00005151-200606000-00003

Effect of adding impression material to mandibular denture

space in Piezography

K. IKEBE, I . OKUNO & T. NOKUBI Division of Oromaxillofacial Regeneration, Osaka University Graduate School of

Dentistry, 1–8 Yamadaoka Suita Osaka, Japan

SUMMARY The purpose of the study was to examine

the effect of adding impression material on denture

space using a piezographical record. Subjects were

ten voluntary edentulous patients, aged from61 to 84

years old. A maxillary trial denture with anterior

artificial teeth and a mandibular base plate with a

keel were inserted into the oral cavity. Three ml of

tissue-conditioning materials was injected on the

base plate for each trial. Afterwards, the patients

were instructed to pronounce various phonemes, so

that tongue, cheeks and lips conformed to the

denture space. The impression complexes were cut

at the level of the estimated occlusal plane. Occlusal

analogues were made by duplicating the impression

complexes. Measurements were performed for five

analogues from the first to fifth additions for each

subject. The data were compared using analysis of

variance (ANOVA), and a Friedman’s test followed

by a Bonferroni test for multiple comparisons with a

level of significance at 5%.At themolar andpremolar

positions, the bucco-lingual widths of the occlusal

table increased significantly at incremental injection

of impressionmaterials fromP1 to P4. Themidpoints

of the analogues were located at a distance of 1.5 mm

buccally at the molar position and at a distance of

1.9 mm buccally at the premolar position from the

top of the alveolar crest, independent of the addition

of impression material. It was concluded that den-

ture space was regulated by volume of material and

was located slightly on the buccal side from the crest

of the residual alveolar ridge.

KEYWORDS: complete denture, denture space, pro-

nunciation, polished surface, artificial teeth arrange-

ment

Accepted for publication 10 September 2005

Introduction

In the past, most patients became edentulous at a

sufficiently young age that good adaptation to complete

dentures was possible, even when the dentures differed

from accepted design standards (1). However, recently

patients are experiencing tooth loss later in life, when

the ability of the patient to develop the neuromuscular

skills necessary to wear dentures successfully has

already been physiologically reduced. In particular,

wearing dentures is often difficult for cases in which the

residual ridge is atrophic.

The arrangement of teeth in complete dentures has

been based on mechanical principles. The biology and

physiology of the stomatognatic muscles surrounding

the prosthetic appliance tend not to be considered during

various functions (2, 3). The mandibular complete

denture is usually less stable than themaxillary complete

denture (1, 4), and several dentists have proposed

different techniques for solving this problem (1, 5–7).

The neutral zone philosophy is based on the concept

of a specific space that is considered to exist for each

individual patient, where the tongue forces pressing

outward are neutralized by the contraction of lip and

cheek muscles pressing inward and where the function

of the intra-oral muscles will not dislodge complete

dentures (8–10).

Piezography, a technique used to record shapes by

means of pressure, is a method for recording a patient’s

denture space in relation to oral function (11, 12). This

method provides a mandibular denture with a piezo-

graphically produced lingual surface, which customizes

the contour and precludes over-extension (1). This

technique involves introduction of a mouldable mater-

ª 2006 Blackwell Publishing Ltd doi: 10.1111/j.1365-2842.2005.01582.x

Journal of Oral Rehabilitation 2006 33; 409–415

ial into the mouth to allow unique shaping by various

functional muscle forces. Speech is one function that

can be employed as a selected variable using this

technique. Heath reported that the recording of denture

space morphology varies according to the volume of

material used (13).

The purpose of the present study was to examine,

through Piezography, the effect of the addition of

impression material on the morphology of the man-

dibular denture space, as related to both the polished

surface and arrangement of artificial teeth of complete

dentures.

Materials and methods

Subjects

Ten volunteer edentulous patients (three males and

seven females), ranging in age from 61 to 84 years,

were randomly selected from among outpatients of the

Osaka University Dental Clinic attached to the Dental

School. All patients were free from oral pathologies and

compromised medical conditions. Informed consent

was obtained from each participant, and the protocol

was approved by the Institutional Review Board of the

Osaka University Graduate School of Dentistry.

Construction of maxillary complete denture and mandibular

base plate with a keel

One dentist performed all the clinical and laboratory

work. Maxillary trial complete dentures were manufac-

tured by a conventional method. The maxillary anterior

artificial teeth were arranged so as to restore appearance

and the ability to produce accurate speech. The appro-

priate location and dimensions of the posterior occlusal

rims were given to eachmaxillary trial complete denture

beforehand to record the polished surface of mandibular

dentures using phonetics. The tentative occlusal plane

was made to coincide with the Camper’s plane (14). The

palatal form of the denture was obtained by utilizing a

palatogram (15). Vertical dimension of occlusion was

determined by facial measurement with a Willis Bite

Gauge* and use of a vertical dimension of rest and

intraocclusal rest space (1, 16).

