Introduction

Obstructive sleep apnea (OSA) is a common sleep disorder characterized by recurrent upper airway narrowing or obstruction, resulting in apnea or hypopnea episodes and oxygen desaturation [1]. It is more prevalent in males over the age of 50 and in individuals with a high body mass index (BMI) [2]. Polysomnography (PSG) remains the gold standard for diagnosis, and the Apnea–Hypopnea Index (AHI), representing the number of apnea and hypopnea events per hour of sleep, is widely used to assess disease severity [3, 4].

Although several studies have reported a positive correlation between higher AHI values and the presence of sleep bruxism (SB), this association remains controversial, as indicated by a recent meta-analysis [5, 6]. SB is classified as a sleep-related movement disorder characterized by rhythmic (phasic) or sustained (tonic) contractions of the masticatory muscles [7]. This involuntary activity can lead to adverse oral health outcomes such as restoration fractures, masticatory muscle pain, and temporomandibular disorders [8].

The association between OSA and SB is thought to result from micro-arousals and sympathetic activation triggered by airway obstruction during sleep. These responses may induce rhythmic masticatory muscle activity (RMMA) as a protective reflex to reopen the airway [9, 10]. Increasing OSA severity is associated with more frequent respiratory events and arousals, which are thought to contribute to repetitive masticatory muscle activity. Repeated activation can lead to adaptive changes in the masticatory muscles, such as hypertrophy or fatigue-related thinning. Moreover, repeated nocturnal muscle activation, combined with intermittent hypoxia and sympathetic overactivity in OSA, may promote structural remodeling and altered mechanical properties of the masticatory muscles, reflected as changes in muscle stiffness and thickness [11].

This relationship has been evaluated using various diagnostic approaches, including PSG, electromyography (EMG), video recordings, questionnaires, and clinical assessments [12,13,14]. Recently, ultrasonography (USG) has been proposed as a noninvasive, radiation-free, portable, and cost-effective alternative for assessing masticatory muscle structure and function in patients with OSA [15, 16]. Advanced techniques such as Shear Wave Elastography (SWE) and Brightness Mode (B-mode) USG enable objective evaluation of muscle stiffness and thickness, providing insight into structural adaptations related to OSA severity [17, 18].

Although EMG has been used to evaluate masticatory muscle activity in patients with OSA, no studies have yet applied SWE for this purpose [7, 19, 20]. Therefore, this study aimed to assess the stiffness and thickness of the masticatory muscles in individuals with OSA and to investigate their relationship with disease severity.

Methods

Participants

The study included 68 adult patients diagnosed with OSA who were consecutively recruited from the Epilepsy Monitoring and Sleep Disorders Center, Dokuz Eylül University, between January 2024 and January 2025. All participants were over 18 years of age and had completed overnight PSG confirming the diagnosis of OSA. All patients were evaluated prior to the initiation of any OSA-related treatment to avoid potential treatment effects on masticatory muscle characteristics.

Patients at different levels of disease severity (mild, moderate, and severe) who had not yet initiated any form of OSA treatment were included in the study. To minimize potential confounding factors, patients with systemic comorbidities or conditions that could influence OSA or masticatory muscle function were excluded. These included individuals with high BMI, edentulism, use of intraoral or sleep-related devices, history of head or neck surgery or trauma, craniofacial abnormalities, connective tissue disorders, neurological or musculoskeletal conditions, endocrine diseases, occlusal dysfunctions, or psychiatric disorders [21].

Additionally, patients who were aware of wake bruxism were excluded to ensure that only sleep-related bruxism was assessed [22]. All participants provided written informed consent prior to participation. The study was conducted after obtaining ethical approval from the Non-Interventional Research Ethics Committee (No: 2025/01–10, Date: 08/01/2025), and all procedures were carried out in accordance with the principles of the Declaration of Helsinki. Clinical and ultrasound examinations were performed after participants signed the informed consent form.

Study design

Sample size calculation

To our knowledge, no study has evaluated the stiffness of the masseter and temporalis muscles in patients with obstructive sleep apnea using SWE. Therefore, sample size calculations were performed using the GPower 3.1 program. Within the t-test family, a minimum of 68 participants was determined based on an effect size of d = 0.35, a significant level (α = 0.05), a 95% confidence interval, and a power of 80% (1-β = 0.80).

Clinical data collection and evaluation

The participants' sex, age, and BMI were recorded, with BMI calculated using the method established by the World Health Organization. Maximum mouth opening, including any instances of painful mouth opening, was measured with a precision caliper [23].

