The influence of double flask investing on tooth displacement in dentures processed by microwave...

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The influence of double flask investing on tooth displacementin dentures processed by microwave irradiation

Arcelino Farias Neto, Rodrigo L. dos Santos Sousa and Celia M. Rizzatti-BarbosaDepartment of Prosthesis and Periodontology, Piracicaba Dental School, State University of Campinas, Piracicaba, SP, Brazil

Gerodontology 2011; doi: 10.1111/j.1741-2358.2011.00587.x

The influence of double flask investing on tooth displacement in dentures processed by microwaveirradiation

Objective: This study evaluated the influence of the bimaxillary flask (BMF) and two different investing

materials on first molar inclination in dentures processed by microwave irradiation.

Background: The BMF may minimise tooth displacement, saving time and improving occlusion.

Methods and materials: Forty pairs of dentures were randomised into four groups: stone wall in

monomaxillary flask; silicone wall in BMF; stone wall in BMF; acrylic resin retentions and silicone in BMF.

Dentures were processed by microwave irradiation. Two referential points were established on tooth

surface. A microscope and a digital pachymeter were used to measure the distance between these points,

and the angles a (right maxillary molar), b (left maxillary molar), a¢ (right mandibular molar) and b¢ (left

mandibular molar) were calculated by the law of cosines. Data were submitted to Kruskal–Wallis (5%

significance).

Results: No difference was observed among the groups (p > 0.05). In the intra-group analysis, a was

significantly different for groups I, II and III; a¢, for groups II and IV; b, for all groups; b¢, for groups III and

IV.

Conclusion: First molar inclination was similar for monomaxillary and BMFs. The use of stone or silicone

as investing materials presented the same effect on tooth inclination.

Keywords: complete denture, microwave irradiation, occlusion, polymerisation.

Accepted 4 August 2011

Introduction

Occlusal harmony is an important feature in com-

plete denture treatment, as it influences denture

efficacy and stabilisation1. However, the dental

arrangement established during setting up of teeth

may change during denture flasking and process-

ing. Dimensional changes and distortion of the

denture because of the investing stone mould and

the heating of the acrylic resin can promote teeth

movement and, consequently alterations in occlu-

sal contacts and occlusal vertical dimension2,3.

These changes may result in traumatic occlusion,

irregular distribution of occlusal stresses on

underlying tissues, and alterations of dental func-

tion, denture comfort, and chewing efficiency4.

Many studies have investigated tooth displacement

and its relationship with the variables involved

in denture construction, as flask closure5,6, flask

cooling method and deflasking procedure7, post-

pressing time8, water storage5 and polymerisation

techniques8, but an ideal protocol that leads to

minimum changes has not been established.

The bimaxillary flask (BMF) is a new device

which was designed to solve tooth displacement

during denture processing. Maxillary and mandib-

ular dentures are polymerised at the same time

with teeth in occlusion, which may result in

minimising tooth displacement, saving time and

improving harmony of the occlusal vertical

dimension9,10. Previous investigations have shown

that dentures processed in the BMF exhibit a

reduction in vertical dimensional changes11 and

linear movement of the artificial teeth12 when

compared with those processed in monomaxillary

flask (MMF). Laboratorial analysis revealed that

the BMF does not promote changes in resin prop-

erties such as surface roughness, hardness, and

� 2011 The Gerodontology Society and John Wiley & Sons A/S 1

superficial porosity13. In addition, polymerisation

of both dentures in one investment may save up

20% of material for each denture10. Although

promising, more studies that compare the BMF to

the traditional MMF are necessary before double

flasking become a usual procedure in complete

denture construction.

Therefore, the aim of this study was to evaluate

the influence of the BMF and two different

investing materials (stone and silicone) on first

molar inclination in dentures processed by micro-

wave irradiation. The research hypothesis is that

the BMF may reduce tooth inclination during

denture processing when compared with the MMF

and differences between investing materials are

expected.

