effect of pre-processing methods on bond strength between acrylic resin teeth and acrylic denture...

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Effect of pre-processing methods on bond strength betweenacrylic resin teeth and acrylic denture base resin

Murat Kurt, Yakup Sinasi Sarac, Cagrı Ural and Duygu SaracDepartment of Prosthodontics, Faculty of Dentistry, University of Ondokuz Mayis, Samsun, Turkey

Gerodontology 2011; doi: 10.1111/j.1741-2358.2011.00480.x

Effect of pre-processing methods on bond strength between acrylic resin teeth and acrylic denturebase resin

Objectives: This study evaluated the effects of various pre-processing methods on the bond strength

between resin and denture teeth.

Backgrounds: Debonding of acrylic resin teeth from denture base material is a problem for patients

wearing complete dentures.

Materials and Methods: Four experimental groups (n = 30) were investigated by subjecting tooth–resin

bonding to tensile loading. Specimens were prepared and tested according to the methods of the Inter-

national Standards Organization (ISO 22112:2005) using a special assembly. Four pre-processing surface

treatments of teeth were applied: (i) ST1, no treatment applied (control); (ii) ST2, wax solvent (Dewaks,

Faber Kimya & Ilac, Turkey); (iii) ST3, boiling water followed by conditioning with methyl methacrylate

(MMA) monomer (Meliodent, Bayer Dental, Germany); (iv) ST4, boiling water followed by wax solvent

agent and finally MMA monomer application. Bond strength test was performed using a universal testing

machine.

Results: All the strength values of the test groups were within clinically acceptable limits. The lowest

values were from the ST1 group and the highest values were in the ST4 group.

Conclusions: Wax elimination methods affected bonding strength. Application of wax solvent and MMA

monomer to the ridge lap surfaces of the teeth gave the best results. In clinical practice, this application

procedure may decrease the bonding failure of denture teeth.

Keywords: acrylic denture teeth, denture base resin, bonding strength, wax elimination.

Accepted 14 November 2010

Introduction

Acrylic resin denture base material has been

available to the dental profession for over 60 years,

and although materials with superior properties

have been on the market for some time, it still re-

mains the most popular choice of clinicians. The

requirement of the simple processing equipment

and low cost of the fabrication process are the

reasons for its continued popularity1,2. Acrylic resin

denture base material is therefore particularly rel-

evant considering that the most common reason for

the elderly to seek dental treatment is for the

replacement of missing teeth, either partial or

complete3. However, debonding of acrylic teeth

from denture base resins remains a problem for

both patients and clinicians. The failure rate of

acrylic dentures resulting from fracture has been

reported to be unacceptably high, with the most

prevalent type of failure documented being deb-

onding or fractures of the teeth2,4. It is reported

that approximately 30% of all denture repairs

undertaken by commercial dental laboratories in-

volved faults attributable to the failure of bonding

between the teeth and the denture base resin2.

The variables such as tooth/alveolar ridge position,

occlusion, processing, different denture base

materials and preparation of ridge lap of surfaces of

the teeth are the factors that may also effect the

This research has been presented at ‘‘International Sci-

entific Congress and Exhibition’’ held at _Izmir, Turkey,

during 6–8 November 2009.

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

failure of bonding between denture base materials

and acrylic teeth.

Adequate bonding of acrylic resin teeth to denture

base resin is necessary because it increases the

strength and durability of the denture since the

teeth become an integral part of the prosthesis4.

Artificial teeth in an acrylic structure are often pre-

ferred because they chemically bond to denture base

materials and are easier to adjust. The combination

of acrylic teeth and denture base acrylic is mediated

by polymethyl methacrylate (PMMA), which is

copolymerised with a cross-bonding substance. To

reduce the fracturing of acrylic teeth, a cross-

bonding substance is used in a high proportion5.