The assessment of SB was based on the presence of the following clinical signs and and findings: Muscle pain and transient trismus in the morning, history of repeated fracture of sound teeth or restorations, grinding witnessed by bed partner, abnormal tooth wear, impressions of teeth in the buccal mucosa and/or tongue, and masseter muscle hypertrophy [24]. Based on these findings, the study population was divided into two groups, as shown in the study flow chart: OSA patients with SB and those without SB (Fig. 1).

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.
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Flowchart of participants selection

Measurement of apnea severity and daytime sleepiness

A full-night standard polysomnographic evaluation was conducted for all participants in the sleep laboratory using the Alice 6 system (Philips Respironics, Murrysville, PA, USA), in accordance with the current guidelines for the diagnosis and management of OSA [25].

OSA severity was evaluated using the AHI derived from overnight PSG recordings. PSG was performed in a sleep laboratory in accordance with standardized protocols, and the data were analyzed by a specialist in sleep medicine. OSA severity was categorized based on AHI values as follows: AHI < 5 events/hour: Normal, AHI 5–15: Mild OSA, AHI 15–30: Moderate OSA, and AHI > 30: Severe OSA [26].

Daytime sleepiness was assessed using the Epworth Sleepiness Scale (ESS), a validated self-reported questionnaire in which participants rated their likelihood of dozing off in eight different daily situations. Each response was scored on a scale from 0 (no chance of dozing) to 3 (high chance of dozing), resulting in a total score ranging from 0 to 24. A score above 10 indicated excessive daytime sleepiness [27].

Ultrasonography measurements

Muscle thickness and stiffness were assessed using a LOGIQ P9 ultrasound device (GE Healthcare, WI, USA) equipped with a linear probe (L3–12t, 2–12 MHz). All examinations were performed with participants in the supine position during both resting and contraction phases. During the procedure, participants were instructed to remain motionless, avoid turning their heads or swallowing, and follow specific breathing and occlusion instructions depending on the measurement phase.

All ultrasound evaluations were performed by a right-handed operator with eight years of clinical experience. To assess intraobserver reliability, all measurements were repeated by the same operator under identical conditions. In addition, a subgroup of 20 patients who had not yet initiated treatment was recalled, and the same measurements were independently repeated by another experienced operator to evaluate interobserver reliability. A generous amount of standard water-based ultrasound gel was applied between the probe and the skin surface to avoid any compression or pressure on the underlying tissues.

Muscle thickness measurements

Resting measurements were obtained while participants were instructed to close their lips, swallow their saliva, take a deep breath, and relax the jaw, avoiding any occlusal contact between the teeth. For contraction measurements, participants were instructed to clench their teeth maximally in centric occlusion [27].

The most reliable site for measuring the masseter was identified as the midpoint of the mediolateral line over the mandibular ramus. The ultrasound probe was placed perpendicular to the muscle fibers and parallel to the long axis of the mandible, at the midpoint between the zygomatic arch and the gonial angle. The probe was adjusted to obtain a clear image. Muscle thickness was measured with reference to the masseteric fascia (Fig. 2a).

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.
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An edited ultrasound image with the masseter muscle thickness highlighted in yellow, providing clearer identification. Thickness measurements of the masseter muscle in resting and clenching states (a1/a2). SWE of the masseter muscle in the resting state (b). SWE of the masseter muscle in the clenching state (c)

Only the anterior region of the temporalis muscle was accessible for measurement. The linear probe was positioned between the lateral canthus and the anterior hairline. Muscle thickness was measured at rest and during contraction as the maximum distance between the inner and outer fascia (Fig. 3a).

Fig. 3
Fig. 3The alternative text for this image may have been generated using AI.
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Edited ultrasound image highlighting the thickness of the anterior region of the temporalis muscle in yellow for clearer identification. Thickness measurements of the anterior region of the temporalis muscle in resting and clenching states (a1/a2). SWE of the anterior region of the temporalis muscle in the resting state (b). SWE of the temporal muscle in the clenching state (c)

Muscle stiffness measurements

Muscle stiffness was evaluated using the SWE mode of the ultrasound system. The transducer was positioned perpendicular to the longitudinal axis of each muscle, targeting the widest segment at the mid-belly region as visualized in B-mode imaging. Once the optimal imaging plane was identified, an elastographic frame of appropriate size was created for each muscle.