Methods and materials

Study design

A sample size of 10 pairs of dentures per group had

been calculated using standard statistical criteria

(a = 0.05, b = 0.20), yielding a power of 80% for

the primary outcome of the study, tooth inclina-

tion. Forty pairs of dentures were randomly divided

into four groups, according to flask type and

investing material: group I – flasking with type III

stone wall (Herodent; Vigodent, Rio de Janeiro, RJ,

Brazil) in MMF (Classico, Sao Paulo, SP, Brazil);

group II – flasking with silicone rubber wall (Labor

Mass; Dental Manufacturing, Rovigo, Italia) and

type III stone (Herodent; Vigodent) in BMF; group

III – flasking with type III stone wall (Herodent;

Vigodent) in BMF; group IV – acrylic resin

(Duralay; Reliance, Worth, IL, USA) retentions

done on buccal first molar surfaces and flasked with

silicone rubber wall (Labor Mass; Dental Manu-

facturing) and type III stone (Herodent; Vigodent)

in BMF. In group I, mandibular and maxillary

dentures were flasked separately in MMF, while in

groups II, III and IV, both dentures were flasked at

the same time in BMF (Dental Vipi, Pirassununga,

SP, Brazil) as previously described (Fig. 1)10. Cya-

noacrylate (Henkel Loctite, Rocky Hill, CT, USA)

was employed to help keep teeth articulated during

flasking. All groups were processed with thermo-

cured acrylic resin (Vipi Wave; Dental Vipi) and

cured by microwave energy (Model AW-42; BSH

Continental, Sao Paulo, SP, Brazil) for 20 min at

20% power, followed by 5 min at 60% power.

After denture curing, flasks were placed on the

bench until reach room temperature. Then, they

were opened and dentures carefully deflasked and

cleaned. All dentures were fixed on their respective

casts with cyanoacrylate and kept under 1 kgf

weight for 1 min. Tooth inclination angles were

measured before and after flasking by a second

operator who was blinded to sample groups.

Sample preparation

Silicone moulds (Rema Sil; Dentaurum, Ispringen,

Germany) were made from two master casts sim-

ulating the edentulous maxillary and mandibular

arches without irregularities in the alveolar ridge

walls. Forty pairs of similar working casts were

poured in type III dental stone (Herodent Soli-

Hock; Vigodent) in a ratio of 30 ml water to 100 g

powder. To construct the maxillary and mandibular

(a) (b)

(c) (d)

Figure 1 (a) I, Polyvinyl-chloride

flask base; II, inferior intermediate

part; III, superior intermediate part;

IV, flask lid; V, screws for microwave

irradiation processing used for

simultaneous polymerisation of

maxillary and mandibular prostheses

with teeth in occlusion. (b) Mandib-

ular cast invested in the flask base

with dentures in occlusion.

(c) Placement of the inferior inter-

mediate part of the flask. (d) After the

stone is set, petroleum jelly was

applied on the stone surface, the

superior intermediate part was placed

together with the flask lid, and it was

then completely filled with the type

III stone.

� 2011 The Gerodontology Society and John Wiley & Sons A/S

2 A. Farias Neto et al.

denture matrixes, metallic spheres with 2 mm

diameter were placed side by side with sticky wax

(Pasom, Sao Paulo, SP, Brazil) over the denture-

bearing area on the master casts (Fig. 2). Then, the

denture bases were waxed to obtain a smooth

surface and uniform width. The mean height of the

occlusion wax rims was 10 mm with anteroposte-

rior inclination. The master casts were mounted in

Bio art 4000 semi-adjustable articulator (Bio Art,

Sao Carlos, SP, Brazil) and artificial teeth model

A25 and 32 M (Vipi-Dent Plus; Dental Vipi) ar-

ranged in bilateral balanced occlusion.

Devices of type IV dental stone (Vel Mix Stone;

Kerr, Orange, CA, USA) were constructed to

duplicate the denture matrixes and obtain forty

pairs of complete dentures with standardised width

and teeth arrangement (Fig. 3). The master cast

and its denture matrix were fixed in a wax plate

(Epoxiglass, Diadema, SP, Brazil) with the occlusal

third of the artificial teeth stuck in the wax and

invested by a wax wall to confine the stone. The

stone was poured in three steps. Briefly, the first

one covered the lateral and anterior surfaces from

the buccal side of one of the cast and denture

halves. After the stone was set, the external sur-

faces of the first part were trimmed and finished.

Splits were created on the surface of the plaster to

guide the fit of the other parts. Petroleum jelly

(Labsynth, Diadema, SP, Brazil) was used as a

separating medium and the stone was poured on

the other half. After the stone was set, the proce-

dures were repeated as the opposite side. The base

of the device was obtained by covering the upper

and posterior surfaces of the whole set. A funnelled

perforation for insertion of liquefied wax was made

at the base of the maxillary and mandibular matrix

devices in the posterior and anterior surfaces,

respectively. Finally, after the denture matrix was

removed, the negative impression was obtained to

place the artificial teeth in the base of the device,

the splits for fitting the working cast, and an empty

space between the device and the cast for waxing

the dentures.

