Factors Associated With Paradoxical Masseteric Bulging After Botulinum Toxin Injection for Masseter Hypertrophy: A Retrospective Analysis

BACKGROUND: Botulinum neurotoxin (BoNT) injection is the preferred minimally invasive treatment for masseter hypertrophy, but paradoxical masseteric bulging (PMB) is a distressing complication. How to clinically prevent and avoid the development of PMB remains a key concern for injecting physicians. METHODS: This study collected ultrasound and injection data from 22 PMB masseter muscles and 66 non-PMB masseter muscles between September 2024 and January 2025. Univariate analysis was used to compare imaging and injection-related parameters between the two groups, including masseter prominence, masseter thickness, deep inferior tendon (DIT) type, DIT thickness, intraoperative tactile sensation, and injected agent. RESULTS: Univariate analysis showed significant intergroup differences in masseter prominence grade, DIT type, DIT thickness, masseter thickness, injection dosage, and intraoperative tactile sensation (all p < 0.05). The PMB group had greater masseter thickness (13.30 ± 0.171 mm vs. 10.32 ± 0.169 mm), thicker DIT (0.85 [0.348] mm vs. 0.60 [0.208] mm), and a higher incidence of fascial penetration sensation than the control group. CONCLUSION: Preoperative ultrasound assessment of masseter and DIT characteristics, combined with individualized layered injection and intraoperative tactile feedback, effectively mitigates PMB risk. Ultrasound-guided precise supplementary injection is the preferred intervention for PMB management.