The relationship of masseter muscle thickness with face morphology and parafunctional habits: an ultrasound study

Abstract

Objective:

The aim of this study was to determine the normal range of masseter muscle thickness by ultrasonographic measurement in individuals over 15 years of age, and to evaluate its relationship with age, gender, facial morphology, body mass index and parafunctional habits.

Methods:

The study was conducted on 115 volunteers whose lateral cephalometric radiography was performed within the indication in Gazi University Faculty of Dentistry, Department of Dentomaxillofacial Radiology. The participants in the study were asked questions about their parafunctional habits, age, height and weight. Individuals were grouped as hypodivergent (n = 28), normdivergent (n = 55), or hyperdivergent (n = 32) according to vertical face morphology by making measurements on lateral cephalometric films. Right and left masseter muscle thicknesses of individuals were measured by ultrasonography while at rest and in contraction.

Results:

The mean value of masseter muscle thickness was found to be 13.57 ± 2.57 mm. The rest and contracted muscle thicknesses were significantly higher in males than in females for the right and left masseter muscles (p < 0.05). When the masseter muscle was at rest and contracted, its thickness was higher in individuals with hypodivergent facial morphology, than in the other groups. No statistically significant difference was found in terms of masseter muscle thickness between individuals having parafunctional habits and those who did not have parafunctional habits (p > 0.05).

Conclusion:

Although masseter muscle thickness varied according to vertical facial morphology, this was not the case for parafunctional habits.

Introduction

According to a law proposed by Julius Wolf in 1870 and accepted in the field of biomechanics, it is argued that the size and activities of skeletal muscles are effective on the morphology of the bone structures in the region where the origin and insertion of the relevant muscle are attached. ¹ Experimental studies found significant correlations between mechanical stress and morphological changes of bone tissue. Based on this result, the opinion that similar interactions occur between bone shape and muscle activity in the craniofacial structure comes to the fore. ² Contrary to this hypothesis, there are also studies indicating that muscle activity is not associated with vertical facial morphology. ^3,4 It is widely accepted in the literature that the function, shape, and thickness of the masticatory muscles have important effects on facial morphology. ^5–7 When the relationship between facial morphology and muscle thickness was evaluated, it was determined that the masseter muscle was the most related to the muscles of the masticatory system. ^2,8–13 In addition, it is known that facial morphology is related to the activity of the masticatory muscles at rest and the activity during biting. ¹⁴

Activities of the masticatory muscles are divided into functional and parafunctional activities. ¹⁵ As a result of parafunctional activities, hyperactivity occurs in the masticatory muscles. ¹⁵ The most common parafunctional habit is bruxism. In previous studies, the relationship between bruxism and facial morphology was evaluated, and it was stated that the masseter muscle showed adaptive changes in the presence of bruxism and people had rectangular facial morphology. ¹⁶ The activities of masticatory muscles are determined by electromyography. However, recently, muscle thickness has been accepted as an indicator of jaw muscle activity. ¹⁷

CT, MRI, and ultrasonography can be used to measure masseter muscle thickness. ¹⁸ Ultrasonography is a non-invasive imaging technique that evaluates the parameters of the masticatory muscles. Ultrasonography has advantages such as being relatively cheaper, repeatable, and easier to use compared to CT and MRI. In addition, ultrasonography has no known cumulative biological effects. ⁹ This study aimed to determine the normal range of masseter muscle thickness by ultrasonographic measurement in individuals over 15 years of age and to evaluate its relationship with age, gender, facial morphology, body mass index, and parafunctional habits.

Methods and materials

Ethics committee approval was obtained from Gazi University Clinical Research Ethics Committee before starting the study (Research No: 2019-034). In this study, 115 volunteers aged 15 years and over, whose lateral cephalometric radiographs were taken within the indications at Gazi University Faculty of Dentistry, Department of Dentomaxillofacial Radiology were included.

The following criteria were considered in the included individuals:

1. No history of any congenital and/or acquired anomalies of the lips, mouth, and face (cleft lip-palate, trauma, etc.)

2. No history of trauma in the head and neck region.

3. Not having undergone a surgical operation on the head and neck region

4. No previous orthodontic treatment

5. An absence of posterior teeth missing other than third molars

Consent forms were obtained from the parents of individuals under the age of 18 and from individuals aged 18 and over who agreed to participate in the study.

