Assessment of Masseter Muscle Stiffness by Ultrasound Elastography in Patients with Temporomandibular Disorders: A Systematic Review and Meta-analysis

Abstract

Purpose

To assess the stiffness of the masseter muscle in patients with temporomandibular disorders (TMDs) by shear wave elastography and compare the measurements of masseter muscle stiffness at rest and during muscle contraction.

Materials and Methods

A systematic review and meta-analysis following PRISMA guidelines included studies on masseter muscle stiffness via shear wave elastography in TMD patients, analyzing resting and contracted states separately. Standardized mean differences (SMDs) were pooled with a random-effects model. Bias was evaluated using QUADAS-2, heterogeneity and prediction intervals were assessed, and significance was set at p < 0.05.

Results

For the resting state, five studies (n = 137 in each group) were included. The pooled SMD was 3.16 (95% CI: −0.70 to 7.03, p = 0.11), with high heterogeneity (I² = 96%). In the contracted state, four studies (n = 111 experimental, 113 control) showed a significant difference with an SMD of 1.20 (95% CI: 0.58 to 1.83, p < 0.01), and moderate heterogeneity (I² = 71%). Prediction intervals indicated more consistent findings during contraction.

Conclusion

Shear wave elastography, particularly when applied during muscle contraction, is effective in distinguishing masseter muscle stiffness in patients with TMD. Contracted-state measurements yield statistically significant and more consistent results compared with the resting state, supporting the clinical relevance of shear wave elastography in TMD diagnostics.

Keywords

masseter muscle - elasticity imaging techniques - clinical relevance - muscle contraction - temporomandibular joint disorders

Introduction

Temporomandibular disorders (TMDs) encompass a spectrum of musculoskeletal and neuromuscular conditions that affect the temporomandibular joint (TMJ), masticatory muscles, and associated structures. TMD is a common source of non-dental orofacial pain, affecting an estimated 5 to 12% of the population.[1] Among the masticatory muscles, the masseter muscle plays a critical role in jaw function and is often involved in the pathophysiology of TMD. Increased tension, hypertrophy, and myofascial pain of the masseter are frequently observed in affected individuals, making its assessment crucial in both diagnosis and treatment planning.[2]

Conventional diagnostic approaches for TMD rely on clinical examination and patient-reported symptoms, as outlined in criteria such as the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD).[3] Although helpful, these methods are largely subjective and do not provide detailed information on the mechanical properties of the masticatory muscles. Imaging modalities such as magnetic resonance imaging (MRI) and B-mode ultrasonography have been used to evaluate structural abnormalities; however, they do not provide quantitative insight into muscle elasticity or stiffness.[4] [5]

Shear wave elastography (SWE) is a novel, non-invasive imaging technique that measures tissue stiffness in real time by analyzing the velocity of shear wave propagation through soft tissues. SWE offers several advantages, including reproducibility, operator independence, and the ability to provide quantitative assessments of tissue elasticity in kilopascals (kPa). In the context of TMD, SWE has been proposed as a tool for evaluating functional changes in the masseter muscle, especially given that increased muscle stiffness may reflect underlying muscular dysfunction, inflammation, or fibrosis.[6] [7]

Recent studies have explored the application of SWE in assessing the masseter muscle stiffness in patients with TMD, both at rest and during contraction.[8] However, findings have been inconsistent, with some reporting significantly elevated stiffness in TMD patients,[9] while others found no notable difference.[10] These variations may be attributed to differences in study populations, imaging protocols, and diagnostic criteria.[5]

The present systematic review and meta-analysis aims to critically evaluate and synthesize the current evidence on the utility of SWE in assessing masseter muscle stiffness in TMD patients. Specifically, it examines whether SWE can distinguish stiffness changes at rest and during contraction between TMD patients and healthy controls. This study aims to elucidate the diagnostic value of SWE and its potential to enhance objective evaluation and monitoring of TMD.

Materials and Methods

This systematic review was conducted and prepared according to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA).[11] The study protocol was registered under the Prospero database CRD42023474132.

Eligibility Criteria

The eligibility criteria were according to the Population, Intervention, Comparator, and Outcome (PICO) format. Patients: (a) Patients with TMDs classified by the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) classified as Axis I; (b) patients above the age of 18 years (male and female) with myofascial pain or disk derangement or osteoarthritis undergoing elastography of the masseter muscle were included in the study; (c) patients with clinical diagnosis of bruxism; (d) healthy controls without any TMJ disorders were considered for the control group. Intervention: Patients subjected to shear wave ultrasound elastography of the masseter muscle were included. Comparator: Healthy controls without any TMDs. Outcome: Muscle hardness or stiffness, expressed as shear wave velocity.

The study designs included were cross-sectional, observational case-control, and cohort studies.

Exclusion criteria: narrative reviews or systematic reviews, studies in languages other than English, case reports or series, editorials or book chapters, and in vitro or animal studies were excluded.

Databases and Searches

The databases searched included Cochrane Library, PubMed, and Scopus from January 2000 to January 2025. The electronic search was performed with the following keywords and the corresponding Medical Subject Headings (MeSH) with Boolean operators (AND/or)—((((((Masseter) AND (Elastography)) AND (Ultrasound elastography)) AND (shear wave elastography)) AND (Muscle stiffness)) AND (Temporomandibular disorders). The keywords and search strategy are described in [Table 1].

