Effe

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Murat Alkurt, DD

This study was supported by ProjecInternational Congress of the Acade

aResearch Assistant, Department ofbProfessor, Department of ProsthodcResearch Assistant, Department of

Alkurt et al

ct of repair resin type and surface

tment on the repair strength of

t-polymerized denture base resin

S,a Zeynep Yesil Duymus, DDS, PhD,b andMustafa Gundogdu, DDSc

Faculty of Dentistry, Atatürk University, Erzurum, Turkey; Facultyof Dentistry, Recep Tayyip Erdo�gan University, Rize, Turkey

Statement of Problem. Acrylic resin denture fracture is common in prosthodontic practice. When fractured denture bases arerepaired, recurrent fractures frequently occur at the repair surface interface or adjacent areas.

Purpose. The purpose of this study was to evaluate the effect of different surface treatments on the flexural strength ofthe acrylic resin denture base repaired with heat-polymerized acrylic resin, autopolymerizing resin, and light-polymerizedacrylic resin.

Material and Methods. Ninety-six specimens of heat-polymerized acrylic resin were prepared according to the AmericanDental Association Specification No. 12 (65.0 � 10.0 � 2.5 mm) and sectioned into halves to create a repair gap (3.0 �10 � 2.5 mm). The sectioned specimens were divided into 3 groups according to their repair materials. The specimens fromeach group were divided into 4 subgroups according to their surface treatments: a control group without any surfacetreatment; an experimental group treated with methyl methacrylate monomer (MMA group); an experimental group treatedwith airborne-particle abrasion with aluminum oxide particles of 250-mm particle size (abrasion group); and an experimentalgroup treated with erbium:yttrium-aluminum-garnet laser (laser group). After the surface treatments, the 3 materials wereplaced into the repair gaps and then polymerized. After all of the specimens had been ground and polished, they were storedin distilled water at 37�C for 1 week and subjected to a 3-point bend test. Data were analyzed with a 2-way analysis ofvariance, and the Tukey honestly significant difference test was performed to identify significant differences (a¼.05). Theeffects of the surface treatments and repair resins on the surface of the denture base resin were examined with scanningelectron microscopy.

Results. Significant differences were found among the groups in terms of repair resin type (P<.001). All surface-treatedspecimens had higher flexural strength than controls, except the surface treated with the methyl methacrylate in the heat-polymerized group. A significant difference between the control and abrasion groups (P¼.013) was found. The scanningelectron microscopy observations showed that the application of surface treatments modified the surface of the denturebase resin.

Conclusions. The repair procedure with heat-polymerized resin exhibited significantly higher flexural strength thanthat of the autopolymerized and light-polymerized resins. In addition, the airborne-particle abrasion with aluminumoxide particles of 250-mm particle size improved the flexural strength of the specimens tested. (J Prosthet Dent2014;111:71-78)

t No. 2005/102 (Atatürk University). A poster presentation of this study was presented at the 10thmy of Prosthodontics and Gnathological Society, April 2012, Antakya, Turkey.

Prosthodontics, Faculty of Dentistry, Atatürk University.ontics, Faculty of Dentistry, Recep Tayyip Erdo�gan University.Prosthodontics, Faculty of Dentistry, Atatürk University.

72 Volume 111 Issue 1

The Journal of

Clinical Implications

Based on the results of this study, heat-polymerized acrylic resin canbe used to repair acrylic resin denture base. Autopolymerizing resin orlight-polymerized acrylic resin may be the method of choice in a dentalpractice. This study also suggests that airborne-particle abrading thesurface of acrylic resin denture base with 250-mm aluminum oxideparticles enhances the repair.

