Surface Electromyographic Analysis of Masticatory and Neck Muscles in Skeletal Class II Division 1 Malocclusion with EMG Indices - A In Vivo Study

Introduction

Mandibular muscles constitute the main biomechanical challenge of the skull through their action on the bony appendages, the teeth, and the mandibular joint load. Interactions within and between masticatory muscles draw attention to whether and how mastication is impaired if a muscle or even a single muscle compartment is unfit.¹ Electromyography (EMG) is the most objective and reliable tool to assess the function and efficiency of muscles by monitoring their electrical potential. This allows the range and duration of muscle activity to be calibrated. One type of EMG is surface electromyography (sEMG), which uses surface electrodes and detects superimposed motor unit action potentials from fibers, as opposed to single action potentials recorded from of intramuscular EMG.² Several features are associated with Class II skeletal malocclusions, including Angle's Class II molar relation, increased ANB angle, and increased overjet. Due to the important role of the muscles of mastication, there is still controversy over the effectiveness and recurrence of Class II malocclusions. The muscles of mastication have been reported to be involved in Class II malocclusion. Surface EMG of head and neck musculature has been reported as a valid objective method for the functional assessment of oromandibular apparatus and neck with good reproducibility and reliability when a standardized protocol is used. In the clinical situation, the simultaneous evaluation of the jaw and neck muscles contributes to a better understanding of the patient’s problems, allowing the individualization of changes in neuromuscular coordination, as well as a quantitative follow-up of the effects of treatment.³ This is usually done using the EMG index, which allows for better quantification and reproducibility. Having an insight into all these parameters with respect to musculature, surface EMG, and malocclusion, we have put forward this study with a main aim of evaluating the activity of oro-facial and neck musculature in skeletal classdivision 1 malocclusion using surface electromyography in horizontal and vertical growth patterns.

Aims and Objectives

The aim of the study was to evaluate and compare the activity of oro-facial and neck musculature in skeletal class II division 1 malocclusion subjects having horizontal and vertical growth patterns using surface electromyography and to co-relate the muscle activity with electromyographic indices.

Inclusion criteria

1. Non-growing subjects comprising males and females between the age group of 18 - 30 years requiring orthodontic treatment.
2. Skeletal class II malocclusion with horizontal and vertical growth patterns. 
3. Angle’s class II molar relation with increased overjet and crowding.

Exclusion criteria

1. Patients with congenitally or clinically missing permanent first molar. 
2. Patients with prosthetic replacement.
3. Patients with temporomandibular joint problems.
4. Patients with crossbite and scissor–bite.
5. Patients with craniofacial syndromes, neurological problems, and medically compromised patients.

Materials and Methodology

Materials

A total of twenty non-growing patients comprising males and females in the age group 18-30 years with skeletal class II malocclusion showing horizontal and vertical growth patterns were selected from the Department of Orthodontics and Dentofacial Orthopaedics.

Methodology

Lateral cephalograms of the subjects were taken with a film/focus distance of 1.50 meters and a cathode voltage of 75 kV. All lateral cephalograms were traced and measured manually by a single investigator under standardized conditions. The cephalometric measurements were traced according to Bjork, Jarabak & Down’s and Steiner’s analyses (Figure 1).

The given sample was divided into two groups based on Bjork sum values, Jarabak ratio and Mandibular plane angle.

Jarabak’s Ratio

Posterior facial height /Anterior facial height x 100

A value below 62% indicates vertical growth pattern, and that exceeding 65% indicates a horizontal growth pattern.

Y Axis (Growth Axis)

This angle is obtained by joining Sella-Gnathion plane with the Frankfort horizontal plane. The mean value is 59º with a range of 53 to 66 degrees. If the angle is greater than normal, it represents a vertical growth pattern and if the angle is lesser than normal, it indicates a horizontal growth pattern.

Mandibular Plane Angle

According to Steiner’s

The angle formed between SN plane and mandibular plane. Mandibular plane is the line connecting Gonion and Gnathion. The average value is 32º. This angle gives an indication of the growth pattern of an individual. A lower angle is indicative of horizontal growth pattern while an increased angle is suggestive of a vertical growth pattern.

According to Down’s

The mandibular plane angle is obtained by intersecting the mandibular plane (Gonion to Menton) with the Frankfort horizontal plane. The mean value is 21.9º while the range is between 17º to 28º. An increased mandibular plane angle infers a vertical grower with a hyper-divergent facial pattern.

