INSTRUMENTED SPLINT FOR THE DIAGNOSIS OF BRUXISM

Pilar Lafont Morgado, Andrés Díaz Lantada, Alexander Martínez Álvarez, Antonio Barrientos Cruz

Héctor Lorenzo-Yustos, Pedro Luis Castedo Cepeda, Roberto González Herranz

Julio Muñoz García and Javier Echavarri Otero

Universidad Politécnica de Madrid

C/ José Gutiérrez Abascal, nº 2. 28006 – Madrid, Spain

Keywords: Telemedicine, Electroactive Polymers (EAPs), Biomaterials, Bruxism, Temporomandibular Joint.

Abstract: Bruxism is a health problem consisting in grinding or tightly clenching the upper and lower teeth. Both the

grinding and sliding lead to wear of the teeth and produce a noise during the night that is sufficiently loud to

disturb the sleep of anyone sharing the bedroom. The tension produced causes problems in the muscles,

tissues and other structures surrounding the jaw, ear pain, headaches, lesions to the teeth and disorders in the

jaw joints.

For an early, rapid, effective and economical diagnosis of bruxism, we propose the use of instrumented

splints to detect and record the intensity and duration of interdental pressure episodes. This paper sets out

the design, manufacture and testing of an instrumented splint for diagnosing the signs of bruxism.

The system stands out for its use of piezoelectric polymeric sensors which, because of their reduced

thickness, do not cause any alteration to the patient’s bite. It lets a quantitative assessment of intraoral

pressure be made and bruxism behaviour be diagnosed at an early stage, so as to being able to programme

corrective actions before irreversible dental wear appears. The first “in vitro” simulations and “in vivo”

trials performed served to demonstrate the feasibility of the system in accordance with the initial objectives.

1 BRUXISM:

CHARACTERISTICS AND

PREVALENCE

Bruxism is a health problem consisting in grinding

or tightly clenching the upper and lower teeth. Both

the grinding and sliding lead to wear of the teeth and

produce a noise during the night that is sufficiently

loud to disturb the sleep of anyone sharing the

bedroom. The tension produced causes problems in

the muscles, tissues and other structures surrounding

the jaw, ear pain, headaches, lesions to the teeth and

disorders in the jaw joints. All these symptoms as a

whole are usually described as temporomandibular

joint problems (TMJ) or also as Craniomandibular

Disfuntion Pain Syndrome.

The phenomenon was introduced to dental

literature as bruxomania by Marie and Pietkiewkz in

1907. They described the habit of grinding the teeth.

The term bruxism was introduced by Frohman in

1931. In 1936 Miller proposed using the term

bruxomania for daytime grinding and bruxism for

night time grinding. The terms traumatic neuralgia,

the Karolyi effect and occlusive habit neurosis have

all been used to refer to some form of teeth grinding

or clenching.

According to a study by the Canadian Sleep

Society nocturnal bruxism affects 8% of the adult

population and 14% of the child population. A

decrease in the population affected can be

appreciated with age, attaining 3% for people over

60. However, for researchers like Melis and Granada

prevalence is around 25%.

As to differences by sex there is no general

agreement since there are publications that describe

a greater bruxism activity in men (Quirch, Ozaki),

others in women (Barreto, De los Santos) while

others deem it to be an insignificant factor (Hayden,

Kononean).

To summarise Nishigawa’s study on the bite

force produced during bruxism episodes, this can

frequently reach 1100 N exceeding the maximal

voluntary bite force. Pressures reached on the teeth

surface can reach 40 MPa, high enough to cause

high levels of wear and even breakages.

As to the duration of bruxism episodes, an

average time of around 7 seconds has been found

and when developing sensors it is necessary to

216

Lafont Morgado P., Díaz Lantada A., Martínez Álvarez A., Barrientos Cruz A., Lorenzo-Yustos H., Luis Castedo Cepeda P., González Herranz R., Muñoz

García J. and Echavarri Otero J. (2008).

