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

Masseter Muscle Stiffness in Bruxism and Myofascial Pain–Related Temporomandibular Disorders: A Systematic Review and Meta-Analysis of Quantitative Ultrasound Elastography Studies | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Masseter Muscle Stiffness in Bruxism and Myofascial Pain–Related Temporomandibular Disorders: A Systematic Review and Meta-Analysis of Quantitative Ultrasound Elastography Studies Muhammed Enes NARALAN, Ömer HATİPOĞLU This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8925974/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 24 Mar, 2026 Read the published version in Oral Radiology → Version 1 posted You are reading this latest preprint version Abstract 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. Bruxism Temporomandibular Disorders Myofascial Pain Masseter Muscle Shear Wave Elastography Acoustic Radiation Force Impulse Figures Figure 1 Figure 2 Figure 3 Figure 4 1. INTRODUCTION Temporomandibular disorders (TMDs) are a broad and clinically heterogeneous group of conditions that commonly present with pain, restricted jaw movement, and reduced quality of life, involving the temporomandibular joint, masticatory muscles, and surrounding structures 1 . Although TMD is widely reported as prevalent in clinical samples, estimates vary substantially across studies, largely due to differences in diagnostic approaches and case definitions 2 . Recent evidence syntheses continue to underline TMD as a major burden across populations and reinforce the need for standardized assessment frameworks to support more consistent clinical decision-making 3 . Bruxism is currently defined as a repetitive masticatory muscle activity characterised by clenching or grinding of the teeth and/or by bracing or thrusting of the mandible. It has two distinct circadian phenotypes—sleep bruxism (rhythmic or non-rhythmic activity during sleep) and awake bruxism (repetitive or sustained tooth contact and/or bracing or thrusting during wakefulness). 4 For clinical and research operationalisation, a pragmatic grading framework (possible/probable/definite) has been proposed to reflect the certainty of assessment, and contemporary consensus emphasises that, in otherwise healthy individuals, bruxism should not be framed as a disorder but rather as a behaviour that may act as a risk (or protective) factor along a continuum; therefore, universal cut-off points are not recommended. Interpretation of the literature remains challenging because definitions, subtypes, and measurement strategies vary considerably across studies. 5 Despite this variability, several investigations report an association between self-reported bruxism and TMD-related symptoms, and neurobiological models highlight potential roles of stress-related pathways and central nervous system mechanisms that may contribute to inter-individual variability in clinical expression. 4 , 6 In routine practice, masticatory muscle assessment relies on symptom history, palpation, and functional examination. While essential, these methods do not directly quantify the mechanical properties of muscle tissue, creating a need for objective and reproducible tools capable of tracking tissue-level change over time, including treatment response 7 . As quantitative approaches expand, methodological variability and measurement error become increasingly important determinants of observed effects. Ultrasonography has long been used as an accessible modality for evaluating masseter morphology, most commonly via thickness measurements 8 . However, morphology and mechanical stiffness do not necessarily change in parallel; muscle architecture and functional state can complicate this relationship 9 . Thickness alone is therefore insufficient to explain the full clinical picture in symptomatic patients 10 . More recent work has pursued a more systematic evaluation of the morphology–function relationship to strengthen interpretation of masseter findings 11 . Ultrasound elastography addresses this gap by enabling quantitative assessment of muscle mechanical properties. Shear wave elastography (SWE) provides outputs such as shear wave velocity or Young’s modulus, allowing stiffness reporting in m/s or kPa. Yet muscle tissue is anisotropic, and factors such as probe orientation and imaging plane can meaningfully influence measured values 12 with plane-dependent variability repeatedly highlighted as a major rationale for protocol standardization 13 . Device-related factors also matter different ultrasound systems and proprietary processing algorithms can introduce systematic differences in SWE estimates 14 . In response, semi-automated region of interest (ROI) strategies has been adopted to improve reproducibility and standardization 15 , and evidence suggests acceptable reliability when appropriate protocols are applied 16. Age is another key determinant, since muscle stiffness changes with aging, and imbalances in age distribution can confound between-group comparisons in bruxism and TMD samples 16 , 17 , including in more recent datasets 18 . Biologically, increased stiffness in bruxism reflects more than mechanical loading alone. Aberrant repair processes and fibrotic remodelling may contribute, indicating tissue-level adaptation beyond simple overuse 19 . Reviews of muscle fibrosis further describe connective tissue–mediated pathways that can elevate stiffness 20 , and experimental studies support the concept that repeated overuse may induce micro-damage followed by remodelling 21 , 22 . This study systematically reviews and meta-analyses masseter stiffness in individuals with bruxism and/or myofascial pain–related TMD versus healthy controls using quantitative ultrasound elastography, and evaluates how methodological heterogeneity (devices, protocols, and populations) may influence pooled estimates. 2. Materials And Methods 2.1. Guidelines and eligibility criteria The research conducted in this study strictly adhered to the guidelines outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement, a comprehensive framework that outlines the minimum set of essential items for reporting systematic reviews and meta-analyses. The study's target population encompassed individuals diagnosed with bruxism or TMD presenting with myofascial pain, without any gender, socioeconomic, or ethnic restrictions, across various countries. Eligible studies included human observational research such as longitudinal, cohort, cross-sectional, and case-control studies. Specifically, the study seeks to answer the following PICO question: Population (P): Adults and adolescents diagnosed with bruxism or myofascial pain affecting the masticatory muscles. Intervention (I): Assessment of masseter muscle stiffness using SWE or Acoustic ARFI imaging. Comparison (C): Healthy control groups (asymptomatic individuals) without bruxism or TMD signs. Outcome (O): Quantitative differences in masseter muscle stiffness, reported as Young’s modulus (kPa) or Shear Wave Velocity (m/s). Exclusion criteria were meticulously defined, excluding studies that utilized Strain Elastography (which provides qualitative or semi-quantitative strain ratios rather than absolute stiffness values). Furthermore, descriptive studies, reviews, case reports, protocols, personal opinions, letters, posters, animal studies, and studies lacking a healthy control group were excluded. Additionally, studies with inaccessible full texts, or those with insufficient data for extraction (e.g., missing mean or standard deviation) where authors could not be contacted, were also excluded from the systematic review. 2.2. Information sources and search strategy The two researchers (M.E.N. and O.H) conducted a comprehensive literature search across three major electronic databases: PubMed, Web of Science, Scopus, and Open Grey, covering the period from inception to November 2025. The search strategy was designed to identify studies evaluating masseter muscle stiffness using quantitative ultrasound elastography (Supplemental File 1). In addition, the reference lists of included studies and relevant review articles were manually screened to identify any additional eligible studies providing normative values. The researchers manually reviewed the reference lists of all relevant papers gathered during the search process to ensure no significant studies were overlooked. Identified studies were saved and organized in EndNote 20 software, which facilitated efficient management, deduplication, and systematic screening. The most recent search was completed on November 26th, 2025. 2.3. Study selection and data extraction Two independent reviewers (M.E.N. and O.H) screened the titles and abstracts of retrieved records. Potentially relevant articles were subjected to full-text review. Disagreements were resolved by consensus. Data were extracted using a standardized form, including author, year, country, study design, sample size, age/gender distribution, ultrasound device (manufacturer/model), probe frequency, patient positioning, and stiffness values (Mean ± Standard Deviation). 2.4. Assessment of the risk of bias within the studies The methodological quality of the included studies was evaluated using the Newcastle-Ottawa Scale (NOS) adapted for cross-sectional and case-control studies. This tool assesses three domains: (1) Selection of study groups (0–4 stars), (2) Comparability of the groups (0–2 stars), and (3) Ascertainment of the Outcome/Exposure (0–3 stars). The maximum possible score is 9 stars. Studies scoring ≥ 7 stars were considered "high quality," 4–6 stars "moderate quality," and < 4 stars "low quality." Two independent reviewers (M.E.N. and O.H) conducted the evaluations, and discrepancies were resolved by consensus. 2.5. Risk of bias across studies Small-study effects/publication bias were explored using funnel plot inspection and Egger’s regression test. However, because fewer than ten datasets were available, these methods have limited power and may fail to detect asymmetry; therefore, findings were interpreted cautiously and were not considered definitive evidence for the absence of publication bias. 2.6. Summary measures The primary effect size was the standardized mean difference (SMD; Hedges g) with 95% confidence intervals, selected to improve comparability across different ultrasound systems, acquisition protocols, and reporting units (kPa vs m/s). Mean differences (MD) in kPa were additionally synthesised as a sensitivity analysis after unit harmonisation. 2.7. Data Transformation 2.8. Synthesis of results A priori, we considered bruxism and myofascial pain–related TMD as clinically distinct phenotypes. Accordingly, phenotype-specific synthesis (bruxism vs controls; myofascial pain–related TMD vs controls) was planned to support clinically coherent interpretation. Because the available evidence base is small and reporting is not fully harmonised across studies, we additionally present an overall pooled estimate across phenotypes as an exploratory summary of the average direction and magnitude of masseter stiffness differences across the broader spectrum of masticatory muscle dysfunction. This exploratory pooled estimate is interpreted cautiously and is not intended to imply that bruxism and myofascial pain–related TMD represent a single unified clinical entity. Random-effects models were fitted using REML, and statistical inference was based on Hartung–Knapp adjustment. Forest plots were generated for the primary SMD outcome; the kPa-based MD synthesis was reported as a sensitivity analysis. In cases where significant heterogeneity was identified in a model, the use of a random-effects model was recommended over a fixed-effects model if methodological or clinical heterogeneity was present. A random-effects model with a 95% confidence interval (CI) was chosen as the preferred meta-analysis model due to the absence of methodological, clinical, and statistical homogeneity. The alpha value for significance was set at 0.05, and all statistical tests were conducted using this threshold. 2.9. Grade Analysis The Grades of Recommendation, Assessment, Development, and Evaluation (GRADE) system was employed to ensure a transparent and structured evaluation of the importance of outcomes in our study. This system provides a comprehensive framework for rating the quality of evidence and strength of recommendations in healthcare. The GRADE system uses specific criteria to downgrade or upgrade the certainty of evidence based on factors such as risk of bias, inconsistency, indirectness, imprecision, and publication bias. 2.10. Sensitivity Analysis Sensitivity analyses included leave-one-out diagnostics for the primary SMD model. In addition, a unit-restricted sensitivity analysis was performed by pooling only studies reporting stiffness directly in kPa (i.e., without m/s-to-kPa conversion). Because all included studies were rated as high quality, exclusion based on risk-of-bias categories was not undertaken. 