d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 530–534

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New theoretical model to measure pressure producedduring impression procedure for complete dentures—Visualinspection of impression material flow

G. Nishigawaa,∗, Y. Maruoa, M. Irieb, M. Okac, Y. Tamadaa, S. Minagi c

a Occlusion and Removable Prosthodontics, Okayama University Hospital, 2-5-1, Shikata-cho, Okayama 700-8525, Japanb Department of Biomaterials, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1,Shikata-cho, Kitaku, Okayama City 700-8525, Japanc Department of Occlusal and Oral Functional Rehabilitation, Okayama University Graduate School of Medicine, Dentistry andPharmaceutical Sciences, 2-5-1, Shikata-cho, Kitaku, Okayama City 700-8525, Japan

a r t i c l e i n f o

Article history:

Received 24 July 2012

Received in revised form

6 November 2012

Accepted 13 February 2013

Keywords:

Impression

Pressure

Complete denture

a b s t r a c t

Objective. A theoretical model, based on fluid dynamics, was developed to measure impres-

sion pressure. The purpose of this study was to evaluate the validity of this theoretical model

by comparing its theoretical analysis against actual pressure measurements conducted

using an impression tray and edentulous oral mucosa analog embedded with pressure

sensors.

Methods. In the theoretical model, a hollow tube was mounted onto an impression tray

by penetrating through the tray. When force was applied to the tray, pressure was pro-

duced which then caused the impression material to flow into the hollow tube. Length of

impression material which flowed into tube was denoted as l. In the calculation formula

for theoretical model, pressure impulse I was expressed as a function of impression flow

length l. For actual pressure measurements, four electric pressure sensors were embedded

in an experimental edentulous arch. To visually observe and measure length of impression

material flow, four transparent silicon tubes were mounted vertically at different positions

on tray. During tray seating, impression material flowed into tubes and pressure which

caused material flow movement was measured by the embedded sensor at each tube’s

position.

Results. Based on actual pressure measurements under one experimental condition,

regression analysis of pressure data acquired from electric sensors yielded the formula,

Y = 0.056X2 + 0.124X. Based on theoretical analysis using a particular viscosity value, the

numerical formula yielded was Y = 0.057X2, which resembled that of the regression formula.

Significance. Theoretical model presented in this paper augured well for clinical application

as an easy and economical means to examine magnitude and distribution of impression

pressure by measuring lengths of impression material flow in tubes fixed to impression

tray.

© 2013 Academy

∗ Corresponding author at: Occlusion and Removable Prosthodontics, OKitaku, Okayama City 700-8525, Japan. Tel.: +81 86 235 6687; fax: +81 86

E-mail address: goro@md.okayama-u.ac.jp (G. Nishigawa).0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Puhttp://dx.doi.org/10.1016/j.dental.2013.02.005

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

kayama University Hospital, 2-5-1, Shikata-cho, 235 6689.

blished by Elsevier Ltd. All rights reserved.

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. Introduction

.1. Pressure involved in making impressions foromplete dentures and pressure measurement usingensors

o design a complete denture, the contour of denture founda-ion area is obtained by using an impression material and anmpression tray. During the impression procedure, pressure isxerted by the impression material on basal seat mucosa whileeating the impression tray. Basal seat mucosa is compressiveissue, and pressure applied upon it by the impression mate-ial causes slight distortion. Therefore, the magnitude andistribution of impression material-produced pressure hasn important impact on impression making, because it mayesult in an inaccurate record of a distorted denture-bearingrea [1–5].

In dental clinics, escape holes and/or relief space in thempression tray provide relief for the pressure built up in thempression material [6–10]. To evaluate the changes in impres-ion pressure caused by relief space and escape holes, severalnvestigations were carried out using electric pressure sensors2,11,12].

Apart from electric pressure sensors, few reports havemerged on using alternative means to measure impressionressure. Nishigawa et al. [13] used video camera recording toisually examine the effects of escape holes and relief spacen the dynamics of the impression material during impressionray seating. In another study, Rihani [14] used fluid move-

ents in a manometer to measure the pressure exerted onpper denture-bearing area during impression making.

Today, the use of electric pressure sensors remains theainstream method of measuring pressure produced during

mpression making. In many studies, they are either mountedn impression trays or on experimental jaw models to measureressure [11,15–17]. However, as these sensors and measuringevices must be custom-made, they are usually complex andostly. The magnitude and distribution of pressure producedn the edentulous mucosa play a decisive role in final impres-ion making, as the mucosa was distorted easily by excessiveressure during impression taking. For this reason, pressureeasurement should not be viewed by patients as a cost-

rohibitive task or by dental practitioners as an intimidatingask due to the need for many measuring points. Thus, hereinies the impetus to develop an easy and economical methodo measure impression pressure.

