An abscess was surgically removed in a 26 year old man from the middle of the posterior triangle on the right side. During recovery the patient noticed that her shoulder drooped and she could no longer raise her right hand above her head to brush her hair. Which nerve has been accidentaly cut during the surgery?

If the tip of the shoulder is drooping, trapezius is denervated, and the accessory nerve has been damaged. The accessory nerve (CN XI) runs through the posterior triangle, so it is likely to be injured in posterior neck triangle operations. If the accessory nerve is damaged there, the trapezius will be denervated, but the sternocleidomastoid will be fine. Ansa Cervicalis is a structure of the cervical plexus that innervates the strap muscles; if this structure was injured, you would note weakness in the strap muscles, not the trapezius. The facial nerve innervates the muscles of facial expression; it is not found in the posterior triangle. The hypoglossal nerve provides motor innervation to the tongue. It is also not in the posterior triangle.