Prosthodont Res Pract 5 : 166-170, 2006

ORIGINAL ARTICLE

Characteristics of the Perioral Muscle Electromyographic Activities during Jaw Functions in Healthy Young Adults Yoshimi Inoko, DDS, PhD,a Ken Yoshimura, DDS,b PhD, and Osami Morita, DDS, PhDa aDepartment of Complete Denture, bDepartment of Anatomy, The Nippon Dental University School of Life Den- tistry at Niigata, Niigata, Japan

Clinical significance various jaw functions. The results of the present study indicated that the muscle (Ment) acted in coordination with the Key words; mentalis muscle, masticatory muscles, actions of the masticatory muscles during various jaw electromyogram (EMG), jaw function functions. Further study is considered to be necessary to explore the relationship between the stability of mandibular dentures and the EMG activity of Ment Introduction during various jaw functions. The synchronized activity of the lower orbicularis oris and mentalis muscle (Ment) in function Abstract 1-6 Purpose: The aim of the present study was to record has been clarified by many studies. These mus- cles seem to closely affect the retainment reten- the electromyographic (EMG) activity of the mentalis 7,8 muscle (Ment) during various jaw functions in healthy tion and stabilization of the lower denture. The young subjects. mandibular labial flange is situated in the labial Methods: Twelve healthy males were enrolled for the vestibule. The function of this vestibule is affect- study. A portable EMG recording device was used to ed by contraction of the Ment and orbicular oris record the EMG activity from the Ment, anterior belly muscles. Ment provides a supportive function to of the (Dig), and the masseter (Mm) the orbicular oris muscle.6 The muscles of the muscles. The EMG activities of these muscles were re- have a critical role in the successful use of corded during maximal voluntary clenching (MVC), dentures.9 Ment arises from the anterior alveolar maximal voluntary jaw opening (MVO), and the mas- bone of the mandibular bone. From the origin of ticatory process. The EMG activity of Ment was com- pared with the EMG activities of Dig and Mm during Ment, Ment fibers diverge in their course toward various jaw functions. the skin and at the same time, the fibers con- Results: (1) The EMG activities of Ment and Dig were verge toward and interlace with those of the con- significantly greater during MVO than during MVC tralateral muscles after crossing the midline of (P<0.05). A strong correlation was found between the the skin in the mentalis region. Only the most activities of Ment and Mm during MVO (r =0.739, lateral fibers end in the skin of the same side.10 P<0.01), and between those of Ment and Dig during Ment elevates the skin of the and causes MVC (r=0.664, P<0.05). (2) It was observed that dur- protrusion of the lower lip outward. Since its ori- ing mastication of peanuts, Ment showed synchronous gin is at a higher level than the fornix of the ves- activity with that of Dig and reciprocal activity with tibule, Ment pushes the lower vestibule superfi- that of Mm. cially during contraction.9 Contraction of this Conclusion: These results indicate that Ment acts in coordination with the actions of Dig and Mm during muscle can, therefore, dislodge mandibular den- tures, particularly when the residual ridge in the anterior region is at the same height as the for- Corresponding to: Dr Yoshimi Inoko 11 Department of Complete Denture, nix of the vestibule. Itsuki reported significant School of Life Dentistry at Niigata, increased EMG activity of the orbicularis oris The Nippon Dental University, muscles and Ment in open bite patients during 1-8 Hamaura-cho, Niigata 951-8580, Japan swallowing. Stavridi12 showed that the use of lip Tel: +81-25-267-1500, Fax: +81-25-267-8906 E-mail: [email protected] pads, Ment activity increased during lip closure. However, the relationship between contraction of Received on January 6, 2006 / Accepted on June 14, 2006 Ment and the jaw functions has not yet been