INSTRUMENTED SPLINT FOR THE DIAGNOSIS OF BRUXISM.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 216-221

DOI: 10.5220/0001051802160221

 SciTePress

distinguish bruxism episodes from mioclonus or

rapid contractions (< 0.5 s) of the jaw muscles.

However, it is important to point out that

everybody subconsciously clenches their teeth at

some time of the day and this could be considered as

bruxism activity. The term bruxism is only used

though when the duration and intensity of this

activity has a bearing on dental wear and the

appearance of TMJ problems.

One of the main problems associated with the

traditional diagnosis of bruxism is that it is

frequently made when the teeth are already highly

worn and the prognosis of the illness is more severe.

Bruxism activity can also be recorded by an EEG

(electroencephalogram) as well as by EMG

(electromiography) and S-EMG (surface

electromiography). In many cases video-cameras are

used in the study to distinguish the bruxism episodes

of the mioclonus or rapid contractions (< 0.5 s) of

the jaw muscles.

However, in order to be able to make an earlier,

more rapid, more effective and economical diagnosis

of bruxism, the research team that have written this

paper propose using instrumented splints for

detecting and recording the intensity and duration

of interdental pressure episodes. Explained below

are the design, manufacture and trials of an

instrumented splint for the diagnosis of bruxism

activity. It has been developed by researchers at

Universidad Politécnica de Madrid in collaboration

with Ibex Estética Dental S.L..

2 DESIGN OF THE DEVICE FOR

DIAGNOSING BRUXISM

USING ELECTROACTIVE

POLYMERS AS SENSORS

Traditionally discharge splints or protection devices

are used to treat bruxism and prevent the associated

dental wear. As a diagnostic device we propose

introducing pressure sensors into a splint so that

patients’ bite episodes can be recorded and the

extent of their pathology be assessed. Piezoelectric

polymers are used as pressure sensors for the

reasons set out below.

Piezoelectric Electroactive Polymers as

Pressure Transducers: PVDF (Polyvinylidene

Fluoride)

Piezoelectricity was discovered in 1880 by

Pierre and Paul-Jaques Curie, who observed that

when certain crystals were compressed, like quartz

or tourmaline, depending on certain directions they

produced a voltage between zones on their surface.

When force was applied the relative positions of the

crystal molecules changed producing an internal

displacement of charges which was the cause of this

voltage.

These crystals also underwent the inverse effect

since they became deformed when a voltage

difference was applied. This property is found in

materials lacking a centre of symmetry and the

phenomenon is called ferroelectricity when a non-

conductor crystal or dielectric material exhibits

spontaneous electric polarisation.

Polyvinylidene fluoride or PVDF -(CH

-CF

and its co-polymers such as poly(vinilydenefluoride-

trifluoroethylene) or P(VDF-TrFE), are the polymers

of this kind with the largest number of industrial

applications. They posses partial crystalinity with an

inactive amorphous phase and an elastic modulus

close to between 1 and 10 GPa. Their use as

actuators is limited by the need to apply high electric

fields (around 20 V/μm for a 3% deformation), but

their use as pressure sensors is taking the place of

traditionally used piezoelectric ceramic materials.

Figure 1: Metallized PVDF sheets. Piezotech S.A..

The use of this type of sensor was considered

because of its reduced thickness, which does not

cause any alteration to the patient’s bite and because

of its greater resistance and sensitivity compared to

ceramic piezoelectrics. To make the sensors, we

took PVDF 40 μm thick sheets from Piezotech S.A.

with Au-Pt coated electrodes. These sheets were cut,

joined to the connecting wires and suitably

encapsulated to protect them and be inserted into the

splint (see manufacturing process). The sensors

obtained are shown below, together with the

behaviour model allowing them to be simulated and

the first results obtained in the trials carried out.

INSTRUMENTED SPLINT FOR THE DIAGNOSIS OF BRUXISM

217

Figure 2: Piezoelectric sensors manufactured. Product

Development Laboratory. Universidad Politécnica de

Madrid.

Figure 3: a) shows the piezoelectric sensor

layout. The charge displacement produced when a

force is applied to the piezoelectric sensor can be

represented using the equivalent electric circuit

depicted in Figure 3 b).

