A patient has surgery on the mandibular cheek to remove a squamous cell carcinoma. After the surgery, the patient appears normal at rest but has a crooked smile. Damage to which of the following structures has most likely occurred?

A. Temporal nerve

B. Marginal mandibular nerve

C. Buccal nerve

D. Spinal accessory nerve

Rationale: Understanding the normal anatomy of surgical sites is critical to avoiding inadvertent damage to important underlying structures. 

Correct answer: B. Marginal mandibular nerve

The marginal mandibular nerve innervates the lower lip muscles (orbicular oris, depressor anguli oris, depressor labii inferioris and mentalis). Damage to this nerve results in an inability to pull the lip downward and laterally resulting in a crooked smile and normal appearance at rest.  

Incorrect answers:

A. The temporal nerve innervates the following muscles: frontalis, corrugator supercilii, orbicularis oculi, and portions of the auricular muscle. Damage to this nerve results in the inability to raise the eyebrow and close the eye completely.

C. The buccal nerve innervates the buccinator, depressor septi nasi, nasalis, and upper lip muscles including (zygomaticus major and minor, levator labii superioris, orbicularis oris, and levator anguli oris). Damage to the buccal nerve results in inability to purse lips, uneven facial expressed both as rest and with smiling, drooling, muffled speech, and food accumulation. 

D. The spinal accessory nerve innervates the sternocleidomastoid and trapezius muscles. Damage to the spinal accessory nerve leads to winged scapula and the inability to elevated the shoulder. This nerve is located 1cm above Erb's point and caution must be taken when performing surgeries in this location. 

Additional reading at Fitzpatrick's Dermatology Chapter 201: Cutaneous Surgical Anatomy