For each measurement, three optimal SWE frames were selected. Within each frame, a 6-mm circular region of interest (ROI) was manually placed at the center of the muscle tissue. The mean value of these three measurements were recorded as the final stiffness value in kilopascal (kPa) [28].

Representative SWE images of the masseter and temporalis muscles, both at rest and during contraction, are presented in Figs. 2b/2c and 3b/3c, respectively.

Statistical analysis

All continuous variables—including demographic measures (age, BMI) and biomechanical outcomes (muscle thickness and stiffness, AHI, ESS)—were first examined for normality by the Shapiro–Wilk test and by inspection of skewness and kurtosis values, and for homogeneity of variances using Levene’s test. Variables that conformed to a normal distribution were summarized as mean ± SD and compared between groups (OSA severity; bruxism vs. non‐bruxism) using the independent‐samples t‐tests. Categorical variables were expressed as counts and percentages and assessed by Pearson’s chi‐square test. Analyses were executed in IBM SPSS Statistics version 30.0 (IBM Corp., Armonk, NY, USA). All statistical tests were two‐tailed, and p < 0.05 was considered to denote statistical significance.

Inter- and intra-rater reliability analyses were conducted to evaluate the consistency of measurements obtained by two independent raters and by the same rater across repeated assessments. For continuous variables, the intraclass correlation coefficient (ICC) was calculated to assess both inter- and intra-rater reliability. The inter-rater reliability was analyzed using a two-way mixed-effects model based on absolute agreement, while the intra-rater reliability was assessed using a one-way random-effects model reflecting measurement reproducibility within the same evaluator. The interpretation of ICC values followed the classification proposed by Koo and Li, where values less than 0.50 indicate poor, between 0.50 and 0.75 moderate, between 0.75 and 0.90 good, and above 0.90 excellent reliability [29].

Results

SB was identified in 35 out of 68 patients with OSA, accounting for 51.5% of the study population. No statistically significant differences were observed between patients with and without SB in terms of age, sex, BMI, maximum mouth opening, AHI, or ESS scores. Based on OSA severity, 36 patients (52.9%) were classified as having mild to moderate OSA, while 32 patients (47.1%) had severe OSA. The severe OSA group exhibited significantly higher BMI values compared to the mild to moderate group (p = 0.007). Detailed demographic and clinical characteristics of the groups are summarized in Table 1.

Table 1 Demographic and clinical characteristics of OSA patients according to the presence of SB and disease severity (n = 68)
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Patients with SB had significantly lower right masseter muscle thickness compared to those without SB, both at rest (p = 0.036) and during clenching (p = 0.019). In addition, individuals with severe OSA had greater thickness in the left temporalis muscle at rest (p = 0.017) and during clenching (p = 0.026), as well as in the right masseter muscle at rest (p = 0.009) and during clenching (p = 0.022). These findings are summarized in Tables 2 and 3.

Table 2 Comparison of Masticatory Muscle Thickness According to the Presence of SB and OSA Severity
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Table 3 Comparison of masticatory muscle stiffness according to the presence of SB and OSA Severity
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The stiffness values of the masticatory muscles increased during the clenching condition; however, no statistically significant difference was observed in muscle stiffness between the presence of SB and different OSA severity.

Tables 4 and 5 present the intra- and inter-rater reliability of masticatory muscle thickness and stiffness measurements. Intra-rater analysis, based on three repeated acquisitions per muscle (n = 68), demonstrated excellent consistency, with ICC values ranging from 0.876 to 0.987 (p < 0.001) across all parameters. Inter-rater reliability, evaluated in a subset of 20 randomly selected untreated patients, also showed good to excellent agreement, with ICCs between 0.854 and 0.980 (p < 0.001). The narrow 95% confidence intervals in both analyses confirm high precision and reproducibility of the ultrasound protocol for assessing masticatory muscle properties.

Table 4 Intraclass correlation coefficients based on three independent measurements of masticatory muscle thickness and stiffness
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Table 5 Interclass correlation coefficients based on two independent observer of masticatory muscle thickness and stiffness
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Discussion

This study utilized SWE to assess the biomechanical properties of the masseter and temporalis muscles in patients with OSA and to explore their association with SB and disease severity. To the best of our knowledge, this is the first study to apply SWE for assessing masticatory muscles in OSA patients. SWE is a noninvasive, objective, and reproducible imaging modality that enables quantitative evaluation of muscle stiffness and provides potential diagnostic value for characterizing structural changes related to sleep disorders [17, 18, 21].