Several factors affect bonding, such as wax

remaining on the ridge lap area of the teeth, care-

less application of the separating medium during

processing, insufficient monomer use during pro-

cessing and the polymerisation method used in the

processing of the denture base resin2,6. Spratley7

investigated the effects of contamination by wax,

petroleum jelly and sodium alginate on bond

strength. Wax was found to be the principal con-

taminant and a cause of adhesive failure. In further

research, Cunningham and Benington6 evaluated

some of the variables that affect the bond between

acrylic teeth and denture base resin, including the

effects of resin dough time, tooth surface condition,

processing variables, monomer cementation and

acrylic resin cement. They found that the most

important steps in obtaining high bond strength

were the thorough dewaxing of the tooth surface

and the application of resin cement. The lack of

adequate bonding is considered to be the result of

incompatible surface conditions at the tooth/base

interface8. One probable explanation for bonding

failure could be the presence of impurities on the

tooth surface. Impurities could include residual

wax8 because of incomplete elimination or con-

tamination of the ridge lap surfaces with tin-foil

substitutes9. These materials can prevent chemical

bonding between acrylic teeth and denture base

resins10. Papazoglu and Vasilas11 investigated the

tensile bond strengths of composite and autop-

olymerising acrylic resins to acrylic teeth. They

reported that acrylic resin and acrylic teeth pre-

wetted with MMA monomer demonstrated high

bonding strength. Yanikoglu et al.5 investigated the

shear bond strength between light cured composite

and autopolymerised acrylic resin and acrylic resin

teeth. In that study, the surface treatments used for

the acrylic teeth included wetting with MMA

monomer, composite bonding agent and acid

etching. The highest bond strength was obtained

for wetting with monomer.

Cunningham and Benington8 reported that

using a wax eliminator is an effective method for

wax elimination. A series of studies6–8,12,13 have

evaluated wax elimination methods. However,

there is a deficiency of published reports evaluating

the combined effects of the application of MMA

monomer and wax solvent on bonding strength

between acrylic base resin and acrylic teeth.

The purpose of this study was to investigate the

effects of pre-bonding preparation on the bond

strength between acrylic teeth and denture base

resin. The research hypothesis tested was that the

surfaces of the acrylic teeth to which wax solvent

had been applied prior to MMA monomer wetting

would be more retentive than untreated surfaces.

Materials and methods

Twenty sets containing six upper anterior cross-

linked, acrylic denture teeth (Enigma, Davis

Schottlander & Davis Ltd, Letchworth, Herts, Eng-

land) and a conventional heat-polymerised PMMA

denture base material (Meliodent, Bayer Dental,

Germany) were used. Care was taken to ensure

that the teeth moulds were comparable in size and

shape. A metal former (70 · 25 · 7 mm) of a

design specified by the International Standards

Organization (ISO 22112:2005)14 and incorporat-

ing a trough 5 mm wide by 1.5 mm deep for

mounting the teeth was used for the preparation of

the test specimens as shown in Fig. 1. A standar-

dised silicon positioning device with an open

square arrangement (5 · 5 mm) was prepared to

demarcate the ridge lap surfaces of the teeth.

Within the defined area, they were ground with a

carbide burr, followed by finishing with silicone

Figure 1 Schematic view of the metal former that used

in the study.

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

2 M. Kurt et al.

carbide papers with grits of 600, 800 and 1000 in

succession under water cooling to create a flat

surface. Flattering the surface of the cross-linked

teeth does not affect the cross-linking within the

teeth, because the cross-linking is part of the matrix

of the acrylic resin polymer tooth15. The samples

were ultrasonically cleaned in a distilled water bath

for 10 min to remove trapped residue and dried

before further processing. The teeth were subse-

quently mounted on the metal former using base

plate wax (Truwax Base Plate Wax, Dentsply,

Canada). No wax material was allowed to envelop

the necks of teeth. After placing the metal former

with the teeth in a denture base flask (Hanau

Engineering Company Inc., Buffalo, NY, USA) by

using dental stone (Kerr Model Stone, Romulus,

MI, USA), the metal former was removed and the

wax was flushed from the teeth with 90�C tap

water for 30 s. The twenty sets of six teeth in the

denture base flasks were then randomly allocated

to the four groups for the various surface treat-

ments:

1 ST1 (Control): No treatment was applied.

2 ST2: Wax solvent (Dewaks, Faber Kimya & _Ilac,

Turkey) was applied for 5 min according to the

manufacturer’s recommendations and then rinsed

with boiling water.

3 ST3: Conditioning with MMA monomer (Melio-

dent, Bayer Dental, Germany) for 10 s, air drying

and then recoating with MMA before resin packing.

4 ST4: Wax solvent was applied for 5 min, accord-

ing to the manufacturer’s recommendations. The

residual wax solvent was rinsed off with boiling

water, MMA monomer was applied for 10 s,

allowed to air dry and recoated with MMA before

resin packing.

The stone surfaces were coated with mould seal

varnish isolant (Dentsply, Detrey Division, Surrey,

England) and dried. The specimens were packed

with heat-polymerising acrylic denture base resin,

processed for 9 h at 74�C and bench cooled for 1 h

before deflasking3.

