2021, Number 1
Electromyographic neuromuscular balance of the masseter muscles produced by the use of an occlusal splint
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ABSTRACT
Introduction: Neuromuscular balance is the dynamic muscular coordination, between the relationship and activity of the masticatory muscles. The main objective of the occlusal splint is to produce a stable and reproducible mandibular position. Objective: To analyze possible variations in the neuromuscular balance based on electromyographic recordings of the masseter muscles before and after the use of occlusal splints in patients with malpositioned teeth. Material and methods: A cohort of 52 patients from two dental clinics was followed during the period from August to December 2018, through convenience sampling, the general, regional and local clinical examination was performed. Cephalometric analysis of lateral skull radiographs and electromyography evaluation was performed using specific software. In addition to the elaboration and placement of physiological occlusal splints. A bivariate analysis was performed [χ2 and linear regression, Wilcoxon ranks (STATA)]. Results: The mean age was 22 years, 75% were female and the type of malocclusion was skeletal class II (48%). The use of a splint caused a decrease in the RMS differences of 27 (59%) and increased in 19 (41%) patients. Asymptotic significance of the analysis determined a significant difference in the variation of the RMS difference and the muscle asymmetry index before and after splinting. And based on the contrast statistic (p < 0.05), there is insufficient evidence for this data set that the differences in RMS and muscle asymmetry index are splint dependent. Conclusions: Any dental treatment is complex because it alters the function of the stomatognathic system and these changes influence muscle activity, which should be considered correct after being evaluated by physiological patterns of muscle activity.INTRODUCTION
The main function of the occlusal splint is to produce a stable and reproducible mandibular position, which is not determined by the dental contacts already established.1
Neuromuscular balance is the dynamic muscle coordination of the relationship and the activity of the masticatory muscles. It defines the dynamics of the jaw posture to prevent injuries related to mastication forces and the proper performance of the muscles in their different functions (swallowing, phonation, breathing, mastication, jaw posture, etc.).2
Functional performance is an indicator to determine neuromuscular balance. Recent studies define muscle strength asymmetry as equivalent to more than 15% of the deficit compared to the same muscle on the opposite side.3 Asymmetry index quantifies the difference in activity in an isometric contraction of a bilateral muscle; it is estimated by subtracting the unilateral activity from the contralateral activity and dividing by the sum of both activities.4
In this sense, electromyography (EMG) facilitates group or individual assessment of the masticatory muscles.5 In addition, EMG is a diagnostic screening test to detect muscle abnormalities based on the pattern of electrical activity.
This study aims to analyze possible variations in the neuromuscular balance based on electromyographic recordings of the masseter muscles before and after using an occlusal splint in patients with malpositioned teeth.
MATERIAL AND METHODS
The present study followed a cohort of 52 patients from the orthodontic clinic of the División de Estudios de Posgrado e Investigación de la Facultad de Odontologia de la Universidad Nacional Autónoma de México and the Naucalpan UNAM dental clinic, from August to December 2018. Patients with malpositioned teeth without previous orthodontic treatment and/or use of a splint and permanent dentition were included.
Patients who presented partially edentulous arches with periodontal problems, prosthetic, implant, or surgical treatments, with apparent neurological or sensory disabilities, bruxism or rheumatoid arthritis, and a medical indication for the consumption of myorelaxant drugs were excluded.
The study was carried out in the Laboratory of Physiology of the DEPeI-FO-UNAM and the Universidad de la República, Uruguay.
A convenience sampling was done. After being included, the general, regional and local clinical examinations were carried out. Finally, the cephalometric analysis of the lateral skull x-ray was performed with the JOE® program. The protocol was approved by the Ethics Committee of the FO-UNAM. Likewise, all participants signed a written consent before entering the study.
The recording system (hardware and software) used for the evaluation of electromyography was designed, evaluated, and patented by researchers from CU-UNAM and CINVESTAV-IPN in Laboratory of Physiology, Graduate and Research Division, Dental School, National Autonomous University of Mexico, with the name of Electromyograph 1.2 UNAM-CINVESTAV (Figure 1).6
The clinical electromyographic recording method was standardized, obtaining an intraoperative intraclass correlation coefficient of 0.73 and an intraoperative intraclass correlation coefficient of 0.84. The root means square (RMS) recording was performed at maximum intercuspation and reported in microvolts per second (μV/s) (Figure 1).
The measurement of the electromyographic activity of the superficial masseter muscle was made with the patient sitting in the dental unit. The masseter muscle area was cleaned with a cotton swab and alcohol so that the electrode had a better adherence and the signal less noise. Three Kendall™ Medi-trace®100 conductive adhesive ECG electrodes were placed on each side. The first one in the zygomatic arch (origin of the muscle), the second one in the muscle's insertion, just at the angle of the mandible; and the third electrode on the back of the mastoid apophysis (Figure 1). The electrical activity was recorded at two different times. The first recording (baseline) during the initial examination before using the physiological occlusal splint. After this procedure, the splint was placed; after two months of using it, the patients returned for a reevaluation of the electrical activity (final record), maintaining the same criteria used for the first time. Analysis of EMG recordings in the MI was obtained for 30 seconds, measured in microvolts (µV).
