Surface EMG of jaw-elevator muscles and chewing pattern in complete denture wearers, By Tatiana Araya

M. G. PIANCINO*, D. FARINA†, F. TALPONE*, T. CASTROFLORIO*, G. GASSINO‡,

V. MARGARINO‡ & P. BRACCO* *Cattedra di Ortognatodonzia e Gnatologia-funzione masticatoria, Universita` degli

Studi di Torino, Torino, Italy, †Department of Health Science and Technology, Center for Sensory-Motor Interaction (SMI), Aalborg University,

Aalborg, Denmark and ‡Servizio di Riabilitazione Orele, Maxillo-facciale, Universita` degli Studi di Torino

SUMMARY The aim of this study was to investigate

the adaptation process of masticatory patterns to a

new complete denture in edentulous subjects. For

this purpose, muscle activity and kinematic parameters

of the chewing pattern were simultaneously

assessed in seven patients with complete maxillary

and mandibular denture. The patients were analysed

(i) with the old denture, (ii) with the new

denture at the delivery, (iii) after 1 month and

(iv) after 3 months from the delivery of the new

denture. Surface electromyographic (EMG) signals

were recorded from the masseter and temporalis

anterior muscles of both sides and jaw movements

were tracked measuring the motion of a tiny magnet

attached at the lower inter-incisor point. The subjects

were asked to chew a bolus on the right and

left side. At the delivery of the new denture, peak

EMG amplitude of the masseter of the side of the

bolus was lower than with the old denture and the

masseters of the two sides showed the same intensity

of EMG activity, contrary to the case with the

old denture. EMG amplitude and asymmetry of the

two masseter activities returned as with the old

denture in 3 months. The EMG activity in the

temporalis anterior was larger with the old denture

than in the other conditions. The chewing cycle

width and lateral excursion decreased at the delivery

of the new denture and recovered after

3 months.

KEYWORDS: surface electromyography, chewing, jaw

muscles

Accepted for publication 20 March 2005

Introduction

Mastication is a highly coordinated neuromuscular

function involving fast effective movements of the

jaw and continuous modulation of force (1). It is

characterized by ritmicity and a diversity of patterns of

jaw, tongue and facial movements, that vary depending

on the species and food ingested (2). The commands

underlying the basic rhythmical movements of mastication

are generated centrally but those involving

adaptive control are regulated by afferent information,

particularly related to oral-facial kinestetic inputs (2).

The loss of teeth determines important changes in

the masticatory system, which affect bone, oral mucosa

and muscles. The alveolar bone tends to resorb, the

formation of new bone is loosen, and the overlying

mucosa presents a decreased number of receptors, thus

the afferent inputs are reduced (3, 4). Sensory receptors,

such as muscle spindles, periodontal and intradental

pressoreceptors strongly influence the activity of motor

neurons and, thus, muscle control (5, 6). Much of

the integration of sensory feedback with the centrally

generated drive occurs at the level of the premotoneurons

in the nucleus reticularis parvocellularis and in the

mesencephalic trigeminal nucleus and adjacent nuclei

(5). Most of the cells in these nuclei have mucosal

receptive fields and respond to pressure applied to the

teeth, or to stretch of the jaw muscles (5).

In edentulous subjects sensory feedback is altered. In

these patients, the masticatory cycle amplitude and

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efficiency, and the masticatory force are smaller in

comparison with dentate subjects. Moreover, both

opening and closing velocity of masticatory cycles are

reduced, while the occlusal pause is longer (7, 8).

The change of denture determines a modification of

the peripheral information, with a need of adaptation of

the motor control strategy. Mastication is realized by

modulating the activity of the elevator muscles to

preserve the chewing pattern (9, 10). The investigation

of the adaptation process to a new denture is relevant to

understand the control of masticatory muscles and may

provide essential information for the diagnosis of

dysfunctions of the masticatory system (11, 12). The

analysis of electromyographic (EMG) activity and kinetic

of the movement provides an insight into the motor

control system (13).

