Dual Innervation of the Depressor Labii Inferioris and Implications for Deep Plane Facelift and Structural Neck Contouring: Insights from In Vivo Stimulation of the Facial Nerve Branches.

Injury to the depressor labii inferioris (DLI) is a frequent cause of lower lip asymmetry following deep plane facelift and structural neck contouring. This study characterizes the in vivo innervation of the DLI through intraoperative facial nerve mapping during selective facial neurectomy in 20 consecutive patients. In all cases, dual innervation of the DLI was observed: a superficial branch of the marginal mandibular nerve (MMN) supplies the lateral DLI, while an inferomedial cervical branch innervates the medial DLI. Both branches are located superficially and closely adherent to the deep surface of the superficial musculoaponeurotic system (SMAS) and platysma, placing them at risk during sub-SMAS and subplatysmal dissections. These findings challenge the conventional anatomical teaching that the DLI is solely innervated by the MMN. Incorporating this understanding of the DLI's dual innervation into surgical planning may help reduce the incidence of iatrogenic nerve injury and improve both functional and aesthetic outcomes in lower facial rejuvenation procedures.