Determining the presence/absence of parafunctional habits

A questionnaire about parafunctional habits, age, height, and weight was applied to the individuals participating in this study. ^19,20 Individuals who answered yes to at least one of the questions below were considered as individuals with bruxism

• Do you grind your teeth at night while you sleep?

• Do you grind your teeth during the day?

• Do you have a habit of grinding your teeth?

• Do you feel pain in the jaw when you wake up in the morning?

Calculating body mass index

Body mass index (BMI) was calculated with the method recommended by the World Health Organization. According to this method, BMI was obtained by dividing body weight (in kilograms) by the square of body length (in meter). BMI variable was classified according to the group determined by the World Health Organization (<18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30–39.9 obesity, 40≤ extreme obesity).

Measurements on the lateral cephalometric radiographs

The facial morphology was investigated for lateral cephalometric radiographs. SN/GoGn^o angle (26^o–38^o), SGo/NMe (Jarabak ratio), and Po-Or/SGn^o angle (53^o–66^o) were used to determine vertical facial morphology on lateral cephalometric radiographs. Individuals who matched at least two of these analyzes were classified as hypodivergent, normdivergent, or hyperdivergent. ²¹

Ultrasound procedures

The thickness of the masseter muscles of the individuals participating in the study was measured by ultrasonography. All ultrasonographic examinations were performed at Gazi University Faculty of Dentistry, Department of Dentomaxillofacial Radiology. Fujifilm SonoSite M-Turbo (Fujifilm, WA, USA) ultrasonography device and HFL38 × 13–6 MHz linear probe with a scanning depth of 6 cm were used to determine masseter muscle thickness. In order to provide a good acoustic transition environment, ultrasonic gel (Konix ultrasound gel, Turkey) was applied to the probe surface before imaging. Individuals were asked to lie on their back and stay in the most comfortable position by focusing on a point in front of them for ultrasonographic examination. The thickness of the right and left masseter muscles was measured when the teeth were at rest and during maximum clenching (Figure 1, Figure 2). A total of eight measurements were made at 5-min intervals to evaluate the repeatability of the procedure and the control of the error level in the individual measurement in each individual. This procedure consisted of measuring and repeating right masseter muscle contraction and relaxation, left masseter muscle contraction and relaxation, respectively.

Figure 1.

Ultrasonographic image of the masseter muscle at rest.

Figure 2.

Ultrasonographic image of the masseter muscle in contraction.

Masseter muscle was examined from origin to insertion. During imaging, the probe was held perpendicular to the masseter muscle and the anteroposterior thickness of the muscle was measured at its widest point in the transverse section. During the measurements, care was taken that the probe does not create pressure on the skin surface. While making measurements, care was taken that the probe does not create pressure on the skin surface.

To evaluate the sensitivity of ultrasonographic measurements, method error was calculated for right and left masseter muscle thickness parameters in resting and contracted states. As a result of the evaluation made using the Dahlberg method error formula, it was seen that the standard error calculated in muscle thickness measurements was very low, not exceeding 0.45 (3.1%) at contracted and 0.50 (3.9%) at rest.

Statistical analysis

Statistical analyzes of this study were performed using Statistical Package for Social Science (SPSS Inc, Chicago, IL) 18.0 for Windows software. In order to summarize the data obtained in the study, descriptive statistical techniques such as numbers, percentage distributions, mean, and standard deviation were used. Whether there was a difference between the categories of the categorical variables related to the data obtained at the proportional scale level was determined using the t test and analysis of variance (One-Way ANOVA). While the t-test was used to reveal the difference between variables consisting of two categories, analysis of variance was used to reveal the difference between variables consisting of more than two categories. Multiple comparisons were made using Tamhane T2 test or LSD (least significant difference) test depending on the acceptance or rejection of the assumption of homogeneity of variances according to Levene test results to evaluate which category/categories the difference determined by variance analysis was caused.

It was checked with the Kolmogorov–Smirnov test whether the data, for which variance analysis was performed, provided the assumption of normal distribution, and it was seen that they had a normal distribution.

Pearson correlation coefficient was used in calculating the presence and strength of the relationship between continuous variables. The in-class correlation coefficient between repeated measurements was calculated, and the harmony between these measurements was evaluated.

Method error was calculated with the help of Dahlberg formula for masseter muscle thickness parameters in right and left rest and muscle forms to evaluate the sensitivity of masseter muscle thickness measurements taken using ultrasound. The results were evaluated at a significance level of p < 0.05, with a 95% confidence interval.