Although acrylic resins have goodesthetic properties and are easily man-ipulated, they have low fracture re-sistance,1-4 and fractured acrylic resindenture bases are often encountered.1,5

Midline fractures occur twice as often inmaxillary prostheses as in mandibularprostheses.1,6-8 Extensive relief areas,hard and soft tissue undercuts, unbal-anced occlusion, fatigue during masti-cation, and traumatic events (such asdropping the dental prosthesis) inducefractures.1,2,7-12

Fractured denture bases are com-monly repaired, because it is expen-sive and time consuming to remake thedental prosthesis. A satisfactory repairshould have adequate strength10,13-15

and color10,11,13,14 and should beeasy to undertake,10,11 quick,10,11,13,14

dimensionally stable,10,11,13,15 and costeffective.13,14

Heat-polymerized,16 autopolymeriz-ing,17 and light-polymerized acrylicresins15 are used in the repair processesof base materials. The mechanical andchemical properties of acrylic resinsimprove as the temperature during poly-merization is raised.18 Autopolymerizingresins are usually preferred in the repairprocess because they can be applied ina short time and are inexpensive andeasy to use.11-18

The ratio of the repair area strengthto the original strength is 75% to 80%when heat-polymerized acrylic resinsare used17 and 60% to 69% when au-topolymerizing resins are used.9,16 Al-though some research studies5,9,19 havefound that autopolymerizing resinshave a lower flexural strength than heat-polymerized acrylic resins, other studieshave found that they have similar flex-ural strengths.20-22

Prosthetic Dentis

Light-polymerized acrylic resins havevarious advantages, such as superiorstrength, ease of fabrication, ease ofmanipulation, short polymerizationtime, and absence of liquid mono-mer.23-25 The use of light-polymerizedacrylic resin systems for direct intraoralrelining of removable dental prosthesesand the fabrication of denture baseswithout flasking has recently becomepopular.25,26 Polyzois et al27 reportedthat the repair strength was 13 MPa forautopolymerizing resins, 21 to 34 MPafor heat-polymerized acrylic resins, and40 to 44 MPa for light-polymerizedacrylic resins. The authors concludedthat the repair strength was higher inthe heat-polymerized acrylic resins be-cause the base and repair materialshave similar chemical properties. Razaviet al28 reported that base materialsstiffened by light have sufficient strengthfor use as relining materials in clinicalapplications.

Various mechanical and chemical sur-face treatments have been used to improvethe bond strength between the base andrepair materials.29-31 Bur grinding,32

airborne-particle abrasion with 250-mmaluminum oxide (Al2O3) particles, carbondioxide laser application,33 immersion inmethyl methacrylate,20,34,35 and treat-ments with organic solvents such as chlo-roform,30,31,35 acetone,16,20,29-31,36 andmethylene chloride (dichloromethane)29,30

are among these processes. Recently,lasers have been found to provide arelatively safe and easy means of alteringthe surfaces of materials. Although la-sers have not been used to roughenpolymethyl methacrylate (PMMA) sur-faces before a repair process, they havebeen used to etch metals before theapplication of porcelain.37

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Methylene chloride has a carcino-genic potential38-42; therefore, ethylacetate is used in chemical surfacetreatments as a safer alternative. Therepair strength achieved by applyingethyl acetate for 120 seconds duringthe repair process was equal to thatof applying methylene chloride for 5seconds.43 Shen et al31 reported thatchloroform exposure for 5 secondswas sufficient and that longer chloro-form exposure impaired the structureof the repaired surface of the basematerial.

Some of the studies6,9-11,15,27,44-46

applied a 1.5- to 3-mm distance be-tween the repair surfaces, whereas otherstudies allowed a 10-mm distance be-tween the 2 surfaces.47 Beyli and vonFraunhofer48 reported that the colordifferences between the repair materialand the denture base material de-creased with a decline in the poly-merization contraction level when themaximum distance between the repairsurfaces was 3 mm.

