Effects of Bite Opening on Masseter and Geniohyoid Muscles: A Randomised Clinical Trial

Objective: To evaluate the effects of anterior bite plane on the masseter and geniohyoid muscle thickness. Materials and Methods: 14 subjects who needed bite opening were allocated as a single group with mean age of 17.4± 3.4 years and mean overbite of 5.3±0.2 was treated with a fixed anterior bite plane (ABT). The pre-treatment (T1) and post-treatment results (T2) was compared to study the effect of Bite opening. The ultrasonographic imaging was used to evaluate masseter muscle thickness (clenched and relaxed) and geniohyoid muscle thickness. Results: The study showed, the right masseter muscle thickness (RMT) in (R) at T1 was 8.68 ± 1.13 mm, T2 was 7.68 ± 1.14 mm and in (C) 0.72 ± 1.39 mm and 9.86 ± 1.35 mm respectively While left muscle thickness (LMT) was at 8.54 ± 1.3 mm and 7.68 ± 1.3 mm respectively. The mean geniohyoid muscle thickness before treatment was 6.58 ±0.69 mm and after treatment was 7.40 ± 0.69 mm with an increase in thickness of 0.8 ±0.04 mm. Conclusion: Bite opening procedure influence the muscles thickness, with reduction of masseter muscle thickness and increase in geniohyoid muscle thickness. Original Research Article Abiraj et al.; JPRI, 34(25B): 30-38, 2022; Article no.JPRI.85090 31


INTRODUCTION
Deep overbite is considered to be a common malocclusion which refers to the increased overlap of maxillary anteriors over mandibular anteriors beyond 30-40% [1]. In orthodontics, different techniques are used for deep bite correction. Treatment must be carefully planned for each patient, based on the etiology of malocclusion.
Relapse occurs when no accurate identification of the etiologic factors is performed [2].
Management of deepbite becomes more difficult with the existence of, or the increased severity of, an underlying skeletal discrepancy. Nonsurgical correction of a deep bite includes molar extrusion, incisor intrusion, or a combination of both, [3,4] with a general understanding that intrusion of teeth is more difficult than extrusion.
Common method used to correct deep bite includes the use of anterior bite plane [5] that may affect the various muscles of the jaw. An increase in the vertical dimension may lead to some changes in the orofacial structures. It is stated that such changes in vertical dimension alter the length of the main jaw elevator muscles and the position of the mandibular head in the fossa temporalis. Thus, they may affect the masticatory function, resulting in the bite force values [6].
A study by Lindauer et al. stated that the changes in vertical jaw opening affect the relative contributions of masticatory muscles for bite force production. When bite force was consistent, electromyographic activity increased per unit of force production was relatively high at the smaller degrees of jaw opening [7,8,9].
Various methods could be used to study the activity of muscles [10]. One of the methods is the use of ultrasound scanning. This method enables dynamic visualization of the muscles of the head and neck [11,12]. This method is also considered to be the accurate and rapid method for measuring the thickness of muscles, such as the masseter and geniohyoid, without any known adverse effects when compared to other methods like Computed Tomography and Magnetic Resonance Imaging [13,14].
So far no studies had been reported in the literature which evaluated the effect of bite opening using anterior bite plane, on the thickness of masseter and geniohyoid muscles.
Hence this prospective clinical study was taken up to investigate the outcomes of anterior bite plane on masseter and geniohyoid muscle thickness.

MATERIALS AND METHODS
Data were collected from subjects who underwent fixed orthodontic treatment based on the inclusion criteria such as subjects in the age group of 16-24 years with a minimum of 50 percent of anterior deep bite. All subjects should be either horizontal to average skeletal pattern and also patients who required bite opening by extrusion of posteriors will be indicated as a part of their treatment plan. 14 subjects were allocated as a single group who were treated with fixed anterior bite plane (ABT).

Ultrasonographic Method
Patients were examined by using an Ultrasound scanner (Esaote MyLab Seven, Genova, Italy). a. Masseter muscle area: The measurement was done at the thickest part of the masseter, close to the level of the occlusal plane, approximately in the middle of the mediolateral distance of the ramus. Imaging and measurements were performed bilaterally with the subjects in a supine position under two different conditions, when the teeth oocclude gently with the muscle in a relaxed position and during maximal clenching, with the masseter muscle contracted. The thickness of the masseter muscle was evaluated before and after deep bite correction [15,16] (Fig. 1).  [17,18]. The thickness of the geniohyoid muscle was evaluated before and after deep bite correction (Fig. 2).

Statistical Analysis
Inferential statistics included Paired t-test to check pre-operative and post-operative values. IBM SPSS-20 (IBM Company, Palo Alto, California, US) was used for the analyses of data at a significance level of P<0.05. Table 1, Table 2 shows the effect of anterior bite plane (ABT) on masseter and geniohyoid muscle thickness.

