Anatomical considerations regarding the location and boundary of the depressor anguli oris muscle with reference to botulinum toxin injection

Background: Hyperactivity of the depressor anguli oris muscle can lead to a drooping of the mouth corner, which can give a sad, tired, or almost angry look in some patients. Botulinum toxin type A has recently been used to relax these hyperactive muscles. However, it is difficult to inject botulinum toxin type A into the depressor anguli oris muscle because its medial border overlaps with the depressor labii inferioris, and its lateral border is adjacent to the risorius, zygomaticus major, and platysma muscles. The aims of this study were to determine the topography of the facial muscles at the mouth corner and to provide critical information for determining the safest and most effective depressor anguli oris muscle botulinum toxin type A injection site. Methods: Forty-two hemifaces from Korean and Thai adult cadavers were dissected. Results: The location of the modiolus was 11.0 ± 2.6 mm (mean ± SD) lateral and 8.9 ± 2.8 mm inferior to the cheilion. The angle formed by the sagittal line passing through the modiolus (L_V) and the line connecting the modiolus and the intersection point of the lateral border of the depressor anguli oris muscle and the mandibular border (L_P2) was 44.7 ± 13.7 degrees. The angle formed by L_V and the line connecting the modiolus and the most concave point of the medial border of the depressor anguli oris muscle (L_P3) was 31.8 ± 8.5 degrees. Conclusion: These results suggest that the fan-shaped area bounded by L_P2, L_P3, and the mandibular border is the safest and most effective depressor anguli oris muscle injection site.