Novel anatomical guidelines for botulinum neurotoxin injection in the mentalis muscle: a review

Abstract

The mentalis muscle is a paired muscle originating from the alveolar bone of the mandible. This muscle is the main target muscle for botulinum neurotoxin (BoNT) injection therapy, which aims to treat cobblestone chin caused by mentalis hyperactivity. However, a lack of knowledge on the anatomy of the mentalis muscle and the properties of BoNT can lead to side effects, such as mouth closure insufficiency and smile asymmetry due to ptosis of the lower lip after BoNT injection procedures. Therefore, we have reviewed the anatomical properties associated with BoNT injection into the mentalis muscle. An up-to-date understanding of the localization of the BoNT injection point according to mandibular anatomy leads to better injection localization into the mentalis muscle. Optimal injection sites have been provided for the mentalis muscle and a proper injection technique has been described. We have suggested optimal injection sites based on the external anatomical landmarks of the mandible. The aim of these guidelines is to maximize the effects of BoNT therapy by minimizing the deleterious effects, which can be very useful in clinical settings.

Keywords: Botulinum neurotoxin; Cobblestone chin; Facial wrinkle; Injection point; Mentalis muscle.

Fig. 1

Schematic image of the cobblestone chin. The mental crease also appears marked in the cobblestone chin (A). Schematic image of the mentalis muscle (B). The muscle fiber originates from the alveolar bone of the anterior mandible (2 cm inferior to the intercheilion line), where the structure is inferior to the lateral incisors and inserts to the dermal layer of the chin.

Fig. 2

The external anatomical landmarks of the chin region around depressor anguli oris. Pg, pogonion; Gn, gnathion; MT, mental tubercle; Go, gonion.

Fig. 3

Ultrasonography image of the mentalis muscle. (A) The probe was applied horizontally at the Pg, perpendicular to the skin surface. (B) The hypoechoic mentalis is observed directly above the periosteum. Pg, pogonion; PL, 0.5 cm left to Pg; PR, 0.5 cm right to Pg.

Fig. 4

The injection points for the mentalis muscle. Each point is 0.5 cm lateral to the pogonion, where botulinum neurotoxin injections consist of a deep (3 U) and superficial (1 U) injection.