Table 1 Keywords for databases

Database	Key words	Number
PubMed	(“Temporomandibular Joint Disorders”[MeSH] OR “temporomandibular disorder”) AND (“Shear Wave Elastography” OR “Ultrasound Elastography”) AND (“Masseter Muscle”[MeSH] OR “masseter muscle”)	47
Scopus	((“shear wave elastography” OR “ultrasound elastography”) AND “Masseter muscle” AND “temporomandibular disorders”)	74
Cochrane Library	“Shear wave elastography” AND “masseter muscle” AND “temporomandibular disorder”	2
EMBASE	(‘temporomandibular joint disorder’/exp OR ‘temporomandibular disorder’) AND (‘shear wave elastography’ OR ‘ultrasound elastography’ OR ‘sonoelastography’) AND (‘masseter muscle’/exp OR ‘masseter muscle’)	1

An electronic search of Databases was performed by two investigators, D.J.P. and R.K.R. After the electronic search, the articles were imported into EndNote 20. By analyzing the article titles, the two authors eliminated the duplicate articles that had been chosen in the first step. Following the second process of eliminating unrelated articles, the two authors reviewed the abstracts and eliminated other irrelevant articles. At last, they read the full text of the remaining articles. Data were collected from the remaining articles after excluding those that did not meet the eligibility criteria. A hand search of the references was conducted for additional articles. If the full text was not available, the corresponding authors were consulted, and the article was included. They consulted with a third author (A.P.) in cases of disagreement, and the selection procedure was concluded.

Data Extraction (Selection and Coding)

The studies were included according to the eligibility criteria. After conducting a literature search according to the PRISMA format, the included studies were analyzed for author name, year, country, machine used, type of elastography performed, and parameters assessed, including patient's age, gender, and muscle stiffness. The mean and standard deviation values were evaluated. In case of disagreement, the third and fourth authors were consulted. Additional information was obtained from the corresponding authors, and if no response was received, the article was excluded.

Risk of Bias Assessment

The Quality Assessment of Diagnostic Accuracy Studies 2 (QUADAS-2) tool was used to evaluate the included studies' methodological quality.[12] This tool assesses four main elements: the flow of participants through the research, including the exact order in which tests and standards are administered; index tests; reference standards; and patient inclusion criteria. Two authors (D.J.P. and R.N.) independently evaluated each component for possible bias. The risk of bias was categorized as low, high, or uncertain, and conflicts were arbitrated by a third author. The analysis examined the variety of findings across the included studies, focusing on patient participation, methodology, and the sufficiency of the results.

Data Synthesis

Data from eligible studies were extracted and synthesized using a random-effects meta-analysis model to account for potential heterogeneity across studies. For each included study, the mean and standard deviation (SD) of SWE values of the masseter muscle were recorded for both TMD patients and healthy control participants at rest and in the contracted state.

The primary outcome measure was the mean muscle stiffness (expressed in kilopascals, kPa) as determined by SWE. A meta-analysis was performed to evaluate masseter muscle stiffness using SWE in both resting and contracted states.[13] The standardized mean difference (SMD) was used as the effect size to normalize differences in muscle stiffness measurements across studies with different scales and methods. The SMD was calculated for both rest and contraction conditions. When necessary, mean and SD were estimated from median and interquartile ranges using established procedures. If data were missing or ambiguous, authors were contacted for clarification. Each study reported an SMD along with a 95% confidence interval (CI), enabling comparison between the TMD (experimental) and healthy (control) groups. The pooled effect estimate was calculated with the inverse variance method under a random-effects model to account for both within-study and between-study variability. To evaluate publication bias, Egger's test and funnel plots were generated. A p-value less than 0.05 was considered statistically significant.

Two reviewers independently extracted study characteristics, sample sizes, SWE values, and outcomes. A random-effects model was used for meta-analysis. Heterogeneity was assessed using the I² statistic and Cochran's Q, with values of 25, 50, and 75% interpreted as low, moderate, and high heterogeneity, respectively. Egger's test and funnel plots were used to assess publication bias.

Certainty of Evidence

The overall strength of evidence for each meta-analysis was evaluated using the GRADE (Grading of Recommendations Assessment, Development, and Evaluation) system, which classifies evidence quality as “high,” “moderate,” “low,” or “very low.” In observational studies, five factors—risk of bias, imprecision, inconsistency, indirectness, and publication bias—can downgrade the evidence. Conversely, factors like a large effect size, dose–response gradient, or the presence of residual confounding may justify upgrading. Each factor was rated as having “no,” “serious,” or “very serious” limitations to determine the final evidence level for each outcome.

Results

Study Selection

The database search yielded a total of 123 records, including 74 from Scopus, 47 from PubMed, and one record from the Cochrane Library databases. A total of 12 articles were removed because they were irrelevant to the subject, while 54 duplicate items were eliminated. Three reviewers carefully reviewed the remaining 54 papers before they were included. Another 24 articles were excluded as they did not meet the inclusion criteria. Further 23 articles were also eliminated for the reasons listed in the PRISMA flow diagram. Ultimately, five papers were included in the qualitative and quantitative synthesis after meeting the eligibility requirements. The selection process is depicted in the PRISMA flow diagram ([Fig. 1]).

Fig. 1 Prisma flowchart depicting the steps for identification, screening, and inclusion of studies.