Group I (10 subjects): Skeletal class II malocclusion subjects with horizontal facial growth pattern (Bjork sum less than 390 degrees).

Group II (10 subjects): Skeletal class II malocclusion subjects with vertical growth pattern (Bjork sum more than 402 degrees).

The electromyographic recordings were performed using BioPAC (BioEMG III) machine by two operators as seen in Figure 2. The electrodes were placed parallel to the long axis of the muscle fibers as shown in Figure 3. The subjects were made to sit on a dental chair with their heads unsupported and were asked to maintain a natural head position as shown in Figure 4.

Standardization of Recording

To standardize the surface electromyography, potentials of the four muscles (masseter, anterior temporalis, anterior belly of digastric, sternocleidomastoid) with tooth contact, two 10- mm-thick cotton rolls were positioned on the mandibular first and second molars of each subject, and a 5-second maximum voluntary clench was recorded. During this test, the subject was asked to avoid head and neck movements and to remain in a neutral position as shown in Figure 5.

Sample Size Estimation

SPSS (Statistical Package for Social Sciences) version 20. [IBM SPASS statistics (IBM corp. Armonk, NY, USA released 2011)] was used to perform the statistical analysis. Descriptive statistics of the explanatory and outcome variables were calculated by the mean, standard deviation for quantitative variables, and frequency and proportions for qualitative variables.

Inferential statistics like the independent sample t-test were applied to check the statistical difference in masticatory and neck muscle activity between the groups. • The level of significance was set at 5%

• N=(r+1) (Zoe/2 + Z1-ß)2 (σ)2 / rd2

• Where r =1(n1/ n2=1);

• Zoe/2 = 1.96 (p value at significance level at 0.05)

• Z1-ß = 1.64 for 95% power

• σ = 5 (pooled standard deviation)*

• d is the mean difference (63 – 69 = -6)*

• Substituting the above values to the formula,

• N= (1+1) (1.96+1.64)2 (5)2/ (1) (-6)2

• N= 17 ≈ 20 (10 in each group)

Data analysis

All EMG recordings were interpreted using the EMG indices.

POC (Percentage Overlap Coefficient)

Analyzes the asymmetry of left and right-related muscles during static and dynamic tasks. If the muscle contracts in perfect symmetry, a POC as high as 100% is expected. This coefficient can be calculated for different pairs of muscles: masseter POC (MR/ML); temporalis POC (TR/TL); Mean POC (MR + TR)/(ML + TL).

Asymmetry Index

This index compares right muscle activity to left muscle activity. Positive values indicate right muscle activity, while negative values indicate left muscle activity (ASY = [TR + MR] − [TL + ML]).

Activation Index

It compares the effect of dental contact on the activity of the masseter and temporal muscles. It quantifies the differential activity between the temporal and masseter muscles (ACT = [MR + MS] − [TR + TS]). Generally, higher masseter activity indicates a good inter arch relationship, while the presence of temporal muscle contraction indicates an attempt to achieve correspondence between the upper and lower arches. Positive values were in association with higher masseter muscle contraction, while negative values were associated with higher temporal muscle activity.

Torque Index

It evaluates the presence of abnormal late influences on the mandible during the test given by unbalanced pairs TR and ML and TL and MR. It compares the activity of pairs of musculature (TC = [TR + ML] − [TR + ML]). The appearance of right or left temporal muscle activity generates a torque that indicates the right or left mandibular deviation respectively. Positive values were associated with higher right temporal muscle contraction, while negative values were associated with increased left temporal muscle activity.

Results

Mann Whitney test was performed to compare different indices in horizontal and vertical growth pattern subjects. The level of significance was set at 0.05.

Intra class correlation coefficient was tested and the value recorded was 0.83 suggesting no variation between the two observers.

The percentage overlapping coefficient was calculated for both horizontal and vertical growth patterns to quantify the symmetry between the masseter and anterior temporalis group of muscles on the right and left sides and it was observed that the POC was 112.3% in horizontal and 92.35% in vertical growth patterns. It may be inferred that there is an increased symmetry between muscles in the horizontal growth pattern when compared to the vertical growth pattern. The values were statistically significant with a p value of 0.02 as seen in Table 1.

