Multiparametric Ultrasonographic Phenotyping of the Masseter Muscle in Temporomandibular Disorders and Clinically Defined Probable Bruxism

ABSTRACT Background Temporomandibular disorders (TMDs) and bruxism‐related masticatory muscle activity may influence masseter morphology, mechanical response and B‐mode texture, but their combined ultrasonographic phenotype is unclear. Objective To evaluate whether clinically defined TMD status and probable bruxism‐positive phenotype are associated with multiparametric masseter ultrasound profiles. Methods This prospective single‐centre study included 108 adults classified as TMD‐negative/bruxism‐negative controls (C0, n = 31), TMD‐negative/probable bruxism‐positive (T3, n = 25), TMD‐positive/bruxism‐negative (T2, n = 25) and TMD‐positive/probable bruxism‐positive (T1, n = 27). Sleep and awake bruxism were not separately distinguished. Bilateral thickness, cross‐sectional‐area‐derived sonographic estimated volume, strain elastography and resting fractal dimension (FD) were analysed by an observer blinded to group allocation. Results After false‐discovery‐rate correction, four‐group differences remained significant for resting/clenching thickness, estimated volume, strain elastography and resting FD (all q ≤ 0.001), with medium‐to‐large effects. T3 showed higher morphometric values, but sex‐stratified and sex‐adjusted analyses indicated that this finding required caution because of male predominance. GEE models identified sex, age, functional condition and group‐by‐condition terms as important determinants. FD repeatability was excellent (ICC A,1 = 0.964; 95% CI, 0.941–0.977; coefficient of variation, 0.54%). Conclusion Multiparametric masseter ultrasonography may capture complementary morphology, strain‐response and B‐mode texture phenotypes in TMD and clinically defined probable bruxism. Findings should be interpreted as phenotype‐enrichment rather than diagnostic evidence because TMD subtypes were heterogeneous and sleep/awake bruxism was not separately quantified.