“Optimizing Injection Sites of Botulinum Toxin Type A for Improving Oral Commissure Position: A Two-Stage, Prospective, Double-Blind, Randomized Trial”

Background: Drooping oral commissures can impart a displeased appearance, negatively affecting psychosocial interactions and well-being. Botulinum toxin type A (BTX-A) is an efficacious treatment for mouth corner lift. However, the optimal injection strategy is controversial. Therefore, this study aims to compare the efficacy and safety of BTX-A injection at different sites of depressor anguli oris (DAO) muscle and to assess the added benefit of BTX-A injection into the platysma (PLT). Methods: This two-stage, single-center prospective study included a randomized, double-blind randomized trial (stage 1) and a non-randomized prospective study (stage 2). Primary outcomes were oral commissure position (OCDA, OCLA) and lip asymmetry (DSL) using three-dimensional photogrammetry. Secondary outcomes included patient satisfaction, pain, and side effects. Results: Both methods improved OCDA and OCLA across all time points, with lower DAO injection showing greater effects than the upper site. DAO+PLT further improved OCLA at 1 month and OCDA at 6 months. Satisfaction was higher with DAO+PLT (P < 0.05). DSL differences were <1 mm. No severe adverse events occurred. Conclusion: Lower DAO injection is more effective in inhibiting DAO movement. DAO+PLT yields superior commissure lifting with minimal side effects.