The activity index demonstrates the proportion between two muscle groups. AcI values are established within the range of -100 to +100. Negative (-) values signify the prevalence of the anterior temporalis muscle. Conversely, positive (+) values indicate an advantage of the masseter muscle.

Masticatory muscle symmetry was assessed using asymmetry indices, which range from -100 to +100. Negative (-) values indicate a greater activity of muscles on the left side. Positive (+) values imply an advantage of muscles on the right side of the craniofacial complex. Both indices (Asymmetry and Activation indices), nearing 0, indicate balanced and equal bioelectrical engagement of the masseter and anterior temporalis muscles.

The activity index for horizontal growth pattern subjects was 14.28 µV/sec indicating a predominance of the masseter group of muscles. The activity index for the vertical growth pattern was -8.94 µV/sec indicating a predominance of the anterior group of temporalis muscles. The results were statistically significant with a p value of 0.01 as seen in Table 2.

The asymmetry index for the horizontal growth pattern observed was 17.75 µV/sec, indicating the predominance of right-side muscles compared to the left-side. The asymmetry index for the vertical growth pattern group of muscles was -10.77 µV/sec showing the increased dominance of left-side muscles. Both the values were statistically significant with a p value of 0.03 as seen in table 3.

Since an unbalanced contractile activity of the contralateral masseter and temporalis muscles can give rise to a potential lateral displacing component, the Torque Coefficient (TC%) was assessed. TC ranges between 0% (complete presence of lateral displacing force) and 100% (complete absence of lateral displacing force).⁶

The torque index in horizontal and vertical growth patterns were 24.56 and -43.76, respectively inferring a right-sided mandibular movement in the horizontal growth pattern and left-sided mandibular movement in the vertical growers. Hence the results were statistically significant with a p value of 0.008 as seen in Table 4.

The average value of the sternocleidomastoid muscle was interpreted and it was observed to be 18.74 µV/sec in the horizontal growth pattern and 14.15 µV/sec in the vertical growth pattern with a p value of 0.97 which was statistically insignificant as seen in Table 5. This observation has not been discussed in previous studies conducted.

The mean value of the anterior belly of digastric muscles which represents the group of infrahyoid group muscles was 18.87 µV/sec in horizontal pattern and 19.57 µV/sec in vertical growth pattern subjects. The p value was 0.79 which was statistically insignificant as seen in Table 6.

Discussion

Muscles play an important role in various activities related to the maxilla and mandible. They are further divided into primary and secondary masticatory muscles. From an evolutionary perspective, masticatory muscles have adapted tooth morphology and jaw movement to generate species-specific chewing patterns. In the physiological sense, the muscles of mastication are the main load on the skull, affecting the growth, development, and control of the skull.¹ In our study, masseter, temporalis, sternocleidomastoid muscles, and anterior belly of digastric were evaluated in skeletal class II division 1 malocclusion subjects having horizontal and vertical growth patterns as represented in Table 7.

Surface electromyography (sEMG) uses surface electrodes and detects superimposed motor unit action potential. BIO-PAC-III (Surface EMG machine) was used in our study to evaluate the muscle activity in skeletal class II division 1 malocclusion samples with horizontal and vertical growth patterns.

Growth and development are important in planning orthodontic treatment. Age, therefore, plays an important role and must be considered when assessing muscle activity. The 24-h sEMG recordings described by Ueda et al. ² demonstrated a longer duration of temporal and masseter activity in adults, which has been attributed to hypoplastic dentition and temporomandibular joints, as well as immaturity.² Given this, our study criteria included adult patients over the age of 18 years without temporomandibular joint problems and other muscle disorientation disorders.

Class II, Division 1 malocclusion represents the most common skeletal malocclusion encountered by orthodontists and requires a variety of diagnostic aids, both essential and non-essential. Significant controversy remains in the clinical literature regarding the likely cause of recurrence in such cases.⁴ Moreno et al. ⁵ determined the effect of sagittal malocclusion on the electrical activity of masticatory muscles and noted that Angle class II patients exhibited higher activity of temporal muscles during mastication, while class III subjects achieved maximal activity of the anterior temporalis and masseter muscles.⁵ Based on the above studies, this study assessed the association of skeletal class II division 1 malocclusion with muscle activity, which was further subdivided into horizontal and vertical growth patterns. Quantitative EMG analysis using indices allowed reliable and accurate assessment of muscle electrical activity in our study to overcome shortcomings of surface EMG, such as impedance that affects the accuracy and precision of measurements. Therefore, to extend the quantitative EMG analysis, we included all static and dynamic activities, EMG measurements such as activity Index (Ac), asymmetry Index (percentage overlap coefficient - POC), and the torque coefficients (To,Tc). They enable the evaluation of the activity, coordination, and symmetry among muscles that are interconnected, work together (synergistic), or oppose each other (antagonistic). The percentage overlapping coefficient values showed a negative correlation between the symmetry of the muscles and vertical growth pattern which co-relates with increased bite force and larger alveolar bases during maximal intercuspation in horizontal growers compared to vertical growers.⁶