Figure 3: a) Piezoelectric Sensor. b) Electrical behaviour

circuit diagram of the piezoelectric sensor.

Force F on the sensor acts as a generator of

intensity powering a C capacity condenser. Ec (1).

C = C(F) = ε · (L1·L2) / e (1)

Where:

ε.- The dielectric constant of the sensor.

L1·L2.- The effective area of the sensor.

e.- The thickness of the sensor.

The thickness of the sensor, e, depends on the

initial thickness, e

, on the pressure applied, σ = F /

(L1·L2), and the Young modulus of the material, E,

using the following expression Eq. (2):

e = e

· (1 – σ / E) (2)

Current intensity, I, generated by applying force

F, depends on the transversal piezoelectric

coefficient of sensor d33 according to Eq. (3).

Q = d33 · F Æ I = dQ / dt = d33 · dF / dt (3)

When the sensor is connected to an external

circuit, as is shown in Figure 3 b), it discharges in

accordance with the equivalent R resistance of this

external circuit (the oscilloscope input resistance in

the first trials carried out). The intensity is given by

Eq. (4).

I = d33 · dF / dt = U / R + C · dU / dt (4)

With the above equations and previous data a

model was made in Simulink which permits a rapid

assessment of the effect of modifying the

parameters. The model and the results of the

simulation are shown below, together with the first

real trials carried out with the piezoelectric sensor

connected directly to the oscilloscope when it was

subjected to levels of pressure.

For the first simulations and trials with the

sensors manufactured (Figures 5, 6 and 7) we have:

Piezoelectric coefficient (when applying forces

perpendicular to the sensor plane).- d33 = 24 pC/N

Dielectric constant.- ε = 1,1 · 10

-10

F/m;

Elasticity modulus of the PVDF.- E = 2000

MPa

Effective sensor area.- L · L2 = 4 · 10

-4

Sensor thickness.- 40 μm

Oscilloscope input resistance.- R = 10 MΩ

Figure 4: Simulink model for simulating piezoelectric

polymer behaviour.

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218

Figure 5: Simulation of levels of pressure as simulator

input.

Figure 6: Output obtained (voltage) according to the

simulation.

Figure 7: First trials: The oscilloscope shows sensor output

voltage on applying levels of pressure as input.

3 MANUFACTURING THE

DEVICE FOR DIAGNOSING

BRUXISM

To obtain the instrumented splint some of the steps

followed in the manufacture of thermoformed splints

are followed. A model of the patient’s teeth needs to

be made, usually by shape copying. This model is

put into a vacuum thermoforming machine in which

a polymer wafer heated to a temperature higher than

its softening temperature covers the model and

reproduces the teeth geometry when a vacuum is

applied on cooling.

The piezoelectric sensors are then placed on

this first thermoformed layer and the operation is

repeated with a second polymer wafer, whereupon

the sensors become embedded within the two layers.

It is important to control the thermoforming

temperature since piezoelectric polymers used as

sensors begin to lose their electromechanical

coupling at temperatures above 80 ºC.

Finally the excess parts are trimmed off and the

splint is subjected to an adjustment and polishing

process to adapt it to the patient. In this way a splint

is obtained like the one shown below in Figure 8,

which enables interdental pressures to be detected

for the purpose of diagnosing bruxism.

Figure 8: Splint with piezoelectric sensors for diagnosing

bruxism.

4 FIRST “IN VITRO” AND “IN

VIVO” TRIALS

4.1 “In Vitro” Trials

To simulate biting in “in vitro” trials, a

pneumatically operated system was constructed in

which moulds could be placed that reproduce

patients’ teeth, and on which the instrumented

splints could be placed. The pneumatic system’s

actuators allow both perpendicular and transversal

bruxism to be simulated with operating pressures of

up to 6 bar in the pneumatic actuators providing bite

forces of 750 N. This is shown in Figure 9.

Figure 9: Bruxism simulator with pneumatic operation for

the “in vitro” trials.

