The depressor labii inferioris is innervated by the marginal mandibular branch of the facial nerve, making it one of the muscles most vulnerable in mandibular fracture repair and neck dissection.
| Origin | Anterior surface of the mandible — between the symphysis and mental foramen |
|---|---|
| Insertion | Skin and muscle of the lower lip |
| Nerve Supply | Facial nerve — marginal mandibular branch (VII) |
| Blood Supply | Inferior labial artery |
| Actions | Depresses and everts the lower lip; Contributes to the lower lip pouting expression |
|---|
Marginal mandibular nerve injury during neck dissection or submandibular surgery produces lower lip asymmetry from DLI weakness — the affected side fails to depress during smiling. This is called the "marginal mandibular nerve smile deformity." Contralateral DLI botulinum injection is used to balance the smile in cases of permanent paralysis.
Visible as the lower lip depressor during open-mouthed smiling.
Lower lip asymmetry from DLI denervation after submandibular gland surgery, managed by contralateral botulinum injection for symmetry.
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