Anatomy and innervation
The trapezius muscle is a triangular, flat muscle of the shoulder girdle. It is supplied by the accessory nerve (cranial nerve XI) and the cervical plexus (C2-C4). The muscle itself consists of three parts:
- Descending (or superior) part: runs from the occipital bone and nuchal ligament to the lateral third of the clavicle.
- Transverse (or middle) part: originates from the spinal processes of the first to fourth thoracic vertebrae and inserts at the acromion.
- Ascending (or inferior) part: extends from the spinal processes of the fifth to twelfth thoracic vertebrae to the scapular spine.
The trapezius stabilizes and secures the shoulder blade at the thorax and fulfills numerous tasks. It moves the shoulder blade medially and rotates it outward. In addition, the descending part causes elevation of the scapula while the ascending part depresses this bone. Furthermore a unilateral contraction bends the head to the ipsilateral side (lateral flexion) whereas a bilateral contraction raises the head and the cervical vertebral column (dorsal flexion).
A lesion of the accessory nerve may lead to paresis and atrophy of the trapezius. A classic symptom is the inability of abducting the shoulder. The elevation (lifting the arm above 90°) is restricted as well. The shoulder of the affected side is lowered and the medial border of the shoulder blade lies slightly laterally (scapula alata). Injuries of the accessory nerve occur frequently during interventions at the cervical region such as lymph node biopsy or neck dissection, rarely after traumas and skull base tumors.