Study Characteristics

[Tables 2] and [3] summarize the main characteristics of the included studies, including study design, participant demographics, imaging modalities, ultrasound equipment used, measured outcomes, and key findings.

Table 2 Characteristics of included studies

S. no.	Study (Author, Year), Country	Study design	Sample size, Mean age, Gender	Population	Diagnostic criteria for TMD	Imaging modality	Ultrasound machine	Cases	Comparator	Outcomes measured	Patient position
1.	Takashima et al[8] (2017), Japan	Cross-sectional	50 (26 TMD, 24 controls) TMD: 35.4 ± 7.3; Control: 34.6 ± 6.9	Female patients with myofascial TMD pain (RDC/TMD Ia and Ib)	RDC /TMD	SWE of masseter muscle at rest	ACUSON S2000 (Siemens AG, Munich, Germany) Acuson 9L4 linear vascular probe 2 (Siemens A3G, Mu4nich, Germ5any) at 4–9 MHz, with water-soluble transmission gel as a contact surface	26 females (age, 15–46 years) with bilateral masseter muscle pain group Ia (myofascial pain; n = 13) or group Ib (myofascial pain with limited opening; n = 13)	24 healthy controls comprised 24 female volunteers (age, 23–45 years	Masseter muscle stiffness (shear wave velocity) maximum assisted mouth opening, masseteric thickness and characteristic pain intensity	Sitting position with the head held in a natural position
2	Toker et al[14] (2023), Germany	Pilot cross-sectional	20 (10 bruxism, 10 controls) Bruxism—33.9 years ± 13.81 SD, Male /Female 3/7 Controls—Bruxism-Mean age: 23 years ± 2.28, Male /Female 7/3	Adults with clinically diagnosed bruxism and healthy controls	Bruxism patients in TMD clinic	B-mode US and SWE of masseter at rest, 50% bite force, maximal mouth opening	Canon Aplio i800 (Canon Medical Systems, Neuss, Germany) 14 MHz linear transducer, i14LX5/PLI-1205BX,	10 bruxism patients in TMD clinic	10 healthy controls	Masseter muscle stiffness (shear wave velocity), muscle dimensions	Sitting upright and in a relaxed position
3.	Aydin Aksu et al[15] (2023), Turkey	Cross-sectional	52 (26 TMD, 26 controls) 26 TMD TMD: 28.5 ± 9.8; Control: 29.1 ± 4.7 TMD- Male/Female 7/19	Bruxism patients with myofascial pain (VAS ≥4); healthy controls	DC/TMD)	B-mode US and SWE of masseter at rest and contraction	Applio 500 diagnostic color Doppler ultrasound system (Toshiba Medical Systems Corporation, Tochigi, Japan) with a 17.5 MHz linear transducer water-based transmission gel	26 patients divided into two subgroups as splint users (Group AI) and non-users (Group AII)	26 healthy controls	Masseter muscle (MM) anteroposterior (thickness) and transverse (width) dimensions and shear wave velocity (Vs) Pain intensity (VAS)	Supine position
4.	Chen et al[16] (2023), Taiwan	Cross-sectional	98 (48 controls, 50 with pain) TMD: 49.30 ± 16.57; Control: 51.00 ± 417.16 TMD- Male/Female 13/35 Control: Male/Female 24/24	Adults >20 years with or without orofacial pain	TMD diagnosis based on clinical examination	SWE of masseter, temporalis, lateral pterygoid	Aplio 300 Platinum platform, Toshiba, Tokyo, Japan) along with convex (PVT-375SC, 50 mm wide, 1.5–6 MHz) and linear (PLT-1005BT, 58 mm wide, 3.8–10 MHz) transducers	13 unilateral pain 37 bilateral pain	48 healthy controls	Muscle stiffness and thickness Pain intensity Mouth opening	Not reported
5.	Akkoca et al[17] (2025), Turkey	Cross-sectional	54 (25 TMD, 29 controls) TMD-25 (18–35) Controls- 22.0 (20.0–31.0) TMD- Male/Female 3/22 Controls- Male/Female 9/20	Adults with myofascial TMD pain; age- and sex-matched controls	RDC/TMD	B-mode US and SWE (masseter, temporalis, SCM muscles at rest and clench)	LOGIQ P9 with an XD clear ultrasound device (GE Healthcare, WI, USA) equipped with L3–12t (2–8 MHz) linear probes and a standard water-based acoustic gel	25 patients with myofascial pain from the muscular disorders subgroup of temporomandibular disorders	29 healthy controls	Muscle thickness and stiffness in rest and contracted state Pain intensity Mouth opening	Supine position

Abbreviations: DC/TMD, Diagnostic Criteria for Temporomandibular Disorder; RDC/TMD, Research Diagnostic Criteria for Temporomandibular Disorder; SWE, shear wave elastography; TMD, temporomandibular disorder.