Although after rehabilitation with a new denture

EMG parameters usually approach those observed in

dentate subjects, this is not observed in specific cases

(7, 8). In this context, many factors play a role, such as

age, gender, number of years of being edentulous, oral

conditions, denture mobility and subjective experience

wearing dentures (14). The poor fit and the lack of

stability of the full denture clearly affects the masticatory

function (15).

To provide further insight into the adaptation of

mastication to a new complete denture in edentulous

subjects, we planned a longitudinal study for the

simultaneous evaluation of jaw muscle activity

(through EMG) and functional outcome (kinematic

parameters of the chewing pattern) in patients with

complete denture. The subjects were thus analysed in

four experimental sessions: (i) with the old denture

(used for at least 2 years), (ii) with the new denture at

the delivery, (iii) after 1 month and (iv) after 3 months

from the delivery of the new denture.

Methods

Subjects

Seven subjects (four males; age, mean ± s.d.,

63Æ2 ± 6Æ9 years) wearing complete maxillary and mandibular

denture, referring to the School of Dentistry,

Prosthodontic Department, University of Turin, were

selected for the study out of 62 patients. Patients were

recruited with the following inclusion criteria: (i) full

denture wearers (maxillary and mandibular) since at

least 2 years; (ii) stable denture and (iii) good cooperativeness

in the experiment. The exclusion criteria were

the presence of (i) significant medical problems; (ii) soft

and hard tissue oral pathologic disorders; (iii) mandibular

dysfunction and (iv) any pathology affecting

mandibular movements.

New dentures were manufactured with the structural

standards used by the School of Dentistry, University of

Turin. They were designed considering the residual

structures for each subject. Posterior teeth mounting

was executed with Gerber’s technique (multilocally and

independently stable) (16, 17). With this technique,

each tooth, if functionally stressed, transmits forces to

the respective osteo-mucosal support so that the denture

is not displaced. Moreover, the denture prevents

displacement independently from retention forces.

Teeth are mounted estimating the neutral zone,

i.e. the area where tongue forces, working outward,

are neutralized by cheek and lip forces, working inward.

Finally, the freedom in centric technique (17) was used;

it consists in mounting a short radius pestle (maxillary

teeth) and a long radius mortar (mandibular teeth).

Experimental procedures

The same experimental procedure was repeated (i) with

the old denture, (ii) with the new denture at the

delivery, (iii) after 1 month and (iv) after 3 months

from the delivery of the new denture. In each experimental

session, the subjects were asked to chew a soft

bolus (20 · 20 · 20 mm size), which was prepared

with the procedure described in (18). Briefly, 180 g of

cooking gelatine were dissolved in 1 L of water and the

mix was put in a container and warmed up to 60 _C.

10 mg of fucsin were dissolved into 20 mL of water and

then added to the gelatine; the mix was put in a steel

mould and solidified in about 2 h at room temperature.

Pieces of 20 · 20 · 20 mm size were cut and put in a

solution of formalin and water for 24 h before their use.

During the test, the subjects sat comfortably on a

chair with the EMG electrodes placed over the masseter

and temporalis anterior muscles of the two sides, as

described below. They were asked to fix a target on the

wall, 90 cm far, to avoid lateral movements of the head.

The measures were performed in a silent and comfortable

environment. Each recording began with the jaws

with the largest number of teeth in contact (19). The

subjects were asked to find this starting position by

lightly tapping their opposing teeth together and then

clenching. They were asked to hold this position with

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ª 2005 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 32; 863–870

the test bolus on the tongue, prior to start the

recording. Each test consisted in a 10-s long chewing.

The chewing was on the right and left side. The test was

repeated six times for each side, for a total of

12 mastications.