Previous research investigating SB and masticatory muscle function in OSA has primarily relied on questionnaires, clinical examinations, PSG, EMG, portable sleep monitors, and video recordings [5, 12, 13, 19]. Although these approaches allow the assessment of SB frequency and muscle activity, no prior data are available regarding masticatory muscle stiffness in OSA. In our cohort, the prevalence of SB was 51.5%, which aligns with earlier reports indicating rates between 37 and 70% depending on diagnostic criteria and methods [5, 13, 19]. Consistent with previous findings, SB was more common among male patients, suggesting that sex-related factors may contribute to the occurrence of bruxism in OSA populations [13, 30].

The relationship between OSA severity and SB remains controversial in the literature. Some studies have reported a linear increase in SB prevalence with greater OSA severity, whereas others have found an inverse or nonlinear trend [30,31,32]. In our study, the prevalence of SB was 55.6% in mild to moderate OSA and 41.7% in severe OSA, supporting the notion of a nonlinear association. These discrepancies may be explained by differences in diagnostic criteria, scoring methods, or ethnic and demographic factors. Collectively, these findings suggest that OSA and SB may share overlapping risk factors, such as arousal-related sympathetic activation, rather than a direct causal relationship [33].

Our analysis demonstrated that patients with SB exhibited significantly reduced right masseter muscle thickness both at rest and during clenching compared with those without SB. This finding may reflect unilateral chewing habits that were not detectable during clinical examination, but it is also consistent with previous studies showing reduced muscle volume or function in chronic bruxers [11]. Repeated low-level contractions during SB can induce microtrauma and fatigue in the masticatory muscles, leading to reduced EMG activity and eventual thinning [11]. Moreover, sleep fragmentation and intermittent hypoxia associated with OSA and SB can suppress anabolic hormones such as growth hormone and testosterone, potentially contributing to reduced muscle mass [34].

Conversely, patients with severe OSA exhibited increased thickness in the left temporalis and right masseter muscles. This may be due to compensatory hypertrophy caused by increased rhythmic masticatory muscle activity (RMMA) following airway obstruction events or higher BMI-related muscle loading. These findings emphasize the importance of considering hormonal status, functional activity, and body composition when interpreting muscle morphology in OSA patients.

In contrast to muscle thickness, SWE-derived stiffness values did not differ significantly among groups based on SB presence or OSA severity. This may indicate that intrinsic muscle elasticity remains relatively stable, or it may reflect technical limitations of SWE in facial muscles. In individuals with high BMI, increased subcutaneous fat can attenuate shear wave propagation, reducing the sensitivity of stiffness measurements. Uçar et al. similarly reported no significant baseline differences in masseter stiffness between bruxers and controls, emphasizing the potential impact of soft-tissue interference on SWE accuracy [35]. The lack of a consistent relationship between OSA severity and stiffness suggests that further methodological refinement and larger samples are required before SWE can be used as a reliable diagnostic tool in this context.

Taken together, the results of this study indicate a complex interplay between OSA, SB, and masticatory muscle characteristics. The findings support the hypothesis that these disorders may share common physiological mechanisms—particularly arousal-related sympathetic activation—rather than representing a direct cause–effect relationship. The application of SWE provides a novel perspective by allowing quantitative assessment of muscle structure and mechanical properties, potentially enhancing understanding of the musculoskeletal consequences of sleep-related breathing disorders.

Limitations and strengths

This study aimed to examine the relationship between SB, OSA severity, and masticatory muscle thickness and stiffness. Therefore, the analysis focused on comparisons within the patient group based on OSA severity, without including healthy controls. Further research is needed to assess these parameters in healthy individuals and in OSA patients with and without SB. Undetected parafunctional habits like unilateral chewing may have affected muscle measurements. Although demographic factors were considered, BMI was not fully balanced across OSA severity groups. In addition, the absence of full PSG data (including video, EMG, or thermography) and the reliance on patient self-reports for awake bruxism represent limitations of the study, although these approaches are commonly used in similar research settings.

Conclusions

This study used SWE to evaluate the thickness and stiffness of masseter and temporalis muscles in patients with OSA and investigated their association with SB and disease severity. While increased muscle thickness was observed in specific regions among patients with SB and those with severe OSA, muscle stiffness did not differ significantly across groups. These findings suggest that muscle thickness may be a more sensitive indicator of structural changes in OSA than stiffness. SWE appears to be a reliable and promising tool for assessing masticatory muscle characteristics, but further research with larger, more homogeneous samples is needed to clarify its diagnostic value.