Table 3 Summary findings of the included studies

Authors	Results	Outcome	Key findings
Takashima et al[8] (2017), Japan	Muscle stiffness was 12.5 kPa in 13 patients in group Ia, 13.0 kPa in 13 patients in group Ib and 5.25 kPa in 24 control subjects	Masseter muscle stiffness was positively correlated with pain intensity (p < 0.05) and negatively correlated with maximum assisted mouth opening (p < 0.05) and painless mouth opening (p < 0.05).	Masseter stiffness about twice as high in TMD group and correlated with pain intensity. SWE is useful to quantify masticatory muscle stiffness. Masseter muscle stiffness of females measured using SWE was about 2-fold greater in group Ia and Ib than in the healthy control group
Toker et al[14] (2023), Germany	SWV relaxed position (m/s) Bruxism: 1.91 ± 0.44, Healthy controls: 1.66 ± 0.25, p <0.01 SWV 50% bite force (m/s) Bruxism: 2.77 ± 1.11 Healthy controls: 2.42 ± 0.61, SWV at maximal mouth opening (bruxism 4.46 m/s _ 1.17; controls 5.23 m/s _ 0.43).	Higher SWE at rest in bruxism patients; reduced stiffness during maximal mouth opening; limited jaw opening correlated with stiffness	SWE was significantly increased during relaxed jaw and bite force position whereas for maximal mouth opening, the result was vice versa, which could be due to limited jaw movement in chronic bruxism patients
Aydin Aksu et al[15] (2023), Turkey	Group A-Resting: 39.13 ± 4.52 Clenching state: 44.04 ± 4.99 Group B-Resting: 27.73 ± 1.92 Clenching state: 35.52 ± 2.64	Dimensions of the MMs in painful bruxers was not statistically significant differences in MM thickness and width between the two groups with p-values higher than 0.10. The significant differences in stiffness and thickness of masseter muscle became visible in the relaxation state. Stiffer MMs were measured in Group AII (38.16 ± 3.61 kPa) than in Group AI (26.91 ± 2.13 kPa).	Bruxism patients had thinner and stiffer masseter muscles; splint users had lower stiffness
Chen et al[16] (2023), Taiwan	Masseter stiffness at painful site: 14.68 ± 5.30 (11.48 to 17.89) 11.21 ± 4.31 (8.60 to 13.81) p-value < 0.101	The reliability, quantified by the intraclass correlation coefficient for muscle stiffness measurement, ranged from 0.745 to 0.893.	Orofacial pain at the individual muscle level was significantly associated with masseter muscle stiffness. Increased stiffness in masseter and temporalis muscles on painful sides; no significant association with pain intensity.
Akkoca et al[17] (2025), Turkey	Masseter right side, resting (kPa) 7.74 (5.23–17.14) 7.43 (4.77–11.64) 0.527 Masseter right side, clenching (kPa) 21.03 (15.22–37.35) 17.91 (10.92–34.68) p < 0.00) Masseter left side, resting (kPa) 8.05 (3.78–12.46) 7.08 (4.81–11,48) 0.286 Masseter L, clenching (kPa) 20.69 (12.75–36.29) 16.19 (8.8–31.98) 0.003	A positive correlation between masseter muscle stiffness and pain severity, as well as a negative correlation between stiffness and maximum mouth opening. No significant difference was observed in the muscle thickness values for all three muscles between the asymptomatic control and myofascial TMD groups.	Increased masseter stiffness during clenching in TMD patients; no differences at rest. SWE findings suggest that stiffness may increase before muscle thickness in those with mild to moderate pain, supporting its early use in clinical assessment. While the SCM was unaffected, neck muscles may be involved in more severe TMJ dysfunction.

Abbreviations: kPa, kilopascals, m/s, meters/sec; SWE, shear wave elastography; SWV, shear wave velocity; TMD, temporomandibular disorder.

A total of five studies,[8] [14] [15] [16] [17] conducted between 2017 and 2025 across Turkey, Taiwan, Germany, and Japan, were included in this meta-analysis. All studies employed a cross-sectional or observational design and assessed masseter muscle stiffness using SWE combined with B-mode ultrasonography. Sample sizes ranged from 20 to 98 participants, including patients diagnosed with TMD, bruxism, or orofacial pain, along with matched healthy controls. The total sample size included 137 patients in the experimental group (with TMDs) and 137 in the control group. Diagnostic criteria such as DC/TMD and RDC/TMD were employed by Takashima et al,[8] Aydin Aksu et al,[15] Akkoca et al,[17] and Chen et al[16] to diagnose TMD based on clinical examination, while Toker et al[14] included patients clinically diagnosed with bruxism.

SWE measurements were conducted either at rest, during contraction (bite force), or at maximal mouth opening. The ultrasound equipment used varied across studies and included the ACUSON S2000 (Siemens AG, Germany),[8] Canon Aplio i800,[14] Applio 500 diagnostic color-Doppler ultrasound system (Toshiba Medical Systems Corporation, Tochigi, Japan),[15] Aplio 300 Platinum platform (Toshiba, Tokyo, Japan),[16] and LOGIQ P9 with XD clear (GE Healthcare, WI, USA),[17] and assessed outcomes included masseter muscle thickness, stiffness (shear wave velocity), and in some studies, correlations with pain intensity or maximal mouth opening. All measurements were taken during a single session, with no longitudinal follow-up.

Aydin Aksu et al[15] and Akkoca et al[17] performed SWE in the supine position, whereas Takashima et al[8] and Toker et al[14] conducted the assessment in the sitting position. In all studies, SWE was performed at the thickest region of the masseter muscle. Four of the five studies evaluated stiffness in both resting and contracted states, except for Takashima et al,[8] who assessed stiffness only at rest. Overall, the studies consistently reported increased masseter stiffness in patients compared with healthy controls, with significant correlations observed between muscle stiffness, pain intensity, and mouth opening. Trained radiologists performed ultrasonographic assessments. Notably, Takashima et al[8] also validated device reliability through a phantom study and assessed inter- and intra-person reliability.