Results

In our study, masseter muscle thickness of 115 individuals aged between 15 and 55 years (mean age 22.2 ± 8.7), 64 (55.7%) women, and 51 (44.3%) men, was examined by ultrasonography.

The mean thickness of the right masseter muscle at rest was 12.80 ± 2.72 mm, and the mean thickness in the contracted state was 14.39 ± 2.70 mm; for the left masseter muscle, these values were 12.70 ± 2.67 mm and 14.42 ± 2.63 mm, respectively. There was no statistical difference between the thicknesses of the right and left masseter muscles at rest and when they were contracted (p > 0.05). In addition, the mean value of the masseter muscle thickness was found to be 13.57 ± 2.57 mm regardless of right and left; resting, and contracted states.

When the thickness of the masseter muscle was examined between the genders, the muscle thickness at rest and in the contracted state for the right and left masseter muscles were significantly higher in males than in females (p < 0.05) (Table 1).

Table 1.

Masseter muscle thickness by gender

Masseter muscle	Gender	Masseter muscle thickness (mm)			p-value
		Right	Left	Mean
Rest	Female	12.31 ± 2.73	12.10 ± 2.59	12.20 ± 2.59	0.013*
	Male	13.40 ± 2.60	13.45 ± 5.58	13.42 ± 2.53
Contracted	Female	13.84 ± 2.63	13.76 ± 2.42	13.80 ± 2.45	0.005*
	Male	15.06 ± 2.64	15.24 ± 2.66	15.15 ± 2.56

*p<0.05

There was a significant relationship between muscle thickness and age when the right and left masseter muscles were at rest or contracted (p < 0.05); as age increases, muscle thickness also increased.

A significant correlation was found between BMI and muscle thickness when the right and left masseter muscles were at rest or contracted (p < 0.05). The BMI variable was classified according to the grouping determined by the World Health Organization in order to analyze how the BMI and the thickness of the right and left masseter muscles at rest or in contraction has changed [<18.5 (underweight), 18.5–24.9 (normal), 25–29.9 (overweight), 30< (obesity)]. As a result of the analysis, a statistically significant difference (p < 0.05) was found in terms of masseter muscle thickness between the groups. In individuals with a BMI of less than 18.5 (underweight), masseter muscle thickness was minimal, while in individuals with more than 30 (obesity), it was the most (Table 2).

Table 2.

Masseter muscle thickness according to BMI

BMI	<18.5	18.5–24.9	25–29.9	30<	Mean	p
N (%)	20 (17.4%)	63 (54.8%)	25 (21.7%)	7 (6.1%)
Right Masseter Rest	10.91 ± 1.53	12.61 ± 2.51	13.74 ± 2.72	16.42 ± 2.61	12.80 ± 2.72	0.00*
Right Masseter Contracted	12.45 ± 1.63	14.13 ± 2.50	15.48 ± 2.46	18.32 ± 2.16	14.39 ± 2.70	0.00*
Left Masseter Rest	10.66 ± 1.96	12.68 ± 2.53	13.57 ± 2.44	15.64 ± 2.44	12.70 ± 2.67	0.00*
Left Masseter Contracted	12.51 ± 1.85	14.30 ± 2.53	15.38 ± 2.15	17.55 ± 2.84	14.42 ± 2.63	0.00*

BMI, body mass index.

*p<0.05

Measurements were made on the lateral cephalometric radiographs of the individuals included in our study, and the individuals were classified into three groups as hypodivergent, hyperdivergent, and normdivergent according to their vertical facial morphology. Of the individuals, 28 (24.4%) were hypodivergent, 32 (27.8%) hyperdivergent, and 55 (47.8%) normdivergent. There were 64 (55.6%) women of which 11 (39.3%) were hypodivergent, 24 (75%) hyperdivergent, and 29 (52.7%) were normdivergent. There were 51 (44.4%) men of which 17 (60.7%) were hypodivergent, 8 (25%) hyperdivergent, and 26 (47.3%) were normdivergent. There was a statistically significant relationship between facial morphology and gender (p < 0.05).

There was a statistically significant difference in right masseter muscle thickness between the groups determined according to vertical facial morphologies (p < 0.05). In individuals with hypodivergent facial morphology, the thickness of the right masseter muscle was significantly different from those in other groups when the muscle was at rest and contracted (p < 0.05). There was no statistically significant difference between the groups determined for facial morphology in terms of left masseter muscle thickness. However, when the left masseter muscle was at rest and contracted, its thickness was higher in individuals with hypodivergent facial morphology than in the other group (Table 3).

