Masseter Muscle Stiffness in Bruxism and Myofascial Pain–Related Temporomandibular Disorders: A Systematic Review and Meta-Analysis of Quantitative Ultrasound Elastography Studies

Objectives To systematically review quantitative ultrasound elastography evidence and meta-analyze whether masseter muscle stiffness differs between individuals with bruxism and/or myofascial pain–related temporomandibular disorders (TMD) and healthy controls. Methods This systematic review followed PRISMA 2020. PubMed, Web of Science, Scopus, and OpenGrey were searched from inception to November 2025. Eligible studies were observational human studies that quantified masseter stiffness using shear wave elastography (SWE) or acoustic radiation force impulse (ARFI) imaging and included a healthy control group. The primary meta-analysis pooled standardized mean differences (Hedges g) using a random-effects model (REML) with Hartung–Knapp adjustment. A supportive sensitivity analysis pooled mean differences (MD) in kPa after unit harmonization. Risk of bias was assessed with the Newcastle–Ottawa Scale, small-study effects were explored with funnel plot inspection and Egger’s test (noting limited power with < 10 datasets), and certainty of evidence was graded using GRADE. Results Seven studies (eight datasets) including 190 patients and 202 controls (n = 392) were analyzed. Masseter stiffness was higher in patients than controls in the primary analysis (Hedges g = 1.485; 95% CI 0.763–2.207; p = 0.00183), with substantial heterogeneity (I²=84.1%). The kPa-based sensitivity analysis showed a similar direction and significance (MD = 6.16 kPa; 95% CI 3.67–8.65; p = 0.000628; I²=84.4%). Leave-one-out analyses indicated robust findings. Evidence certainty was very low. Conclusions Masseter stiffness is increased in bruxism and/or myofascial pain–related TMD versus healthy controls. High heterogeneity and methodological variability limit universal cutoff values; elastography is best suited for within-centre comparisons and longitudinal monitoring.