Surface EMG recordings

Surface EMG signals were recorded with an eightchannel

electromyograph (model EM2; bandwidth

45–430 Hz per channel)*, which is part of the K6-I

WIN Diagnostic System (20). The relative large highpass

frequency in EMG recordings was selected to

reduce low-frequency movement artefacts. Two electrodes

(Duotrode silver/silver chloride EMG electrodes)*

were located on the masseter and temporalis muscles of

both sides, according to the anatomical landmarks

described by Castroflorio et al. (21), with an interelectrode

distance of 20 mm. This electrode arrangement

and placement provides small sensitivity to

electrode displacements and, thus, good repeatability

of EMG variables (21). Before electrode placement, the

skin was slightly abraded with abrasive paste and

cleaned with ethanol.

Kinematic parameters of the chewing cycle

The mandibular motion was measured with a kinesiograph

(K6-I)* (Fig. 1). The instrument measures jaw

movements with an accuracy of 0.1 mm. Multiple

sensors (Hall effect) in a light weight (four ounce) array

track the motion of a tiny magnet attached at the lower

inter-incisor point. The magnet was put at the level of

the vestibular resin flange on the mandibular denture,

in correspondence to the central incisors, in the fornix

deepest point, without interference with the teeth. The

magnet was fixed to the denture by adhesive paste and

was encircled with dental wax to reduce asperities and

avoid pain in the mandibular lip mucosa. Before

removing the magnet, an impression, with silicone

material, of the mandibular denture frontal area was

taken. Thus, the magnet position could be reproduced

in subsequent experimental sessions. The kinesiograph

was interfaced with a computer for data storage and

subsequent analysis. Kinematic and EMG data were

collected simultaneously.

Signal analysis

The envelope of the surface EMG was computed by

signal rectification and low-pass filtering. The maximum

value of the envelope was used as an index of

muscle activity. The raw kinematic data were analysed

with a custom-made software (Chewing Cycles Analyser,

CCA)† and based on the approximation of the

chewing cycle by Bezier curves. The mean cycle (on

three dimensions) of the set of mastications in each test

was used for further analysis. The first cycle, during

which the bolus was transferred from the tongue to the

dental arches, was excluded from the computation of

the mean cycle. Other cycles were excluded if they

presented at least one of the following characteristics:

(i) minimum opening smaller than 4 mm; (ii) duration

shorter than 300 ms; or (iii) vertical opening smaller

than 3 mm.

From the mean cycle, the following variables were

extracted: (i) pattern width; (ii) sagittal angle (19);

Fig. 1. Experimental set-up. The kinesiograph K6 and the surface

electromyographic electrodes are mounted on the subject.

*Myotronics Research Inc., Tukwila, WA, USA. †University of Torino, Torino, Italy.

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(iii) vertical range (9, 22); (iv) lateral excursion;

(v) angulation of the frontal pathway; (vi) maximum

opening velocity and (vii) maximum closing velocity.

Statistical analysis

Data were analysed using three- and four-way repeated

measures analysis of variance (ANOVA). Significant

interactions were followed by post hoc Student–Newman–

Keuls (SNK) pair-wise comparisons. The alphalevel

for statistical significance was set to P £ 0Æ05. Data

are presented as mean ± s.e.

Results

All patients answered a questionnaire in which they

indicated that after 3 months of use the new dentures

were more comfortable and efficient than the old

ones.

EMG activity

Masseter muscle. A four-way ANOVA was used to analyse

the dependence of peak EMG envelope on the following

factors and the interaction between factors: trial (six

trials for each recording configuration), side of chewing,

experimental session (old denture, new denture

when delivered, new denture after 1 month, new

denture after 3 months), and recorded side. EMG

amplitude in the masseter was not significantly affected

by any of the factors when considered independently

from each other. However, there was a significant

interaction between side of chewing and recorded side

(F ¼ 63Æ64, P < 0Æ001). EMG amplitude from the

muscle of the side of chewing was significantly higher

than that of the other side (SNK: P < 0Æ01) (Fig. 2a).