3. Results 3.1. Study Selection A comprehensive search of PubMed/MEDLINE (n = 65), Web of Science (n = 70), and Scopus (n = 40), supplemented by manual searching (n = 3), yielded 178 records. After removal of duplicates (n = 73), 105 records were screened by title and abstract, and 78 records were excluded. Full texts were sought for 27 reports, and all were retrieved. Of these, 20 reports were excluded after full-text assessment for the following reasons: wrong outcome measure (n = 2), no healthy control group (n = 10), inappropriate study design (review/case report) (n = 5), and wrong population (n = 3). Consequently, 7 studies (8 datasets) were included in the qualitative synthesis and meta-analysis (Fig. 1 ) (Supplemental File 2). 3.2. Characteristics of the Included Studies The seven studies (comprising eight datasets) included in this meta-analysis were published between 2017 and 2025, reflecting the most up-to-date literature. The geographic distribution of the studies was diverse, with three studies from Türkiye (Aksu et al., 2023; Erdur et al., 2025; Akkoca et al., 2025) 23 – 25 and one study each from Poland (Obuchowicz et al., 2024) 26 , Taiwan (Chen et al., 2023) 27 , Germany (Toker et al., 2023) 28 , and Japan (Takashima et al., 2017) 29 . The total participant pool consisted of 190 patients with bruxism and/or myofascial pain–related TMD and 202 healthy controls matched for age and sex (total n = 392). The age distribution varied across studies; while Erdur et al. 25 examined an adolescent population (mean age 14.5), the remaining studies focused on adult subjects with mean ages ranging between 28 and 45 years. From a clinical perspective, the included studies can be categorized into two main groups: (1) those examining patients with myofascial pain–related TMD or bruxism (Aksu et al., 2023 24 ; Takashima et al., 2017 29 ; Akkoca et al., 2025 23 ; Chen et al., 2023 27 ), and (2) those evaluating painless or unspecified bruxism cases 25 , 28 (Toker et al., 2023 28 ; Erdur et al., 2025 25 ). Regarding technical aspects, all studies employed SWE to assess masseter muscle stiffness; however, the ultrasound devices varied across studies (Toshiba Aplio, Canon Aplio, GE Logiq, Supersonic Aixplorer, and Samsung systems). Five studies reported outcomes directly as Young’s modulus (kPa), whereas two studies reported shear wave velocity (m/s), which was standardized for the meta-analysis. Detailed characteristics of all included studies are presented in Table 1 . Table 1 Characteristics of the Included Studies Study (Year) Country Clinical Population Device/System N (Patient / Control) Age Group Original Unit Aydin Aksu et al. (2023) 24 Türkiye Myofascial TMD/Bruxism Toshiba Aplio 500 26 / 26 Adult kPa Erdur et al. (2025) 25 Türkiye Adolescent Bruxism Canon Aplio 500 33 / 33 Adolescent kPa Akkoca et al. (2025) 23 Türkiye Myofascial TMD/Bruxism Samsung HS70A 25 / 29 Adult kPa Obuchowicz et al. (2024) 26 Poland TMD / Various Disease Conditions Supersonic Aixplorer 33 / 32 Adult kPa Chen et al. (2023) 27 Taiwan Orofacial Pain / TMD Canon Aplio 500 37 / 48 Adult kPa Toker et al. (2023) 28 Germany Bruxism Canon Aplio 1800 10 / 10 Adult m/s Takashima et al. – Myofascial Group (2017) 29 Japan Myofascial TMD GE Logiq E9 13 / 12 Adult m/s Takashima et al. – Limited Group (2017) 29 Japan Myofascial TMD GE Logiq E9 13 / 12 Adult m/s 3.3. Results of Individual Studies In this systematic review, most studies comparing patients with bruxism or TMD to healthy controls reported significant differences in masseter muscle stiffness. Specifically, five studies (Aksu et al., 2023 24 ; Erdur et al., 2025 25 ; Takashima et al., 2017 29 ; Chen et al., 2023 27 ; Obuchowicz et al., 2024 26 ) found that resting masseter stiffness (reported in kPa or m/s) was significantly higher in patients than in healthy individuals. In contrast, Akkoca et al. (2025) 23 observed no significant difference in resting stiffness between groups in patients with myofascial pain; however, they reported a significant increase in stiffness during maximal clenching among the patient group. When functional conditions were compared, Toker et al. 28 identified higher stiffness values in bruxism patients at rest yet reported that stiffness during maximal mouth opening was significantly lower in patients compared with controls. With respect to age groups, Erdur et al. 25 found that adolescent bruxism patients exhibited increased stiffness in both closed-mouth and open-mouth positions, similar to findings in adult populations, while no changes in muscle thickness were reported (Table 2 ). Table 2 Summary of Stiffness Outcomes Reported in Individual Studies Study Year Design N (Patient / Control) Mean Age Device Stiffness (Patient) Stiffness (Control) Aydin Aksu et al. (2023) 24 2023 Case–Control 26 / 26 28.0 Toshiba Aplio 39.13 ± 4.52 kPa 27.73 ± 1.92 kPa Erdur et al. (2025) 25 2025 Case–Control 33 / 33 14.5 Canon Aplio 25.10 ± 2.90 kPa 21.50 ± 2.80 kPa Toker et al. (2023) 28 2023 Case–Control 10 / 10 33.9 Canon Aplio 1.91 ± 0.44 m/s 1.66 ± 0.25 m/s Takashima et al. – Myofascial Group (2017) 29 2017 Case–Control 13 / 12 29.5 GE Logiq E9 1.96 ± 0.48 m/s 1.27 ± 0.38 m/s Takashima et al. – Limited Group (2017) 29 2017 Case–Control 13 / 12 29.5 GE Logiq E9 2.00 ± 0.55 m/s 1.27 ± 0.38 m/s Obuchowicz et al. (2024) 26 2024 Cross-Sectional 33 / 32* 45.0 Supersonic Aixplorer 35.00 ± 4.00 kPa 27.00 ± 3.00 kPa Akkoca et al. (2025) 23 2025 Case–Control 25 / 29 29.3 Samsung HS70A 23.65 ± 5.53 kPa 20.35 ± 5.94 kPa Chen et al. (2023) 27 2023 Cross-Sectional 37 / 48 35.0 Canon Aplio 15.64 ± 7.50 kPa 9.90 ± 2.56 kPa * Data from Obuchowicz et al. were obtained from the male subgroup aged 30–40 years. 3.4. Risk of Bias Within Studies All seven studies included in the analysis demonstrated a low risk of bias (high quality). Each study employed clear diagnostic criteria for participant selection (RDC/TMD or DC/TMD) and used objective methods for measuring masseter stiffness (SWE). Notably, the matching of control groups to patient groups in terms of age and sex, as well as the use of and used quantitative SWE protocols for measuring masseter stiffness, were key factors contributing to the internal validity of these studies. No study with moderate or high risk of bias was identified (Table 3 ). Table 3 Quality assessment of the included studies according to the Newcastle-Ottawa Scale (NOS). Study Year Design Selection (Max 4★) Comparability (Max 2★) Outcome (Max 3★) Total Score Quality Level Aydin Aksu et al. (2023) 24 2023 Case-Control ★★★★ ★★ ★★★ 9 High Obuchowicz et al. (2024) 26 2024 Cross-Sectional ★★★★ ★★ ★★★ 9 High Akkoca et al. (2025) 23 2025 Case-Control ★★★★ ★★ ★★★ 9 High Chen et al. (2023) 27 2023 Cross-Sectional ★★★★ ★ ★★★ 8 High Erdur et al. (2025) 25 2025 Case-Control ★★★★ ★★ ★★ 8 High Takashima et al. – Myofascial Group (2017) 29 2017 Case-Control ★★★ ★★ ★★★ 8 High Takashima et al. – Limited Group (2017) 29 2017 Case-Control ★★★ ★★ ★★★ 8 High Toker et al. (2023) 28 2023 Case-Control ★★★ ★★ ★★ 7 High 3.5. Synthesis of Results The primary meta-analysis using standardized mean differences (Hedges g) showed higher masseter stiffness in patients than in controls (g = 1.485, 95% CI 0.763–2.207; p = 0.00183), with substantial heterogeneity (I²=84.1%) (Fig. 2 ). In a secondary sensitivity analysis pooling mean differences in kPa after converting m/s to kPa where applicable, stiffness remained higher in patients (MD 6.16 kPa, 95% CI 3.67–8.65; p = 0.000628; I²=84.4%) The pooled estimate suggests that bruxism and myofascial TMD are associated with a marked increase in masseter muscle stiffness compared with healthy controls. In an exploratory subgroup restricted to bruxism-only datasets (n = 2), the pooled estimate was imprecise and not statistically conclusive (MD 3.48 kPa, 95% CI − 4.82 to 11.77; I² = 0%). 3.6. Sensitivity Analysis Leave-one-out analyses showed that omitting any single dataset did not materially change the pooled effect in either the primary SMD model (Hedges g) or the unit-harmonised MD (kPa) model; statistical significance was maintained across all omissions (Table 4 ). As a unit-restricted sensitivity analysis, restricting the meta-analysis to studies reporting stiffness directly in kPa (without m/s-to-kPa conversion) yielded a comparable effect (MD 6.47 kPa; 95% CI 2.27–10.67), supporting that the overall conclusion was not driven by unit conversion. Table 4 Leave-One-Out Analyses for Masseter Muscle Stiffness Excluded dataset Pooled Hedges g (SMD) 95% CI p-value Pooled MD (kPa) 95% CI p-value Aydin Aksu et al. (2023) 24 1.247 0.717 to 1.777 0.00120 5.315 3.354 to 7.276 0.000567 Erdur et al. (2025) 25 1.525 0.662 to 2.388 0.00496 6.605 3.821 to 9.389 0.00115 Obuchowicz et al. (2024) 26 1.373 0.565 to 2.181 0.00595 5.849 2.964 to 8.734 0.00255 Akkoca et al. (2025) 23 1.627 0.854 to 2.401 0.00212 6.544 3.800 to 9.288 0.00112 Chen et al. (2023) 27 1.553 0.702 to 2.405 0.00427 6.208 3.243 to 9.174 0.00217 Toker et al. (2023) 28 1.591 0.792 to 2.390 0.00279 6.594 3.944 to 9.244 0.000893 Takashima et al. – Myofascial Group (2017) 29 1.482 0.622 to 2.341 0.00556 6.082 3.144 to 9.019 0.00230 Takashima et al. – Limited Group (2017) 29 1.488 0.629 to 2.348 0.00546 6.033 3.124 to 8.942 0.00228 Random-effects models were fitted using REML with Hartung–Knapp adjustment. MD (kPa) analyses used unit harmonization (m/s→kPa) as described in Methods. 3.7. Publication Bias Publication bias was assessed using funnel plots and Egger’s regression test. Visual inspection did not suggest marked asymmetry. Egger’s test was not statistically significant (p = 0.2496). With only eight datasets, funnel plots and regression-based tests have limited power; therefore, these findings should be interpreted cautiously and do not exclude small-study effects (Fig. 3 ). 3.8. Grade Analysis The certainty of the evidence for the association between bruxism and increased Young’s modulus was rated as very low. Although the pooled estimate indicated a higher muscle stiffness in individuals with bruxism, the evidence was downgraded due to the non-randomised design of the included studies, and indirectness arising from differences in study populations, ultrasound elastography systems, acquisition protocols, and outcome measurement units. As a result, confidence in the pooled effect estimate is limited, and the true effect may be different from the observed estimate. 4. DISCUSSION Using a random-effects model pooling eight datasets from seven studies, the primary standardized analysis indicated higher masseter stiffness in patients than in controls (Hedges g = 1.485, 95% CI 0.763–2.207; p = 0.00183). However, substantial between-study heterogeneity (I²=84.1%) and the “very low” certainty of evidence under GRADE require cautious interpretation, and the findings should not be used for diagnostic inference or universal cut-off development. Across the included literature, the overall direction of effect is consistent: most studies report higher resting stiffness in the patient group 24 – 29 . One study separates from this pattern by reporting no clear difference at rest, with a between-group difference emerging only during maximal clenching 23 . This contrast is clinically informative because it implies that stiffness differences may not be uniformly detectable under a single measurement condition. Instead, they may become apparent only when the assessment is aligned with the patient’s functional state or symptom expression (Tables 1 – 2 ). A key clinical implication of the high heterogeneity is that this meta-analysis does not support receiving a universal cutoff value. That limitation should not be interpreted as a failure of elastography itself. A more defensible clinical role is to use elastography for within-centre comparisons—patient versus control under a consistent protocol—and for objectively tracking changes over time, including pre- and post-treatment assessments. In addition, interpreting elastography outputs alongside both resting and functional measurements may improve their correspondence with the clinical picture. This variability should not be interpreted as an artefact of insufficient evidence or limited sample size. If fundamental differences between elastography devices, acquisition parameters, and stiffness computation algorithms persist, statistical heterogeneity is expected to remain high. Under such conditions, the inclusion of additional studies employing heterogeneous methodologies would be unlikely to reduce heterogeneity or modify the direction of the pooled estimates. Thus, the observed inconsistency reflects structural methodological diversity rather than instability of the underlying biomechanical signal. Bruxism and myofascial pain–related