Ultrasonographic and Shear Wave Elastographic Assessment of the Mentalis Muscle: Associations with Mandibular Symphysis and Craniofacial Morphology

Background/Objectives: This study aimed to evaluate the ultrasonographic thickness and shear wave elastography (SWE)-derived stiffness of the mentalis muscle across different sagittal skeletal patterns and to investigate their relationships with mandibular symphysis morphology and cephalometric variables. Methods: Forty-eight adults aged 18–35 years were classified according to ANB angle into Class I, Class II, and Class III groups, with 16 participants in each group. Mentalis muscle thickness and stiffness were assessed bilaterally at lateral, intermediate, and medial regions using ultrasonography and SWE. Cephalometric skeletal, dental, soft-tissue, and symphyseal measurements were evaluated. Group comparisons were performed using one-way ANOVA or Welch’s ANOVA, as appropriate, and within-group associations were assessed using Pearson correlation analysis with Benjamini–Hochberg false discovery rate correction applied separately to elastography and muscle-thickness correlation families. Results: Mentalis muscle thickness did not differ significantly among the skeletal groups. Right intermediate elastography was the only ultrasonographic parameter showing a significant overall intergroup difference (p = 0.011, η2 = 0.182), with Bonferroni-adjusted pairwise comparisons showing significantly higher values in Class II than in Class I (adjusted p = 0.012). After FDR correction, selected associations between elastographic stiffness and skeletal, dental, or soft-tissue variables remained significant, whereas no muscle-thickness correlations remained significant. These associations varied across skeletal groups and were considered exploratory. Conclusions: Resting ultrasonographic characteristics of the mentalis muscle showed no consistent association with mandibular symphysis morphology across sagittal skeletal patterns. Mentalis muscle thickness was comparable among the groups, whereas elastographic stiffness demonstrated only a limited and region-specific between-group difference. Selected associations with skeletal and soft-tissue variables were observed; however, these exploratory findings should not be interpreted as evidence of skeletal-class-specific biological mechanisms. Larger studies incorporating functional assessments of the perioral musculature are needed to confirm these findings.