Adhesion between the denture basematerial and the repair material mayincrease depending on the chemicalsapplied to the acrylic resin surface.These chemicals cause morphologicchanges by roughening the surface.Normally, this roughness can be createdby immersing the surface in an acrylicresin monomer.16,31,49

In spite of the high frequency ofdenture fractures, little information isavailable about the effects of surfacetreatments on the repaired prostheses.Therefore, the purpose of this studywas to investigate the effects of var-ious surface treatments on the repairstrength between the baseplate andheat-polymerized, autopolymerizing, and

Alkurt et al

Table I. Two-way analysis of variance to evaluate significant differencesamong groups

Source Sum of Squares Df Mean Square F P

Repair resin 162098 2 81049 273 <.001

Surface treatment 3184 3 1061 4 .017

Repair resin� surface treatment 5208 6 868 3 .012

Error 24905 84 296

Total 497077 96

Table II. Mean flexural strength and standard deviation for repaired acrylic resinspecimens subjected to different surface treatments and use of various repair resins

Treatment Groups

HP AP LP

X (MPa) SD X (MPa) SD X (MPa) SD

Control, no treatment 106 21 21 9 14 4

Abrasion* 118 17 53 22 17 6

MMA 100 26 36 13 27 13

Lasery 127 29 34 10 15 4

HP, repaired with heat-polymerized acrylic resin; AP, repaired with autopolymerizing resin; LP, repairedwith light-polymerized acrylic resin; MMA, treated with methyl methacrylate monomer; X, mean;SD, standard deviation.*Airborne-particle abrasion with aluminum oxide particles.yErbium:yttrium-aluminum-garnet laser.

January 2014 73

light-polymerized acrylic resins used asrepair materials. The first null hypoth-esis was that the repair resin typewould increase the repair strength ofthe heat polymerized denture baseresin. The second null hypothesis wasthat the chemical and mechanical sur-face treatments would increase therepair strength of the heat-polymerizeddenture base resin.

MATERIAL AND METHODS

A power analysis found that 96specimens were needed to detect asignificant difference among 3 repairresins and 4 surface treatments (a totalof 12 groups) on the repair strength ofa heat-polymerized denture base resin.Ninety-six rectangular heat-polymerizedacrylic resin (DeTrey QC-20; DentsplyLtd) specimens (65.0 � 10.0 � 2.5mm) were prepared according toAmerican Dental Association Specifi-cation No. 1250 with conventionalprocessing methods.

1 Scanning electron microscope image (�2000 magnification) of surface-treated heat-polymerized acrylic resins beforebonding. A, Control. B, Treated with methyl methacrylate monomer. C, Treated with abrasion. D, Treated witherbium:yttrium-aluminum-garnet laser.

Alkurt et al

2 Scanning electronmicroscope image (�2000magnification) of fractured heat-polymerized specimens. A, Control. B, Treatedwith methyl methacrylate monomer. C, Treated with abrasion. D, Treated with erbium:yttrium-aluminum-garnet laser.

74 Volume 111 Issue 1

The specimens were sectioned into 2halves to create a repair gap (3.0 �10 � 2.5 mm) and then divided into 3equal groups (32 specimens per group)according to the repair materials: re-paired with heat-polymerized acrylicresin (HP); repaired with autopolyme-rizing resin (AP) (Takilon; Rodent); andrepaired with light-polymerized acrylicresin (LP) (Versyo.com HD; HeraeusKulzer). The specimens in each group(HP, AP, and LP) were divided into 4subgroups (8 specimens per group)according to the surface treatments: agroup without any surface treatments(control group); an experimental grouptreated with methyl methacrylate mo-nomer for 120 seconds (MMA group);an experimental group treated withairborne-particle abrasion with 250-mmAl2O3 for 10 seconds at a pressure of0.2 MPa, from a distance of 10 mm(abrasion group); and an experimentalgroup processed with an erbium:yttrium-aluminum-garnet (Er:YAG) laser (Doctor

The Journal of Prosthetic Dentis

Smile Erbium & Diode Laser; LambdaSpA) at a wavelength of 2940 nm, a spotsize of 0.8 mm, a pulse frequency of 10Hz, a pulse energy of 150 mJ, and apulse duration of 100 ms (laser group).The laser was applied (scanning) for 60seconds under water irrigation. Duringthe laser application, the distance fromthe laser tip to the specimen was 10 mm.