Evaluation of tooth inclination

Briefly, to evaluate the inclination of the first

maxillary molar, a groove parallel to the inclination

of the distolingual cusp grinding surface was made

with a cone round bur no 701 (Maillefer,

Tulsa, OK, USA) at low-speed rotation. The groove

extended from the distolingual cusp tip to the

cervical area of the mesiobuccal cusp. For the first

mandibular molar, the groove was made from the

distobuccal to the mesiolingual cusp. Metallic pins

(Cadena; Coats Textil, Sao Paulo, SP, Brazil) were

fixed with cyanoacrylate at the extremities of the

groove to establish two referential points: A

(occlusal) and B (cervical) (Fig. 4). As shown in

Fig. 4, by the law of cosines the cosine of the angles

a (right maxillary molar), b (left maxillary molar),

a¢ (right mandibular molar) and b¢ (left mandibular

molar) were calculated through the division of the

adjacent cathetus (D) by the hypotenuse (H),

consequently leading to tooth inclination angles.

The hypotenuse (H) was identified as the greatest

linear distance between A and B, and measured by

a digital pachymeter (Starrett, Itu, SP, Brazil) with

an accuracy of 0.01 mm. The distance between A

and B also was measured in the STM microscope

(Olympus Optical, Tokyo, Japan) with an accuracy

of 0.0005 mm and identified as the cathetus (D).

During measurement, the microscope projects A

and B on the same imaginary horizontal plane

through straight lines which are perpendicular to

that plane (Fig. 5a). If tooth rotates, while the

hypotenuse (H) will remain with no changes,

the relative position between A and B observed in

the microscope will be affected, consequently

leading to different tooth inclination angles

(Fig. 5b). Measurements were accomplished by a

Figure 2 Metallic spheres with 2 mm diameter placed

side by side with sticky wax to construct the denture

matrixes.

Figure 3 Stone device constructed to duplicate the

maxillary denture matrix with artificial teeth and work-

ing cast positioned on it.


Flask investing and tooth displacement 3

second operator, characterising a blinded experi-

ment. The values for a, b, a¢ and b¢ were measured

before and after denture processing. Data were

submitted to Kruskal–Wallis at 5% confidence le-

vel. Statistical analysis was carried out with SPSS

version 16.0 (SPSS Inc, Chicago, IL, USA).

Results

The mean values of each angle before and after

denture flasking for all groups are presented in

Tables 1–4. The mean values of the angles a, b,

a¢ and b¢, before and after denture processing,

were not statistically different among the groups

(p > 0.05). Furthermore, intra-group analysis

revealed that after denture processing, the angle awas statistically different (p < 0.05) for groups I, II

and III. The angle a¢ was statistically different

(p < 0.05) for groups II and IV. The angle b was

statistically different (p < 0.05) for all groups. Fi-

nally, the angle b¢ was statistically different

(p < 0.05) for groups III and IV.

Discussion

Studies about the effect of denture processing on

tooth displacement are important to ensure a more

stable occlusal pattern, retention, and functional

quality of complete dentures14. The processing

warpage and base distortion that occur when the

polymerised dentures are removed from the cast

are considered the major disadvantages of acrylic

resin, and these factors can also modify the teeth

position15.

The first BMF designed was metallic for the

polymerisation of both maxillary and mandibular

dentures in warm water bath (Dental Vipi)11. The

polyvinyl-chloride BMF used in this study allows

simultaneous polymerisation of maxillary and

Figure 5 (a) During the measurement of the cathetus D,

the microscope projects the referential points on the

same imaginary horizontal plane through straight lines

which are perpendicular to that plane. (b) If tooth

rotates, the relative position between the referential

points observed in the microscope will be affected,

consequently leading to different tooth inclination

angles.

Figure 4 By the law of cosines, the cosine of the angle awas calculated (right maxillary molar), through the

division of the adjacent cathetus (D) by the hypotenuse

(H).

Table 1 Means and standard deviations for angle a.

Group

Time

Before After

I 39.64 (1.38) Aa 40.79 (1.47) Ab

II 38.20 (2.02) Aa 39.66 (1.94) Ab

III 39.14 (2.35) Aa 40.12 (1.93) Ab

IV 39.49 (4.72) Aa 40.73 (2.75) Aa

Means followed by the same upper case letters in each

column and same lower case letters in each row do not

differ significantly at p < 0.05.



mandibular dentures with teeth in occlusion by

microwave irradiation. This may result in mini-

mising tooth displacement12, saving time, and

improving harmony of occlusion and vertical

dimension10,11. The advantage of microwave irra-

diation is that it reduces polymerisation time and

dough-forming time, provides more homogeneous

dough, and results in improved adaptation of

the denture base16. Microwave polymerisation

provided a lower degree of artificial tooth move-

ment17, although some authors found no differ-

ence between conventional water bath and

microwave energy18,19. The disadvantage of

microwave polymerisation is that it involves the

use of a special flask made specifically for this

technique13.