Table 3.

Masseter muscle thickness and statistical analysis results according to facial morphologies

Masseter muscle thickness		Normdivergent (n:55)	Hypodivergent (n:28)	Hyperdivergent (n:32)	F	p
Right Masseter Rest	Female	12.21 ± 2.77	13.17 ± 2.19	12.04 ± 2.93
	Male	13.34 ± 2.17	14.23 ± 2.54	11.85 ± 2.53
	Total	12.75 ± 2.55A	13.81 ± 2.42B	11.99 ± 3.02A	3.493	0.034*
Right Masseter Contracted	Female	13.75 ± 2.78	14.53 ± 2.05	13.65 ± 2.72
	Male	14.98 ± 2.29	15.88 ± 2.48	13.59 ± 3.63
	Total	14.33 ± 2.61A	15.35 ± 2.37B	13.64 ± 2.91A	3.146	0.047*
Left Masseter Rest	Female	11.82 ± 2.75	12.91 ± 2.24	12.06 ± 2.59
	Male	13.63 ± 2.21	13.80 ± 2.84	12.13 ± 3.08
	Total	12.75 ± 2.55	13.45 ± 2.62	12.08 ± 2.67	2.007	0.139
Left Masseter Contracted	Female	13.47 ± 2.72	14.52 ± 1.99	13.76 ± 2.22
	Male	15.36 ± 2.49	15.41 ± 3.08	14.52 ± 2.46
	Total	14.37 ± 2.76	15.06 ± 2.70	13.95 ± 2.27	1.360	0.261

*p<0.05

The distribution of individuals according to their parafunctional habits and time of awareness is given in Table 4.

Table 4.

Distribution of individuals according to their awareness of parafunctional habits

Question	Absence N(%)	<1 year N(%)	1–5 years N(%)	5 years< N(%)	Total
Do you chew unilaterally?	72 (62.6%)	10 (8.7%)	11 (9.6%)	22 (19.1%)	115 (100%)
Do you grind your teeth during the day?	77 (67%)	8 (7%)	19 (16.5%)	11 (9.6%)	115 (100%)
Do you grind your teeth while sleeping at night?	90 (78.3%)	6 (5.2%)	8 (7%)	11 (9.6%)	115 (100%)
Do you have a habit of grinding your teeth?	101 (87.8%)	2 (1.7%)	6 (5.2%)	6 (5.2%)	115 (100%)
Do you have a habit of nail biting?	93 (80.9%)	2 (1.7%)	5 (4.3%)	15 (13%)	115 (100%)
Do you have a habit of biting your tongue?	106 (90.2%)	3 (2.6%)	3 (2.6%)	3 (2.6%)	115 (100%)
Do you have a habit of biting your lips?	78 (67.8%)	8 (7%)	16 (13.9%)	13 (11.3%)	115 (100%)
Do you have a habit of biting objects (pencils etc.)?	92 (80%)	5 (4.3%)	7 (6.1%)	11 (9.6%)	115 (100%)

Individuals with parafunctional habits were divided into three groups according to their awareness of these habits. Individuals with more than one parafunctional habit were included in the grouping considering the maximum time period. Those without parafunctional habits were also considered as Group 4. There was no statistical difference between the groups in terms of masseter muscle thickness (p > 0.05) (Table 5).

Table 5.

Masseter muscle thickness according to parafunctional habits

Parafunctional habits	Absence	Presence			p
		<1 year	1–5 years	5 years<
Right Masseter Rest	13.42 ± 2.67	13.03 ± 2.68	12.21 ± 3.03	12.68 ± 2.62	0.468
Right Masseter Contracted	14.86 ± 2.90	14.63 ± 2.26	13.76 ± 2.92	14.36 ± 2.63	0.547
Left Masseter Rest	13.34 ± 2.62	13.36 ± 2.44	11.92 ± 2.99	12.55 ± 2.56	0.215
Left Masseter Contracted	15.03 ± 2.97	14.89 ± 2.47	13.69 ± 2.88	14.31 ± 2.36	0.302

*p<0.05

It was evaluated whether there was a statistical difference between the right and left masseter muscle thicknesses of the individuals who chewed unilaterally when they were at rest and in a contracted state. There was no statistical difference between the mean of right masseter muscle thickness and left masseter muscle thickness at rest and contracted state in individuals who chew unilaterally with the right side (p > 0.05). However, the mean right masseter muscle thickness was higher than the left masseter muscle at rest and in contraction. Similarly, no statistically significant difference was found in individuals chewing with the left side (p > 0.05), but the mean left masseter muscle thickness was higher than the right masseter muscle at rest and in the contracted state.

