Background: The depressor anguli oris muscle (DAO) is a pivotal treatment target in creation of harmonic jawline. However, evidence of its live morphology remains scarce.
Objectives: This study aimed to reevaluate DAO by a facile ultrasound analysis and hereby guide safer and more effective botulinum toxin type A (BTX-A) injection.
Methods: A prospective ultrasound assessment was conducted in 41 patients. Morphology of DAO and its relative position with neighboring structures were appraised at the ubiquitous facial landmark, labiomandibular fold (LMF). Three-dimensional images were captured before and after receiving BTX-A injection based on sonographic evidence.
Results: The skin-to-muscle depths of DAO on average (measured from the medial to lateral border) were 5.26, 5.61, and 8.42 mm. DAO becomes thinner and wider from zone 1 to 3 (p < 0.001). Overlapping lengths between DAO and DLI increased from zone 1 to 3: 4.74, 9.68, 14.54 mm (p < 0.001). The medial border of DAO was located at 4.33, 6.12, 8.90 mm medial to LMF (zone 1-3), and no muscle fibers of DAO was observed at zone 1 and 2 in near one-third of patients. Mouth corner downturn angle improvement upon receiving BTX-A injection at zone 2 and 3 were 88.3%, 32.3%, and 14.7% for the neutral, maximum smile, and down-pulling mouth corner expressions.
Conclusions: This work established an informative ultrasound portrait of the DAO and structures in the perioral region, which suggests LMF as a convenient landmark to locate DAO. Injections at the middle and lower thirds of LMF at a 4-5 mm depth is recommended.
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