TMD are clinically distinct phenotypes and should not be treated as interchangeable labels. To preserve clinical interpretability, our primary intent was phenotype-specific interpretation. However, because the available evidence base is small and reporting is not fully harmonised across studies, we also present an overall pooled estimate across phenotypes as an exploratory summary of the average direction and magnitude of stiffness differences across the broader spectrum of masticatory muscle dysfunction. Importantly, this exploratory pooled estimate should not be interpreted as implying a single unified clinical entity and is not suitable for diagnostic inference or universal cut-off development, particularly in the presence of substantial heterogeneity and device/protocol variability. Importantly, SWE technology does not measure the diagnostic label assigned to the patient (bruxism or TMD), but rather the biomechanical response of muscle tissue to pathological loading, reflected as increased stiffness or tone 30 , 31 . In this context, Chen et al. 27 reported that, in contrast to B-mode ultrasonography, SWE is capable of detecting microstructural alterations and stiffness changes in muscle tissue at an earlier stage than overt morphological changes such as hypertrophy. Consequently, increased muscle stiffness represents a shared and biologically plausible outcome in both bruxism 25 and myofascial TMD 23 . From this perspective, pooling data from these two groups in the present analysis does not introduce conceptual noise; rather, it provides a coherent and integrative representation of the mechanical alterations associated with masticatory muscle dysfunction 32 . From a mechanical standpoint, increased stiffness in bruxism is related to repetitive activity and chronic loading. The fact that several studies detect differences even at rest may indicate that a subset of patients exhibits persistently elevated basal muscle tone 24 , 25 , 29 . Yet the literature also suggests that functional tasks can modify the observed contrast between groups. In some datasets, the difference becomes more evident under activity, which points to an interaction between clinical phenotype—pain, functional limitation, or habit severity—and the measurement condition 23 , 28 . For that reason, elastography values are better understood as supportive quantitative findings rather than as a stand-alone biomarker. Given the clinical and technical variability across the included studies, a high degree of heterogeneity is expected. Clinically, the samples include both adolescents and adults. In adolescents, increased stiffness in the absence of thickness changes suggests that measurable alterations in tone or rigidity may occur early before hypertrophy develops 25 . In adults—particularly those with myofascial pain/TMD—the wider spread of effect sizes may reflect differences in symptom burden and severity that are captured to some extent by stiffness measurements 24 , 27 , 29 . On the technical side, the studies used devices from multiple manufacturers (e.g., Toshiba, Canon, GE, Samsung, Supersonic), and two studies reported outcomes in m/s, requiring conversion to kPa for the meta-analysis (Tables 1 – 2 ). While conversion improves comparability in a pooled analysis, it may also add uncertainty, particularly given the direction-dependent nature of muscle tissue and protocol differences across settings. This methodological diversity—together with population variability—was among the central reasons for downgrading evidence certainty for indirectness in the GRADE assessment. Within this context, employing a random-effects model is methodologically appropriate because it estimates an average effect while acknowledging true between-study variability rather than assuming a single common effect across all conditions. When the datasets are considered together, a pattern emerges that may be clinically relevant: stiffness appears more clearly elevated within the myofascial pain/TMD spectrum, while the difference may be smaller in painless or milder bruxism phenotypes 24 , 25 , 27 , 29 . The finding of a pronounced increase during maximal clenching despite no resting difference in one study further suggests that functional tasks can be more discriminative in selected phenotypes 23 . Conversely, lower stiffness during maximal mouth opening in the patient group could reflect pain or functional limitation that constrains muscle behaviour under that condition 28 . However, because the number of eligible studies is limited, these observations should remain hypothesis-generating rather than being treated as definitive subgroup conclusions. The leave-one-out sensitivity analysis supports the stability of the pooled result. Excluding any single study did not materially change the estimate; the pooled effect size remained within 5.31–6.59 kPa, and significance persisted (Table 4 ). This reduces concern that the overall finding is disproportionately driven by an individual dataset. Regarding publication bias, the funnel plot did not show marked asymmetry and Egger’s test was not statistically significant (p = 0.2496). Still, with a small number of studies, formal tests have limited power; therefore, these results should be interpreted as reassuring rather than conclusive. All included studies were rated “high quality” by the NOS (Table 3 ), reflecting strengths in selection, comparability, and outcome measurement. This does not conflict with the “very low” certainty rating under GRADE. GRADE weighs the totality of evidence and, importantly, begins observational designs at a lower certainty level. In this review, the non-randomized nature of the evidence base, the high heterogeneity, and indirectness linked to population and protocol/device variability contributed to the downgrading. As a result, although the direction of effect is aligned across studies, confidence in the true magnitude remains limited and the true effect may differ from the pooled estimate. This review has several limitations. All included studies were observational, limiting causal inference and aligning with the “very low” certainty of evidence. Heterogeneity was substantial (I² = 84.1%), influenced by age differences (adolescent vs adult), phenotype differences (painful myofascial TMD vs painless bruxism), and varied measurement conditions (rest, mouth opening, maximal clenching). Device and protocol diversity—and the conversion of m/s to kPa in two studies—may constrain direct comparison of absolute values across settings. The limited number of studies reduces the strength of publication bias assessments, and potentially relevant confounders (e.g., stress, sleep characteristics, medication use) may not have been standardized consistently across the evidence base. With only eight datasets, funnel plot inspection and Egger’s test were underpowered; thus, publication bias cannot be confidently excluded. Future studies should focus on improving methodological consistency and clinical interpretability of masseter muscle elastography findings. Prospective and multicentre designs using standardized acquisition protocols, including probe orientation, region-of-interest definition, and reporting units, are required to reduce device- and protocol-related variability. Stratification of study populations according to clearly defined clinical phenotypes, such as painful myofascial TMD versus painless bruxism, may help clarify phenotype-specific stiffness patterns. Longitudinal studies assessing pre- and post-treatment changes are also needed to determine the utility of elastography as a monitoring tool rather than a diagnostic cutoff-based method. Finally, integration of elastography outcomes with clinical measures of pain, function, and psychosocial factors may provide a more comprehensive understanding of masticatory muscle dysfunction. 5. CONCLUSION This systematic review and meta-analysis demonstrated that masseter muscle stiffness is higher in individuals with bruxism and/or TMD associated with myofascial pain compared with healthy controls. However, the high level of between-study heterogeneity and the classification of evidence certainty as “very low” according to GRADE limit confidence in the magnitude of this effect. When applied with appropriate protocols and within a relevant clinical context, elastography represents a practical method capable of quantitatively capturing changes in the masseter muscle and may complement clinical examination. Declarations Conflicts of Interest The authors declare no conflicts of interest. Funding The authors have nothing to report. Author Contribution M.E.N. and Ö.H. designed the study. M.E.N. conducted the searches, screening, and data extraction. M.E.N. and Ö.H. performed the analyses and prepared the figures/tables. M.E.N. drafted the manuscript, and Ö.H. critically revised it. Both authors approved the final manuscript and take responsibility for the work. Acknowledgements The authors have nothing to report. Data Availability All data supporting the findings of this study are included within the article and its supporting information. Ethics Statement The authors have nothing to report. References Valesan LF, Da-Cas CD, Réus JC et al. Prevalence of temporomandibular joint disorders: a systematic review and meta-analysis. Clin Oral Invest. 2021/02/01 2021;25(2):441–53. 10.1007/s00784-020-03710-w Homeida L, Felemban E, Kassar W, Ameen M, Aldahlawi S. Temporomandibular joints disorders (TMDs) prevalence and their relation to anxiety in dental students. F1000Res. 2022;11:271. 10.12688/f1000research.76178.2 . Rentsch M, Zumbrunn Wojczyńska A, Gallo LM, Colombo V. Prevalence of Temporomandibular Disorders Based on a Shortened Symptom Questionnaire of the Diagnostic Criteria for Temporomandibular Disorders and Its Screening Reliability for Children and Adolescents Aged 7–14 Years. J Clin Med Jun. 2023;17(12). 10.3390/jcm12124109 . Lobbezoo F, Ahlberg J, Glaros AG, et al. Bruxism defined and graded: an international consensus. J Oral Rehabil Jan. 2013;40(1):2–4. 10.1111/joor.12011 . Melo G, Duarte J, Pauletto P, et al. Bruxism: An umbrella review of systematic reviews. J Oral Rehabil Jul. 2019;46(7):666–90. 10.1111/joor.12801 . Lobbezoo F, Ahlberg J, Raphael KG, et al. International consensus on the assessment of bruxism: Report of a work in progress. J Oral Rehabil Nov. 2018;45(11):837–44. 10.1111/joor.12663 . Ashir A, Jerban S, Barrère V, et al. Skeletal Muscle Assessment Using Quantitative Ultrasound: A Narrative Review. Sensors. 2023;23(10):4763. Kiliaridis S, Kälebo P. Masseter muscle thickness measured by ultrasonography and its relation to facial morphology. J Dent Res Sep. 1991;70(9):1262–5. 10.1177/00220345910700090601 . Kiliaridis S, Engvall M, Tzakis MG. Ultrasound imaging of the masseter muscle in myotonic dystrophy patients. J Oral Rehabil Aug. 1995;22(8):619–25. 10.1111/j.1365-2842.1995.tb01059.x . Reis Durão AP, Morosolli A, Brown J, Jacobs R. Masseter muscle measurement performed by ultrasound: a systematic review. Dentomaxillofac Radiol Aug. 2017;46(6):20170052. 10.1259/dmfr.20170052 . Shen Y, Li X, Feng X, Zhang C, Shang Y, Lin J. The effects of masseter muscle morphology on three-dimensional occlusion and temporomandibular joint in adult patients with skeletal class II malocclusion: a CBCT study. BMC Oral Health . 2025/08/06 2025;25(1):1296. 10.1186/s12903-025-06674-z Dubois G, Kheireddine W, Vergari C, et al. Reliable protocol for shear wave elastography of lower limb muscles at rest and during passive stretching. Ultrasound Med Biol Sep. 2015;41(9):2284–91. 10.1016/j.ultrasmedbio.2015.04.020 . Adamczewski Z, Stasiak M, Stasiak B, Adamczewska M, Lewiński A. Interobserver Agreement and Plane-Dependent Intraobserver Variability of Shear Wave Sonoelastography in the Differential Diagnosis of Ectopic Thymus Tissue. J Clin Med Jan. 2021;9(2). 10.3390/jcm10020214 . Neto T, Johannsson J, Andrade RJ. Using ultrasound shear wave elastography to characterize peripheral nerve mechanics: a systematic review on the normative reference values in healthy individuals. Ultrasonography May. 2024;43(3):169–78. 10.14366/usg.23211 . Haueise A, Carvalho GF, Azan M, Gehring D, Skerl K, Dieterich AV. Development and validation of a semi-automated algorithm to analyze shear wave elastography clips in muscle tissue. Sci Rep Jun. 2025;20(1):20147. 10.1038/s41598-025-05154-2 . Brown M, Fisher JS, Salsich G. Stiffness and muscle function with age and reduced muscle use. J Orthop Res May. 1999;17(3):409–14. 10.1002/jor.1100170317 . Gajdosik RL, Vander Linden DW, Williams AK. Influence of age on length and passive elastic stiffness characteristics of the calf muscle-tendon unit of women. Phys Ther Sep. 1999;79(9):827–38. Jacob I, Johnson MI, Jones G, Jones A, Francis P. Age-related differences of vastus lateralis muscle morphology, contractile properties, upper body grip strength and lower extremity functional capability in healthy adults aged 18 to 70 years. BMC Geriatr Jun. 2022;29(1):538. 