This meta-analysis concentrated exclusively on muscle stiffness values, as muscle thickness was not consistently reported as statistically significant across the included studies and was, therefore, excluded from the quantitative synthesis. [Table 4] summarizes the list of studies excluded after the analysis.[6] [18] [19] [20] [21] [22] [23] [24] [25] [26]

Table 4 List of studies excluded

	Author	Title	Technique	Reason for exclusion
1.	Olchowy et al,[18] 2020	Potential of Using Shear Wave Elastography in the Clinical Evaluation and Monitoring of Changes in Masseter Muscle Stiffness	Shear wave elastography	Monitor the effect of massage on the masseter muscle by SWE
2.	Olchowy et al,[19] 2022	Assessment of the Masseter Stiffness in Patients during Conservative Therapy for Masticatory Muscle Disorders with Shear Wave Elastography	Shear wave elastography	Evaluating muscle stiffness after treatment; no control group
3.	Guo et al,[20] 2022	Effects of Two Botulinum Toxin Type a Evaluated by Shear Wave Elastography and Electromyographic Measurements of Masseter Reduction	Shear wave elastography	Evaluation of efficacy and safety of onabotulinumtoxin A and Chinese botulinum toxin type A for masseter reduction using elastography
4.	Arda et al,[21] 2011	Quantitative Assessment of Normal Soft-Tissue Elasticity Using Shear-Wave Ultrasound Elastography	Shear wave elastography	Study conducted on healthy adults
5.	Habibi et al,[6] 2022	Quantitative Assessment of Temporomandibular Disc and Masseter Muscle with Shear Wave Elastography	Shear wave elastography	Evaluation of stiffness of disc in healthy population
6.	Olchowy et al,[22] 2021	Determination of Reference Values of the Masseter Muscle Stiffness in Healthy Adults Using Shear Wave Elastography	Shear wave elastography	Study conducted on healthy adults
7.	Koruyucu et al,[23] 2024	Determination of Masseter and Temporal Muscle Thickness by Ultrasound and Muscle Hardness by Shear Wave Elastography in Healthy Adults as Reference Values	Shear wave elastography	Evaluation of muscle hardness in healthy adults
8.	Olchowy et al,[24] 2021	Dentists Can Provide Reliable Shear Wave Elastography Measurements of the Stiffness of Masseter Muscles: A Possible Scenario for a Faster Diagnostic Process	Shear wave elastography	Training for evaluation of muscle stiffness
9.	Ariji et al,[25] 2016	Shear-Wave Sonoelastography for Assessing Masseter Muscle Hardness in Comparison with Strain Sonoelastography: Study with Phantoms and Healthy Volunteers	Shear wave elastography	Evaluation of muscle stiffness in healthy adults and comparison of shear and strain wave elastography

Risk of Bias

The methodological quality of the included studies was assessed using the QUADAS-2 tool ([Table 5]). All five studies demonstrated a low risk of bias in the domains of patient selection, index test (SWE), and flow and timing, indicating appropriate patient recruitment, correct application and interpretation of the index test, and proper management of participant flow and timing. However, concerns were noted in the reference standard domain. Specifically, two studies (Chen et al[16] and Toker et al[14]) did not use an appropriate or clearly defined reference standard for eligibility criteria in TMD diagnosis, which introduces potential bias in the verification of results. In contrast, Takashima et al,[8] Aydin Aksu et al,[15] and Akkoca et al[17] employed valid reference standards (DC/TMD, RDC/TMD) for TMD diagnosis. Overall, the studies were judged to be of acceptable quality, with minor limitations primarily related to the reference standard domain.

Table 5 Risk of bias assessment by QUADAS-2 tool of the included studies

	Patient selection	Index test (SWE)	Reference standard	Flow and timing
Takashima et al, 2017[8]	Yes	Yes	Yes	Yes
Toker et al, 2023[14]	Yes	Yes	No	Yes
Aydin Aksu et al, 2023[15]	Yes	Yes	Yes	Yes
Chen et al, 2023[16]	Yes	Yes	No	Yes
Akkoca et al, 2025[17]	Yes	Yes	Yes	Yes

Abbreviation: SWE, shear wave elastography.

Quantitative Synthesis

All included studies reported higher masseter muscle stiffness values in TMD patients compared with healthy controls. SWE was performed with patients in the resting mandibular position, and most studies used kilopascals (kPa) as the unit of measurement. Variations were noted in ultrasound systems used, region of interest (ROI) size, probe orientation, and measurement protocol.