.2. Novel theoretical model to measure pressure usingmpression material flow

uring the seating of an impression tray with escape holesr grooves, pressure buildup within the impression materialhen trapped between the tray and edentulous mucosa forces

he impression material to extrude through the escape holesr grooves. On this premise, the authors of this paper hypoth-

sized that the volume of impression material extrudeds a function of the pressure exerted by the impression

aterial on basal seat mucosa. If this hypothesis were con-rmed, it could be used to develop an apparatus to be an

( 2 0 1 3 ) 530–534 531

alternative – and a more affordable – means than electric pres-sure sensors to measure pressure.

To confirm this hypothesis, theoretical analysis was per-formed based on a fluid dynamics model of exerted pressureversus impression material flow.

1.3. Hypothesis evaluation using electric pressuresensors embedded in edentulous oral mucosa analog andactual impression material flow

To evaluate the hypothesis of this study, an experimen-tal model was constructed in the laboratory. It comprisedan edentulous oral mucosa analog embedded with electricpressure sensors to measure exerted pressure. At the sametime, the actual volume of impression material extruded inresponse to each pressure magnitude was also recorded.

1.4. Aim of this study

The aim of this study was to prove the hypothesis by usingthe constructed experimental model. Based on the theoreti-cal model, a theoretical formula between pressure exerted byimpression material and the volume of extruded impressionmaterial was derived. On the other hand, regression equationwas calculated for the pressure measured by electric pres-sure sensors and the volume of impression material actuallyextruded.

If statistical similarity could be observed between thetheoretical formula derived from theoretical model and theregression equation calculated from actual measurementsacquired from the experimental model, it would prove thatimpression pressure could be measured by measuring the vol-ume of extruded impression material.

2. Materials and methods

2.1. Theoretical model of impression pressure versusimpression material flow

A theoretical analysis was performed using the fluid dynamicsmodel shown in Fig. 1. In this model, impression material ofviscosity � was assumed to be a Newtonian viscoelastic fluid.Force F applied to impression material caused the latter toproduce pressure P, which then caused the impression mate-rial to move through silicone tube of diameter d at speed ω.The eventual length of impression material which flowed intosilicon tube was l.

It was assumed that the numerical formula for flow of a vis-cous fluid in a channel [18] could be applied to the theoreticalanalysis in this study. The calculation formula for the theo-retical model given in Table 1 shows that pressure impulse Icould be expressed as a function of the length of impressionmaterial, l, flowing into the silicon tube.

2.2. Experimental edentulous arch model and

impression material flow

The experimental edentulous arch was a flat, circular-shaped,acrylic model of 60 mm diameter. Four electric pressure

532 d e n t a l m a t e r i a l s 2

F

Impression Material

(viscosit y: μ)

d

l

P

ω: speed of impress ion materia l

moved through si licone tube

Fig. 1 – Theoretical model of impression pressure versusimpression material flow. Force F applied to impressionmaterial (viscosity �) caused pressure P to build up withinimpression material. P forced material to flow into siliconetube (diameter d) with speed ω. Length of material flow intotube is l.

Table 1 – Calculation formula for theoretical model andanalysis based on fluid dynamics.

P = (�/2)�(l/d)ω2 = (�/2)(64�/dω�)(l/d)ω2

=32�(l/d2)ω(�: coefficient of friction of tube, �: density of fluid).

Next,ω = dl/dt = (P/32�)d2/ll dl = (d2/32�)P dt

Integrating once, we obtain,

l2/2 = (d2/32�)∫ t

0P dt

I = (16�/d2)l2

From these results, impression pressure I

(=

∫ tP dt

)can be

theoretical model: Y = 0.057X , 0.065X , and 0.073X (Y: pres-

0

expressed as a function of length of impression materialstreamed into tube “l”.

sensors (PS10KB, Kyowa Dengyo, Japan) were embedded inthe edentulous oral mucosa analog to measure the pressure

actually exerted by the impression material (Fig. 2).

Flat impression tray (2 mm thickness), designed to fitclosely with the experimental edentulous arch model, was

Fig. 2 – Measurement of actual impression flow and pressure uselectrical pressure sensors.

9 ( 2 0 1 3 ) 530–534

made from transparent acrylic. To enable visual observationand recording of the volume of impression material extrudedfrom holes in tray, four transparent silicone tubes (diame-ter: 1.0 mm, length: 60 mm) were vertically mounted on theimpression tray and positioned at the center of each of the fourpressure sensors embedded in the experimental arch model(Fig. 2). With this setup, the volume of impression materialextruded from the inside of tray could be visually observed; atthe same time, the pressure which caused the extrusion couldbe recorded from the corresponding pressure sensor embed-ded in the experimental arch model.

2.3. Measurement of impression pressure and volumeof impression material extruded from tray intotransparent tubes

A polysulfide rubber impression material (Surflex F Regular,GC Corp., Japan) was mixed and placed into the impres-sion tray. Two tray spacer thicknesses were used: 0.5 and1.0 mm. Tray was then seated on the experimental edentulousarch model at a speed of 10 or 20 mm/min using a univer-sal testing machine (Autograph DCSC-2000, Shimadzu, Japan).Seating continued until each tray generated, respectively, 0.5or 1.0 mm of impression material movement.