The mandibular base plate was fabricated from self-

polymerizing resin, and the denture base and bilateral

keels were trimmed. The height of the molar part of the

mandibular denture was determined from the estima-

ted occlusal plane and occlusal vertical dimension. The

keels, made of self-polymerizing resin to hold the

impression material, were attached to both the right

and left sides of the denture base (Fig. 1). Keels were

designed so as not to interfere with oral function.

The shape of the polished surface of dentures was

built up as the patients pronounced certain phonemes.

Piezography was used to produce the completed man-

dibular denture space. First, patients were required to

practice the phonemes. Then the maxillary trial denture

and the mandibular base plate with keels were inserted

Keel KeelBase plate

Fig. 1. Maxillary wax denture and mandibular base plate with

keels in oral cavity.

Fig. 2. Tissue conditioning material is injected onto mandibular

base plate using a syringe.*SS White Manufacturing Ltd., Gloucester, UK.

K . I K E B E et al.410

ª 2006 Blackwell Publishing Ltd, Journal of Oral Rehabilitation 33; 409–415

in the oral cavity. Powder and liquid tissue conditioning

material† were mixed and immediately injected onto

the base plate using a dental impression syringe

(Fig. 2). In the present study, the volume of tissue

conditioning material injected each time was 3 mL.

The patients were then asked to pronounce various

sounds so that tongue, cheeks and lips would conform

to the future polished surface of the denture for selected

Japanese sounds. The labial sounds [m], [b] and [p], the

dental sound [s], and the alveolar sounds [t] and [d]

were used in the present study. The patients were

instructed to pronounce the sounds repeatedly for 90 s

before the material set. The register complex with the

base plate, keels and tissue conditioning material was

then removed from the oral cavity. The excessive tissue

conditioning material was trimmed, and the piezo-

graphic record was reinserted into the mouth. Addi-

tional tissue conditioning material was injected over

the previous register complex, and the patient was

asked to repeat the phonemes again in the same way.

This further procedure was repeated five times for all

patients. The final piezographic records were those

obtained in the five further procedures.

These five piezographic records per patient were

seated on the working cast, and the investing cores of

the buccal and lingual indexes were manufactured from

a silicone impression material‡ in order to enclose and

capture the piezographically generated profile. The core

indexes were guided to replace with acrylic resin in

order to make the experimental analogues. The register

complexes were cut at the level of the estimated

occlusal plane. Five experimental analogues (P1–P5)

were manufactured for each patient (Fig. 3).

The measured points of the molar area were defined

using the anterior borders of the retromolar pad as

reference points. The points were 10 mm (RM2, LM2),

15 mm (RM1, LM1) and 20 mm (RP, LP) forward from

the reference points on both the left and right sides

(Fig. 4). In addition, the reference of the midline was

determined by a perpendicular line from the incisive

papilla toward the occlusal plane.

In this study, a non-contact three-dimensional digit-

izer§ was used to measure distances on the occlusal

plane of the experimental analogues. The bucco-lingual

or labio-lingual width of each point (Fig. 5a) and

discrepancies between the midpoint of the bucco-

lingual edge and the anatomical crest of the residual

alveolar ridge (Fig. 5b) were measured. In addition, the

distance between the left and right sides of the midpoint

of the bucco-lingual edge (Fig. 5c) and that between

the right and left sides of the lingual edge were

measured (Fig. 5d). Measurements were performed

for five analogues from the first to fifth additions for

each subject.

P1

P2

P3

P4

P5

Fig. 3. A series of the experimental analogues with acrylic resin

(P1–P5).

10 mm

5 mmRM2

LPRP

LM1RM1LM2

Anterior borders of the retromolar pad

MidlineI

5 mm

Fig. 4. Measured points on the occlusal plane. Molar area 2 (M2)

10 mm forward from the anterior borders of the retromolar pads.

Molar area 1 (M1) 15 mm forward from the anterior borders of

the retromolar pads. Premolar area (P) 20 mm forward from the

anterior borders of the retromolar pads. Incisal points (I) incisive

papilla.

†Tissue Conditioner; Shofu Inc., Kyoto, Japan.‡Lab Silicone; Shofu Inc.§VIVID 700; Minolta Inc., Osaka, Japan.

M AND I B U L A R D EN T UR E S P A C E I N P I E Z OG RA P H Y 411


The data analyses were performed using SPSS 13Æ0for Windows¶. The data were compared using an

analysis of variance (ANOVA), and Friedman’s test

followed by a Bonferroni test for multiple comparisons

with a level of significance at 5%.

