Determination of Reference Values of the Masseter Muscle Stiffness in Healthy Adults Using Shear Wave Elastography

Determination of masseter and temporal muscle thickness by ultrasound and muscle hardness by shear wave elastography in healthy adults as reference values

OBJECTIVES

The purpose of this study is to prospectively investigate the reference values of masseter and temporal muscle thicknesses by ultrasonography and muscle hardness values by shear wave elastography in healthy adults.

METHODS

The sample of the study consisted of a total of 160 healthy individuals aged between 18 and 59, including 80 women and 80 men. By examining the right and left sides of each participant, thickness and hardness values were obtained for 320 masseter muscles and 320 temporal muscles in total.

RESULTS

The mean masseter muscle thickness was found to be 1.09 cm at rest and 1.40 cm in contraction. The mean temporal muscle thickness was found to be 0.88 cm at rest and 0.98 cm in contraction. The thickness values of the masseter and temporal muscles were significantly greater in the male participants than in the female participants (P < .001). While there were significant differences between the right and left masseter muscle thickness values at rest and in contraction, the values of the temporal muscles did not show a significant difference between the sides. While the resting hardness (rSWE) of the masseter muscle was transversally 6.91 kPa and longitudinally 8.49 kPa, these values in contraction (cSWE) were found, respectively, 31.40 and 35.65 kPa. The median temporal muscle hardness values were 8.84 kPa at rest and 20.43 kPa in contraction. Masseter and temporal muscle hardness values at rest and in contraction were significantly higher among the male participants compared to the female participants (P < .001).

CONCLUSION

In this study, reference values for the thickness and hardness of the masseter and temporal muscles are reported. Knowing these values will make it easier to assess pain in the masseter and temporal muscles and determine the diagnosis and prognosis of masticatory muscle pathologies by allowing the morphological and functional assessments of these muscles, and it will identify ranges for reference parameters.

Assessing thickness and stiffness of superficial/deep masticatory muscles in orofacial pain: an ultrasound and shear wave elastography study

INTRODUCTION

Sonoelastography has been increasingly used for non-invasive evaluation of the mechanical features of human tissues. The interplay between orofacial pain and regional muscle activity appears clinically paramount, although only few imaging studies have investigated this association. Using shear wave sonoelastography (SWS), this study ascertained whether orofacial pain induced alterations in the stiffness of superficial and deep masticatory muscles.

METHODS

All participants were systematically evaluated for oral/facial-related conditions, including the area and intensity of pain. SWS was applied to measure the stiffness of the bilateral masseter, temporalis, and lateral pterygoid muscles. The association between orofacial pain and muscle stiffness/thickness was investigated using a generalized estimating equation for adjusting the influence of age, sex, laterality, and body mass index on muscle thickness/stiffness.

RESULTS

A total of 98 participants were included in the present study: 48 asymptomatic controls, 13 patients with unilateral pain, and 37 patients with bilateral orofacial pain. 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. A trend of increased stiffness (p = 0.06) was also observed in relation to the painful side of the temporalis muscle. No significant correlation was identified between the numeric rating scales for pain and stiffness measurements.

CONCLUSIONS

SWS provides reliable stiffness measurements for the superficial and deep masticatory muscles. The ipsilateral masseter and temporalis muscles might be stiffer than those on the side without orofacial pain. Future studies using the present sonoelasotography protocol can be designed to investigate the stiffness changes in the target muscles after interventions.

Shear Wave Elastography in Bruxism-Not Yet Ready for Clinical Routine

Ultrasound shear wave elastography (SWE) is an emerging modality for the estimation of stiffness, but it has not been studied in relation to common disorders with altered stiffness, such as bruxism, which affects almost one-third of adults. Because this condition could lead to an increased stiffness of masticatory muscles, we investigated SWE in bruxism according to a proof-of-principle and feasibility study with 10 patients with known bruxism and an age- and gender-matched control group. SWE of the left and right masseter muscles was estimated under three conditions: relaxed jaw, 50% of the subjective maximal bite force, and maximal jaw opening. Rejecting the null hypothesis, SWE was significantly increased during relaxed jaw (bruxism 1.92 m/s ± 0.44; controls 1.66 m/s ± 0.24), whereas for maximal mouth opening, the result was vice versa increased with 2.89 m/s ± 0.93 for bruxism patients compared with 3.53 m/s ± 0.95 in the healthy control, which could be due to limited jaw movement in chronic bruxism patients (bruxism 4.46 m/s ± 1.17; controls 5.23 m/s ± 0.43). We show that SWE in bruxism is feasible and could be of potential use for diagnostics and monitoring, though we also highlight important limitations and necessary methodological considerations for future studies.