During tray seating, signals registered by the four pressuresensors embedded in the experimental edentulous arch modelwere transmitted to a Signal Processor (Nihon Denki San-ei,Japan) (Fig. 2). Pressure impulses detected from each sensorwere calculated by this processor. Lengths of impression mate-rial which were extruded into the transparent silicone tubes,as caused by built-up pressure, were measured.

3. Results

During the tray seating procedure of the experiment, three vis-cosity values of the impression material were measured usinga rheometer: 350, 400, and 450 Pa s. Accordingly, three numer-ical formulae – each corresponding to a viscosity value – wereobtained from the calculation formula of [I = (16 �/d2)l2] for the

2 2 2

sure impulse produced by impression material; X: length ofimpression material in tube; viscosity � of impression mate-rial: 350, 400, or 450 Pa s; d: 1.0 mm).

ing an edentulous oral mucosa analog with embedded

d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 530–534 533

Fig. 3 – Relation between length of impression material extruded into tube and pressure measured by sensors affixed one ickne

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xperimental edentulous arch model surface (tray spacer th

Relations between the length of impression materialxtruded into the transparent silicon tubes (X-axis) and theagnitude of pressure measured by the sensors (Y-axis) are

hown in Figs. 3 and 4. Using regression analysis, four regres-ion formulae were obtained from two seating speeds and tworay spacer/impression material thicknesses.

As seen in Figs. 3 and 4, coefficients of the theoretical andegression formulae did not show proximate values. Nonethe-ess, curves of the four regression formulae calculated fromhe experimental edentulous arch model were drawn withinhe area of the curves of the three theoretical formulae for the

ost part. Therefore, both the experimental and theoreticalesults seemed to show some similarity.

. Discussion

.1. Theoretical model versus experimental edentulous

rch model to measure pressure

he authors of this paper hypothesized that the volume ofmpression material extruded from escape holes in tray is a

ig. 4 – Relation between length of impression material extrudedxperimental edentulous arch model surface (tray spacer thickne

ss: 0.5 mm).

function of the pressure built up during tray seating. In theconceptualizing of the theoretical model, volume of extrudedimpression was represented by the length of impression mate-rial extruded into transparent tube to simplify the model –and thus the vertical mounting of silicon tubes into tray onthe experimental model. From the theoretical model, a cal-culation formula of [I = (16 �/d2)l2] showing the relationshipbetween pressure impulse and volume of extruded impressionmaterial was obtained.

At the experimental edentulous arch model, electric pres-sure sensors were affixed on the experimental jaw modelsurface, not on the tray surface. During impression tray sea-ting, pressure signals registered by the sensors were recorded.Transparent silicon tubes were fixed in the tray right abovethe center of each pressure sensor, ensuring that the pres-sure built up in the impression and at each tube’s positionagreed with the pressure at sensor surface. On this premise,

relation between length of impression material extrudedinto tube and pressure magnitude at each tube’s positioncould be examined using the experimental edentulous archmodel.

into tube and pressure measured by sensors affixed onss: 1.0 mm).

l s 2

r

534 d e n t a l m a t e r i a

4.2. Comparison of numerical formulae obtained fromtheoretical and experimental models

Despite differences in tray seating speed (10 or 2 mm/min) andtray spacer thickness (0.5 or 1.0 mm), similar quadratic curvesshowing the relation between actual pressure measured andlength of impression material extruded into tubes were drawn(Figs. 3 and 4). These results showed that the relation betweenimpression material and pressure was approximately consis-tent regardless of tray seating speed or relief space thickness.

Viscosity of the impression material increased after mix-ing, and it changed continuously during the entire pressuremeasurement period. Viscosity at the beginning of pressuremeasurement was approximately 350 Pa s, and it increased to450 Pa s at the end of tray seating procedure. Therefore, threenumerical formulae according to the viscosity values of 350,400, and 450 Pa s were used for validity evaluation of the the-oretical model.

The three numerical formulae obtained from the theoreti-cal model (I = (16 �/d2)l2; Table 1) seemed to bear resemblanceto the four regression formulae obtained from the experimen-tal model (Figs. 3 and 4). These results suggested that thetheoretical model presented in this paper might have univer-sal validity. Therefore, the numerical formula obtained fromtheoretical analysis could tolerate variations in tray seatingspeed and tray spacer thickness.

4.3. Application of theoretical model for pressuremeasurement in clinical situations

Only one kind of impression material was evaluated in thisstudy. Further investigations are needed to clarify the relation-ship between impression pressure and volume of extrudedmaterial from tray for different kinds of impression materialswith different rheological properties.

Within the limitations of this study, it was concluded thatmeasuring the impression material extruded from escapeholes and grooves in tray could potentially be an alternativemethod of measuring impression pressure. The transparenttubes enabled clear visual inspection of the magnitude anddistribution of pressure during impression tray seating on theedentulous arch.

In terms of clinical significance and implications, the find-ings of this study could be leveraged to develop an easy andeconomical apparatus that does not require electric sensorsto measure pressure during impression making in the future.

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