Lower Facial Nerve Nomenclature Clarification: Cervical Branch Controls Smile-Associated Lower Lip Depression and Dental Display

Abstract

Objective: In head and neck ablative surgery, traditional teaching is that the key facial nerve branch to preserve along the plane of the lower border of the mandible is the marginal mandibular branch (MMb), which is considered to control all lower lip musculature. The depressor labii inferioris (DLI) is the muscle responsible for pleasing lower lip displacement and lower dental display during natural emotive smiling. Study Design: To understand the structure/function relationships of the distal lower facial nerve branches and lower lip musculature. Setting: In vivo extensive facial nerve dissections under general anesthesia. Methods: Intraoperative mapping was performed in 60 cases, using branch stimulation and simultaneous movement videography. Results: In nearly all cases, the MMb innervated the depressor anguli oris, lower orbicularis oris, and mentalis muscles. The nerve branches controlling DLI function were identified 2 ± 0.5 cm below the angle of the mandible, originating from a cervical branch, separately and inferior to MMb. In half of the cases, we identified at least 2 independent branches activating the DLI, both within the cervical region. Conclusion: An appreciation of this anatomical finding may help prevent lower lip weakness following neck surgery. Avoiding the functional and cosmetic consequences that accompany loss of DLI function would have a significant impact on the burden of potentially preventable sequelae that the head and neck surgical patient frequently carries.