The number of individuals with bruxism was 55 (47.8%), while those without bruxism were 60 (52.2%). Only 15 (27.3%) of the individuals with bruxism had no parafunctional habits other than bruxism. The other 40 (72.7%) individuals had other parafunctional habits, including at least one, together with bruxism. To evaluate the masseter muscle thickness in terms of bruxism, four groups were formed: with only bruxism, with parafunctional habits other than bruxism, with parafunctional habits including bruxism, and without parafunctional habits. No statistically significant difference was found between the groups in terms of masseter muscle thickness (p > 0.05) (Table 6).

Table 6.

Masseter muscle thickness between groups according to bruxism

Bruxism	Only bruxism (N:15)	Parafunctional habits other than bruxism (N:35)	Parafunctional habits including bruxism (N:40)	Without parafunctional habits (N:25)	p
Right Masseter Rest	12.56 ± 2.40	12.82 ± 2.94	12.47 ± 2.68	13.42 ± 2.67	0.576
Right Masseter Contracted	14.37 ± 2.21	14.36 ± 2.94	14.11 ± 2.55	14.86 ± 2.90	0.761
Left Masseter Rest	12.56 ± 2.63	12.87 ± 2.56	12.19 ± 2.79	13.34 ± 2.62	0.383
Left Masseter Contracted	14.21 ± 2.75	14.49 ± 2.36	13.91 ± 2.82	15.03 ± 2.97	0.529

*p<0.05

Discussion

Masticatory muscle activities are thought to have a systematic effect on craniofacial morphology. ^{2,5,8,9,17,22,23} Muscle thickness is also evaluated as an indicator of the activities of the masticatory muscles. ¹⁷ According to Wolf’s law, the bone-forming skeletal structure develops and remodels in accordance with the genetic model. This development and remodeling take place thanks to the electromagnetic field created by the stress forces around the long axis of the bone. ^1,8,18 The masseter is the muscle most associated with facial morphology in research on the effect of facial muscles on craniofacial morphology. ^8–11,13

In addition to the effect of the masseter muscle, one of the masticatory muscles on facial morphology, the increase in thickness seen in this muscle also causes aesthetic anxiety in patients recently. The change in facial appearance, especially as a result of masseter hypertrophy, is one of the causes of complaints in patients, and various treatment methods related to muscle thickness are applied for this. ²⁴ In addition, it is common to think that relapses are seen due to masseter muscle activity after treatment in individuals treated orthodontically due to malocclusion and individuals treated with orthognathic surgery. ^25,26 For these reasons, masseter muscle has an important place in diagnosis and treatment planning in various fields of medicine and dentistry. Considering this situation, in this study, muscle thickness was measured to determine masseter muscle activity, and the relationship of masseter muscle thickness with facial morphology, parafunctional habits, and BMI was evaluated.

Depending on the developing technology, masseter muscle thickness can be measured with various imaging methods. These imaging modalities are CT, MRI, and ultrasonography. CT was used by Weijs and Hillen ²² to measure masseter muscle thickness on 16 adult volunteers. However, due to ethical reasons, the use of CT is limited nowadays. MRI and ultrasonography are more commonly used imaging modalities to measure masseter muscle thickness. ^3,9 MRI has disadvantages such as long examination time, expensive equipment and materials, inability to obtain images in cases such as claustrophobia, sensitivity to motion, and therefore a high probability of artifact formation. ²⁷ Because of these disadvantages, the clinical use of MRI for the measurement of muscle thickness does not seem practical. Ultrasonography has advantages, such as ease of use, better acceptability in terms of patient comfort, obtaining a large number of images in a short time, and not having a cumulative effect. ²⁷ Because of these advantages, the use of ultrasonography was preferred for the measurement and evaluation of masseter muscle thickness in this study. Ultrasonography and MRI were evaluated in terms of determining muscle thickness when compared in various human and animal studies. It has been reported that there was no significant differences between the two imaging modalities. ^28,29 Ultrasonography has been used in many studies to measure masseter muscle thickness. ^{8,9,12,17,30–33} In addition, it was stated that ultrasonography was a repeatable, simple and inexpensive method to accurately measure muscle thickness, provided that the operator adhered to the imaging protocol. ¹⁷ In the previous studies, it was emphasized that the ultrasonography method was quite reliable in the determination of masseter muscle thickness. ^{8,9,12,17,30–33} In this study, the repeatability of masseter muscle thickness measurement of 115 patients was found to be statistically significant in line with these studies. Kiliaridis and Kalebo ⁹ stated that the repeatability method error of masseter muscle thickness was 7.1% in the resting state and 4% in the contracted state in the measurements they made on the ultrasonography images obtained with a 7 MHz probe. Bakke et al ¹⁰ indicated that the repeatability method error of masseter muscle thickness was between 3 and 7% in the measurement they performed ultrasonography imaging with a 7.5 MHz probe. Kubota et al, ² on the other hand, found this margin of error as 3.2–3.6% in the measurements they made on the ultrasonographic images obtained with a 7.5 MHz probe. In this study, the repeatability method error of masseter muscle thickness in the measurement on the ultrasonography images obtained using a 13-6 MHz probe was at a very low level, consistent with previous studies.