INSTRUMENTED SPLINT FOR THE DIAGNOSIS OF BRUXISM

219

To carry out the “in vitro” trials moulds of the

teeth of 3 patients taking part in the research were

done. Resin reproductions of these teeth were made

as a support for manufacturing the splints, which

could also be placed in the bite simulator to

artificially operate these instrumented splints.

Figures 10 and 11 show the response of the

instrumented splints on being placed in the bite

simulator and subjected to pneumatic operation. The

connectors coming from the splint attached to the

sensors were connected to a charge amplifier and an

analogical-digital converter. This system’s output

was recorded using a data acquisition card

commercially available known as “Measurement

Computing USB 1208-FS”.

The response to prolonged 10-second bites was

studied for with 10-second relaxation between bites.

The sensor response capacity and that of the A/D

amplifier-converter system were also assessed, with

successive bite episodes of different frecuencies.

Response vs. Time

0,5

1,5

2,5

0 20406080100

Figure 10: Simulation of 3 successive bites: Operation at 1

bar ≈ 150 N of bite during 10 s and relaxation during

another 10 s.

Response vs. Time

0,5

1,5

2,5

0 20 40 60 80 100 120 140

Figure 11: Simulation of successive bite episodes

(operation at 1 bar ≈ 150 N of bite): First episode.- 8 bites

with 2 s clenching and relaxation time. Second episode.-

10 bites with 1 s clenching and relaxation time. Third

episode.- 10 bites with 0.2 s clenching and relaxation time.

After a positive assessment of the properties of

the intrabucal pressure detection system and signal

adjustment, processing and recording, “in vivo”

trials were carried out, the results of which are

presented below.

4.2 “In Vivo” Trials

The splints used for the previous trials were used

again with the 3 patients taking part in the research

for the first “in vivo” trials. They enabled the

response of splints in actual mouths to be assessed

and their resistance and duration to be tested, as well

as their water-tightness to avoid any deterioration of

the sensors. The responses recorded both of sudden

and prolonged bites are shown in Figures 12 and 13.

Response vs Time

0,5

1,5

2,5

0 102030405060708090

Figure 12: Output voltage in response to bite impacts

every 15 seconds.

Response vs Time

0,5

1,5

2,5

0 20 40 60 80 100 120

Figure 13: Output voltage with prolonged bites every 15

seconds and 15 seconds of relaxation.

The results of the trials carried out with the

patients’ splints show that it is possible to detect

bruxism episodes of different intensities and

duration which, combined with the ability to record

and store the data, converts the system into a

“Holter” for diagnosing bruxism and evaluating

intrabucal pressures.

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5 RESULTS ASSESSMENT AND

FUTURE ACTIONS

The complete development of a splint to assess

intrabucal pressure and diagnose bruxism and other

occlusive pathologies has been presented. The

system stands out for its use of polymeric

piezoelectric sensors which, on account of their

reduced size, do not produce any alterations to the

patient’s bite. The design process, modelling,

simulation, manufacture and first trials have been

described in detail, both in the pneumatic simulator

and in 3 patients taking part in the research.

Currently, additional trials are being carried out

with a total of 15 patients, with similar results to

those shown in Figure 7. The electronics used

(analogical-digital converter module and charge

amplifier) need to be improved in order to optimise

system response. The results of the “in vivo” trials

are being compared with the behaviour models set

out in order to improve control over the factors

influencing the diagnosis of bruxism using

instrumented splints.

However, what should be highlighted is the

possibility to obtain a device that will enable

intrabucal pressure to be quantitatively assessed and

bruxism behaviour to be diagnosed at an early stage,

so that corrective actions can be programmed before

the appearance of irreversible dental wear. The first

“in vitro” simulations and “in vivo” trials carried out

serve to demonstrate the feasibility of the system in

accordance with the initial objectives.

This work was partial result of “FEMAB

Project: Micro-instrumented Anti-bruxism Splint”

subsidised by the Spanish Ministry of Education and

Science with Reference PROFIT (Promotion of

Technical Research) CIT-020400-2005-17. It has

been carried out in collaboration between

Universidad Politécnica de Madrid and Ibex Estética

Dental S.L..

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