10.1186/s12877-022-03183-4 . Mann CJ, Perdiguero E, Kharraz Y, et al. Aberrant repair and fibrosis development in skeletal muscle. Skelet Muscle May. 2011;4(1):21. 10.1186/2044-5040-1-21 . Mahdy MAA. Skeletal muscle fibrosis: an overview. Cell Tissue Res Mar. 2019;375(3):575–88. 10.1007/s00441-018-2955-2 . Barbe MF, Hilliard BA, Amin M, et al. Blocking CTGF/CCN2 reduces established skeletal muscle fibrosis in a rat model of overuse injury. FASEB J May. 2020;34(5):6554–69. 10.1096/fj.202000240RR . Patel PR, Tamas IP, Van Der Bas M, et al. Repetitive Overuse Injury Causes Entheseal Damage and Palmar Muscle Fibrosis in Older Rats. Int J Mol Sci Dec. 2024;18(24). 10.3390/ijms252413546 . Akkoca F, Özyürek S, Ilhan G, Koyuncu E, Ozdede M. Role of the masseter, anterior temporalis, and sternocleidomastoid muscles in myofascial temporomandibular disorder pain: evaluation of thickness and stiffness by ultrasonography. Article. Oral Radiol. 2025;41(3):363–71. 10.1007/s11282-025-00807-4 . Akkoca Obuchowicz S, Kursoglu P, Turker I, et al. Dynamic Quantitative Imaging of the Masseter Muscles in Bruxism Patients with Myofascial Pain: Could It Be an Objective Biomarker? J Pers Med Oct. 2023;6(10). 10.3390/jpm13101467 . Erdur EA, Öztürk M, Dag N, Erdur Ö, Altindag A. Usability of shear wave elastography in the quantitative evaluation of masseter muscle stiffness in adolescents with bruxism. Dentomaxillofac Radiol . 2025 MAY 29 2025; 10.1093/dmfr/twaf012 Obuchowicz R, Obuchowicz B, Nurzynska K, Urbanik A, Pihut M. Population Analysis of Masseter Muscle Tension Using Shear Wave Ultrasonography across Different Disease States. J Clin Med Sep. 2024;5(17). 10.3390/jcm13175259 . Chen YJ, Lin HY, Chu CA, et al. Assessing thickness and stiffness of superficial/deep masticatory muscles in orofacial pain: an ultrasound and shear wave elastography study. Article. Ann Med. 2023;55(2):2261116. 10.1080/07853890.2023.2261116 . Toker C, Marquetand J, Symmank J, et al. Shear Wave Elastography in Bruxism-Not Yet Ready for Clinical Routine. Diagnostics (Basel) Jan. 2023;11(2). 10.3390/diagnostics13020276 . Takashima M, Arai Y, Kawamura A, Hayashi T, Takagi R. Quantitative evaluation of masseter muscle stiffness in patients with temporomandibular disorders using shear wave elastography. Article. J Prosthodont Res. 2017;61(4):432–8. 10.1016/j.jpor.2017.01.003 . Stiver ML, Mirjalili SA, Agur AMR. Measuring Shear Wave Velocity in Adult Skeletal Muscle with Ultrasound 2-D Shear Wave Elastography: A Scoping Review. Ultrasound Med Biol Jun. 2023;49(6):1353–62. 10.1016/j.ultrasmedbio.2023.02.005 . Olchowy A, Seweryn P, Olchowy C, Wiȩckiewicz M. Assessment of the masseter stiffness in patients during conservative therapy for masticatory muscle disorders with shear wave elastography. Article. BMC Musculoskelet Disord. 2022;23(1):439. 10.1186/s12891-022-05392-9 . Bisht SR, Paul A, Patel P, Thareja P, Mercado-Shekhar KP. Systematic quantification of differences in shear wave elastography estimates between linear-elastic and viscoelastic material assumptionsa). J Acoust Soc Am. Mar 2024;1(3):2025–36. 10.1121/10.0025291 . Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8925974","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":596727311,"identity":"52ce20fc-1b7d-47da-aa27-af281f001f23","order_by":0,"name":"Muhammed Enes NARALAN","email":"data:image/png;base64,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","orcid":"","institution":"Recep Tayyip Erdoğan University","correspondingAuthor":true,"prefix":"","firstName":"Muhammed","middleName":"Enes","lastName":"NARALAN","suffix":""},{"id":596727312,"identity":"4ff7ae5a-9d25-412e-a590-646ef3df7bb2","order_by":1,"name":"Ömer HATİPOĞLU","email":"","orcid":"","institution":"Recep Tayyip Erdoğan University","correspondingAuthor":false,"prefix":"","firstName":"Ömer","middleName":"","lastName":"HATİPOĞLU","suffix":""}],"badges":[],"createdAt":"2026-02-20 12:38:25","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8925974/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8925974/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11282-026-00914-w","type":"published","date":"2026-03-24T16:11:16+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":103842116,"identity":"f9aa9318-f3fb-424c-b402-2d41fe882185","added_by":"auto","created_at":"2026-03-03 14:57:06","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":41079,"visible":true,"origin":"","legend":"\u003cp\u003ePRISMA 2020 flow diagram of study identification, screening, eligibility, and inclusion.\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-8925974/v1/a49bd296049cb148e2cf8703.png"},{"id":103842165,"identity":"f5cc3a3f-48f9-458d-b188-dc0ac2a3f933","added_by":"auto","created_at":"2026-03-03 14:57:12","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":54858,"visible":true,"origin":"","legend":"\u003cp\u003eForest plot (Hedges g) of masseter stiffness in bruxism and/or myofascial pain–related TMD vs controls. Positive values indicate higher stiffness in patients. Random-effects model: REML with Hartung–Knapp; diamond = pooled effect; bar = 95% prediction interval.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-8925974/v1/5a459eca74d80e68e6c01297.png"},{"id":103842105,"identity":"e18a189f-fbad-4140-a738-396e554f74ab","added_by":"auto","created_at":"2026-03-03 14:57:01","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":62015,"visible":true,"origin":"","legend":"\u003cp\u003eFunnel plot (MD, kPa) for sensitivity analysis. With only eight datasets, asymmetry tests have low power; interpret cautiously.\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-8925974/v1/24b865ce451329953ebcf5bb.png"},{"id":103842119,"identity":"642b1a37-a30a-4bf2-866a-3f99b53a40cc","added_by":"auto","created_at":"2026-03-03 14:57:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":66140,"visible":true,"origin":"","legend":"\u003cp\u003eGRADE summary of findings (kPa). Evidence rated very low due to non-randomized design, heterogeneity, indirectness (device/protocol/unit differences), and imprecision.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-8925974/v1/871891fb7e4225d584e61853.png"},{"id":105755913,"identity":"8e0e5994-38f2-4401-afe6-f7207e5e6dda","added_by":"auto","created_at":"2026-03-30 16:32:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1309778,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8925974/v1/7fd919ac-3e32-4c66-9376-0d6aba1828ae.pdf"},{"id":103842167,"identity":"ce94add3-72f9-47c1-a87a-4251e3f208a8","added_by":"auto","created_at":"2026-03-03 14:57:13","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":14530,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementalFile1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8925974/v1/79951eb53c24e7fe2f652263.docx"},{"id":103842113,"identity":"aedfb982-503b-4f0f-a78c-cb32ea763b99","added_by":"auto","created_at":"2026-03-03 14:57:03","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":51551,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementalFile2.docx","url":"https://assets-eu.researchsquare.com/files/rs-8925974/v1/ec7a011c7151823647e80d7d.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Masseter Muscle Stiffness in Bruxism and Myofascial Pain–Related Temporomandibular Disorders: A Systematic Review and Meta-Analysis of Quantitative Ultrasound Elastography Studies","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eTemporomandibular disorders (TMDs) are a broad and clinically heterogeneous group of conditions that commonly present with pain, restricted jaw movement, and reduced quality of life, involving the temporomandibular joint, masticatory muscles, and surrounding structures \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Although TMD is widely reported as prevalent in clinical samples, estimates vary substantially across studies, largely due to differences in diagnostic approaches and case definitions \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Recent evidence syntheses continue to underline TMD as a major burden across populations and reinforce the need for standardized assessment frameworks to support more consistent clinical decision-making \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBruxism is currently defined as a repetitive masticatory muscle activity characterised by clenching or grinding of the teeth and/or by bracing or thrusting of the mandible. It has two distinct circadian phenotypes\u0026mdash;sleep bruxism (rhythmic or non-rhythmic activity during sleep) and awake bruxism (repetitive or sustained tooth contact and/or bracing or thrusting during wakefulness).\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e For clinical and research operationalisation, a pragmatic grading framework (possible/probable/definite) has been proposed to reflect the certainty of assessment, and contemporary consensus emphasises that, in otherwise healthy individuals, bruxism should not be framed as a disorder but rather as a behaviour that may act as a risk (or protective) factor along a continuum; therefore, universal cut-off points are not recommended. Interpretation of the literature remains challenging because definitions, subtypes, and measurement strategies vary considerably across studies.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e Despite this variability, several investigations report an association between self-reported bruxism and TMD-related symptoms, and neurobiological models highlight potential roles of stress-related pathways and central nervous system mechanisms that may contribute to inter-individual variability in clinical expression.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eIn routine practice, masticatory muscle assessment relies on symptom history, palpation, and functional examination. While essential, these methods do not directly quantify the mechanical properties of muscle tissue, creating a need for objective and reproducible tools capable of tracking tissue-level change over time, including treatment response \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. As quantitative approaches expand, methodological variability and measurement error become increasingly important determinants of observed effects.\u003c/p\u003e \u003cp\u003eUltrasonography has long been used as an accessible modality for evaluating masseter morphology, most commonly via thickness measurements \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. However, morphology and mechanical stiffness do not necessarily change in parallel; muscle architecture and functional state can complicate this relationship \u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Thickness alone is therefore insufficient to explain the full clinical picture in symptomatic patients \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. More recent work has pursued a more systematic evaluation of the morphology\u0026ndash;function relationship to strengthen interpretation of masseter findings \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. Ultrasound elastography addresses this gap by enabling quantitative assessment of muscle mechanical properties. Shear wave elastography (SWE) provides outputs such as shear wave velocity or Young\u0026rsquo;s modulus, allowing stiffness reporting in m/s or kPa. Yet muscle tissue is anisotropic, and factors such as probe orientation and imaging plane can meaningfully influence measured values \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e with plane-dependent variability repeatedly highlighted as a major rationale for protocol standardization \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDevice-related factors also matter different ultrasound systems and proprietary processing algorithms can introduce systematic differences in SWE estimates \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. In response, semi-automated region of interest (ROI) strategies has been adopted to improve reproducibility and standardization \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, and evidence suggests acceptable reliability when appropriate protocols are applied 16. Age is another key determinant, since muscle stiffness changes with aging, and imbalances in age distribution can confound between-group comparisons in bruxism and TMD samples \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, including in more recent datasets \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eBiologically, increased stiffness in bruxism reflects more than mechanical loading alone. Aberrant repair processes and fibrotic remodelling may contribute, indicating tissue-level adaptation beyond simple overuse \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Reviews of muscle fibrosis further describe connective tissue\u0026ndash;mediated pathways that can elevate stiffness \u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e, and experimental studies support the concept that repeated overuse may induce micro-damage followed by remodelling \u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThis study systematically reviews and meta-analyses masseter stiffness in individuals with bruxism and/or myofascial pain\u0026ndash;related TMD versus healthy controls using quantitative ultrasound elastography, and evaluates how methodological heterogeneity (devices, protocols, and populations) may influence pooled estimates.