The marginal mandibular nerve is a branch of the facial nerve (CN VII) arising from the lower division. It runs along the inferior border of the mandible, deep to the platysma, and supplies the depressor anguli oris, depressor labii inferioris, and mentalis muscles. It typically crosses the facial vessels at the inferior border of the mandible at the anterior edge of the masseter.
The marginal mandibular nerve is the most commonly injured branch of the facial nerve in neck, submandibular, and parotid surgery, producing the characteristic lower lip droop and inability to depress the corner of the mouth. Its injury is recognized immediately when the patient attempts to smile or bare their teeth. It is protected during submandibular gland excision by an incision 2-3 cm below the mandibular border, and during neck dissection by dissecting in a subfascial plane. Even temporary neuropraxia causes significant functional and cosmetic deficit.
Injury during submandibular gland excision, parotidectomy, or neck dissection causes weakness of the lower lip depressors producing a smile asymmetry with the corner of the mouth drawn upward on the affected side; most neuropraxias recover within 3-6 months but transection produces permanent weakness requiring possible reanimation procedures.
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