The surface-treated and controlspecimens that were sectioned into 2halves were embedded in the dentureflask, with a metal mold (3.0 � 10 �2.5 mm) placed in the center of therepair gap for standardization. After thedenture flask was opened, the metalmold that formed the repair gap wasremoved. HP, AP, and LP were appliedto the repair gap of the surface-treatedand control specimens.

The specimens in the HP group werepolymerized by keeping the dentureflask in a thermal chamber (TermotronP-100; Termotron do Brazil Ltda) for9 hours once it reached boiling

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temperature (74�C) by using the longboiling method. The specimens in theAP group were polymerized by keepingthem under pressure at 55�C for 15minutes. This process was carried outto enhance strength and decreaseporosity. The specimens in the LP groupwere polymerized by reapplying lightwith a UniXS polymerizing box (Her-aeus Kulzer) for 3 minutes for a finalpolymerization after a light applicationof 60 seconds for prepolymerization.When the polymerization processeswere complete, the specimens werecarefully removed from the dentureflask, and residual acrylic resin wasremoved with a tungsten carbide bur atlow speed. The specimens were moldedto the final shape with 600-grit abrasivepaper under running water. All speci-mens were stored in distilled water at37�C for 1 week before testing.

A 3-point bend test was performedimmediately after removing the speci-mens from the distilled water and

Alkurt et al

3 Scanning electron microscope image (�2000 magnification) of fractured autopolymerized specimens. A, Control.B, Treated with methyl methacrylate monomer. C, Treated with abrasion. D, Treated with erbium:yttrium-aluminum-garnet laser.

January 2014 75

without drying the specimens. This testwas carried out on a universal testingmachine (Model 2519-106; InstronCorp). A custom-made stainless steeldevice with a 50-mm span distancebetween the 2 supports was used, andthe crosshead speed was set at 5 mm/min. A load was applied in the centerof the specimens (center of the repairarea). The specimens were loadeduntil the first sound of a crack wasdetected, and the load (N) wasrecorded.

The flexural strength values ofeach specimen were calculated withthe following formula: S¼3WL/2bd2,where S is the flexural strength (inmegapascals), W is the fracture load (innewtons), L is the distance between thesupports (50 mm), b is the specimenwidth (10 mm), and d is the specimenthickness (2.5 mm).

To evaluate the effects of the surfacetreatments and repair resins on the

Alkurt et al

surface of the denture base resin, 4specimens (1 specimen each from thecontrol, MMA, abrasion, and lasergroups) were selected before repair, andrepresentative fractured specimens fromeach group were selected after the 3-point bend test. These selected speci-mens were gold-sputtered and examinedunder a field emission scanning electronmicroscope (SEM) (Zeiss EVO LS 10;Carl Zeiss) at 10.0 kV. The SEM photo-micrographs were made with �2000magnification for visual inspection. Inaddition, the nature of the failure wasnoted as adhesive (interface), cohesive(only at the repair material), or mixed(interface and repair material).

A 2-way analysis of variance(ANOVA) was used to study the effectsof the repair resin type, surface treat-ments, and their interaction on theflexural strength, followed by the Tukeyhonestly significant difference test witha confidence level of .05 to determine

the mean differences. The statisticalanalysis was performed with statisticalsoftware (SPSS v19.0; IBM Inc).

RESULTS

The 2-way ANOVA results are pre-sented in Table I. Significant differenceswere found for repair resin type(P<.001), surface treatments (P<.05),and their interaction (P<.05). Themean and standard deviation values offlexural strength for each of the experi-mental groups are presented in Table II.

The flexural strength of the speci-mens repaired with HP, AP, and LP wasdifferent (P<.001). The lowest flexuralstrength values were observed in the LPgroup (14 MPa to 27 MPa) and thehighest in the HP group (100 MPa to127 MPa).

All surface-treated specimens hadhigher flexural strength than the con-trols, except those treated with the

4 Scanning electron microscope image (�2000 magnification) of fractured light-polymerized specimens. A, Control.B, Treated with methyl methacrylate monomer. C, Treated with abrasion. D, Treated with erbium:yttrium-aluminum-garnet laser.