Results

Bucco-lingual or labio-lingual width of occlusal plane

In the molar area (M2 and M1), the mean of the

bucco-lingual widths on the occlusal plane was 3Æ2 mm

for P1. The mean increased significantly with each

impression material addition to reach 7Æ2–8Æ8 mm,

whereas the width of P5 showed no significant

difference from that of P4 (Fig. 6). In the premolar

area (P), the mean of the bucco-lingual widths of the

occlusal plane also increased significantly from P1

(2Æ5 mm) to P4 (7Æ0 mm) with each addition, whereas

that of P5 showed no significant difference from that

of P4.

In the incisal area, as six of the ten cases did not

reach the estimated occlusal plane in P1, the labio-

lingual widths from P2 to P5 were measured and

compared. Consequently, there was no significant

difference in width among the experimental analogues

(Fig. 7).

Frontal aspects

Anatomical crest ofresidual alveolar ridge

Buccolingual center of the denture spaceon the occlusal table

(b)(a)

RM2

LPLM1RM1LM2

I

RP

(c)

RM2

LPLM1RM1LM2

RP

(d)

RM2

LPLM1RM1LM2

RP

Fig. 5. Measured items on the

occlusal plane. (a) Bucco-lingual or

labio-lingual width of each point.

(b) Discrepancy between the

midpoint of the bucco-lingual width

of the recorded occlusal table and the

anatomical crest of the residual

alveolar ridge. (c) Distance between

left and right sides of the midpoint of

the bucco-lingual width of the

recorded occlusal table. (d) Distance

between left and right sides of the

lingual edge of the recorded occlusal

table.

P1 P2 P3 P4 P5

Right side Left side(mm) (mm)

0

2

4

6

8

10

Buc

co-li

ngua

l wid

th

0

2

4

6

8

10

Buc

co-li

ngua

l wid

thMeasured points

RM2 RM1 RP LM1LP LM2

Fig. 6. Bucco-lingual width of molar area (mean and standard

deviation).

(mm)

0

2

4

Labi

o-lin

gual

wid

th

P2 P3 P4 P5

Measured points

Fig. 7. Labio-lingual width of incisal area (mean and standard

deviation).¶SPSS Inc., Chicago, IL, USA.



Discrepancy between the midpoint of the bucco-lingual edge

and the crest of the alveolar ridge on the occlusal plane

Compared with the top of the alveolar crest, the

midpoints of the bucco-lingual edge on the occlusal

plane were located 1Æ5 mm buccally on average at both

M2 and M1 and 1Æ9 mm buccally in all cases at point P

(Fig. 8). However, for the same measurement points,

no significant discrepancy was found among the

experimental analogues from P1 to P5.

Distance between right and left sides of the midpoints of the

bucco-lingual edges of the occlusal plane

At each measurement point, the distances between the

left and right sides of the midpoint of the bucco-lingual

edge (Fig. 9) were quite consistent (50 mm at M2,

48 mm at M1, and 40 mm at P) even if the number of

impression material additions was increased, and no

significant difference was found among any of the

registers of P1–P5.

Distance between right and left sides of the lingual edge

of occlusal plane

Distances between the right and left sides of the lingual

edge of the occlusal plane (Fig. 10), i.e. the width of

tongue space, at M2, M1 and P, decreased significantly

from P1 (42, 40 and 37 mm respectively) to P4 (47, 42

and 39 mm respectively) with the addition of impres-

sion material, whereas that of P5 showed no significant

difference from that of P4.

RM2

RM1

RP

02 13

P1P2P3P4P5

Right side

0 21 3

Left side

LM2

LM1

LP

Mea

sure

d po

ints

Distance (mm) Distance (mm)Anatomicalalveolar ridge

Anatomical alveolar ridge

Buccalside

Lingualside

Buccalside

P1P2P3P4P5

Fig. 8. Discrepancy between the

midpoint of the bucco-lingual width

of the recorded occlusal table and the

crest of the alveolar ridge (mean and

standard deviation).

P1 P2 P3 P4 P5

M2 M1 P0

10

20

30

40

50

60D

ista

nce

(mm)

Measured points

Fig. 9. Distance between right and left sides of the midpoints of

the bucco-lingual width of the recorded occlusal table (mean and

standard deviation).

M1M2 P

(mm)

0

10

20

30

40

50

60

Dis

tanc

e

Measured points

P1 P2 P3 P4 P5

Fig. 10. Distance between right and left sides of the lingual edge

of the recorded occlusal table (mean and standard deviation).

M AND I B U L A R D EN T UR E S P A C E I N P I E Z OG RA P H Y 413


Discussion

Regarding complete denture treatment, several meth-

ods that take physiological function into account have

been developed since the 1930s. These studies have

clarified that the bucco-lingual tooth position and the

contour of the polished surface are important for

denture retention and stability (2, 17). Fahmy and

Kharat reported that artificial teeth were arranged over

the center of the alveolar ridges in conventional

dentures, which was found to be better for mastication.