A meta-analysis of five studies evaluated the stiffness of the masseter muscle at rest in patients with TMDs compared with healthy controls using elastography, encompassing a total of 137 patients in the experimental group (with TMDs) and 137 in the control group. The forest plot ([Fig. 2]) displays the pooled SMD was 3.16 (95% CI: −0.70 to 7.03), indicating an overall increase in resting masseter stiffness among TMD patients; however, the result was not statistically significant (Z = 1.61, p = 0.11). The prediction interval (−12.08 to 18.41) was notably wide, reflecting a high degree of variability and uncertainty about the effect size in future studies. Substantial heterogeneity was observed among the included studies, with an I² value of 96%, a Tau² of 19.0613, and a significant Cochran's Q test (Chi² = 108.49, df = 4, p < 0.01). The considerable heterogeneity suggests substantial differences in study design, population characteristics, or elastographic techniques, which may explain the broad range of effect estimates. Although the direction of effect favors increased resting masseter stiffness in TMD patients, the variability limits the confidence in the pooled estimate. The funnel plot ([Fig. 3]) does not demonstrate a potential publication bias. The Egger's test discards the presence of funnel plot asymmetry (intercept: 8.39, 95% CI: 0.66–16.13, t: 2.128, p-value: 0.123). Most studies cluster near the top of the plot, indicating high precision; however, one study (Takashima et al[8]) lies far to the right, exerting a disproportionate influence on the overall effect size.

Fig. 2 Forest plot depicting the pooled standardized mean difference (SMD) of masseter muscle stiffness in the resting state.

Fig. 3 Funnel plot of the five studies assessing the masseter muscle stiffness in the resting state.

In the analysis of masseter muscle stiffness in the contracted state, four studies were included in the forest plot ([Fig. 4]), encompassing a total of 111 patients in the experimental group (with TMDs) and 113 in the control group. The pooled SMD was 1.20 with a 95% confidence interval of 0.58 to 1.83, indicating a statistically significant increase in masseter muscle stiffness among TMD patients compared with healthy controls (Z = 3.76, p < 0.01). This suggests that elastography can reliably detect increased stiffness of the masseter muscle during contraction in individuals with TMD. However, moderate heterogeneity was observed among the included studies, with an I² value of 71%, Tau² = 0.3014, and a statistically significant Chi² test (χ² = 10.18, df = 3, p = 0.02). The prediction interval ranged from −1.53 to 3.94, indicating some uncertainty in future outcomes, but the overall direction of effect remained positive. The funnel plot did not indicate a potential publication bias ([Fig. 5]). The Egger's test does not support the presence of funnel plot asymmetry (intercept: −0.41, 95% CI: −8.97 to −8.15, t: −0.095, p-value: 0.933). No significant publication bias was detected in studies assessing stiffness during contraction.

Fig. 4 Forest plot depicting the pooled standardized mean difference (SMD) of masseter muscle stiffness in the contracted state.

Fig. 5 Funnel plot of the four studies assessing the masseter muscle stiffness in the resting state.

These findings suggest that elastographic measurements of the masseter muscle during contraction provide more consistent and statistically significant differences between TMD patients and controls, compared with measurements taken at rest. The improved effect size and reduced heterogeneity in the contracted state support the utility of contraction-phase elastography as a more reliable diagnostic marker in the evaluation of TMD.

Certainty of Evidence

The certainty of evidence for each outcome was assessed using the GRADE framework and was rated as low to moderate ([Table 6]). For the comparison of masseter stiffness in TMD patients versus controls (Forest Plot 1, [Fig. 2]), the certainty was rated as very low due to substantial heterogeneity and imprecision caused by a wide confidence interval and one outlier study with a substantial effect. An alternative meta-analysis (Forest Plot 2, [Fig. 4]), including fewer studies, showed low certainty evidence, with moderate heterogeneity and improved precision. For resting stiffness in TMD patients only, the certainty was also rated low, primarily due to inconsistency in ultrasonography protocols and equipment across studies. The assessment of inter-assessor reliability of SWE showed moderate certainty, supported by consistent findings in two studies and a high intraclass correlation coefficient (ICC). However, the limited number of studies warranted downgrading.

Table 6 GRADE table of evidence analysis

Outcome	No. of studies	Study design	Effect estimate (95% CI)	Certainty of evidence (GRADE)	Comments
Masseter stiffness in TMD versus controls (Forest Plot 1, [Fig. 2])	5	Observational	SMD = 3.16 [–0.70, 7.03]	⬤◯◯◯ Very Low	Downgraded for very high heterogeneity and serious imprecision. One study with an extreme effect size.
Masseter stiffness in TMD versus controls (Forest Plot 2, [Fig. 4])	4	Observational	SMD = 1.20 [0.58, 1.83]	⬤⬤◯◯ Low	Downgraded for moderate heterogeneity. Improved precision compared with Forest Plot 1.
Masseter stiffness at rest (TMD patients only)	3	Observational	Mean difference = 2.5 kPa [1.1, 3.9]	⬤⬤◯◯ Low	Downgraded for inconsistency across devices and protocols. Clinically meaningful difference observed.
SWE reliability between assessors	2	Observational	ICC = 0.89 [0.80, 0.96]	⬤⬤⬤◯ Moderate	Consistent results and large effect size. Downgraded for limited number of studies.

Abbreviations: SWE, shear wave elastography; TMD, temporomandibular disorder.

The quality of evidence across the included outcomes, as assessed by the GRADE approach, varied from very low to moderate. Notably, the comparison of masseter stiffness between TMD patients and healthy controls yielded a wide effect range with high heterogeneity, resulting in very low certainty. However, when a more refined subset of studies was analyzed (Forest Plot 2, [Fig. 4]), the evidence quality improved slightly to low, indicating more reliable but still cautious interpretation. For within-group assessments of TMD patients, the evidence remained low due to methodological variability. Encouragingly, the inter-assessor reliability of SWE supported by consistent findings in two studies and a high intraclass correlation coefficient (ICC) showed moderate certainty, though the limited number of studies warranted downgrading.

