Experimental study of the center of the intramuscular nerve dense region of the masseter muscle as the optimal botulinum neurotoxin A injection site.

[BACKGROUND] Masseter hypertrophy impacts facial aesthetics. Botulinum neurotoxin type A(BoNT-A) injection is common, but the best site is unclear. This study seeks the optimal injection site for better facial contouring.

[MATERIAL] Sixty rabbits were used to assess BoNT-A injection efficacy at the center of the intramuscular nerve dense region (CINDR). The CINDR of the masseter muscle was localized in 24 adult cadavers using Sihler's staining and spiral computed tomography scanning. The line connecting the mouth corner to the earlobe base (horizontal reference) and the intersection of the line perpendicular to the mandibular angle and the zygomatic arch lower edge (longitudinal reference) were used as reference lines.

[RESULTS] One week after BoNT-A injection, reduced muscle thickness and muscle fiber cross-sectional area, and increased muscle atrophy factor (muscle ring finger-1) expression were observed in rabbits from both conventional and CINDR injection groups, but the changes were more pronounced in the CINDR injection group. The surface position of CINDR in human masseter muscle was located at 66.56 %±1.55 % along the horizontal reference line and 73.04 %±1.29 % along the longitudinal reference line with a puncture depth of 13.26 ± 0.44 mm.

[CONCLUSIONS] The CINDR of the masseter muscle is an optimal BoNT-A injection site for cosmetic applications enhancing the targeting accuracy and efficacy of BoNT-A injections.