This study evaluated tooth displacement after

denture polymerisation through the analysis of the

first molar inclination. Based on the results of

intergroup analysis, there was no significant

difference for the mean value of the angles a, b, a¢and b¢ among the groups I, II, III and IV, before and

after denture processing. These results are in

contrast to previous studies where dentures pro-

cessed in the BMF exhibit a reduction in vertical

dimensional changes11 and linear movement of the

artificial teeth12. We suppose that these differences

are related to the methodology applied to measure

tooth displacement in the present study, which was

based on tooth inclination angles, instead of linear

measurements. Also, it is possible that other

variables present during denture processing, as

flask closure method6,7, post-pressing time6, flask

cooling method7, water storage5, and flasking and

polymerisation techniques8, may have exerted

more influence on polymerisation shrinkage, relief

of internal strains after denture deflasking, and

consequently molar teeth displacement, than the

effects because of differences among investing

materials and flask designs evaluated in this study.

Interestingly, the intragroup analysis revealed a

systematic reduction of the standard deviation for

the angle a, after denture processing, in groups II,

III and IV (Table 1). For the last one, this reduction

was quite significant. The same behaviour was

observed for a¢ (Table 2) in groups I, III and IV; for

b (Table 3) in groups III and IV; and for b¢ (Table 4)

in groups I, II and III. As the values of the angles for

the BMF were more uniform, these results of the

intragroup analysis suggest that the BMF may

affect tooth position in a lower degree than the

MMF as previously shown11,12.

The difference between the mean values for an-

gle b before and after denture processing was

statistically significant for all groups (Table 3). The

same results were also verified for angle a in groups

I, II and III (Table 1), for angle a¢ in groups II and

IV (Table 2), and for angle b¢ in groups III e IV

(Table 4). Thus, it is not possible to state that one of

the groups promoted less tooth displacement than

the other. These findings are in agreement with

other studies that did not observe any difference in

tooth displacement when flasking with stone or

silicone20,21, although some studies have found

better results with silicone8,22.

To summarise, the results of this study showed

that the use of the BMF may be associated with

stone or silicone with similar efficiency to the

Table 2 Means and standard deviations for angle a¢.

Group

Time

Before After

I 38.11 (2.25) Aa 38.22 (1.75) Aa

II 37.92 (2.75) Aa 38.47 (2.98) Ab

III 39.46 (1.83) Aa 39.60 (1.33) Aa

IV 39.54 (2.88) Aa 40.22 (2.72) Ab




Table 3 Means and standard deviations for angle b.

Group

Time

Before After

I 38.89 (1.59) Aa 39.95 (2.29) Ab

II 39.47 (2.27) Aa 40.26 (2.72) Ab

III 38.45 (1.93) Aa 40.11 (1.76) Ab

IV 41.98 (5.52) Aa 43.04 (4.25) Ab




Table 4 Means and standard deviations for angle b¢.

Group

Time

Before After

I 39.57 (2.61) Aa 40.34 (2.28) Aa

II 38.84 (2.26) Aa 39.69 (1.88) Aa

III 40.35 (2.03) Aa 39.81 (1.82) Ab

IV 39.82 (2.03) Aa 40.25 (2.18) Ab





Flask investing and tooth displacement 5

MMF. The advantage of using BMF over MMF is

that this new technique makes it possible to poly-

merise both prostheses in dental occlusion in one

investment, saving up 20% of material for each

denture. This procedure is efficient and simple,

maintaining the dental occlusion of the artificial

teeth10, and is also associated with the absence

of changes in resin properties such as surface

roughness, hardness, and superficial porosity, as

demonstrated in a previous experiment13. Thus,

denture processing with BMF by microwave

irradiation seems to be an appropriate procedure.

Conclusion

The research hypothesis of this study was not

confirmed. The inclination of first molar after

denture processing was similar for monomaxillary

and BMFs. In addition, the use of stone or silicone

as investing materials presented the same effect on

tooth inclination.

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Correspondence to:

Arcelino Farias Neto, Faculdade de Odontologia de

Piracicaba, Departamento de Protese e Periodontia,

Av. Limeira, 901, Caixa Postal 52, Piracicaba, CEP:

13414-903, SP, Brazil.

Tel.: 41 19 2106 5211

Fax: 410 19 2106 5218

E-mail: [email protected]



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