\u003c/p\u003e"},{"header":"2. Materials And Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1. Guidelines and eligibility criteria\u003c/h2\u003e\n \u003cp\u003eThe research conducted in this study strictly adhered to the guidelines outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement, a comprehensive framework that outlines the minimum set of essential items for reporting systematic reviews and meta-analyses. The study\u0026apos;s target population encompassed individuals diagnosed with bruxism or TMD presenting with myofascial pain, without any gender, socioeconomic, or ethnic restrictions, across various countries. Eligible studies included human observational research such as longitudinal, cohort, cross-sectional, and case-control studies.\u003c/p\u003e\n \u003cp\u003eSpecifically, the study seeks to answer the following PICO question:\u003c/p\u003e\n \u003cul\u003e\n \u003cli\u003e\n \u003cp\u003ePopulation (P): Adults and adolescents diagnosed with bruxism or myofascial pain affecting the masticatory muscles.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eIntervention (I): Assessment of masseter muscle stiffness using SWE or Acoustic ARFI imaging.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eComparison (C): Healthy control groups (asymptomatic individuals) without bruxism or TMD signs.\u003c/p\u003e\n \u003c/li\u003e\n \u003cli\u003e\n \u003cp\u003eOutcome (O): Quantitative differences in masseter muscle stiffness, reported as Young\u0026rsquo;s modulus (kPa) or Shear Wave Velocity (m/s).\u003c/p\u003e\n \u003c/li\u003e\n \u003c/ul\u003e\n \u003cp\u003eExclusion criteria were meticulously defined, excluding studies that utilized Strain Elastography (which provides qualitative or semi-quantitative strain ratios rather than absolute stiffness values). Furthermore, descriptive studies, reviews, case reports, protocols, personal opinions, letters, posters, animal studies, and studies lacking a healthy control group were excluded. Additionally, studies with inaccessible full texts, or those with insufficient data for extraction (e.g., missing mean or standard deviation) where authors could not be contacted, were also excluded from the systematic review.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2. Information sources and search strategy\u003c/h2\u003e\n \u003cp\u003eThe two researchers (M.E.N. and O.H) conducted a comprehensive literature search across three major electronic databases: PubMed, Web of Science, Scopus, and Open Grey, covering the period from inception to November 2025. The search strategy was designed to identify studies evaluating masseter muscle stiffness using quantitative ultrasound elastography (Supplemental File 1).\u003c/p\u003e\n \u003cp\u003eIn addition, the reference lists of included studies and relevant review articles were manually screened to identify any additional eligible studies providing normative values. The researchers manually reviewed the reference lists of all relevant papers gathered during the search process to ensure no significant studies were overlooked. Identified studies were saved and organized in EndNote 20 software, which facilitated efficient management, deduplication, and systematic screening. The most recent search was completed on November 26th, 2025.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3. Study selection and data extraction\u003c/h2\u003e\n \u003cp\u003eTwo independent reviewers (M.E.N. and O.H) screened the titles and abstracts of retrieved records. Potentially relevant articles were subjected to full-text review. Disagreements were resolved by consensus.\u003c/p\u003e\n \u003cp\u003eData were extracted using a standardized form, including author, year, country, study design, sample size, age/gender distribution, ultrasound device (manufacturer/model), probe frequency, patient positioning, and stiffness values (Mean\u0026thinsp;\u0026plusmn;\u0026thinsp;Standard Deviation).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4. Assessment of the risk of bias within the studies\u003c/h2\u003e\n \u003cp\u003eThe methodological quality of the included studies was evaluated using the Newcastle-Ottawa Scale (NOS) adapted for cross-sectional and case-control studies. This tool assesses three domains: (1) Selection of study groups (0\u0026ndash;4 stars), (2) Comparability of the groups (0\u0026ndash;2 stars), and (3) Ascertainment of the Outcome/Exposure (0\u0026ndash;3 stars). The maximum possible score is 9 stars. Studies scoring\u0026thinsp;\u0026ge;\u0026thinsp;7 stars were considered \u0026quot;high quality,\u0026quot; 4\u0026ndash;6 stars \u0026quot;moderate quality,\u0026quot; and \u0026lt;\u0026thinsp;4 stars \u0026quot;low quality.\u0026quot; Two independent reviewers (M.E.N. and O.H) conducted the evaluations, and discrepancies were resolved by consensus.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e2.5. Risk of bias across studies\u003c/h2\u003e\n \u003cp\u003eSmall-study effects/publication bias were explored using funnel plot inspection and Egger\u0026rsquo;s regression test. However, because fewer than ten datasets were available, these methods have limited power and may fail to detect asymmetry; therefore, findings were interpreted cautiously and were not considered definitive evidence for the absence of publication bias.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e2.6. Summary measures\u003c/h2\u003e\n \u003cp\u003eThe primary effect size was the standardized mean difference (SMD; Hedges g) with 95% confidence intervals, selected to improve comparability across different ultrasound systems, acquisition protocols, and reporting units (kPa vs m/s). Mean differences (MD) in kPa were additionally synthesised as a sensitivity analysis after unit harmonisation.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e2.7. Data Transformation\u003c/h2\u003e\n \u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img1772448882.png\" width=\"746\" height=\"184\"\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e2.8. Synthesis of results\u003c/h2\u003e\n \u003cp\u003eA priori, we considered bruxism and myofascial pain\u0026ndash;related TMD as clinically distinct phenotypes. Accordingly, phenotype-specific synthesis (bruxism vs controls; myofascial pain\u0026ndash;related TMD vs controls) was planned to support clinically coherent interpretation. Because the available evidence base is small and reporting is not fully harmonised across studies, we additionally present an overall pooled estimate across phenotypes as an exploratory summary of the average direction and magnitude of masseter stiffness differences across the broader spectrum of masticatory muscle dysfunction. This exploratory pooled estimate is interpreted cautiously and is not intended to imply that bruxism and myofascial pain\u0026ndash;related TMD represent a single unified clinical entity.\u003c/p\u003e\n \u003cp\u003eRandom-effects models were fitted using REML, and statistical inference was based on Hartung\u0026ndash;Knapp adjustment. Forest plots were generated for the primary SMD outcome; the kPa-based MD synthesis was reported as a sensitivity analysis.\u003c/p\u003e\n \u003cp\u003eIn cases where significant heterogeneity was identified in a model, the use of a random-effects model was recommended over a fixed-effects model if methodological or clinical heterogeneity was present. A random-effects model with a 95% confidence interval (CI) was chosen as the preferred meta-analysis model due to the absence of methodological, clinical, and statistical homogeneity. The alpha value for significance was set at 0.05, and all statistical tests were conducted using this threshold.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e2.9. Grade Analysis\u003c/h2\u003e\n \u003cp\u003eThe Grades of Recommendation, Assessment, Development, and Evaluation (GRADE) system was employed to ensure a transparent and structured evaluation of the importance of outcomes in our study. This system provides a comprehensive framework for rating the quality of evidence and strength of recommendations in healthcare. The GRADE system uses specific criteria to downgrade or upgrade the certainty of evidence based on factors such as risk of bias, inconsistency, indirectness, imprecision, and publication bias.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e2.10. Sensitivity Analysis\u003c/h2\u003e\n \u003cp\u003eSensitivity analyses included leave-one-out diagnostics for the primary SMD model. In addition, a unit-restricted sensitivity analysis was performed by pooling only studies reporting stiffness directly in kPa (i.e., without m/s-to-kPa conversion). Because all included studies were rated as high quality, exclusion based on risk-of-bias categories was not undertaken.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Study Selection\u003c/h2\u003e \u003cp\u003eA comprehensive search of PubMed/MEDLINE (n\u0026thinsp;=\u0026thinsp;65), Web of Science (n\u0026thinsp;=\u0026thinsp;70), and Scopus (n\u0026thinsp;=\u0026thinsp;40), supplemented by manual searching (n\u0026thinsp;=\u0026thinsp;3), yielded 178 records. After removal of duplicates (n\u0026thinsp;=\u0026thinsp;73), 105 records were screened by title and abstract, and 78 records were excluded. Full texts were sought for 27 reports, and all were retrieved. Of these, 20 reports were excluded after full-text assessment for the following reasons: wrong outcome measure (n\u0026thinsp;=\u0026thinsp;2), no healthy control group (n\u0026thinsp;=\u0026thinsp;10), inappropriate study design (review/case report) (n\u0026thinsp;=\u0026thinsp;5), and wrong population (n\u0026thinsp;=\u0026thinsp;3). Consequently, 7 studies (8 datasets) were included in the qualitative synthesis and meta-analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) (Supplemental File 2).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Characteristics of the Included Studies\u003c/h2\u003e \u003cp\u003eThe seven studies (comprising eight datasets) included in this meta-analysis were published between 2017 and 2025, reflecting the most up-to-date literature. The geographic distribution of the studies was diverse, with three studies from T\u0026uuml;rkiye (Aksu et al., 2023; Erdur et al., 2025; Akkoca et al., 2025)\u003csup\u003e\u003cspan additionalcitationids=\"CR24\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e and one study each from Poland (Obuchowicz et al., 2024)\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, Taiwan (Chen et al., 2023)\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, Germany (Toker et al., 2023)\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e, and Japan (Takashima et al., 2017)\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe total participant pool consisted of 190 patients with bruxism and/or myofascial pain\u0026ndash;related TMD and 202 healthy controls matched for age and sex (total n\u0026thinsp;=\u0026thinsp;392). The age distribution varied across studies; while Erdur et al. \u003csup\u003e25\u003c/sup\u003e examined an adolescent population (mean age 14.5), the remaining studies focused on adult subjects with mean ages ranging between 28 and 45 years.