Determination of masseter muscle thickness by ultrasonography has been the subject of many studies. ^{2,8–12,17,34} Bakke et al ¹⁰ stated in their study in which they measured masseter muscle thickness from different parts of the muscle, and stated that the maximum muscle thickness was 11.58 ± 1.2 mm at rest and 13.34 ± 1.46 mm when contracted. Kubota et al ² reported masseter muscle thickness as 15.8 ± 3 mm at rest and 16.7 ± 2.7 mm when contracted. In the present study, the mean masseter muscle thickness was 12.75 ± 2.63 mm at rest and 14.48 ± 2.59 mm at contracted. The reason for the differences in these studies may be that each study included individuals from different populations and numbers.

In previous studies, they stated that the masseter muscle thickness was higher in males than in females. ^9,11,34 In addition, Park et al, ³³ when they evaluated the right and left masseter muscle thicknesses in terms of genders in their research, they stated that the thickness of both right and left masseter muscles at rest and in contraction was significantly less in females than in males, which supports this study. The reason for the difference in muscle thickness between the sexes can be explained by the type, size, and the number of fibers in the muscle structure. Tuxen et al ³⁵ in their study stated that Type I and IM fibers in female masseter muscle were more numerous and formed a larger cross-sectional area, while in male masseter muscle, Type II fibers formed a larger cross-sectional area than female masseter muscle. In addition, the type, size and number of fibers that made up skeletal muscles could be affected by various factors. These factors were physical activity, ³⁶ genetic factors, ³⁷ and sex hormones. ³⁸ More studies can be done on this subject in the future since there were not enough studies on which of these factors affected masseter muscle thickness more.

In this study, there was a similar but weak relationship between muscle thickness and age when the right and left masseter muscles were at rest or contracted. Newton ^39,40 stated in two studies with CT that there were a strong inverse correlation between age and masseter muscle thickness, but individuals were included in these studies without distinguishing with or without teeth. On the contrary, Kiliaridis and Kalebo ⁹ argued that there was no relationship between age and masseter muscle thickness. The differences between this study and other studies may be due to different age ranges and differences in dentition. In studies on loss of muscle mass and decrease in activity, it was stated that the loss in muscle mass and decrease in activity were 5–13% between the ages of 60–70, and 11–50% over the age of 80. ⁴¹

Satıroğlu et al ⁸ argued that there was a significant relationship between masseter muscle thickness and BMI. The finding of this study is consistent with the previous study. However, in this previous study, only the relationship between masseter muscle and BMI was examined without BMI classification. As far as we know, there is no other literature on this subject. Some research evaluated the relationship of masseter muscle thickness with height and weight and found no relationship between masseter muscle thickness and these two variables. ^9,33 In addition, Raadsheer et al ¹¹ stated that both weight and height and masseter muscle thickness were associated, and that masseter muscle thickness decreased with the reduction of these two variables. The difference between the studies may be related to the dietary habits, number of individuals, and ethnicity of the individuals included in the study. However, no research has been found in the literature on nutritional habits and masseter muscle thickness or activity.

