ARTÍCULO CIENTÍFICO

 

Electromyography Evaluations of the masticator muscles during the maximum bite force

Evaluación electromiográfica de los músculos masticadores durante la fuerza máxima de mordedura

 

 

M.J.P. Coelho-Ferraz¹, F. Bérzin², C. Amorim³

1 Professor, Department of Orthodontics, Uniararas School, Araras, São Paulo, Brasil.
2 Fausto Bérzin. Professor and Department Chair, Department of Morphology, Piracicaba Dental School, UNICAMP, São Paulo, Brasil.
3 Cesar Amorim. Engineering, São José dos Campos, São Paulo, Brasil.

Dirección para correspondencia

 

 

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ABSTRACT

Healthy individuals were examined in terms of the pattern of activity of the masseter and temporal muscles in their anterior portion of both right and left sides, respectively, with the maximum bite force. The study consisted in seventeen adult volunteers with no sign of apparent temporomandibular dysfunction, of both genders, connected to the School of Dentistry of Piracicaba, with average age of 25 years old. The electromyography data were obtained, bilaterally, of the masseter, anterior portion of temporal and suprahyoid muscles in the postural and isometric positions. Were utilized surface passive electrodes for kids of Ag/AgCl, of circular format and dischargeable of Meditrace^® Kendall-LTP, model Chicopee MA01, attached to a pre-amplifier with gain of 20 times forming a differential circuit. The records of the electrical signs were captured by the equipment EMG-8OOC of EMG System of Brazil Ltd. of eight channels, with frequency of 2 KHz and 16 bits of resolution, digital filter with band pass of 20 to 500Hz. It was utilized also a pressurized transductor which consists of a pressurized rubber tube connected to a sensor element (MPX 5700) to obtain the maximum bite force. The statistic tests used were linear correlation, test t in pair and analysis of variance. Probability of p<0, 05 were significant considered statistically. The results showed a certain degree of muscular asymmetry for the masseter and anterior portion of temporal muscles that depended on the task performed or contraction level. The index of asymmetry was lower in the postural position than in the maximum bite force.

Key words: Electromyography; Mandibular biomechanics.

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RESUMEN

La actividad de los músculos masetero y de la porción anterior temporal de ambos lados, derecho e izquierdo, respectivamente, durante la fuerza máxima de mordedura fue estudiada en voluntarios sanos. El estudio incluyó a 17 voluntarios adultos de ambos sexos, edad promedia de 25 años, que no evidenciaban ningún indicio de disfunción temporomandibular y eran relacionados con la Facultad de Odontología de Piracicaba. Se registraron los datos electromiográficos en ambos lados de la cara del masetero y de la porción anterior de los músculos temporal y suprahioideo en las posiciones postural e isométrica. Se utilizaron electrodos de superficie pasivos para niños, de Ag/AgCl, con forma circular y descargables de Meditrace^® Kendall-LTP, modelo Chicopee MA01. Éstos se conectaron a un preamplificador con una ganancia de 20x que formaba un circuito de diferenciales. Se captaron los registros de las señales eléctricas utilizando un equipo EMG-8OOC de EMG System of Brazil, Ltd., de ocho canales, a una frecuencia de 2 KHz con 16 bitios de resolución y un filtro digital con un paso de banda de 20 a 500 Hz. Se utilizó también un transductor de presión que consistía en un tubo de goma con un sensor de presión (MPX 5700)* (Motorola SPS, Austin, TX, EE.UU.) para registrar la fuerza máxima de mordedura. El análisis estadístico incluyó la correlación lineal, la prueba t emparejada y el análisis de la varianza. Se consideró estadísticamente significativa una probabilidad de p<0,05. Los resultados pusieron en evidencia cierto grado de asimetría muscular para el masetero y la porción anterior del músculo temporal que variaba en relación con la tarea realizada y la intensidad de la contracción. El índice de asimetría fue menor en la posición postural que en la de fuerza máxima de mordedura.

Palabras clave: Electromiografía; Biomecánica mandibular.

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Introduction

The jaw dynamics has been the focus of many scientific investigations and many aspects of biomechanics are not fully understood. The pattern of the activities of the masticator muscles during the maximum aperture depends on some occlusal factors (Helkimo et al., 1977), premature contacts (Bakke & Moller,1980), facial morphology, general muscular strength and sexual differences (Killiaridis et al., 1995) and also the phase of the dentition, location of the bite force, emotional aspects of the patient as well as the attitude of the researcher and the patient, signals and symptoms of temporomandibular dysfunction(Fields et al., 1986; Bakke et al., 1990).