Technique for the elaboration of the physiological occlusal splint. To this purpose a 0.060'' caliber acetate was placed in a vacuum thermoplastic machine. Later a 0.080'' caliber acetate was added. It was trimmed over the middle of all the upper teeth. A relining was carried out with self-curing acrylic on the occlusal splint surface, leveling the material. An occlusal adjustment was made, and the dimension of the splint was verified (interdental unocclusion) and that it did not interfere with any mandibular movement.
Using indications of the physiological occlusal splint was the same for all patients: usage for 24 hours a day removing it only for eating and brushing teeth washing it with neutral liquid soap for hands, without scrubbing study variables. The dependent variables were estimated based on the electromyographic value of RMS (basal and final RMS difference, neuromuscular balance, basal and final symmetry index).
The RMS difference was obtained by subtracting the RMR values of the right masseter muscle from the RMS of the left masseter muscle obtained during the first recording (baseline). The final RMS difference was estimated with the same process but with the RMS values obtained after two months of use of the physiological occlusal splint for each patient.
Neuromuscular balance was dichotomously determined (1 = decreased 2 = increased) through the behavior of the baseline RMS difference compared to the final RMS difference, assuming that the decrease in the electromyographic difference between the masseter muscles after two months of use of the occlusal splint is obtained in presence of neuromuscular balance.
The muscle activity asymmetry index was determined for each individual using the equation proposed by Naeije, McCarrol, and Weijs.7
Assymmetry index = [(right RMS - left RMS) / (right RMS + left RMS)] ×100
Independent variables: skeletal malocclusion (class I, II, and III) and facial biotype (dolichofacial, mesofacial, and brachifacial).
Confounding variables: facial pain and rheumatoid arthritis, both determined dichotomously (presence and absence by self-report) and the body mass index (BMI), categorized (underweight, normal weight, overweight).
Statistical analysis: statistical analysis was performed using the Statistical Software for Data Science (STATA) program, version 14, Windows platform. Bivariate analysis (χ2 and linear regression according to the nature of the independent variables) was performed to determine the relationship and distribution of neuromuscular balance. In addition, using the nonparametric Wilcoxon rank-sum test, the change in the response variables differences in RMS (baseline and final) and muscle asymmetry index (baseline and final), produced by the effect of deprogramming therapy, were analyzed. For all tests in this study, an alpha significance level of 5% (p = 0.05) was used, thus establishing the maximum error probability to reject the null hypothesis of Ho.
RESULTS
The mean age was 22 years and 75% of the sample was women. The type of skeletal malocclusion class II (48%) was the most frequent and the dolichofacial biotype (67%). In addition, 54% of the patients did not report having facial pain and 67% had a standard BMI.
Neuromuscular electrical activity before using the splint was 190 µV of the right masseter muscles and 148 µV of the left ones. However, after two months of using the occlusal splint, it was 173.5 µV for the right masseter muscles and 146 µV for the left ones.
The RMS differences of the masseter muscles were estimated. The median and interquartile range of the baseline RMS difference was [42.00 µV (1-60)] and the final RMS difference was (26.4 µV (0.9-60)) (Table 1). Unfortunately, four patients did not complete the study adequately and timely.
Neuromuscular balance was determined assuming that the difference in electromyographic activity decreases. The use of a splint caused a decrease in the RMS differences of 27 (59%) patients and increased in 19 (41%) patients.
The muscle asymmetry index estimates a percentage result that is why the magnitude is smaller than the difference in RMS. However, behavior is similar (Table 1)
The bivariate analysis determined that for each year of age, the neuromuscular balance decreases 0.014 µV (Table 2).
The patients who presented an opposite neuromuscular effect (increase in the RMS difference) with the use of the occlusal splint were patients with class II skeletal malocclusion (19%), patients who did not report facial pain (22%) and patients who had a normal mass index (26%).
The asymptotic significance of the analysis determined a significant difference in the variation of the RMS difference (significance of the RMS difference = 0.275) and the muscle asymmetry index (significance of the asymmetry index = 0.769) before and after the use of the splint (Table 3).
Based on the contrast statistic (p < 0.05) there is insufficient evidence for this data set that the differences in RMS and the muscle asymmetry index depend on the use of a splint. In this sense, it impacts the reduction of differences in RMS and asymmetry index, bringing the masseter muscles closer to a balance function. This association will depend on three aspects to consider: i) correct usage of a physiological occlusal splint as indicated at the beginning of the treatment, ii) attend to the corresponding controls to make adjustments according to the muscle response and symptoms reported by the patient every two weeks and, iii) the morphological and clinical characteristics of each patient (Figure 2).
