Anatomical Mapping of the Masseter for Safe Botulinum Toxin Injection: A Cadaveric Study.

[BACKGROUND] Botulinum toxin (BTX) is commonly used in masseter injections to enhance facial contour. However, the injection may harm the surrounding structure due to insufficient research on the anatomical structure of the masseter region.

[OBJECTIVES] This study is aimed at providing anatomical evidence for facial injections in clinical practice by dissecting the superficial masseter structure and analyzing it by classifying the masseteric nerve and structure.

[METHODS] Select 24 adult gross specimens with intact facial characteristics and dissect the masseter regions to expose the surface structure of the masseter muscle. After that, draw a heatmap according to the frequency of occurrence of the above structures. Then, dissect and document the structure relationship of the masseteric nerves and each layer of the masseter.

[RESULTS] The masseter region has a safe triangular zone (△LMF). The course probability of the facial nerve, parotid gland, parotid duct, facial artery, and facial vein is the lowest in this region. The masseteric structure is classified into three types: overlapped type, juxtaposed type, and parallel type, of which the juxtaposed type is the most common. In the safe triangular zone, the small branches of the masseteric nerve are mainly distributed in the connection range of line LM.

[CONCLUSION] The three-point injection approach is presented after a comprehensive analysis incorporating the masseter injection's safe triangular zone and the masseter's internal structure. It provides a more accurate treatment strategy for the masseter microplastic injection.