The measure of the maximum bite force is an attempt to quantify the total force of the muscles of mandibular closeness (Wang et al., 1990). Castro folio et al. (2005) concluded that the variables of the surface electromyography measured in the isometric contraction of the lifter mandibular muscles with the recording of the bite force showed good reproducibility to clinical application, denoting the importance of the knowledge that the reception of load in the left and right during the mandibular opening and closing in the isometric bite, the head of the mandible of the balancing side receives, in average, more load than the head of the mandible in the side of work that depends on the pattern of muscular recruiting (if the bite point stays constant) and the mechanical properties of the food (Hylander, 1975, 1977, 1985).

The aim of this study was to examine in individuals considered healthy, the activity pattern of the masseter and anterior portion of both left and right sides, respectively, with the maximum bite force.

 

Material and Method

The study consisted in the selection of seventeen volunteers, from both genders, connected to the School of Dentistry of Piracicaba, with average age of 25. The current work started only after the approval of the Ethical Committee in Research with Human Beings (CEP), of FOP-UNICAMP, according to documentation required by the Resolution 196/96 of the National Commission of Ethics in Researches (CONEP) of the National Council of Health. For the selection of the sample of the current study, the criteria that were followed are: dental relation of Class I of Angle and permanent dentition, no functional orthodontic and/or orthopedic treatment of the maxilla in the last 10 years, no occurrence of large cavities and no apparent signals or symptoms of masticator disorders.

The electromyography records were obtained from the masseter muscle and the anterior portion of the temporal muscle. It was used the Electromyography EMG-800C of EMG System of Brazil Ltd. of eight channels, with five bands of amplification gain, rejection of common mode > 120 dB, analogical-digital converter (A/D) of 16 bits of resolution of dynamic band, connected to PC using a net converter Ethernet 10Mbits with a connector RJ45 (10BASE T) utilizing protocol TCP/IP; filter type Butterworth, of low-pass of 500Hz and high-pass of 20Hz; software of acquisition and analysis of electromyographic signals platform Windows XP/ 98 / NT for simultaneously presentation of the signals of various channels and treatment of the signal (value of RMS, medium, minimum, maximum and desvio padrão, FFT on line) with rate of acquisition (sampling) up to 4800 samples / second per programmable channel per software. It was used passive bipolar surface electrodes for kids of Ag/AgCl, circular format and dischargeable of Meditrace^® Kendall- LTP, model Chicopee MA01, which capture the electrical activity of various motor units, at the same time attached to a pre amplifier with gain of 20 times, supplying a general performance of the muscular dynamics. A reference electrode (earth) was positioned in the right wrist greased with electro conductor gel to reduce the unwanted electrical noises to the electromyographic signal. Surface electrodes were fixed, bilaterally, in the anterior portion of the temporal muscle, superficial portion of the masseter muscle, according to anatomical references and procedures according to Vitti e Basmajian (1997). After this selection, the signal were processed through the software EMG Analysis V1.01 of EMG System of Brazil Ltd., which determine the value of the RMS in isometric and the maximum bite force in Kilogramforce( Kgf).

The bite force was determined with a pressurized transductor which consists of a pressurized rubber tube connected to a sensor element (MPX 5700)* . The tube and the sensor were connected to an analogical converter / electronic digital circuit, fed by an analogical signal coming from an element of sensitive pressure. The system was connected to a computer and the software to read the bite force was developed in Basic language. This software generates a text file in column with the data of the bite force easily read by Excel.

Three evaluations of the bite force were operated in each volunteer, who bit the tube with maximum force three times successively for 5 seconds, with an interval of 1 minute. The tube was placed among the superior and inferior molar teeth, bilaterally. To obtain the maximum intercuspidation, the volunteers were trained before the operation of the device.

The numeric values of each evaluation were calculated by the difference between maximum and minimum pressure and the average value of the three takes, for each patient selected. The values of the pressurized tube were transferred to the software Excel where were obtained in pounds per square inch (psi) and later converted in Newtons(N) taking in consideration the area of the tube (1 psi= 29 792).

For both, the eletromyographic records and the acquisition of the bite force, the volunteers were kept seated, with the Plan of Frankfurt.

 

Statistical Analysis

In order to quantitatively compare the contribution of the masseter and anterior portion of the temporal muscles in the bite force, the average of the masseter and temporal muscles, right and left, respectively were calculated in each volunteer. The activity index (Naeije et al., 1989) was used to indicate the relative contribution of the masseter and temporal in maximum intercuspidation :

The Activity Index varies between +100 and -100

• Activity of the masseter muscles only +100

• Equal activity of the muscles 0

• Activity of the temporal muscles only -100

To quantitatively describe the asymmetry in the activity of the masticator muscles, the asymmetry index was followed which is mathematically equivalent to the activity index:

The asymmetry index also varies from + 100 e – 100

• Activity of the right muscle only +100

• Equal activity 0

• Activity of the left muscle only -100

The statistical tests used were of linear correlation, pared test t and analysis of variance. Probability of p<0,05 were considered statistically significant.

