Cosmetic Effects of Botulinum Toxin in the Lower Face
Botulinum toxin injection into the depressor anguli oris (DAO) and mentalis muscles produces temporary paralysis that lifts downturned mouth corners, smooths chin dimpling, and creates a more youthful lower facial contour by blocking acetylcholine release at the neuromuscular junction. 1, 2
Mechanism of Action
Botulinum toxin blocks acetylcholine transmission at the neuromuscular junction by inhibiting acetylcholine release from presynaptic motor neuron terminals, resulting in temporary muscle paralysis that lasts weeks to months until new nerve terminals sprout. 1
Specific Muscle Effects
Depressor Anguli Oris (DAO)
- The DAO pulls the corners of the mouth downward, creating a sad or aged appearance; paralyzing this muscle allows unopposed action of the elevators, lifting the mouth corners upward. 3, 2
- Treatment of the DAO is now routinely performed and helps improve overall facial appearance by counteracting the downward pull on the oral commissures. 2
Mentalis Muscle
- The mentalis causes chin dimpling or "cobblestone chin" when hyperactive; botulinum toxin injection smooths this texture and reduces excessive chin wrinkling. 4
- The mentalis originates from the alveolar bone of the mandible and is the main target for treating cobblestone chin caused by mentalis hyperactivity. 4
Clinical Considerations and Pitfalls
Common Adverse Effects
- Medial injection or diffusion into the DAO can cause paralysis of the depressor labii inferioris muscle, resulting in smile asymmetry with excessive lowering of the unaffected side of the lower lip. 3
- Lack of anatomical knowledge can lead to mouth closure insufficiency and smile asymmetry due to lower lip ptosis after injection. 4
Correction Strategy
- Smile asymmetry from unilateral DAO paralysis can be corrected by injecting 3 U of botulinum toxin into the hyperactive depressor labii inferioris muscle on the contralateral side to restore symmetry. 3
Dosing Considerations
- The lower third of the face typically requires approximately 20 U of botulinum toxin type A for comprehensive treatment. 3
- Men require more units of botulinum toxin than women for equivalent effects. 2
Treatment Integration
- Combination therapy using botulinum toxin along with lasers or filler substances is ideal for comprehensive facial rejuvenation. 2
- Treatment of the orbicularis oris, mentalis, and depressor anguli oris together helps improve overall facial appearance beyond isolated muscle treatment. 2
Recovery Timeline
Recovery from paralysis takes weeks to months and occurs after sprouting of new nerve terminals, with toxin type A producing the most prolonged effects. 5, 1