## Abstract

## Objective

A theoretical model, based on fluid dynamics, was developed to measure impression 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 produced 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 *= 0.056 *X *^{2 }+ 0.124 *X *. Based on theoretical analysis using a particular viscosity value, the numerical formula yielded was *Y *= 0.057 *X *^{2 }, 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.

## 1

## Introduction

## 1.1

## Pressure involved in making impressions for complete dentures and pressure measurement using sensors

To design a complete denture, the contour of denture foundation area is obtained by using an impression material and an impression tray. During the impression procedure, pressure is exerted by the impression material on basal seat mucosa while seating the impression tray. Basal seat mucosa is compressive tissue, and pressure applied upon it by the impression material causes slight distortion. Therefore, the magnitude and distribution of impression material-produced pressure has an important impact on impression making, because it may result in an inaccurate record of a distorted denture-bearing area .

In dental clinics, escape holes and/or relief space in the impression tray provide relief for the pressure built up in the impression material . To evaluate the changes in impression pressure caused by relief space and escape holes, several investigations were carried out using electric pressure sensors .

Apart from electric pressure sensors, few reports have emerged on using alternative means to measure impression pressure. Nishigawa et al. used video camera recording to visually examine the effects of escape holes and relief space on the dynamics of the impression material during impression tray seating. In another study, Rihani used fluid movements in a manometer to measure the pressure exerted on upper denture-bearing area during impression making.

Today, the use of electric pressure sensors remains the mainstream method of measuring pressure produced during impression making. In many studies, they are either mounted in impression trays or on experimental jaw models to measure pressure . However, as these sensors and measuring devices must be custom-made, they are usually complex and costly. The magnitude and distribution of pressure produced on the edentulous mucosa play a decisive role in final impression making, as the mucosa was distorted easily by excessive pressure during impression taking. For this reason, pressure measurement should not be viewed by patients as a cost-prohibitive task or by dental practitioners as an intimidating task due to the need for many measuring points. Thus, herein lies the impetus to develop an easy and economical method to measure impression pressure.

## 1.2

## Novel theoretical model to measure pressure using impression material flow

During the seating of an impression tray with escape holes or grooves, pressure buildup within the impression material when trapped between the tray and edentulous mucosa forces the impression material to extrude through the escape holes or grooves. On this premise, the authors of this paper hypothesized that the volume of impression material extruded is a function of the pressure exerted by the impression material on basal seat mucosa. If this hypothesis were confirmed, it could be used to develop an apparatus to be an alternative – and a more affordable – means than electric pressure sensors to measure pressure.

To confirm this hypothesis, theoretical analysis was performed based on a fluid dynamics model of exerted pressure versus impression material flow.

## 1.3

## Hypothesis evaluation using electric pressure sensors embedded in edentulous oral mucosa analog and actual impression material flow

To evaluate the hypothesis of this study, an experimental model was constructed in the laboratory. It comprised an edentulous oral mucosa analog embedded with electric pressure sensors to measure exerted pressure. At the same time, the actual volume of impression material extruded in response to each pressure magnitude was also recorded.

## 1.4

## Aim of this study

The aim of this study was to prove the hypothesis by using the constructed experimental model. Based on the theoretical model, a theoretical formula between pressure exerted by impression material and the volume of extruded impression material was derived. On the other hand, regression equation was calculated for the pressure measured by electric pressure sensors and the volume of impression material actually extruded.

If statistical similarity could be observed between the theoretical formula derived from theoretical model and the regression equation calculated from actual measurements acquired from the experimental model, it would prove that impression pressure could be measured by measuring the volume of extruded impression material.

## 2

## Materials and methods

## 2.1

## Theoretical model of impression pressure versus impression material flow

A theoretical analysis was performed using the fluid dynamics model shown in Fig. 1 . In this model, impression material of viscosity *μ *was assumed to be a Newtonian viscoelastic fluid. Force *F *applied to impression material caused the latter to produce pressure *P *, which then caused the impression material to move through silicone tube of diameter *d *at speed *ω *. The eventual length of impression material which flowed into silicon tube was *l *.

It was assumed that the numerical formula for flow of a viscous fluid in a channel could be applied to the theoretical analysis in this study. The calculation formula for the theoretical model given in Table 1 shows that pressure impulse *I *could be expressed as a function of the length of impression material, *l *, flowing into the silicon tube.

P = ( ρ /2) λ (l/ d ) ω ^{2 }= ( ρ /2)(64 μ /d ωρ )(l/ d ) ω ^{2 } |

=32 μ (l/ d ^{2 }) ω |

( λ : coefficient of friction of tube, ρ : density of fluid). |

Next, |

ω = dl / dt = ( P /32 μ ) d ^{2 }/ l |

l dl = ( d ^{2 }/32 μ ) P dt |

Integrating once, we obtain, |

l 2 / 2 = ( d 2 / 32 μ ) ∫ 0 t P d t |

I = (16 μ / d ^{2 }) l ^{2 } |

From these results, impression pressure I = ∫ 0 t P d t can be expressed as a function of length of impression material streamed into tube “ l ”. |

## 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 sensors (PS10KB, Kyowa Dengyo, Japan) were embedded in the edentulous oral mucosa analog to measure the pressure actually exerted by the impression material ( Fig. 2 ).