The relationship between the thickness of masticatory muscles and facial morphology has been studied by many researchers. ^2,8–11,18 In some studies, the relationship between the thickness of the masticatory muscles and various parameters that show the vertical face dimensions without dividing the vertical face morphology into groups has been investigated. ^2,9–12 Some studies have shown an inverse correlation between the gonial angle and the SN/mandibular plane (MP) angle with the thickness of the masseter muscle. ^2,10 In some studies, it has been observed that there is an negative correlation between anterior face height and masseter muscle thickness. ^9,11 In another study, it has been stated that there is a possitive correlation between posterior face height, Jarabak ratio and thickness of the masseter muscle. ¹² All these studies showed that the masseter muscle thickness was thicker in individuals with a vertically shorter face and thinner in individuals with a longer face. In studies that divide vertical facial morphology into groups, ^8,17,18 similarly and in line with this study, it has been stated that masseter muscle thickness was highest in individuals with hypodivergent facial structure and least in individuals with hyperdivergent facial morphology. The possible explanation for the relationship between vertical facial morphology and masseter muscle thickness was thought to be related to the weak forces produced by passive stretching of the masseter muscle to affect skeletal growth pattern and tooth eruption. ⁴² These weak forces, produced by passive stretching of hypofunctional muscles, may result in greater eruption of the maxillary molars and less inhibition of periosteal bone apposition in the angular region, resulting in vertical growth in individuals, and thus hyperdivergent facial morphology. ⁴² Contrary to this situation, it has been stated in the literature that there was no relationship between the masseter muscle and facial morphology. ⁴³ The reason for this variation may be due to technical, racial, and ethnic differences.

In this study, individuals with parafunctional habits were divided into groups according to the time they were aware of these habits and those who did not have any parafunctional habits. There was no statistically significant difference in masseter muscle thicknesses between these groups. As far as we know, there is no previous study examining masseter muscle thickness by forming groups according to the time of parafunctional habits. However, there were studies examining the relationship between masseter muscle thickness or activity and parafunctional habits. ^44,45 These studies stated that there was no statistical relationship between masseter muscle activity or muscle thickness and parafunctional habits. ^44,45

In a study on rats, unilateral teeth were extracted, and when the masseter and temporal muscles volume in the toothed and edentulous region was evaluated 6 weeks and 12 weeks after the extraction, the masseter muscle volume was statistically significantly less than the muscle volume on the toothed side due to the inability to chew on the extraction side. ⁴⁶ In the same study, it was emphasized that there was no difference between the right and left muscle thicknesses of the rats whose teeth were not extracted in the control group. ⁴⁶ In another study evaluating the masseter and temporal muscle activity in unilateral posterior edentulous, bilateral posterior edentulous, fully edentulous patients, it was stated that the masseter muscle activity of unilateral edentulous individuals increased after prosthesis was made, but was lower than the toothed side. ⁴⁷ It was emphasized that masseter muscle activity was approaching normal as the adaptation of the prosthesis side increased over time after the prosthesis was made, and double-sided chewing was ensured. ⁴⁷ It has been known that masseter muscle thickness is compatible with muscle activity, and in this study, the fact that the masseter muscle thickness on the one side used by individuals chewing on one side was thicker than the other side was an indication that the chewing side was more active. Unlike other studies, the reason why we could not find a difference between the two sides in our study may be due to the absence of posterior tooth deficiencies in the patients.

There were very few studies examining the relationship between bruxism and masseter muscle thickness. ^44,48 In a study on the comparison of masseter muscle activity and the thickness of individuals with and without nocturnal bruxism, no statistically significant difference was found between those with and without bruxism in terms of masseter muscle thickness. ⁴⁴ The report of the previous study was consistent with the present study. As can be understood from these studies, the reason for the increase in muscle thickness may not be due to bruxism. Masticatory muscle hyperactivity or parafunctions could not be confirmed in all cases of hypertrophy. However, masticatory muscle hyperactivity or parafunctions could be identified as a possible cause. ⁴⁹ There was also a study stating that the masseter muscle was thicker in individuals with bruxism than in individuals without bruxism. ⁴⁸ More research is needed on this topic.

Conclusion

There were several factors that affect masseter muscle thickness. In the results of this study, masseter muscle thickness varies according to vertical facial morphology, but this was not the case for parafunctional habits. However, different results can be achieved by increasing the sample size. In Dentomaxillofacial Radiology, it is known that the etiology behind every dental and facial ailment is important, and it is argued that the cause must be eliminated first in order to ensure the continuity of the treatment. Therefore, more research is needed on the factors that cause hypertrophy in the masseter.