Ultrasonographic analyses of Crow's feet and novel guideline for botulinum toxin injection

Background: Crow's feet are bilateral orbital wrinkles formed by the orbital portion of the orbicularis oculi muscle, which is the target muscle for botulinum neurotoxin (BoNT) injection. Objectives: This study's aim was to demonstrate a novel BoNT injection guideline by assessing muscle width, thickness, and dynamic features using ultrasonography. Methods: Twenty healthy Korean volunteers (10 men, 10 women; mean age, 25.6) participated. The width, thickness, and dynamic movement of the orbicularis oculi muscle were measured using ultrasonography. Two volunteers were selected to receive BoNT injections. Injections were administered using a novel method with two curved reference lines passing a point 15 mm lateral to the lateral canthus (conventional injection) and a point 5 mm lateral to the lateral margin of the frontal process of zygomatic bone (additional injection). Result: At the lateral canthus level, the distance between the lateral margin of the frontal process and the most lateral margin of the orbicularis oculi muscle was 12.5 ± 1.3 mm. The thickness of the orbicularis oculi muscle at the midpoint of the frontal process, the lateral marginal of the frontal process, and 5 mm lateral to the lateral marginal of the frontal process was 0.7 ± 0.3 mm, 1.1 ± 0.3 mm, and 1.2 ± 0.3 mm, respectively. The crow's feet of the two volunteers began to disappear from day 3 and completely disappeared on day 7 after the injection. Conclusion: The novel injection technique based on the ultrasonographic anatomy resulted in improvements in the appearance of crow's feet.