166 EMG Activity of the Mentalis Muscle during Jaw Movements

entation at an interelectrode distance of 15 mm (Fig.1). The impedance of each active electrode was maintained at under 10 kΩ. Reference elec- trodes for the muscles were attached to the back of the neck. The EMG was recorded at a sam- pling frequency of 1 kHz. The mean values of the rectified EMG data for every 0.1 second were stored in a personal computer, and the integrated EMG values (μVs) were measured after the re- cording. During the EMG recording, the subjects were Fig. 1 Schematic drawing of positioning of bipolar surface comfortably seated upright on a chair without electrodes of the Ment. head support, and asked to carry out the follow- ing tasks: (1) Maximal voluntary clenching (MVC): to keep clarified. Furthermore, it has also not been clari- their teeth clenched as strongly possible at fied as to contraction of Ment associated with the intercuspal position for a duration of ap- which jaw functions is capable of dislodging man- proximately 3 seconds. dibular dentures. (2) Maximal voluntary jaw opening (MVO): to The aim of this study was to record the electro- keep their jaws at the maximum myographic (EMG) activity of Ment during masticato- opening position for a duration of approxi- ry movements, maximal voluntary clenching and max- mately 3 seconds. imal voluntary jaw opening in healthy young subjects. (3) Mastication: to freely masticate on 3 grams of peanuts as a test food prior to swallowing. All the tasks were required to be repeated 3 times. Between the tasks, the subjects were in- Materials and methods structed to relax to avoid muscle fatigue. 1. Subjects The subjects consisted of 12 adult male volun- 3. Quantitative data analysis teers with a mean age was 26 years and 6 After the recording of the EMG activities, data months. All had complete natural dentition (ex- analysis was performed with an accessory soft- cept for the third molars), a healthy periodontal ware (Muscle Tester ME3000P Software v.2.1- condition, no nasal disease, respiratory disease program, Mega Electronics Ltd, Kuopio, Fin- or parafunctional habits (e.g.: bruxisms, land), and the following variable were calculated: habit, repetitive swallowing, lip biting). The The integrated EMG values for Ment, Dig and study was approved by the ethical committee of Mm at MVC and MVO were calculated for 3 sec- our university. The purpose and methods of the onds of each test. In each subject, the mean inte- study were explained to all the volunteers, in- gral values of the measurements were calculated formed consent was obtained from each of them for each muscle. prior to their participation in the study. 4. Statistics analysis 2. Experimental procedures The statistical analysis was performed using a In this experiment, a digital EMG recording de- statistical software (Smart Mate III ). The EMG vice (Muscle Tester ME3000P, Mega Electronics activities of individual muscles during MVO and Ltd, Kuopio, Finland) and bipolar surface Ag-Ag- MVC were compared by Wilcoxon’s rank test. Cl electrodes with a diameter of 5mm (Blue sen- Spearman’s rank correlation test was used to sor, type-N-00-S, Medico test A/S. Osyka, Den- analyze the relationship among the activities of mark) were used to record the EMG activities of Ment and Dig, and Mm during MVC and MVO. the Ment, the anterior belly of the digastric mus- cle (Dig), and masseter (Mm) muscles. After care- ful preparation of the skin, each pair of elec- trodes was placed in the main direction of the muscle fibers according to their anatomical ori-

167 Inoko et al., Prosthodont Res Pract 5 : 166-170, 2006

200 MVC ）

V 60.0 μ r=0.664 （ 150 P <0.05

MVC 100 MVO

50 ） 40.0 V μ EMG median value （ g 0 Di Mm Ment Dig Fig. 2 EMG median value at MVO and MVC.

20.0 MVO 600.0 r=0.739 P <0.01 500.0 0.0 0.0 20.0 40.0 60.0 80.0

400.0 Ment（μV） ）

V Fig. 4

μ Correlation between EMG means integrated value of （ 300.0 Ment and Dig during MVC. nt Me

200.0 Ment 200μV

100.0

Dig 0.0 0.0 20.0 40.0 60.0 80.0 Mm（μV） Fig. 3 Correlation between EMG means integrated value of Ment and Mm during MVO. Mm

Results Fig. 5 The raw EMG during peanuts mastication. 1. EMG activity during MVC and MVO The EMG activity of Ment and Dig during MVO were significantly greater (P<0.05) than those during MVC (Fig. 2). A strong correlation was Discussion found between the activities of Ment and Mm Many researchers4,6,11,13 have reported that the during MVO (r=0.739, P<0.01) (Fig. 3), and be- morphological characteristics of the maxillofacial tween those of Ment and Dig during MVC area may affect the perioral muscle EMG activi- (r=0.664, P<0.05) (Fig. 4). ties. Therefore, young healthy males with normal occlusion or and maxillofacial morphology were 2. EMG activity during peanut mastication enrolled for this study. Mandibular movements Figure 5 shows an example of a raw EMG record- are carried out mainly by the closing and open- ing during peanut mastication. It was observed ing muscles of the mouth, but are also affected by that the EMG activity of Ment during peanut the acting on mandibular move- mastication was synchronous with that of Dig ments. Most notably, the opening and closing and reciprocal to that of Mm. muscles of the act antagonistically and have a cooperative relationship with each of the

