Longitudinal Computed Tomographic Evaluation of Mandibular Bone Morphology Following OnabotulinumtoxinA Treatment of Masseter Muscle Prominence: Results of a 12-Month, Repeat-Treatment, Placebo-Controlled Study in Healthy Adults

BACKGROUND: Botulinum neurotoxins (eg, onabotulinumtoxinA) are used off-label to treat masseter muscle prominence (MMP), a benign enlargement of the masseter muscles that may be aesthetically undesirable. Limited evidence suggests botulinum neurotoxin injections into masticatory muscles may adversely affect mandibular morphology, but this has not been rigorously investigated in context of MMP. OBJECTIVE: To evaluate whether mandibular morphology changes occur in individuals with MMP following bilateral masseter treatment with onabotulinumtoxinA. METHODS: In a 12-month, double-blind, placebo-controlled, Phase 2 study, 187 healthy adults with bilateral MMP received 1 or 2 onabotulinumtoxinA treatments (24, 48, 72, or 96 U) or placebo. Multidetector computed tomography (MDCT) scans (baseline, days 90 and 360) were evaluated using quantitative and qualitative mandibular morphology indicators. Adverse events (AEs) were monitored throughout. RESULTS: Quantitative assessments of bigonial width, cortical thickness, mandibular flare, and gonial angles found no differences between onabotulinumtoxinA (n=150) and placebo (n=37). Most participants (94%) showed no qualitative changes to mandibular condylar head (shape, position, cortical surface, and cortical thickness), glenoid fossa, or articular eminence (appearance, cortical surface). No clinically significant changes were identified after repeated qualitative assessments in 12 participants with findings suggestive of post-baseline osteopathic change. The most reported treatment-related AE, mastication disorder (6.0%), was consistent with prior reports following onabotulinumtoxinA masseter treatment for MMP. CONCLUSION: Quantitative and qualitative MDCT evaluations showed no detectable differences in predefined mandibular morphometric measures versus placebo after 1 or 2 onabotulinumtoxinA treatments for MMP over 1 year. Safety events were localized and consistent with drug mechanism. Late-emerging effects beyond 12 months cannot be excluded.