The test specimens were removed from the stone

by using a walnut blaster and cleaned. The bases of

the specimens were then ground flat to form a

stable base for testing. All specimens were subse-

quently stored in a distilled water bath at 37�C for

7 days and then thermocycled between 4 and 60�Cwith 30-s dwell times for 5000 cycles. The acrylic

blocks with the teeth were mounted in a universal

testing machine (Lloyd LRX, Llyod Instruments

PIC., England) according to the ISO 22112:2005

directions. Tensile strength testing was then per-

formed with a crosshead speed of 5 mm/min)1

until failure occurred. The load at fracture was

recorded in Newtons (N), which was converted to

MPa. The specimen surfaces were examined under

a magnifying glass to assess whether the mode of

failure was cohesive, adhesive or a combination of

the previous two modes.

To test for normal distribution, the tensile bond

strength values were evaluated with the Kol-

mogorov–Simirnov test. Mean tensile bond

strength values were then analysed by one-way

analysis of variance (ANOVA). One specimen from

each group was imaged and evaluated by scanning

(a) (b)

(c) (d)

Figure 2 SEM view of tested

groups. (a), No surface treatment;

(b), Monomer; (c), Wax Solvent

Agent; (d), Wax solvent

agent-monomer.


Bond strength of resin teeth to denture base 3

electron microscope (SEM) (Jeol JSM 6335-F; Jeol

Ltd, Portland, OR, USA) after the application of

surface treatments (Fig. 2).

Results

All the tensile bond strength values were evaluated

with the Kolmogorov–Simirnov test and were

normally distributed (p > 0.05). Values were also

analysed with one-way ANOVA and post-hoc Tukey.

One-way ANOVA of the data is presented in Table 1,

and the mean tensile bond strengths and standard

deviations are shown in Table 2. Surface treatment

methods affected the tensile bond strength values.

The highest mean values were from the ST4 group

(35.28 MPa), and the lowest values were obtained

from the ST1 (control) group (17.65 MPa). All the

other groups showed significant differences from

the control group (p < 0.05). Most adhesive failures

in the control group and most cohesive failures in

the denture base resin were seen in the ST4 group.

Percentages of failure types of the test specimens

are listed in Table 3.

Discussion

A series of studies have attempted to determine the

quality of denture tooth bonding produced by

commonly employed dental laboratory conditions.

Modification of denture tooth ridge lap surface by

sandblasting, grinding, cutting grooves, preparation

of diatoric and the teeth are the methods that have

been proposed in the literature to increase the

bond strength between denture base materials and

acrylic teeth4,6,16,17.

In the present study, the wax elimination

methods significantly affected the bonding values

of acrylic teeth to their denture base so the research

hypothesis was accepted. Although a good chemi-

cal bonding is usually achieved between acrylic

teeth and denture base materials, separation does

occur, as does the fracturing of teeth. This may be

attributed to traces of wax remaining on the ridge

laps of the teeth or to the careless application of the

separating medium during processing5.

In the present study, the highest tensile bond

strengths were obtained in the ST4 group and the

lowest values were obtained in the control group.

Mean bond strength of the ST4 group was higher

than the stated ISO-3336 minimum of 31 MPa13.

Wax elimination from the surfaces of acrylic teeth

at a range of water temperatures showed wax

retention up to 90�C8. Although in the control

group wax was eliminated with only 90�C tap

water, this procedure was inadequate for wax

elimination. It was determined that teeth with no

prior wax contact had a bond strength of

35.36 MPa, while those which were dewaxed with

boiling water only reached a value of only

14.28 MPa3. These strengths are in approximate

agreement with the ST4 group (35.28 MPa) and

the control (17.65 MPa), respectively, from this

study. The effect on bond strength of monomer

coating of the tooth ridge lap has been studied, and

the results suggest that the modifications before

packing the resin increased the adhesive bond

strength, whereas conflicting results for the use of

monomer have also been reported in two stud-

ies7,9. In a related study, a comparison was made of

the shear bond strength of microwave-polymerised

and conventional water bath-cured polymethyl

Table 1 One-way ANOVA test.Sum of squares df Mean square F Sig.

Between groups 5335.344 3 1778.448 54.795 0.000

Within groups 3764.946 116 32.456

Total 9100.290 119

Table 2 Mean and standard deviations of tensile bond

strength values in each of four groups.

Test

groups n Mean (MPa)

Standard

deviation

ST1 30 17.65a ±3.86

ST2 30 29.41b ±5.14

ST3 30 22.74c ±5.20

ST4 30 35.28d ±7.84

Same letter are not statistically different (p > 0.05).

Table 3 Percentage of failure types of tested specimens.

Adhesive Cohesive

Adhesive +

Cohesive

ST1 (Control) 57 6 37

ST2 (Dewax) 7 27 66

ST3 (Monomer) 10 13 77

ST4 (Dewax-Monomer) 7 43 50


4 M. Kurt et al.

methacrylate (PMMA) bonding to denture teeth.

