Background: The frontalis is a representative target muscle for botulinum neurotoxin type A injections aimed at treating horizontal wrinkles in the forehead region. However, a lack of information regarding the shape and thickness of the frontalis may lead to unexpected side effects. Methods: This study dissected hemifaces of 44 embalmed Korean and Thai cadavers and performed ultrasound examinations on 20 Korean volunteers. Two anatomical types were identified: (1) the lateral portion of the frontalis covered the superior temporal line in type I, and (2) the lateral border of the frontalis and the superior temporal line almost coincided in type II. A horizontal line was drawn laterally from the midpoint between the metopion and the glabella, and landmarks F1, F2, and F3 were defined as points where this horizontal reference line intersected with vertical lines from the midpoint of the pupil, the lateral canthus, and the lateral orbital rim, respectively. Results: Type I was more common than type II [84 percent (37 of 44) versus 16 percent (seven of 44)]. When the lateral border of the frontalis ran along the border, there were no cases in which the superior temporal line was not visible. The mean minimum distance in type I was 10.53 mm. The muscle thicknesses at F1, F2, and F3 were 1.80 ± 0.44 mm (mean ± SD), 1.61 ± 0.37 mm, and 0.11 ± 0.04 mm, respectively. Conclusions: This study yielded data on the location and thickness of the lateral border of the frontalis. An anatomical study-based, ultrasound-guided injection technique can achieve reliable results when noninvasive treatment is applied to the forehead area.
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