Botox Therapy for Hypertrophy of the Masseter Muscle Causes a Compensatory Increase of Stiffness of Other Muscles of Masticatory Apparatus

Little is known about the nature of masseter muscle hypertrophy. We investigated the masseter muscle stiffness change after a single intra-masseteric session of Botox injections in people with benign bilateral masseter hypertrophy and the effect of the treatment on the stiffness of the temporalis muscle. Stiffness of the muscles was measured with shear-wave elastography at baseline and 3 weeks after Botox injections in 22 otherwise healthy people. Before the treatment, the stiffness of the masseter was lower than of the temporalis muscle (10.18 ± 1.67 kPa vs. 11.59 ± 1.54 kPa; p = 0.002). After the treatment, this difference increased (6.38 ± 1.34 vs. 13.10 ± 1.92; p < 0.0001). The drop in the stiffness of the masseter muscle was symmetrical (left side by 3.78 kPa; right side by 3.83 kPa). No differences between the left and right sides of the face in terms of muscle stiffness were observed. The study shows that Botox injections reduce stiffness of the masticatory muscles which, in turn, increases the stiffness of the temporalis muscles. Due to the knock-on effect of the change in the masseter function on the other masticatory muscles, simultaneous evaluation and treatment of the temporalis muscle may be required to ensure the desired functional and cosmetic effect.

Assessment of the masseter stiffness in patients during conservative therapy for masticatory muscle disorders with shear wave elastography

Background

The complex structure of the stomatognathic system plays a vital role in chewing, digestion, speaking, breathing, facial expression and swallowing. Its complexity is the primary reason for creating multidisciplinary teams to manage temporomandibular disorders (TMD). We aimed to assess the masseter stiffness in patients undergoing conservative therapy for masticatory muscle disorders and evaluate the efficacy of manual therapy and stabilization occlusal splint in the treatment of masticatory muscle disorders.

Methods

This uncontrolled prospective cohort study included 35 patients with masticatory muscle disorders. The study lasted for eight weeks. The patients were treated with manual therapy and stabilization occlusal splint and evaluated using shear wave elastography of the masseter muscles and patient-reported outcome measures (PROMs) to assess pain, anxiety, quality of sleep, satisfaction with life and perceived stress.

Results

After the treatment, the stiffness of both masseter muscles decreased significantly (by 4.21 kPa). The patients reported a significant reduction in pain. At baseline, the median scores ranged from 5 to 8; after treatment, they ranged from 0 to 1 (p < 0.0001). The patients also reported significant improvement in terms of all patient-reported outcome measures. The reduction in stiffness corresponded to the improvement in pain and PROMs, as shown by correlations which were insignificant for all measures.

Conclusions

Conservative therapy of masticatory muscle disorders involving manual therapy and stabilization occlusal splint is effective. It reduces the masseter stiffness as objectively shown in shear wave elastography and improves subjective PROMs scores, including numerical pain assessment and selected questionnaires. Shear wave elastography has the potential for broad application in clinical practice to monitor masticatory muscle disorders treatment effects due to its objectivity and non-invasive character. Further research is recommended on larger patient populations and longer follow-up.

Trial registration

The study was registered at clinicaltrials.gov (NCT03844854). First posted date: 19/02/2019.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05392-9.

Gender-Related Biomechanical Properties of Masseter Muscle among Patients with Self-Assessment of Bruxism: A Comparative Study

It seems extremely important to know the biomechanical properties of the orofacial tissues among patients with increased activity of masticatory muscles, such as bruxism. The aim of this study was to evaluate biomechanical properties of the masseter muscle by using MyotonPRO in adults with probable bruxism and to define gender relations. This study was conducted in the Temporomandibular Disorders Department at Poznan University of Medical Sciences, Poland (June 2021-November 2021) among patients that reported bruxism symptoms The patients underwent a clinical examination, then the biomechanical properties of the masseter muscles were assessed. The MyotonPro measured masseter tone, stiffness and elasticity in 36 patients with a self-assessment of bruxism (18 women and 18 men). Data were collected from relaxed and contracted muscles. In relaxed masseter muscles there were no statistically significant differences between the sexes in muscle tone, stiffness and elasticity. During contraction significant differences were found between the sexes in the tension and the stiffness of the masseter muscles. Moreover, women often experienced headaches in the temporal region and pain in the masseter muscles during palpation. Among patients with a self-assessment of bruxism, accompanying pain was significantly more frequent in women. Male gender was associated with increased muscle tension and stiffness of the contracted masseter muscle.

Stiffness of the Masseter Muscle in Children-Establishing the Reference Values in the Pediatric Population Using Shear-Wave Elastography

In children, the quality and muscle function are altered in many pathologic conditions, including temporomandibular disorders. Although several methods have been used to evaluate muscle tonus, none became a golden standard. Moreover, the masseter muscle characteristics in children have not been investigated to date. This study aimed to measure the stiffness of the masseter muscle using shear-wave elastography in healthy children. We enrolled 30 healthy children (mean age 10.87 ± 3.38 years). The stiffness of masseter muscles was measured with shear wave elastography. Stiffness for the total sample was 6.37 ± 0.77 kPa. A comparison of the measurements did not show significant differences between the right and the left masseter muscles (left—6.47 ± 0.78 kPa; right—6.24 ± 0.76 kPa; p = 0.3546). A significant difference was seen between boys and girls (boys—5.94 ± 0.50 kPa; girls—6.63 ± 0.80; p = 0.0006). Shear-wave elastography is a promising diagnostic tool. It may help to detect changes in the stiffness of the masseter muscle and draw attention to pathological processes within the jaw muscles. Directions for further research shall include determining stiffness values in pathological conditions and the impact of biological and functional factors on the stiffness of the masseter muscle.