoid shape. It elevates, retracts, depresses and upwardly rotates the scapula.

Overview

Trapezius has a long, continuous origin from the skull to the lower thoracic spine. It arises from the medial third of the superior nuchal line, the external occipital protuberance, the nuchal ligament and the spinous processes of C7–T12. Its fibres converge on the shoulder girdle: the lateral third of the clavicle, the acromion and the spine of the scapula. It has three functional parts.

- Descending (upper) fibres: elevate the scapula (shrugging) and, with the lower fibres, rotate it upward.
- Transverse (middle) fibres: retract the scapula.
- Ascending (lower) fibres: depress the scapula.

The upper and lower fibres act as a force couple with serratus anterior to upwardly rotate the scapula, which makes overhead abduction beyond 90° possible.

Its motor nerve is the spinal accessory nerve (CN XI). CN XI emerges from behind SCM, crosses the posterior triangle just under the investing fascia, where it is very superficial and easily injured, and runs on the deep surface of trapezius. Fibres from C3–C4 carry proprioception. The superficial cervical (transverse cervical) artery accompanies the nerve.

Location

Superficial upper back and posterior neck, from the occiput to T12 and laterally to the shoulder.

Parts & subdivisions

Descending (upper) part

Occiput, nuchal ligament → lateral third of the clavicle. Elevates the scapula and upwardly rotates it.

Transverse (middle) part

C7–T3 spines → acromion and the spine of the scapula. Retracts the scapula.

Ascending (lower) part

T4–T12 spines → medial end of the scapular spine. Depresses the scapula and assists upward rotation.

Anatomy

Origin

Medial third of the superior nuchal line, external occipital protuberance, nuchal ligament, and spinous processes and supraspinous ligaments of C7–T12.

Insertion

Lateral third of the clavicle, acromion and spine of the scapula.

Action

Upper fibres elevate the scapula; middle fibres retract it; lower fibres depress it. Upper and lower fibres together upwardly rotate the scapula. Bilaterally, it extends the neck.

Innervation

Spinal accessory nerve (CN XI) for motor supply; C3–C4 anterior rami (cervical plexus), mainly proprioceptive.

Nerve roots

CN XI (spinal accessory nucleus, C1–C5); C3–C4 (proprioception)

Arterial supply

Superficial branch of the transverse cervical artery (superficial cervical artery); dorsal scapular artery; occipital artery; dorsal branches of the posterior intercostal arteries.

Relations

Superficial to the rhomboids, levator scapulae, splenius, supraspinatus and erector spinae. It forms the posterior border of the posterior triangle of the neck, and its lateral border bounds the triangle of auscultation.

Clinical significance

Accessory nerve injury

Usually iatrogenic, during lymph node biopsy or neck dissection in the posterior triangle. Patients have a drooping shoulder, weak shrugging, difficulty abducting above 90°, lateral scapular winging and later wasting. SCM is spared if the injury lies in the posterior triangle.

Triangle of auscultation

Bounded by trapezius, latissimus dorsi and the medial border of the scapula. Here the chest wall is covered only by thin muscle, so breath sounds are heard clearly when the patient leans forward with the arms folded.