However, all of the participants in their study expressed

a definite sense of superior comfort and speech ability

with the neutral zone denture and selected the neutral

zone denture over the conventional one (6).

Positioning artificial teeth in the neutral zone has two

objectives. First, the teeth will not interfere with

normal muscle function, and second, the forces exerted

by the musculature against the dentures are more

favourable for stability and retention (8).

In this study, Piezography (1) was used to record

denture space by means of the speech function of each

patient. There are several advantages to using speech

for recording the denture space, e.g. the patients can

practice before the impression is taken; the procedure is

easy to understand, especially for the elderly; it is easy

to inspect for proper oral function while the patients

pronounce the phonemes.

However, it remains unclear exactly when the

procedure for obtaining the piezographic record is

complete. For example, in the flange technique (2, 3),

recording of the denture space is complete when the

resoftened flange wax no longer flows toward the

occlusal surface of the occlusal rims. The recording of

denture space morphology has been reported to vary

according to the volume of the material used (1, 13).

In order to address this volumetric variable, a slowly

setting gel, such as tissue conditioning material, was

used. Tissue conditioner** is a soft material used

originally in the conditioning of denture bearing

tissue and in dynamic impression (18). The use of

Tissue Conditioner** for Piezography is advantageous

because it has a suitable viscoelastic property and

setting time and can be injected gradually over several

applications. With regard to initial flow of Tissue

Conditioner, the widths of the rheometer trace at

37 �C are approximately 70% at 3 min and 20% at

15 min (19).

In this study, the impression material was injected

into the oral cavity several times in order to determine

the suitable denture space. To date, the volume and

number of additions of impression materials have not

been clarified. The present study examined the effect of

incremental injections of impression material on the

resultant denture space. At the molar and premolar

positions, the bucco-lingual widths of the experimental

analogues increased significantly with each impression

material addition of 3 mL from the first to the fourth

trial, 12 mL in total.

In this study, except for the bucco-lingual width of

the occlusal table, which increased significantly with

incremental injections of impression material, the rest

of the resultant morphology of the denture space is

believed to be repeatable despite additional introduc-

tions of impression material.

The bucco-lingual widths of commercially available

acrylic resin teeth of the first molar range from 7Æ7 to

9Æ0 mm, and therefore some of the bucco-lingual

widths are larger than the width of the denture space

determined at point M1 in this study. In these cases,

dentists have to select smaller commercially available

teeth, grind the buccal and/or lingual surface, or make

customized teeth to replace artificial teeth in order not

to disturb the function of the removable dentures.

The bucco-lingual center of the occlusal table

obtained by Piezography in this study was located

slightly to the buccal of the residual alveolar ridge.

Morikawa et al. reported that the centerline of the

neutral zone was located 1Æ9 mm to the buccal side of

the alveolar crest (20). Fahmi stated that the longer the

period of edentulousness, the more buccally located the

neutral zone was from the crest of the alveolar ridge

(21). The present results are in agreement with these

previous reports.

The distance between the left and right sides of the

center of the occlusal surface of the experimental

analogue was approximately the same, independent

of the number of recordings, while the width of the

tongue space decreased gradually. This result indicates

that the recorded denture space was expanded both

buccally and lingually, because muscle pressure might

be recorded equally and the horizontal center of the

recorded space was consistent at the occlusal plane in

the different records compared to the horizontal loca-

tion of the crest of the residual ridge.**Shofu Inc.



Conclusions

We examined the effect of the addition of an impression

material on the morphology of the mandibular denture

space by a piezographic technique, as related to both

the polished surface and artificial teeth arrangement

of complete dentures. The denture space was regulated

by volume of material. The horizontal center of the

recorded space at the occlusal plane was located slightly

on the buccal side compared with the horizontal

location of the crest of the residual alveolar ridge,

however, it was consistent in the different records.

Acknowledgments

We greatly appreciate the grammatical correction of the

manuscript by Joanne Madsen, MA. We are especially

grateful to Professor Susumu Nisizaki, Faculty of

Dentistry, University of Uruguay for commenting and

advising on the manuscript. This research was sup-

ported by a Grant-in-Aid for Scientific Research (No.

16390555 and No. 17791388) from Japan Society for

the Promotion of Science.

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Correspondence: Kazunori Ikebe, DDS, PhD, Assistant Professor,

Division of Oromaxillofacial Regeneration, Osaka University Graduate

School of Dentistry, 1-8 Yamadaoka Suita, Osaka, 565-0871, Japan.

E-mail: [email protected]

MAND I B U L A R D EN T UR E S P A C E I N P I E Z OG RA P H Y 415


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