Commentary on: Key Anatomical Clarifications for the Marginal Mandibular Branch of the Facial Nerve: Clinical Significance for the Plastic Surgeon

Anatomic studies of the facial nerve are important to educate the surgeon about normal anatomy and variants of the nerve branches in order to reduce the risk of iatrogenic injury. In “Key Anatomical Clarifications for the Marginal Mandibular Branch of the Facial Nerve: Clinical Significance for the Plastic Surgeon,” ¹ the authors seek to determine if the cadaveric preservation process changes the anatomy of the marginal mandibular branch of the facial nerve from its anatomy in a fresh cadaver.

The marginal mandibular branch of the facial nerve courses along the inferior border of the mandible and is vulnerable to iatrogenic injury when surgery is performed in this region. In particular, surgical approaches for facial rejuvenation, facial fracture management, or salivary gland procedures may involve working in close proximity to the nerve’s course. An injury to this nerve is problematic in that it results in a paralysis of the depressor anguli oris and the depressor labii inferioris, leading to cosmetic impairment by disabling inferolateral lateral lip movement of the ipsilateral lower lip. The most important method to mitigate the risk of iatrogenic nerve damage is a thorough understanding of the regional anatomy, the purpose of the authors’ study. However, other methods that are employed to reduce risk warrant a discussion as well.

Prior to the procedure the anesthesiologist should be made aware that the patient should not be relaxed for surgery and that 4 nerve twitches should be present at the time of surgery. Short-acting depolarizing or nondepolarizing relaxants can be used to provide relaxation for intubation but allow nerve function to rapidly return after induction. The use of a nerve integrity monitor or a battery-powered nerve stimulator is also useful. If a battery-powered stimulator is used, it should be checked repeatedly during surgery to make sure it is working. This is easily done by touching the stimulator tip to the grounding wire and verifying that the red light in the handle illuminates. In this author’s experience it is not uncommon to notice the stimulator has failed and needs to be replaced. The inferior border of the mandible can be marked, and an incision line 2 cm below is also marked for an external approach to the mandible. It is important to do this because the relation of the nerve to the inferior mandible has been shown to change with head rotation. No such change is noted with flexion or extension.²

Several valuable anatomic studies have been published in the Aesthetic Surgery Journal regarding the anatomy of the mandibular branch as it relates to rhytidectomy. Wilhelmi et al found that the mandibular branch is superficial to the facial artery, making it susceptible to damage during superficial musculoaponeurotic system (SMAS) undermining or even subcutaneous dissection if an inadvertent sub-SMAS plane is entered.³ They found that the facial artery is about 3 cm anterior to the masseteric tuberosity or one-quarter the distance from the masseteric tuberosity to the mandibular midline. Zins et al also demonstrated that the distance one-quarter from the posterior mandible is a facial danger zone.⁴ They evaluated the proximity of the mandibular branch of the facial nerve to the mandibular osseocutaneous ligament and the platysma mandibular ligament and found that the mandibular branch is superficial to the facial artery, making the nerve more susceptible to injury when dissection is performed in this region. Although they felt careful subcutaneous dissection could be performed safely, they advocated against sub-SMAS dissection in the region.

The majority of anatomic studies have been performed on preserved human cadavers. Many studies of the facial nerve have been published but few compare fresh to cadaveric human anatomic specimens. Given the previously demonstrated importance in the understanding of this nerve to reduce injuries, it is important to ensure that the previous reported studies are accurate and that the embalming process does not alter the anatomy. The authors are to be congratulated for their efforts to ensure that the previous studies actually reflect the nerve anatomy in living patients.

Disclosures

The author declared no potential conflicts of interest with respect to the research, authorship, and publication of this article.

Funding

The author received no financial support for the research, authorship, and publication of this article.

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