Posterior triangle of the neck

• Borders

  The anterior boundary of the posterior triangle is the posterior border of sternocleidomastoid, the base is formed by the clavicle and the posterior boundary is the anterior border of trapezius. The triangle contains several neurovascular structures and has a muscular floor.

  
• Nerves

  Some of the sensory nerves of the cervical plexus lie in the posterior triangle. The lesser occipital nerve passes from beneath sternocleidomastoid to ascend the neck towards the suboccipital region. The supraclavicular nerves supply the skin in the lateral side of the neck and the clavicle.

  The great auricular nerve passes from beneath sternocleidomastoid to cross the anterior surface of this muscle accompanied by the external jugular vein.

  
• Cutaneous nerves

  
• Veins

  •External jugular anterior to SCM (post triangle)

  •Internal jugular posterior to SCM (ant triangle)

  •Anterior jugular midline, may be absent

  The external jugular vein is formed by the union of the posterior auricular vein with the posterior division of the retromandibular vein .The external jugular vein crosses the anterior surface of sternocleidomastoid to pass into the posterior triangle and cross the anterior surface of the posterior belly of omohyoid to reach the subclavian vein deep in the root of the neck.

  
• the external jugular vein and the supraclavicular nerves. It also shows a nerve near the posterior boundary of the triangle passing between sternocleidomastoid and trapezius. This is cranial nerve eleven, the spinal accessory nerve. This nerve supplies both trapezius and sternocleidomastoid with motor innervation

  
• Deep structures

  The posterior belly of omohyoid crosses the posterior triangle so the muscle is found in both triangles of the neck. Small vessels cross the posterior triangle immediately posterior to omohyoid – the transverse cervical and suprascapular vessels. The roots of the brachial plexus and the phrenic nerve are also exposed.

  
• The phrenic nerve supplies the diaphragm and passes from C3,4 and 5 nerve roots to descend the neck. It crosses the anterior surface of anterior scalene muscle in the root of the neck before passing into the thorax. The roots of the brachial plexus arise from C5-T1 and cross through the posterior triangle to pass under the clavicle and into the axilla. The nerves of the brachial plexus supply the structures of the upper limb.

  
• Sternocleidomastoid (SCM)

  motor innervation = spinal accessory nerve (CN XI)

  proprioception = cervical plexus  C2, C3

  Sternocleidomastoid forms the border between the triangles of the neck so does not lie in either triangle. It is a very large muscle that attaches to the mastoid process of the temporal bone and crosses the neck to attach to both the clavicle and sternum. It is crossed by the external jugular vein and the great auricular nerve.

  
• SCM movements

  Lateral flexion brings the head closer to the shoulder, extension pulls the head backwards and flexion of the neck pulls the chin towards the body wall.

  
• Trapezius

  innervation = spinal accessory nerve (CN XI), cervical plexus C3,C4 (proprioception)

  Trapezius is a large superficial muscle of the back that can elevate, depress, retract and rotate the scapula. It forms the posterior boundary of the posterior triangle so also does not lie within the triangle itself.

  
• Floor

  -Omohyoid – depress hyoid bone  ansa cervicalis

  -Splenius capitis – extend the neck, rotate  dorsal rami middle cervical

  The scalene muscles, levator scapulae and splenius capitis lie in the floor of the triangle.

  
• Levator scapulae

  •Transverse processes of atlas and axis, 3rd and 4th C vertebrae to medial scapular border between superior angle and medial end of scapular spine

  •Help control scapular position, elevate shoulder (with trapezius), shoulder fixed can laterally flex neck to the same side

  •Direct C3, 4 and C5 via dorsal scapular nerve

  
• Scalenes

  -From transverse processes of C3-6 (anterior), C2-7 (middle) and C4-6 (posterior)

  -To ribs 1 (scalene tubercle = anterior; and behind subclavian artery groove = middle) and 2 (posterior)

  flex neck (bilateral), tilt head to same side (unilateral) and elevate upper ribs

  Anterior scalene is crossed by the phrenic nerve. The roots of the brachial plexus lie between anterior and middle scalene.

  
• arrow shows anterior scalene with the phrenic nerve