Masseter Muscle Morphometric Changes After Botulinum Toxin Type A Injection: A Prospective 3-Month Study

Myofascial pain of the masticatory muscles, frequently associated with bruxism, is a prevalent condition with substantial impact on quality of life. Botulinum toxin type A (BoNT-A) is used as an adjunctive treatment for refractory cases, but the temporal pattern of masseter morphometric change after injection remained incompletely characterised. This prospective observational pilot study was designed and reported strictly as a 3D imaging and morphometric methodology study; clinical pain and patient-reported outcomes were outside its scope and were not assessed. Eleven adults with myofascial pain and bruxism received BoNT-A (100 U total: 30 U per masseter side, 20 U per anterior temporalis side). Structured-light 3D facial scans (EinScan H, Shining 3D) were acquired at baseline (T0), one month (T1), and three months (T2). A standardised masseter region of interest ("TGL area") was defined using cutaneous landmarks (tragus, gonion, labial commissure). Measurement precision was confirmed by intra-session repeatability (0.083 ± 0.020 mm), registration accuracy (RMS error 0.189-0.200 mm), and excellent landmark reliability (mean ICC [3, 1] = 0.955, 95% CI: 0.843-0.991). Repeated-measures ANOVA showed a significant effect of time on bilateral surface displacement (F(1.85,18.52) = 14.39, p < 0.001, η²G = 0.44), with significant reductions at T1 (-0.716 ± 0.480 mm; p = 0.002, Hedges' g = 2.03) and T2 (-0.847 ± 0.588 mm; p = 0.002, g = 1.96) relative to baseline, and no significant change between T1 and T2 (p = 1.000). Changes were bilaterally symmetric (symmetry index at T1: 0.231 ± 0.154 mm). These findings indicate that 3D structured-light scanning reliably detects subtle facial contour changes after BoNT-A injection and may complement clinical and patient-reported assessments; the pilot design and small sample preclude definitive conclusions.