The results above suggest that, with standardized protocols, SWE can be a reproducible method for evaluating masseter stiffness. Overall, the findings underscore the need for higher-quality, standardized research to strengthen confidence in SWE-based diagnostic applications for TMD.

Discussion

This systematic review and meta-analysis examined the role of SWE in quantifying masseter muscle stiffness among patients with TMDs. A total of five studies were included, involving 137 TMD patients and 137 healthy controls in the resting state analysis, and 111 TMD patients with 113 controls in the contracted state analysis. Although the results demonstrate a trend toward increased stiffness in TMD-affected masseter muscle, the statistical significance and consistency of findings differed markedly between rest and contraction conditions. This variability emphasizes the importance of methodological rigor, diagnostic consistency, and protocol standardization in future SWE-based investigations.

This meta-analysis systematically compared the stiffness of the masseter muscle in patients with TMD at rest and during contraction using SWE. The findings demonstrate that stiffness values were elevated in TMD patients across both conditions; however, only the contracted-state measurements yielded statistically significant and consistent results. The high heterogeneity observed in resting-state data likely reflects variability in study protocols, muscle relaxation levels, and patient positioning. On the other hand, during contraction, SWE appears to provide a more standardized and reproducible assessment of muscle biomechanics, likely due to more uniform muscle engagement and tonicity. These findings align with the pathophysiological understanding that TMD often affects muscle function under load or stress, thereby making contracted-state evaluations more clinically meaningful.

Myofascial pain is commonly seen among patients with TMDs. Myofascial pain alone represents 45.3% of TMD diagnoses and is defined as regional muscle pain associated with tenderness on palpation and referred pain, taut bands, and trigger points.[26] The diagnosis of myofascial pain based on the trigger points is routinely performed by palpation. This method is subjective, and there is a need for the development of new imaging biomarkers that have sufficiently high accuracy and reproducibility in detecting pathological processes. Technological advancements have led to the development of US elastography for determining the stiffness of muscles.[19] Application of pressure creates shear waves, which are detected by longitudinal ultrasonography waves, since they move through tissues much more quickly than shear waves do. The SWE is based on the physical principle that a portion of the ultrasonic waves' energy is transformed into force, which pushes the object away from the ultrasonic source. Therefore, a larger value indicates greater stiffness.[27] This non-invasive technique has great repeatability, intra- and inter-operator agreement, and the potential to yield quantitative data. Numerous studies increasingly support the application of SWE in dentistry. This technique is tested by known-hardness phantoms and contrasted with alternative techniques.[28] SWE results were shown to be accurate and reproducible.[24]

Among the included studies, the investigation by Takashima et al[8] quantified masseter muscle stiffness by SWE in patients with masticatory myofascial pain in the resting state. The stiffness was about twice as high in the myofascial group (Ia and Ib) and correlated with pain intensity but not with mouth opening. The study's limitation includes its small sample size and lack of muscle state standardization (e.g., inconsistent rest posture). Moreover, the absence of detailed control for patient-reported pain intensity could be a potential drawback. Takashima et al concluded that masseter stiffness was about twice as high in the TMD group and correlated with pain intensity. They concluded that SWE is a valuable method for quantifying masticatory muscle stiffness.

Toker et al[14] determined muscle stiffness at rest and in the contracted state in patients with bruxism. The study suggested that SWE diagnostic techniques could be a valuable tool for functional analysis in patients with bruxism. The shear wave velocity could be used to quantify tension in the masticatory musculature, particularly the masseter muscle, both in a relaxed and functional state. In patients with bruxism, SWE values were significantly elevated during both relaxed jaw and bite force positions, whereas stiffness decreased during maximal mouth opening. This inverse finding may be attributed to restricted jaw mobility commonly observed in chronic bruxism, suggesting a potential correlation between limited opening and increased resting muscle stiffness. Despite a straightforward methodological approach, this study concluded that SWE had limited reproducibility and clinical translatability due to operator-dependent variability. Although it supported the presence of higher stiffness in TMD patients during contraction, it cautioned against interpreting SWE as a standalone diagnostic tool. These findings are reflected in our meta-analysis, which shows moderate heterogeneity (I² = 71%) during contraction and very high heterogeneity at rest (I² = 96%). The results of this study underscore the importance of training, machine calibration, and standardized probe positioning in enhancing measurement reliability.

The investigation by Aydin Aksu et al[15] uniquely targeted bruxism within the TMD patients, offering a more specific subgroup analysis. They suggested that bruxism patients had stiffer muscles, and patients with splints had lower stiffness values, with the elevated stiffness during contraction, corroborating the significant pooled effect size. Notably, the study highlighted that chronic bruxism may induce myofascial adaptations, including fibrosis and muscle overload, contributing to the elevated stiffness values. This provides a potential explanation for between-study heterogeneity: varying inclusion of bruxism-related myalgia may amplify stiffness in some cohorts compared with broader TMD groups.

Chen et al[16] combined ultrasound and SWE to evaluate both superficial (masseter) and deep muscles (temporalis and lateral pterygoid) in patients with orofacial pain. Their methodology involved measurement at rest and during contraction, showing significantly increased stiffness in TMD patients in both conditions. These results aligned with our meta-analysis findings during contraction (SMD = 1.20, 95% CI: 0.58–1.83), supporting SWE's diagnostic utility in the dynamic state. Notably, this study applied DC/TMD criteria and used ROI averaging, which may explain their lower intra-observer variability and stronger statistical signals.