\u003c/p\u003e \u003cp\u003eFrom a clinical perspective, the included studies can be categorized into two main groups:\u003c/p\u003e \u003cp\u003e(1) those examining patients with myofascial pain\u0026ndash;related TMD or bruxism (Aksu et al., 2023 \u003csup\u003e24\u003c/sup\u003e; Takashima et al., 2017 \u003csup\u003e29\u003c/sup\u003e; Akkoca et al., 2025 \u003csup\u003e23\u003c/sup\u003e; Chen et al., 2023 \u003csup\u003e27\u003c/sup\u003e), and\u003c/p\u003e \u003cp\u003e(2) those evaluating painless or unspecified bruxism cases \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e (Toker et al., 2023 \u003csup\u003e28\u003c/sup\u003e; Erdur et al., 2025 \u003csup\u003e25\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003eRegarding technical aspects, all studies employed SWE to assess masseter muscle stiffness; however, the ultrasound devices varied across studies (Toshiba Aplio, Canon Aplio, GE Logiq, Supersonic Aixplorer, and Samsung systems). Five studies reported outcomes directly as Young\u0026rsquo;s modulus (kPa), whereas two studies reported shear wave velocity (m/s), which was standardized for the meta-analysis. Detailed characteristics of all included studies are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharacteristics of the Included Studies\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStudy (Year)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCountry\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eClinical Population\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDevice/System\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eN (Patient / Control)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAge Group\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eOriginal Unit\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAydin Aksu et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u0026uuml;rkiye\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMyofascial TMD/Bruxism\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eToshiba Aplio 500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e26 / 26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdult\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ekPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eErdur et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u0026uuml;rkiye\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAdolescent Bruxism\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCanon Aplio 500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e33 / 33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdolescent\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ekPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAkkoca et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u0026uuml;rkiye\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMyofascial TMD/Bruxism\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSamsung HS70A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e25 / 29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdult\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ekPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eObuchowicz et al. (2024) \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePoland\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTMD / Various Disease Conditions\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSupersonic Aixplorer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e33 / 32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdult\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ekPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChen et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTaiwan\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOrofacial Pain / TMD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCanon Aplio 500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e37 / 48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdult\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ekPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eToker et al. (2023) \u003csup\u003e28\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGermany\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBruxism\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCanon Aplio 1800\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e10 / 10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdult\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003em/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Myofascial Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eJapan\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMyofascial TMD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGE Logiq E9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13 / 12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdult\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003em/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Limited Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eJapan\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMyofascial TMD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGE Logiq E9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e13 / 12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAdult\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003em/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Results of Individual Studies\u003c/h2\u003e \u003cp\u003eIn this systematic review, most studies comparing patients with bruxism or TMD to healthy controls reported significant differences in masseter muscle stiffness. Specifically, five studies (Aksu et al., 2023 \u003csup\u003e24\u003c/sup\u003e; Erdur et al., 2025 \u003csup\u003e25\u003c/sup\u003e; Takashima et al., 2017 \u003csup\u003e29\u003c/sup\u003e; Chen et al., 2023 \u003csup\u003e27\u003c/sup\u003e; Obuchowicz et al., 2024 \u003csup\u003e26\u003c/sup\u003e) found that resting masseter stiffness (reported in kPa or m/s) was significantly higher in patients than in healthy individuals.\u003c/p\u003e \u003cp\u003eIn contrast, Akkoca et al. (2025)\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e observed no significant difference in resting stiffness between groups in patients with myofascial pain; however, they reported a significant increase in stiffness during maximal clenching among the patient group.\u003c/p\u003e \u003cp\u003eWhen functional conditions were compared, Toker et al. \u003csup\u003e28\u003c/sup\u003e identified higher stiffness values in bruxism patients at rest yet reported that stiffness during maximal mouth opening was significantly lower in patients compared with controls. With respect to age groups, Erdur et al. \u003csup\u003e25\u003c/sup\u003e found that adolescent bruxism patients exhibited increased stiffness in both closed-mouth and open-mouth positions, similar to findings in adult populations, while no changes in muscle thickness were reported (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of Stiffness Outcomes Reported in Individual Studies\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStudy\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYear\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDesign\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN (Patient / Control)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMean Age\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDevice\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eStiffness (Patient)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eStiffness (Control)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAydin Aksu et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase\u0026ndash;Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e26 / 26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e28.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eToshiba Aplio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e39.13\u0026thinsp;\u0026plusmn;\u0026thinsp;4.52 kPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e27.73\u0026thinsp;\u0026plusmn;\u0026thinsp;1.92 kPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eErdur et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2025\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase\u0026ndash;Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33 / 33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e14.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCanon Aplio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e25.10\u0026thinsp;\u0026plusmn;\u0026thinsp;2.90 kPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e21.50\u0026thinsp;\u0026plusmn;\u0026thinsp;2.80 kPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eToker et al. (2023) \u003csup\u003e28\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase\u0026ndash;Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10 / 10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e33.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCanon Aplio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.44 m/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.66\u0026thinsp;\u0026plusmn;\u0026thinsp;0.25 m/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Myofascial Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2017\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase\u0026ndash;Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e13 / 12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e29.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGE Logiq E9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e1.96\u0026thinsp;\u0026plusmn;\u0026thinsp;0.48 m/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38 m/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Limited Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2017\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase\u0026ndash;Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e13 / 12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e29.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eGE Logiq E9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.00\u0026thinsp;\u0026plusmn;\u0026thinsp;0.55 m/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e1.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38 m/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eObuchowicz et al. (2024) \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2024\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCross-Sectional\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33 / 32*\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e45.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSupersonic Aixplorer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35.00\u0026thinsp;\u0026plusmn;\u0026thinsp;4.00 kPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e27.00\u0026thinsp;\u0026plusmn;\u0026thinsp;3.00 kPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAkkoca et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2025\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase\u0026ndash;Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25 / 29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e29.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSamsung HS70A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e23.65\u0026thinsp;\u0026plusmn;\u0026thinsp;5.53 kPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e20.35\u0026thinsp;\u0026plusmn;\u0026thinsp;5.94 kPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChen et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCross-Sectional\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e37 / 48\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e35.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCanon Aplio\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e15.64\u0026thinsp;\u0026plusmn;\u0026thinsp;7.50 kPa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e9.90\u0026thinsp;\u0026plusmn;\u0026thinsp;2.56 kPa\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"8\"\u003e* Data from Obuchowicz et al. were obtained from the male subgroup aged 30\u0026ndash;40 years.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Risk of Bias Within Studies\u003c/h2\u003e \u003cp\u003eAll seven studies included in the analysis demonstrated a low risk of bias (high quality). Each study employed clear diagnostic criteria for participant selection (RDC/TMD or DC/TMD) and used objective methods for measuring masseter stiffness (SWE). Notably, the matching of control groups to patient groups in terms of age and sex, as well as the use of and used quantitative SWE protocols for measuring masseter stiffness, were key factors contributing to the internal validity of these studies. No study with moderate or high risk of bias was identified (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eQuality assessment of the included studies according to the Newcastle-Ottawa Scale (NOS).