 

Results

The table 1 registered the average values of the electromyographic activity of four masticator muscles in postural position and maximum aperture between the male and female genders. In resting there was significant statistical difference only for the right masseter muscle, being the electrical potential higher for the male gender. In the task of maximum intercuspidation , there was significant statistical difference also for the right masseter muscle in the male gender (table 1).

The Asymmetry Index was calculated to quantify the differences between the right and left sides of each volunteer. There was not sexual dimorphism tested by the test t of Student for independent samples, being calculated the total Asymmetry Index (Table 2). Considering the masseter and anterior temporal muscles, the first presented asymmetry index higher on both tasks performed.

 

Activity of the masseter and anterior portion of temporal muscles

The table 3 indicated the record of the activity index of the masseter and temporal muscles for the tasks of resting and isometry. In the resting position more than 58% of the volunteers kept the mandibular position utilizing thee anterior portion of the temporal muscle more than the masseter muscle. The action of the anterior portion of the temporal muscle in average was higher than the masseter muscle (23.59% versus 6.38%). The masseter muscle presented a higher activity in the dental intercuspidation (more than 77%) than the anterior portion of the temporal muscle.

 

The relative contribution of the masseter and anterior portion of the temporal muscles depended on the contraction level (p<0.05). A bigger bite force showed higher contribution of the masseter muscle (Fig. 1).

 

Discussion

The current study has investigated the normal variability of the activity of the masticator muscles, its asymmetries as well as its contributions to generate maximum bite force.

The Asymmetry Index and Muscular Activity were proposed to evaluate patients with muscular craniomandibular disorders (Naeije et al., 1985). Researches showed that the asymmetric muscular activity depended on the bite force and that in each level of contraction the asymmetry of the masseter muscle is higher than the anterior portion of the temporal muscle, being in accordance to (Naeije et al., 1989; Pruim, 1979; Ferrario et al., 1993); corroborating with the results found in this study.

There was sexual dimorphism in the electrical activity of the masseter muscle in the right side in both tasks of resting and maximum bite, being higher in the male gender, once the sexual difference of the cranium mainly refer to the female fragility of the muscles, determining a lower development of the bone superstructures in women (Madeira, 1998; Rizzolo & Madeira, 2004). Throckmorton & Dean (1994) Showed that the mechanical advantage of the masseter muscle in individuals of male gender was higher, as well as the bite force, whose difference could be explained by the size of the transversal section of the muscle.

The volunteers of this essay presented an asymmetric index lower in postural position (-11,4%) than in isometry (14,6%). The anterior temporal muscle showed an asymmetry index lower than the masseter muscle in the postural position. In the maximum bite, the prevalence of the anterior temporal muscle with low level of contraction is according to (Naeije et al., 1989; McCarroll et al., 1989a; McCarroll et al., 1989b) where the masseter muscle increases its contribution in the maximum bite force. This mandibular dynamics also is in accordance to the findings of Throckmorton & Dean (1994), Latif (1957), Bérzin (2004).

Hylander (1975, 1977, 1985) demonstrated the importance in analyzing the mandibular biomechanics in frontal projection, reporting that the muscles of occlusion in the side of the job generate more muscular force than the side of balancing which receive more load. On the other hand, differences in the electromyographic records of the masticator muscles of both sides is questioned in their relation to the maximum bite force, that is, the capacity of the individual in recruiting all the motor units. The criteria of asymmetry to elaborate and compos the process of diagnosis seems to be useful and according to the results of Ferrario et al.(1993), an asymmetry index of 18% can be considered normal.

 

Conclusion

A certain degree of muscular asymmetry can be considered normal which is different to the masseter and anterior portion of temporal muscles and depends on the task performed or contraction level. The asymmetry index was lower in postural position than in maximum aperture. The interdisciplinary treatment among deontology, speech therapy and physical therapy don’t have to be only " symmetrical appearance", once the tissues of the stomatognatic system are capable of adapting to their regional environment due to tissue responses in condition of health or in dysfunction.

 

 

Dirección para correspondencia:
Maria Julia P. Coelho-Ferraz
Alameda João Senra, 148
Colinas de São João
Limeira – SP, Brasil
13481-299
e-mail: mjcoelhoferraz@hotmail.com

Recibido: 22.9.08
Aceptado: 20.11.08

 

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