168 EMG Activity of the Mentalis Muscle during Jaw Movements other muscles. In this study, the EMG activities be appropriate candidates for this study, similar of Ment, Dig and Mm were recorded during to subjects5 in the previous study. MVO, MVC, and mastication, in order to clarify The present study revealed the EMG activities the behavior of Ment. The researches mentioned of Ment and Dig at MVO and MVC. The EMG ac- above examined in activity of Ment using EMG tivities of Ment and Dig at MVO were signifi- recordings, with the surface electrodes placed cantly higher than those at MVC. These findings over the skin of the lower lip. The orientation of may indicate that the activity of Ment during jaw the electrodes is important when examining the opening may be capable of dislodging mandibular activity of these muscles. Stavridi and Ahlgren12 dentures. EMG activities of not only the Dig, but placed the electrodes, 2 cm apart, at the level of also Mm were observed during MVO. This phe- the suprapognion point. Kurashima and Fukui5 nomenon seems to be responsible for the mandib- placed the electrodes over on the skin of origin of ular stretch reflex.15 Furthermore, a strong posi- Ment in relation to the midline of the face. Yama- tive correlation of the EMG activities was found guchi et al4 determined the orientation of the between Ment and Mm during MVO, and be- electrodes through anatomical orientation and tween Ment and Dig during MVC. These findings placed the electrodes over Ment. In our study, the may indicate the relation of Mm activity to wide surface electrodes were placed on the skin of ori- unrestricted opening of the jaw during MVO, and gin of Ment according to the anatomical orienta- that of Ment and the Dig along with the Mm ac- tion; the divergence of the Ment fibers was not tivity during MVO and MVC. confirmed by an objective test. Therefore, there is Masticatory function is known to be influenced some possibility that the EMG activity of Ment by various proprioceptors and inputs from the recorded in this study also includes some EMG central nervous system.16 During mastication, signals from the periphery of Ment. contraction of the jaw opening muscles extends As previously stated,9 contraction of Ment ele- the mandible, and the tongue carries food into vates the skin of the chin and causes the lower the interocclusal space. Moreover, contraction of lip to protrude. In addition, the depressor muscle the jaw closing muscles elevates the mandible to of the lower lip causes the lower lip to be drawn crush or grind the food. The masticatory process outward. In this study, no movements of the low- is structured by the regular jaw opening and er lip to an outward position was observed dur- closing muscle activities. In regard to the EMG ing MVC or MVO in any of the subjects. There- activities during the mastication of peanuts, the fore, we do believe that the surface EMG activity of Ment was synchronous with that of recorded from the skin of the chin is reliably in- Dig, and reciprocal to that of Mm. dicative of the muscle activity of Ment. The labial flanges of mandibular dentures oc- Chewing gum and peanuts are usually used as cupy a potential space bounded by the labial as- the tester objects in this type of studies.14 The pect of the residual ridge, the mucolabial fold, mechanical properties of chewing gum change and the orbicular oris muscle. If the labial flange little during mastication. Evaluation of the mus- of the mandibular denture does not have an ap- cle activities using chewing gum as the tester propriate shape, it is thought that the EMG ac- substance is useful for the examination of masti- tivity of Ment might change during jaw function. catory movements in a fixed condition. Because Therefore, it is necessary to consider the influ- peanuts are crushable, evaluation of the mastica- ence of the labial flange of mandibular dentures tory process using peanuts as the tester sub- on the activity of Ment during jaw functions. stance is also useful for evaluating masticatory From our results, we propose that the Ment may movements. In this study, we selected peanuts as be one of the responsible muscles for the dislodg- the tester substance for evaluating the activity of ing of mandibular dentures. Clinical application Ment during mastication. of these findings must await further study to Prior to the start of the experiments, EMG re- evaluate the effects of the labial flange of man- cordings were carried out for 10 seconds in the dibular dentures on the activity of Ment. physiological rest position. The EMG activity in the resting position ranged from 1~5μV. No un- usual EMG activity during the resting position was observed in any of the subjects. Accordingly, Conclusion the subjects recruited by us were considered to 1. The EMG activities of Ment and Dig at MVO

169 Inoko et al., Prosthodont Res Pract 5 : 166-170, 2006

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