Throckmorton et al.⁷ demonstrated that when the mandibular ramus is in an upright position and the gonial angle is relatively acute, the elevator muscles of the mandible experience an increased mechanical advantage. As the gonial angle increases, the mechanical advantage of the musculature decreases, and an equivalent effort by the muscle would produce less force at the dental occlusion.⁷ It can be inferred that symmetry between muscles increases the vertical growth pattern compared to the horizontal growers and the value is statistically significant with a p value of 0.02, a result consistent with previous studies. The activity index, one of the proposed parameters, shows the relative value of the activity of the center of the masseter muscle compared to the activity of the anterior part of the temporalis muscle, as a suitable muscle. The activity index⁸ value indicates increased masseter muscle activity in horizontal growers in our study, which is inconsistent with the findings of Alabdullah et al.,⁶ Ingervall,9 Tomiyama et al.¹⁰ and Yousefzadeh et al.¹¹ who deduced that horizontal growers have reduced masseter activity, but in concordance with the findings of Ueda et al.¹² The elevated muscle activity can be attributed to the aligned orientation of both the upper and lower jaw bases. This alignment enhances the biting mechanism. Additionally, the muscles in individuals with shorter facial structures exhibit greater thickness compared to those with longer facial structures. This disparity in muscle thickness could potentially account for the heightened muscle activity observed in this particular study.

The present study showed an increased activity of anterior temporalis in vertical growth pattern which translates to increased activity of elevator group of muscles and weaker activity of clenching group of muscles.

Gisele et al.¹³ reported no difference between temporoman dibular disorder (TMD) patients and healthy controls when asymmetry was observed at rest, and it was found that individuals with good health with a clear temporalis muscle dominance are displayed in relation to the masseter muscle. However, this prevalence does not exist while analyzing asymmetry during maximum voluntary contraction. Additionally, dominance between the sides in the same patient was found in both the groups. In our study, the asymmetry index for horizontal growth pattern observed was 17.75 µV/sec, indicating the predominance of right-side muscles compared to the left side. The asymmetry index for vertical growth pattern group of muscles was -10.77 µV/sec showing increased dominance of left side muscles.

The electromyographic (EMG) signals from the masseter and anterior temporalis muscles on both the left and right sides of each participant in the test were subsequently examined. This analysis aimed to determine whether there was a potential lateral deviation effect on the jaw due to the imbalanced couples of transverse (TR) and medial-lateral (ML), as well as the torsional (TL) and medio-lateral (MR) forces applied during the test. The torque coefficient was calculated in our study and it inferred that there is a right-sided mandibular movement in horizontal growth pattern and left sided mandibular movement in vertical growth pattern.

The activity of sternocleidomastoid muscles was assessed at maximum voluntary contraction and it was noted that the mean value was 18.74 µV/sec in horizontal growth pattern and 14.15 µV/sec in vertical growers which was statistically insignificant. These observations were not discussed in previous studies conducted.

The mean values of anterior belly of digastric muscles which represents the group of infrahyoid group muscles was 18.87 µV/sec in horizontal pattern and 19.57 µV/sec in vertical growers which was statistically insignificant. These observations were not interpreted in the previous studies.

Conclusion

1. The horizontal growth pattern displayed symmetry among muscles as opposed to the vertical growth pattern.
2. In individuals with horizontal growth pattern, there was predominance of masseter muscle activity, whereas those with vertical growth pattern exhibited greater activity of anterior temporalis muscle.
3. There was increased activity of right sided muscles in horizontal growth pattern while an increased activity of left sided muscles was noted in vertical growth pattern subjects.
4. There was right sided mandibular movement in horizontally growing subjects and left sided mandibular movements in vertically growing subjects.

Conflict of interest

None