Muscles of the Lower Lip: Functional Anatomy, Innervation Patterns, and Nerve Injury Prevention.

[INTRODUCTION] The depressor-labii-inferioris (DLI), depressor-anguli-oris (DAO), mentalis, and platysma muscles control lower lip position-essential for smiling and oral function-and are at risk during facial reconstructive and aesthetic procedures. Prior anatomical studies, lacking functional analysis, suggested the marginal mandibular nerve (MMn) significantly contributes to mid-lower lip depression through DLI innervation. This study systematically assessed the functional anatomy and innervation of these muscles and evaluated whether MMn injury worsens lower lip position.

[METHODS] A retrospective cohort study of prospectively collected data included adult synkinesis patients undergoing selective neurectomies and myectomies with unilateral facial nerve mapping. Intraoperatively, facial nerve branches were stimulated and categorized by muscle activation patterns. Additional sub-analysis evaluated lower lip position changes using pre- and postoperative photographs in patients who underwent MMn neurectomy without platysma myectomy.

[RESULTS] Of 82 patients, 45 met inclusion (mean age: 51.5; 87% female). An average of 11.3 nerve branches were mapped per patient: zygomatic (2.80), buccal (4.36), MMn (1.16), and cervical (3.02). DAO and mentalis demonstrated mixed innervation patterns. DLI and platysma were predominantly cervical; 95.5% of DLI branches were cervical with 68.8% being pure DLI branches. In the sub-analysis (n=24), MMn neurectomy did not worsen lower lip position.

[CONCLUSIONS] The DAO, mentalis, and platysma muscles are poly-innervated by buccal, MM, and cervical branches, while the DLI is mostly mono-innervated by cervical branches. Isolated MMn injury rarely worsens lower lip position, while injury to cervical branches to the DLI causes abnormal lower lip elevation and can be avoided by more caudal platysma myotomy.