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStudy\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYear\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDesign\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSelection (Max 4★)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eComparability (Max 2★)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eOutcome (Max 3★)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eTotal Score\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eQuality Level\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAydin Aksu et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase-Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003e9\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eObuchowicz et al. (2024) \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2024\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCross-Sectional\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003e9\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAkkoca et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2025\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase-Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003e9\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChen et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCross-Sectional\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003e8\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eErdur et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2025\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase-Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003e8\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Myofascial Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2017\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase-Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Limited Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2017\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase-Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eToker et al. (2023) \u003csup\u003e28\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCase-Control\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e★★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e★★\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e\u003cb\u003e7\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHigh\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Synthesis of Results\u003c/h2\u003e \u003cp\u003eThe primary meta-analysis using standardized mean differences (Hedges g) showed higher masseter stiffness in patients than in controls (g\u0026thinsp;=\u0026thinsp;1.485, 95% CI 0.763\u0026ndash;2.207; p\u0026thinsp;=\u0026thinsp;0.00183), with substantial heterogeneity (I\u0026sup2;=84.1%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). In a secondary sensitivity analysis pooling mean differences in kPa after converting m/s to kPa where applicable, stiffness remained higher in patients (MD 6.16 kPa, 95% CI 3.67\u0026ndash;8.65; p\u0026thinsp;=\u0026thinsp;0.000628; I\u0026sup2;=84.4%)\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe pooled estimate suggests that bruxism and myofascial TMD are associated with a marked increase in masseter muscle stiffness compared with healthy controls.\u003c/p\u003e \u003cp\u003eIn an exploratory subgroup restricted to bruxism-only datasets (n\u0026thinsp;=\u0026thinsp;2), the pooled estimate was imprecise and not statistically conclusive (MD 3.48 kPa, 95% CI\u0026thinsp;\u0026minus;\u0026thinsp;4.82 to 11.77; I\u0026sup2; = 0%).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003e3.6. Sensitivity Analysis\u003c/h2\u003e \u003cp\u003eLeave-one-out analyses showed that omitting any single dataset did not materially change the pooled effect in either the primary SMD model (Hedges g) or the unit-harmonised MD (kPa) model; statistical significance was maintained across all omissions (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). As a unit-restricted sensitivity analysis, restricting the meta-analysis to studies reporting stiffness directly in kPa (without m/s-to-kPa conversion) yielded a comparable effect (MD 6.47 kPa; 95% CI 2.27\u0026ndash;10.67), supporting that the overall conclusion was not driven by unit conversion.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eLeave-One-Out Analyses for Masseter Muscle Stiffness\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExcluded dataset\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePooled Hedges g (SMD)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e95% CI\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePooled MD (kPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e95% CI\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003ep-value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAydin Aksu et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.247\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.717 to 1.777\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.315\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.354 to 7.276\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.000567\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eErdur et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.525\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.662 to 2.388\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00496\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.605\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.821 to 9.389\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.00115\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eObuchowicz et al. (2024) \u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.373\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.565 to 2.181\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00595\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.849\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.964 to 8.734\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.00255\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAkkoca et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.627\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.854 to 2.401\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00212\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.544\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.800 to 9.288\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.00112\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChen et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.553\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.702 to 2.405\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00427\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.208\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.243 to 9.174\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.00217\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eToker et al. (2023) \u003csup\u003e28\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.591\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.792 to 2.390\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00279\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.594\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.944 to 9.244\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.000893\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Myofascial Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.482\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.622 to 2.341\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00556\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.082\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.144 to 9.019\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.00230\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTakashima et al. \u0026ndash; Limited Group (2017) \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.488\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.629 to 2.348\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.00546\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e6.033\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e3.124 to 8.942\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.00228\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eRandom-effects models were fitted using REML with Hartung\u0026ndash;Knapp adjustment. MD (kPa) analyses used unit harmonization (m/s\u0026rarr;kPa) as described in Methods.\u003c/em\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e3.7. Publication Bias\u003c/h2\u003e \u003cp\u003ePublication bias was assessed using funnel plots and Egger\u0026rsquo;s regression test. Visual inspection did not suggest marked asymmetry. Egger\u0026rsquo;s test was not statistically significant (p\u0026thinsp;=\u0026thinsp;0.2496). With only eight datasets, funnel plots and regression-based tests have limited power; therefore, these findings should be interpreted cautiously and do not exclude small-study effects (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e3.8. Grade Analysis\u003c/h2\u003e \u003cp\u003eThe certainty of the evidence for the association between bruxism and increased Young\u0026rsquo;s modulus was rated as very low. Although the pooled estimate indicated a higher muscle stiffness in individuals with bruxism, the evidence was downgraded due to the non-randomised design of the included studies, and indirectness arising from differences in study populations, ultrasound elastography systems, acquisition protocols, and outcome measurement units. As a result, confidence in the pooled effect estimate is limited, and the true effect may be different from the observed estimate.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. DISCUSSION","content":"\u003cp\u003eUsing a random-effects model pooling eight datasets from seven studies, the primary standardized analysis indicated higher masseter stiffness in patients than in controls (Hedges g\u0026thinsp;=\u0026thinsp;1.485, 95% CI 0.763\u0026ndash;2.207; p\u0026thinsp;=\u0026thinsp;0.00183). However, substantial between-study heterogeneity (I\u0026sup2;=84.1%) and the \u0026ldquo;very low\u0026rdquo; certainty of evidence under GRADE require cautious interpretation, and the findings should not be used for diagnostic inference or universal cut-off development.\u003c/p\u003e \u003cp\u003eAcross the included literature, the overall direction of effect is consistent: most studies report higher resting stiffness in the patient group \u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26 CR27 CR28\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. One study separates from this pattern by reporting no clear difference at rest, with a between-group difference emerging only during maximal clenching \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. This contrast is clinically informative because it implies that stiffness differences may not be uniformly detectable under a single measurement condition. Instead, they may become apparent only when the assessment is aligned with the patient\u0026rsquo;s functional state or symptom expression (Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eA key clinical implication of the high heterogeneity is that this meta-analysis does not support receiving a universal cutoff value. That limitation should not be interpreted as a failure of elastography itself. A more defensible clinical role is to use elastography for within-centre comparisons\u0026mdash;patient versus control under a consistent protocol\u0026mdash;and for objectively tracking changes over time, including pre- and post-treatment assessments. In addition, interpreting elastography outputs alongside both resting and functional measurements may improve their correspondence with the clinical picture. This variability should not be interpreted as an artefact of insufficient evidence or limited sample size. If fundamental differences between elastography devices, acquisition parameters, and stiffness computation algorithms persist, statistical heterogeneity is expected to remain high. Under such conditions, the inclusion of additional studies employing heterogeneous methodologies would be unlikely to reduce heterogeneity or modify the direction of the pooled estimates. Thus, the observed inconsistency reflects structural methodological diversity rather than instability of the underlying biomechanical signal.\u003c/p\u003e \u003cp\u003eBruxism and myofascial pain\u0026ndash;related TMD are clinically distinct phenotypes and should not be treated as interchangeable labels. To preserve clinical interpretability, our primary intent was phenotype-specific interpretation. However, because the available evidence base is small and reporting is not fully harmonised across studies, we also present an overall pooled estimate across phenotypes as an exploratory summary of the average direction and magnitude of stiffness differences across the broader spectrum of masticatory muscle dysfunction. Importantly, this exploratory pooled estimate should not be interpreted as implying a single unified clinical entity and is not suitable for diagnostic inference or universal cut-off development, particularly in the presence of substantial heterogeneity and device/protocol variability.