There was also a significant interaction between side of

chewing, recorded side, and experimental session

(F ¼ 3Æ55, P < 0Æ05). With the old denture, EMG

amplitude was significantly higher for the chewing side

than for the other side (SNK: P < 0Æ01) (Fig. 2a). This

did not hold when the new denture was delivered, i.e.

at the delivery of the denture there was no difference

between EMG amplitude of the two sides. The two sides

led to different EMG amplitudes only after 1 month

from the delivery (SNK: P < 0Æ05; Fig. 2a).

Comparing the different experimental sessions, EMG

amplitude was smaller at the delivery of the new

denture than with the old denture only for the side of

chewing and not for the other side (SNK: P < 0Æ05).

After 1 month, EMG amplitude did not change with

respect to delivery and was still smaller, for the side of

chewing, than the amplitude with the old denture.

After 3 months, EMG amplitude increased with respect

to 1 month (SNK: P < 0Æ05) and was not significantly

different from the amplitude recorded with the old

denture (Fig. 2a).

Temporalis anterior muscle. A four-way ANOVA (factors:

trial, side of chewing, experimental session, and recorded

side) of temporalis anterior EMG amplitude was

significant for the interaction between side of chewing

and recorded side (F ¼ 24Æ9, P < 0Æ01) and for the

interaction among the trial, side of chewing, and

experimental session (F ¼ 4Æ5, P < 0Æ05). Post hoc SNK

test revealed that the EMG activity in the side of

chewing was higher than in the other side (P < 0Æ05).

Moreover, the EMG activity of the temporalis muscle

with the old denture was higher than in all other

experimental sessions (SNK: P < 0Æ05; Fig. 2b).

Kinematic parameters

Three-way ANOVA’s were used to analyse the dependence

of kinematic parameters on the following factors

and interaction between factors: trial, side of chewing

and experimental session. Kinematic parameters for

the four recording conditions are reported in Table 1.

The pattern width was significantly affected by the

experimental session (F ¼ 3Æ6, P < 0Æ05). Pattern width

was smaller with the new denture at the delivery than

with the old denture and with the new denture after

1 month (SNK: P < 0Æ05). Pattern width was larger

after 3 months with the new denture than in all other

conditions (SNK: P < 0Æ05). Lateral excursion

depended on the experimental session (F ¼ 3Æ2,

P < 0Æ05), with larger values after 1 and 3 months

with respect to the new denture at the delivery (SNK:

P < 0Æ05). Moreover, lateral excursion was smaller at

the delivery of the new denture than with the old

denture (SNK: P < 0Æ05). The sagittal angle increased

with the new denture at the delivery with respect to

the old denture and was smaller after 3 months than

in the other conditions (ANOVA: F ¼ 3Æ2, P < 0Æ05; SNK:

P < 0Æ05). The vertical range, angulation of the

frontal pathway, maximum opening velocity, and

maximum closing velocity did not depend on any of

the factors.

Surface electromyographic

(EMG) amplitude (maximum of EMG

envelope) (mean ± s.e.) in the four

experimental sessions for the two

recorded sides of the (a) masseter and

(b) temporalis anterior muscle.

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Discussion

In this study we monitored jaw-elevator muscle EMG

activity and kinematic parameters (chewing pattern) in

edentulous patients during chewing, before and after

the delivery of a new complete maxillary and mandibular

denture. This analysis provides indication on

muscle coordination and its functional outcome. Most

previous studies focused separately on EMG amplitude

(9, 10) or chewing pattern (7, 8).

With the old denture, EMG amplitude of the

masseter of the side of the bolus was significantly

higher than in the other side, as in dentate subjects

(23–25). This is because of the adaptation to the

established intra-oral stimuli of the old denture.

However, the masseter muscle activity of the edentulous

is lower in comparison with the activity of the

dentate subjects (23, 26) which is because of the

instability of the complete denture requiring a continuous

control of the dynamic mandibular posture

while chewing.