The dispersion of the differences in RMS (red) and the asymmetry index (blue) of each patient is observed. The behavior of these two variables does not show any pattern or trend of significant behavior over time. However, it is illustrated that half of the patients present negative and positive behavior (Figure 2).
DISCUSSION
The neuromuscular imbalance is the theoretical basis for using the occlusal splint as a diagnostic tool in different specialties. Several studies whose main purpose was to evaluate the effect of the occlusal splint in normal subjects have shown that it reduces the effect of muscular hyperactivity and facilitates mandibular manipulation, helping dentists centralize the condyles with their respective articular discs in the mandibular fossae.7-11
Nassar et al,9 Lund et al,12 and Donegan et al13 conducted studies with subjects without any sign of muscle or joint dysfunction. When the mandible was at rest for long periods, there was an alteration in the electromyographic activity of the masseter muscles, which indicates that the use of an occlusal splint modifies this activity and that over time such results are consistent with ours, in which the deprogramming device changed the electromyographic activity during the specific time. Muscle activation patterns (RMS) over time decreased in most of the patients (class I), in the same way as observed in the Nassar et al9 and Lund et al12 studies.
The authors who showed changes in the functional balance of the stomatognathic system conclude that using a splint is effective, allowing the condyles to settle in a more centralized position in the fossae, thus reducing and balancing the activation of all the musculature.
However, this effect was not observed in the present study; an explanation for such results may be the selected sample, which included individuals with different morphological (skeletal) characteristics that seem to condition the effect of the neuromuscular response, thus providing an alternative response to that previously reported.
Ferrario et al8 and Naeije et al7 showed that ordinary subjects with healthy teeth have a certain degree of muscular asymmetry. Their findings show that a perfect symmetry does not exist and can be misleading since it implies too narrow criteria of ordinary. The concept of asymmetry seems more valuable and is consistent with reality. An asymmetry index of 18% should be considered standard and compatible with a function. The aim of the treatment would not be just a "symmetrical" appearance but a well-functioning system.13-14
We consider that the decrease in muscle activity may be associated with an increase in the number of occlusal contacts in a symmetrical muscular position observed during the first weeks of using the physiological occlusal splint. This decrease in electromyographic activity is not observed in class II skeletal malocclusion patients.
One of the main limitations of the study is the biological diversity that characterizes the response variable (neuromuscular electrical activity), which prevents determining the specific effect of therapy and will always depend on various factors, thus compromising the normalization or changes of electromyographic activity of the superficial masseter muscles during use of the occlusal splint. On the other hand, even with the study's strengths, it is necessary to carry out more analysis to evaluate and interpret neuromuscular balance in an objective and personalized way. The results of the study are reproducible and consistent in other populations.
CONCLUSIONS
The anatomical characteristics of each individual are related to the masticatory function. In the same way, muscle function plays an essential role in the occlusion and its respective modifications.
Surface electromyography is a practical study for evaluating muscles (healthy and dysfunctional) with important clinical contributions. It allows evaluating the bilateral symmetry facilitating the understanding of the physiological conditions of the stomatognathic system, generating an objective evaluation when considering the effect of the treatment.
Neuromuscular balance is limited to several factors. However, assessing neuromuscular electrical activity will help determine the adaptive position of the jaw.
The activity indices can be calculated in different ways considering the morphological and functional limitations of the patients, as it was for this study. However, the index reported more asymmetric patients, thus justifying that deprogramming therapy does modify electrical activity but does not guarantee neuromuscular balance in all patients.
Any dental treatment is complex because it alters the function of the stomatognathic system and these changes influence muscle activity, so it is essential to consider the functional aspect before and during the treatment. Prosthetic reconstructions, orthodontic and orthopedic treatments, among others, must be considered correct after being evaluated through physiological patterns of muscle activity. Likewise, the indices could evaluate balance estimated from functionality and symmetry.
REFERENCES
Fernández Rey LI. Análisis y desarrollo de nuevas variables electromiográficas y refleximétricas para el estudio de la función cráneo-mandibular [Maestría en Ciencias Odontológicas Función Cráneo-mandibular y Dolor Orofacial]. Universidad de la Republica Uruguay; 2018. Disponible en: https://odon.edu.uy/sitios/craneomandibular/wp-content/uploads/sites/42/2019/11/Tesis-Ignacio-Fernandez.pdf
AFFILIATIONS
1 Laboratorio de Fisiología, División de Estudios de Posgrado e Investigación, Facultad de Odontología. Universidad Nacional Autónoma de México, México.
2 Cátedra de Fisiología General y Bucodental, Facultad de Odontología. Universidad de la República, Montevideo, Uruguay.
3 Especialidad en Ortodoncia, Facultad de Estudios Superiores Iztacala. Universidad Nacional Autónoma de México, México.
CORRESPONDENCE
Claudia Ivonne Rodríguez Castañeda. E-mail: dra.ivonnerodriguez@gmail.comReceived: Mayo 2020. Accepted: Enero 2021.