\u003c/p\u003e \u003cp\u003eImportantly, SWE technology does not measure the diagnostic label assigned to the patient (bruxism or TMD), but rather the biomechanical response of muscle tissue to pathological loading, reflected as increased stiffness or tone \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. In this context, Chen et al. \u003csup\u003e27\u003c/sup\u003e reported that, in contrast to B-mode ultrasonography, SWE is capable of detecting microstructural alterations and stiffness changes in muscle tissue at an earlier stage than overt morphological changes such as hypertrophy. Consequently, increased muscle stiffness represents a shared and biologically plausible outcome in both bruxism \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e and myofascial TMD \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. From this perspective, pooling data from these two groups in the present analysis does not introduce conceptual noise; rather, it provides a coherent and integrative representation of the mechanical alterations associated with masticatory muscle dysfunction \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFrom a mechanical standpoint, increased stiffness in bruxism is related to repetitive activity and chronic loading. The fact that several studies detect differences even at rest may indicate that a subset of patients exhibits persistently elevated basal muscle tone \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Yet the literature also suggests that functional tasks can modify the observed contrast between groups. In some datasets, the difference becomes more evident under activity, which points to an interaction between clinical phenotype\u0026mdash;pain, functional limitation, or habit severity\u0026mdash;and the measurement condition \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. For that reason, elastography values are better understood as supportive quantitative findings rather than as a stand-alone biomarker.\u003c/p\u003e \u003cp\u003eGiven the clinical and technical variability across the included studies, a high degree of heterogeneity is expected. Clinically, the samples include both adolescents and adults. In adolescents, increased stiffness in the absence of thickness changes suggests that measurable alterations in tone or rigidity may occur early before hypertrophy develops \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. In adults\u0026mdash;particularly those with myofascial pain/TMD\u0026mdash;the wider spread of effect sizes may reflect differences in symptom burden and severity that are captured to some extent by stiffness measurements \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOn the technical side, the studies used devices from multiple manufacturers (e.g., Toshiba, Canon, GE, Samsung, Supersonic), and two studies reported outcomes in m/s, requiring conversion to kPa for the meta-analysis (Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). While conversion improves comparability in a pooled analysis, it may also add uncertainty, particularly given the direction-dependent nature of muscle tissue and protocol differences across settings. This methodological diversity\u0026mdash;together with population variability\u0026mdash;was among the central reasons for downgrading evidence certainty for indirectness in the GRADE assessment. Within this context, employing a random-effects model is methodologically appropriate because it estimates an average effect while acknowledging true between-study variability rather than assuming a single common effect across all conditions.\u003c/p\u003e \u003cp\u003eWhen the datasets are considered together, a pattern emerges that may be clinically relevant: stiffness appears more clearly elevated within the myofascial pain/TMD spectrum, while the difference may be smaller in painless or milder bruxism phenotypes \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. The finding of a pronounced increase during maximal clenching despite no resting difference in one study further suggests that functional tasks can be more discriminative in selected phenotypes \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Conversely, lower stiffness during maximal mouth opening in the patient group could reflect pain or functional limitation that constrains muscle behaviour under that condition \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. However, because the number of eligible studies is limited, these observations should remain hypothesis-generating rather than being treated as definitive subgroup conclusions.\u003c/p\u003e \u003cp\u003eThe leave-one-out sensitivity analysis supports the stability of the pooled result. Excluding any single study did not materially change the estimate; the pooled effect size remained within 5.31\u0026ndash;6.59 kPa, and significance persisted (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). This reduces concern that the overall finding is disproportionately driven by an individual dataset.\u003c/p\u003e \u003cp\u003eRegarding publication bias, the funnel plot did not show marked asymmetry and Egger\u0026rsquo;s test was not statistically significant (p\u0026thinsp;=\u0026thinsp;0.2496). Still, with a small number of studies, formal tests have limited power; therefore, these results should be interpreted as reassuring rather than conclusive.\u003c/p\u003e \u003cp\u003eAll included studies were rated \u0026ldquo;high quality\u0026rdquo; by the NOS (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), reflecting strengths in selection, comparability, and outcome measurement. This does not conflict with the \u0026ldquo;very low\u0026rdquo; certainty rating under GRADE. GRADE weighs the totality of evidence and, importantly, begins observational designs at a lower certainty level. In this review, the non-randomized nature of the evidence base, the high heterogeneity, and indirectness linked to population and protocol/device variability contributed to the downgrading. As a result, although the direction of effect is aligned across studies, confidence in the true magnitude remains limited and the true effect may differ from the pooled estimate.\u003c/p\u003e \u003cp\u003eThis review has several limitations. All included studies were observational, limiting causal inference and aligning with the \u0026ldquo;very low\u0026rdquo; certainty of evidence. Heterogeneity was substantial (I\u0026sup2; = 84.1%), influenced by age differences (adolescent vs adult), phenotype differences (painful myofascial TMD vs painless bruxism), and varied measurement conditions (rest, mouth opening, maximal clenching). Device and protocol diversity\u0026mdash;and the conversion of m/s to kPa in two studies\u0026mdash;may constrain direct comparison of absolute values across settings. The limited number of studies reduces the strength of publication bias assessments, and potentially relevant confounders (e.g., stress, sleep characteristics, medication use) may not have been standardized consistently across the evidence base. With only eight datasets, funnel plot inspection and Egger\u0026rsquo;s test were underpowered; thus, publication bias cannot be confidently excluded.\u003c/p\u003e \u003cp\u003eFuture studies should focus on improving methodological consistency and clinical interpretability of masseter muscle elastography findings. Prospective and multicentre designs using standardized acquisition protocols, including probe orientation, region-of-interest definition, and reporting units, are required to reduce device- and protocol-related variability. Stratification of study populations according to clearly defined clinical phenotypes, such as painful myofascial TMD versus painless bruxism, may help clarify phenotype-specific stiffness patterns. Longitudinal studies assessing pre- and post-treatment changes are also needed to determine the utility of elastography as a monitoring tool rather than a diagnostic cutoff-based method. Finally, integration of elastography outcomes with clinical measures of pain, function, and psychosocial factors may provide a more comprehensive understanding of masticatory muscle dysfunction.\u003c/p\u003e"},{"header":"5. CONCLUSION","content":"\u003cp\u003eThis systematic review and meta-analysis demonstrated that masseter muscle stiffness is higher in individuals with bruxism and/or TMD associated with myofascial pain compared with healthy controls. However, the high level of between-study heterogeneity and the classification of evidence certainty as \u0026ldquo;very low\u0026rdquo; according to GRADE limit confidence in the magnitude of this effect. When applied with appropriate protocols and within a relevant clinical context, elastography represents a practical method capable of quantitatively capturing changes in the masseter muscle and may complement clinical examination.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eConflicts of Interest\u003c/h2\u003e\n\u003cp\u003eThe authors declare no conflicts of interest.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThe authors have nothing to report.\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\n\u003cp\u003eM.E.N. and \u0026Ouml;.H. designed the study. M.E.N. conducted the searches, screening, and data extraction. M.E.N. and \u0026Ouml;.H. performed the analyses and prepared the figures/tables. M.E.N. drafted the manuscript, and \u0026Ouml;.H. critically revised it. Both authors approved the final manuscript and take responsibility for the work.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThe authors have nothing to report.\u003c/p\u003e\n\u003ch2\u003eData Availability\u003c/h2\u003e\n\u003cp\u003eAll data supporting the findings of this study are included within the article and its supporting information.\u003c/p\u003e\n\u003ch2\u003eEthics Statement\u003c/h2\u003e\n\u003cp\u003eThe authors have nothing to report.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eValesan LF, Da-Cas CD, R\u0026eacute;us JC et al. Prevalence of temporomandibular joint disorders: a systematic review and meta-analysis. 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Mar 2024;1(3):2025\u0026ndash;36. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1121/10.0025291\u003c/span\u003e\u003cspan address=\"10.1121/10.0025291\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Bruxism, Temporomandibular Disorders, Myofascial Pain, Masseter Muscle, Shear Wave Elastography, Acoustic Radiation Force Impulse","lastPublishedDoi":"10.21203/rs.3.rs-8925974/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8925974/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eObjectives\u003c/h2\u003e \u003cp\u003eTo systematically review quantitative ultrasound elastography evidence and meta-analyze whether masseter muscle stiffness differs between individuals with bruxism and/or myofascial pain\u0026ndash;related temporomandibular disorders (TMD) and healthy controls.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003e 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\u0026ndash;Knapp adjustment. A supportive sensitivity analysis pooled mean differences (MD) in kPa after unit harmonization. Risk of bias was assessed with the Newcastle\u0026ndash;Ottawa Scale, small-study effects were explored with funnel plot inspection and Egger\u0026rsquo;s test (noting limited power with \u0026lt;\u0026thinsp;10 datasets), and certainty of evidence was graded using GRADE.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eSeven studies (eight datasets) including 190 patients and 202 controls (n\u0026thinsp;=\u0026thinsp;392) were analyzed. Masseter stiffness was higher in patients than controls in the primary analysis (Hedges g\u0026thinsp;=\u0026thinsp;1.485; 95% CI 0.763\u0026ndash;2.207; p\u0026thinsp;=\u0026thinsp;0.00183), with substantial heterogeneity (I\u0026sup2;=84.1%). The kPa-based sensitivity analysis showed a similar direction and significance (MD\u0026thinsp;=\u0026thinsp;6.16 kPa; 95% CI 3.67\u0026ndash;8.65; p\u0026thinsp;=\u0026thinsp;0.000628; I\u0026sup2;=84.4%). Leave-one-out analyses indicated robust findings. Evidence certainty was very low.\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e \u003cp\u003eMasseter stiffness is increased in bruxism and/or myofascial pain\u0026ndash;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.\u003c/p\u003e","manuscriptTitle":"Masseter Muscle Stiffness in Bruxism and Myofascial Pain–Related Temporomandibular Disorders: A Systematic Review and Meta-Analysis of Quantitative Ultrasound Elastography Studies","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-03 14:56:16","doi":"10.21203/rs.3.rs-8925974/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"92d9aea1-8f3b-47d6-bc02-fb9e920b3f5b","owner":[],"postedDate":"March 3rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-30T16:27:26+00:00","versionOfRecord":{"articleIdentity":"rs-8925974","link":"https://doi.org/10.1007/s11282-026-00914-w","journal":{"identity":"oral-radiology","isVorOnly":false,"title":"Oral Radiology"},"publishedOn":"2026-03-24 16:11:16","publishedOnDateReadable":"March 24th, 2026"},"versionCreatedAt":"2026-03-03 14:56:16","video":"","vorDoi":"10.1007/s11282-026-00914-w","vorDoiUrl":"https://doi.org/10.1007/s11282-026-00914-w","workflowStages":[]},"version":"v1","identity":"rs-8925974","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8925974","identity":"rs-8925974","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}