Priming of the denture tooth surface with mono-

mer liquid produced significantly higher bond

strengths than other surface treatments3. An ear-

lier study by Barpal et al.18 showed that monomer-

treated teeth composed of Ivocap high-impact

acrylic resin had a significantly decreased failure

load.

Cunningham and Benington6 found that mono-

mer application to the tooth surface significantly

improved bond strength. The greatest bond

strength to denture teeth was seen in acrylic resins

polymerised under pressure and wetted with

monomer5. According to Marra et al.10, Lucitone

550 denture base resin showed the highest mean

shear bond strength after surface treatment with

MMA. Saavedra et al.16 recorded similar results

with the same denture base resin after application

of an MMA-based surface treatment. The current

study demonstrated that monomer application to

the tooth surface before packing the resin improved

tensile bond strength (22.74 MPa).

When acrylic resin teeth were wetted with

MMA, it plasticised their surfaces and diffused into

the acrylic resin12,17. The SEM images demon-

strated morphologic features of tooth surface that

differed when treated with wax solvent and

MMA looked more smoother in texture than the

untreated control group.

Although an effective application time for MMA

for increasing repair strength was reported

by Vallittu et al.,12 there are no guidelines in the

literature concerning the most effective applica-

tion time for MMA to improve the bond strength

between denture teeth and acrylic resin denture

base. In this study, the teeth were treated with

monomer for 10 s in the ST3 and ST4 groups. The

application time of the monomer is reflected in

variability in bond strength values amongst related

studies2,12,17.

Higher bond strengths were obtained with the

application of dewaxing agent in the ST2 group

compared to the ST1 and ST3 groups. Wax con-

tamination of teeth has long been known to be a

major cause of denture tooth failure6–8,12,13. That

finding was reaffirmed by this study. Wax solvent

agents have been stated to be essential for complete

wax removal. It was also concluded that no stan-

dard technique existed amongst dental technicians

for the preparation of acrylic teeth prior to acrylic

resin polymerisation and that the use of a wax

solvent was recommended before packing the

resin3. However, the most favourable application

time for wax solvents was not well defined in

earlier studies1,6,8.

In the present study, the tensile bond strength

between acrylic teeth and denture base resin was

significantly increased by the separate use of MMA

and wax solvent agent. The authors of the current

study are not aware of earlier studies comparing

the effectiveness of these two agents (wax sol-

vent + MMA) separately. The current research

showed that the highest tensile bond strength

value (35.28 MPa) was obtained with the applica-

tion of wax solvent agent prior to MMA treatment.

It appears that the wax solvent created a dewaxed

and cleaned surface on which MMA would be

more effective.

In related observations evaluating the quality

of denture tooth bonding to denture base

resin1–6,8,10,16,19–26, there is wide variation in the

materials tested and the methodology used for

constructing and testing the samples. The differ-

ences in bond strength values may be explained by

the size and shape of the specimens or the method

and magnitude of loading used during the test. The

ability of acrylic teeth to bond to denture base resins

may also be affected by the type of tooth material

(conventional or cross-linked) and denture base

resins. Although Takahashi et al.17 reported that

conventional resin teeth bonded more strongly

than cross-linked denture teeth to denture base

resins, Chai et al.19 concluded that there was no

significant difference in bond strength between

conventional resinous and highly cross-linked

denture teeth to a pour-type denture base resin.

Further studies are therefore needed to clarify the

influence of different solvent agents, combinations

of them and application times on the bond strengths

of various types of denture teeth and base resins.

Conclusions

Within the limitations of the current study, the

following conclusions can be drawn:

1 Bonding between acrylic denture teeth and base

resin was improved by the use of MMA and wax

solvent separately.

2 Use of wax solvent prior to monomer application

achieved the highest bond strength.

3 The use of a wax solvent on the ridge lap surfaces

of denture teeth before wetting with MMA

monomer and before packing the acrylic resin may

reduce bonding failures.

Acknowledgement

The authors thank Gregory T. Sullivan of OYDEM,

Ondokuz Mayis University in Samsun, Turkey, for

editing an earlier version of this manuscript.


Bond strength of resin teeth to denture base 5

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Correspondence to:

Dr. Murat Kurt, Ondokuz Mayıs Universitesi Dis

Hekimligi Fakultesi, Protetik Dis Tedavisi Anabilim

Dalı, 55139 Kurupelit-Samsun/Turkiye.

Tel.: +90 362 3121919-3686

Fax: +90 362 4576032

E-mail: [email protected]


6 M. Kurt et al.

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