76 Volume 111 Issue 1

MMA in the HP group. A significantdifference was noted between the con-trol and abrasion groups (P¼.013).However, no significant differences werefound among the control, MMA, andlaser groups.

Representative SEM images of thecontrol, MMA, abrasion, and lasergroup specimens before bonding arepresented in Figure 1. The surfacetreatment resulted in irregularities andmany small pits on the surface of thedenture base resin. The SEM images ofthe representative surfaces of the frac-tured HP specimens are presented inFigure 2; AP specimens are presented inFigure 3; and LP specimens are pre-sented in Figure 4. For all specimens,adhesive failure was observed.

DISCUSSION

This study evaluated the effects ofrepair resin type and surface treatment on

The Journal of Prosthetic Dentis

the repair strength of a heat-polymerizeddenture base resin. For the repair resingroups, significant differences (P<.001)were found. Therefore, the first null hy-pothesiswasnot rejected. For the surface-treatment groups, the analysis found asignificant difference between the controland abrasion (P¼.013) groups. No sig-nificant differences were found amongthe control, MMA, and laser groups.Thus, the second null hypothesis waspartially rejected.

Ogle et al23 reported that light-polymerized acrylic resins had more ac-curate fit and higher strength than heat-polymerized acrylic resins. In this study,the highest repair strength was found inthe HP group, and the lowest repairstrength was in the LP group. Andreo-poulos et al51 found that repairs withlight-polymerized Triad VLC resin(Dentsply Intl Inc) had a much lowerstrength than those with autopolymeriz-ing resins. The authors concluded that

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theflexural strengthwas lower in the light-polymerized Triad VLC acrylic resinbecause this material has high viscosityand poor adhesion as a repair material.The current study found that the speci-mens in the LP group have a lower repairstrength than the specimens in the APgroup. This result is compatible with thefindings of Andreopoulos et al.51 Theseresults may be attributable to a high rateof cross-linkingbetween similar resinbasematerials and to poor interaction andlack of adhesion or cohesion between theLP repairmaterial and the PMMA. The LPmaterial may not penetrate the PMMA.

Some researchers have reported thatexposing the repair surface to monomerincreases the bond strength.32,49 Olveraand de Rijk52 found that a 4-minuteexposure to monomer increased thefracture strength. Vallittu et al34 estab-lished that immersing the repair surfacesfor 180 seconds in methyl methacrylateincreased the transverse strength

Alkurt et al

January 2014 77

compared with shorter exposures. Expo-sure to monomer softens the PMMA,enhances the spread of superficial fis-sures, and forms pits in the bond sur-face.49 As a result, the repair materialdiffuses into the bond surface and de-velops adhesion. The increased repairstrength found with chemical surfacetreatments may be because of monomerinfiltration into the pits and cracks.

The results showed that HP had thehighest repair strength with the laserpretreatment, the AP with the abrasionpretreatment, and the LP with the MMApretreatment. These surface morpho-logic changes may enhance the me-chanical retention between the fracturedsurface and repaired acrylic resin.

The limitations of the study includethe absence of artificial aging withthermal cycling and the use of rectan-gular specimens instead of more com-plex denture shapes.53,54 In vitro studiesare limited in their ability to predict thesuccess of a material or technique in aclinical situation.22 Further in vitrostudies and clinical research are neces-sary to investigate the effects of differentlaser types on the bonding of the repairresin to the denture base resin.

CONCLUSIONS

1. The results showed that the HPgroup exhibited a significantly higherrepair strength than those of the APand LP groups.

2. The surface treatment to the repairsurface improved the flexural strength,except for the surface treated with theMMA in the HP group.

3. Pretreatment with abrasion pro-vided a significantly higher flexural st-rength than that found in the controls.

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Corresponding author:Dr Zeynep Yesil DuymusRecep Tayyip Erdo�gan UniversityFaculty of DentistryDepartment of ProsthodonticsRizeTURKEYE-mail: zyesilz@hotmail.com

Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.

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