The study by Akkoca et al[17] examined stiffness across multiple muscles, including the temporalis and sternocleidomastoid (SCM), in addition to the masseter. The authors found that among the three muscles, the masseter muscle had increased stiffness in TMD, especially under contraction. Their inclusion of muscles outside the masticatory group (e.g., SCM) provides insight into compensatory mechanisms and postural imbalances in chronic TMD. SWE findings indicate that stiffness may precede changes in muscle thickness, particularly in mild to moderate cases, supporting its early clinical application. Their findings supported SWE's ability to detect inter-muscle compensation and systemic myofascial changes, suggesting broader diagnostic applications beyond the masseter alone.

The overall nonsignificant increase in resting-state stiffness (SMD = 3.16, p = 0.11) and its high heterogeneity (I² = 96%) can be attributed to methodological disparities in studies mentioned earlier. There was variability in rest condition definitions (some used seated posture, others supine or relaxed jaw state). The machines used for SWE were different, and the ROI placement (Supersonic Imagine, GE, Siemens) also differed. The definitions of TMD were heterogeneous, with some studies specifying myofascial pain and others including joint dysfunction and bruxism. The presence or absence of comorbid bruxism could have exaggerated the stiffness independently.

In contrast, the more uniform and statistically significant findings during contraction (SMD = 1.20, p < 0.01) likely reflect a better-controlled physiological condition where increased neuromuscular activity amplifies stiffness changes in pathological tissues, enhancing the sensitivity of SWE to detect such abnormalities. The significant findings in the contracted state across four studies reflect the potential utility of SWE as a dynamic biomarker for TMD. Contracted muscle evaluations may better capture pathophysiological changes due to chronic overuse, muscle guarding, or bruxism, as suggested by Manfredini et al.[29] Bruxism-induced myofascial pain, frequently observed in TMD patients, is associated with hyperactivity and increased stiffness of masticatory muscles, further reinforcing the relevance of contraction-based assessments.[22] Central sensitization and neuroplastic adaptations by Capra and Ro[30] could also explain elevated stiffness levels. Chronic TMD is known to induce alterations in motor control and muscular architecture, contributing to muscle hypertrophy, increased tone, and fibrosis—all of which may be detected via SWE.[31] The use of standardized diagnostic protocols, such as the DC/TMD criteria outlined by Schiffman et al,[3] improves comparability across studies. Variations in TMD definitions—ranging from clinical examination to self-reported questionnaires—may account for some of the observed heterogeneity. Notably, Chen et al[16] and Toker et al[14] did not apply DC/TMD or RDC/TMD criteria in their patient selection, which may have influenced diagnostic consistency. The GRADE results corroborated the above findings and suggested that, with standardized protocols, SWE can be a reproducible method for evaluating masseter stiffness. Overall, the findings underscore the need for higher-quality, standardized research to strengthen confidence in SWE-based diagnostic applications for TMD.

Limitations and Future Directions

Although SWE is a promising tool to determine muscle stiffness, several limitations must be addressed. The cross-sectional designs in all included studies limit the ability to make causal inferences. Small sample sizes in the studies and selective reporting may have inflated variability. Lack of inter-machine calibration challenges the generalizability of specific shear modulus cut-offs. Absence of longitudinal follow-up restricts the understanding of SWE as a monitoring tool. Technical challenges such as depth-related signal attenuation, soft tissue anisotropy, and the subjective component of pain intensity complicate SWE interpretation. Moreover, Bercoff et al.[32] and Okeson[33] emphasized the importance of understanding viscoelastic tissue responses and clinical correlates of masticatory muscle dysfunction when interpreting elastographic outcomes.

Future research should prioritize standardized SWE protocols with defined anatomical landmarks. The patients should be included based on unified diagnostic criteria (e.g., DC/TMD) for consistent subject selection. The stiffness thresholds should be explored across the spectrum of TMD (acute, chronic, with/without bruxism). Prospective longitudinal studies should be performed to establish responsiveness to treatment. Furthermore, dynamic elastography could be leveraged for monitoring response to conservative therapy (e.g., physiotherapy, occlusal splints), assessing severity and progression in bruxism-associated myalgia, and identifying compensatory muscular involvement (as shown in SCM and temporalis stiffness). Future studies should aim for methodological standardization, inclusion of larger and more diverse populations, and longitudinal SWE measurements to assess treatment response or disease progression.

The findings from this meta-analysis suggest that SWE, particularly during active muscle contraction, is a promising adjunct in the diagnostic workup of TMD. Elevated stiffness values may serve as a quantitative biomarker for myofascial pain of the masseter muscle, particularly when palpation and symptom self-report are inconclusive.

Conclusion

SWE is a promising, non-invasive modality for assessing masseter muscle stiffness in TMD patients. This meta-analysis supports the diagnostic value of SWE in assessing masseter muscle stiffness, particularly in the contracted state. Although variability in resting state measures currently limits their clinical utility, dynamic assessments appear robust and reproducible. When integrated with clinical findings and other imaging modalities, SWE holds promise as a quantitative, non-invasive tool for diagnosing and monitoring TMD-related myofascial dysfunction.

Conflict of Interest

None declared.

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Publication History

Article published online:
05 February 2026