At the delivery of the new denture, there was no

difference between the activity of the masseter muscles

of the two sides, with a decrease in EMG amplitude for

the bolus side with respect to the condition with the old

denture. These results are in agreement with previous

work (1, 9, 10, 27, 28). The lack of habit to the new

denture may be one of the reasons for the decreased

EMG activity in the bolus side. The new intra-oral

stimuli may have also determined the reduced lateral

displacement and the decreased width of the chewing

cycle at the delivery of the new denture with respect to

the condition with the old denture. Moreover, the new

denture, made of rigid materials, likely activated nociceptive

afferents which inhibited muscular contraction

as a protective reflex (29, 30).

After 1 month of use of the new denture, EMG

amplitude was still lower than with the old denture but

the EMG activity of the masseter of the side of the bolus

was larger than that of the other side. After 3 months,

the masseter EMG amplitude was similar to that with

the old denture. At the same time, kinematic parameters

returned to similar values observed with the old

denture or to values indicating a more efficient chewing

pattern than with the old denture. Thus, adaptation to

the new denture occurred within 3 months and probably

previous experience with denture control during

mastication was re-established with the proper integration

between central drive and afferent information.

It has to be noted, however, that the activity of the

temporalis anterior muscle decreased with the new

denture and did not reach the initial value for all the

experimental sessions. Thus, muscle coordination with

the new denture was different than with the old one

even after 3 months. This probably indicates a more

efficient mandibular posture with the new denture

which required a lower activation of the anterior

temporalis muscles to control the mandibular position

during chewing.

The pattern width was larger after 3 months wearing

the new denture than with the old denture, which

indicated a better control and a better fitting of the new

denture. The masticatory cycle was also more displaced

towards the bolus side during the closing phase which

suggests that the new denture did not inhibit jaw

posturing.

Comparing the EMG and the cycle results, it appears

that the pattern changes were smaller than expected.

For example, despite large modifications in EMG

amplitude with the new denture at the delivery, the

opening and closing velocity did not decrease. Thus,

there was a reorganization of muscle coordination in

order to preserve the functional outcome with the new

denture as close as possible to the old condition (7, 8,

23). There are many degrees of freedom in the control

of mastication which allow for different solutions with

similar outcomes. The old, automated motor control

pattern needed 3 months to recover but the temporary

muscle activation strategy implemented in response to

the sudden change in afferent information allowed for a

functional output similar to the old condition. Thus,

edentulous subjects reacted to the new denture by

decreasing EMG activity of the masseter and temporalis

muscles, increasing the symmetry of the masseter

activity between sides, and inhibiting the mandibular

dynamic posture, while maintaining a similar chewing

cycle with respect to the old condition. This underlines

the importance of an integrated analysis of both

kinematics and EMG activity in the follow up of

patients with new dentures.

In conclusion, the results of this study indicated that

in edentulous subjects (i) with a denture used for

several years, the masseter of the side of the bolus is

significantly more active than that of the opposite side,

as it happens in dentate subjects; (ii) at the delivery of a

new denture, the EMG activity of the masseter of the

side of the bolus decreases and it reaches the values

with the old denture after 3 months; (iii) the activity of

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the temporalis anterior decreases with the new denture

and (iv) the pattern width and lateral excursion return

to the values with the old denture (or higher) after

3 months while no changes are observed for opening

and closing velocities.

Acknowledgments

The authors are sincerely grateful to Professor Arthur

Lewin of the Department of Orthodontics, University of

the Witwatersrand, Johannesburg, for the useful discussion

and suggestions in the interpretation of the

results.

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Correspondence: Dario Farina, Department of Health Science and

Technology, Center for Sensory-Motor Interaction (SMI), Aalborg

University, Fredrik Bajers Vej 7 D-3, DK-9220 Aalborg, Denmark.

E-mail: df@hst.aau.dk 870 M. G . P I A N C I N O et al.