RESULTS
When the overall change in the muscle thickness before and after treatment in the relaxed position was compared, the mean right masseter muscle thickness at T1 was 8.39 ± 0.85 mm and at T2 was 7.75 ± 0.81 mm which was statistically significant with a P-value of 0.000 (Graph 1).
When the overall change in the muscle thickness before and after treatment in relaxed position was compared, the mean left masseter muscle thickness at T1 was 8.57 ± 1.04 mm and at T2 was 8.00 ± 1.16 mm which was statistically significant with a P-value of 0.000. Between the groups, with more reduction in Group 1 (Graph 2).
When the overall change in the muscle thickness before and after treatment in clenched position was compared, the mean right masseter muscle thickness before treatment was 10.74 ± 1.15 mm and after treatment was 10.17± 1.18 mm which was statistically significant with P-value of 0.000 (Graph 3).
When the overall change in the muscle thickness before and after treatment in clenched position was compared, the mean left masseter muscle thickness before treatment was 10.86± 1.36 mm and after treatment was 10.25 ± 1.51 mm which was statistically significant with a P-value of 0.000 (Graph 4).
When the overall change in geniohyoid muscle thickness before and after treatment, mean geniohyoid muscle thickness before treatment was 6.67 ± 0.64 mm and after treatment was 7.21 ± 0.64 mm which was statistically significant with a P-value of 0.000 (Graph 5).

DISCUSSION
The purpose of the present study was to evaluate the effect of anterior bite plane and and its influence on masseter and geniohyoid muscle thickness.
Masseter muscle thickness was found to be reduced in all subjects treated with the anterior bite plane in both relaxed and clenched positions. The higher values obtained during contraction of the muscle compared to relaxation in this study is in agreement with those of previous studies by Kubota et al. [19]. Satiroglu et al. [20]. This disparity between the values in masseter muscle thickness during relaxation and during maximal clenching can be explained by the fact that during the contraction phase, the mandible will be elevated. This cause enlargement and thickening of the muscle fibres which may account for the observed higher thickness in the clenched state.
The right masseter muscle was thicker than that of the left during relaxation and contraction in all the subjects. This finding is in line with the findings of Chan et al. [21] and Satiroglu et al. [20] who reported that the right masseter muscle was much thicker than the left. A possible explanation could be that most of the participants in this study masticated on the right side of their mouth. Exercising the muscle has been known to increase its thickness and the bite force [10], and a significant positive correlation has been found between bite force magnitude and the thickness of the masseter muscle. This is also supported by the previous study of He et al. [22] who reported that reduced activity of the masseter Determining which parameter of the masseter muscle is to be measured must be evaluated, as well as the imaging technique to be performed. In this sense, ultrasonography is more feasible, has a reduced cost and does not emit ionizing radiation, as compared to other imaging techniques such as scanning or magnetic resonance [25].
Individuals with reduced overbite tend to have thinner masseter muscle because the superficial masseter muscle is anteriorly inclined and obliquely oriented relative to the occlusal plane and has a superior positioning of its insertion on the mandible compared to deep overbite individuals who have vertically oriented masseter muscle [26].
Previous studies have reported a correlation between masseter muscle thickness and facial morphology. Regardless of gender, patients with a larger intergonial width and lower facial height tended to have a thicker masseter muscle [27].
The reduction in masseter muscle thickness after bite opening is in agreement with the results of previous studies by Weijs and Hillen, [28] Satıroglu F et al. [20] which showed that the masseter muscle is thicker in individuals with a short face who tend to have a deep overbite, and thinner in those with a long face who tend to have reduced overbite or an anterior open bite. The results of this study clearly indicated that the masseter muscle thickness reduced after bite opening.
It was observed that the greatest thickness of the masseter muscle is related to longer mandibular ramus,lower mandibular inclination and gonial angle less obtuse. Kubota et al found no statistically significant correlation between muscle thickness and anterior facial height [29].
Geniohyoid muscles play an important role during hyoid bone elevation. There was a positive correlation between geniohyoid muscle thickness and jaw-opening strength [30]. A correlation between geniohyoid muscle thickness and the movement of hyoid bone was found and results suggested that the geniohyoid muscle is a key muscle involved in the anterior movement of the hyoid bone [31,32,29].
Submental muscles that perform hyolaryngeal elevation can be easily evaluated with the ultrasonography. Knowing the thickness values of geniohyoid muscle can determine the changes brought by the anterior bite plane. It can also be used in the follow-up of treatment [33].
The activities of the suprahyoid muscles have been reported to be induced by contraction of tongue muscles. The tongue-strengthening exercise was useful to increase the muscle power of the geniohyoid [34].
The present study showed a statistically significant increase in the geniohyoid muscle thickness after bite opening. This could be due to the forward movement of the hyoid bone after bite opening. The results of this study clearly indicated that the geniohyoid muscle thickness increased after bite opening.

CONCLUSION
Within the limitations of the present study, it can be concluded that, 1. Masseter muscle thickness reduced with the studied bite opening procedure with the anterior bite plane. 2. Geniohyoid muscle thickness increased with bite opening in subjects treated with the anterior bite plane.

DISCLAIMER
The products used for this research are commonly and predominantly use products in our area of research and country. There is absolutely no conflict of interest between the authors and producers of the products because we do not intend to use these products as an avenue for any litigation but for the advancement of knowledge. Also, the research was not funded by the producing company rather it was funded by personal efforts of the authors.

ETHICAL APPROVAL
The study was initiated after attaining Institutional ethical approval (