Research

In vitro microieakage of composite restorationsafter nonvital bleaching

Mirela Sanae Shinohara, DDSVJosé Augusto Rodrigues, DDSVLuiz André Freiré Pimenta, DDS,

Objective: After bleaching treatment, esthetic restorations often need to be replaced due to colorchanges. Some papers have shown alterations in the bond of adhesive restorations fo bleached teeth.The purpose ol this study was to evaiuate tooth and resin ccmposite adhesion when submitted to ncnvitaldentai bieaching. Method and materials: One hundred and twenty bovine teeth were assigned to 3groups (n = 40): paste cf sodium perborate and water; 37% carbamide peroxide gei; and no bieaching(control). After 3 weeks cf continuous bleaching treatment, standardized Ciass V cavities were prepared atthe cementoenamei junction and restored with Single Bond adhesive system and Z100 resin composite.The samples were thermocycled 1,500 times (5 ± 1 / 55 ± r C ) with a 1 -minute dwell time. Then, theywere immersed in a 2% méthylène blue solution (pH 7) for 4 hours, sectioned, and anaiyzed by stereomi-oroscopy. Microieakage analyses were done, using scores from 0 to 4, considering ieakage on the incisaiwall (enamel) and the cervical wall (dentin). Data were analyzed by Kruskal-Wallis and Mann-Whitneytests (o = 0,05), Results: The results showed that sodium perborate and carbamide peroxide gel signifi-cantly increase the microieakage in Class V resin composite restorations to dentin but not to enamei mar-gins.Conclusion: The risk of microieakage in dentin margins is increased soon after bleaching treatment.(Quintessence tnt2001.32:413-417)

Key words: bleaching agents, discoiored tooth, microieakage, nonvitai tooth, tocth bieaching, walkingbleach

CLINICAL RELEVANCE: After nonvitai bleaching witheither sodium perborate and water or 37% carbamideperoxide, the seai of new resin composite restorations isjeopardized, resulting in increased microieakage of dentinmargins.

Discolored anterior teeth, whether multiple or indi-vidual, present a serious esthetic prohlem. Dental

bleaching should he the first step taken in a treatment,since it is the most conservative one,

A preparation of sodium perhoratc and water in the"walking hleach" technique, for nonvitai tooth bleach-ing, has heen the most popular and recommended one,since the decomposition reaction of sodium perborate

'Graduate SluOenl, Schooi ol Denlislry of Piracicaba, University of Camp-inas, Piracicaba, Sào Paulo, Brazil.

'Pcslgraduate Student, Department of Restorative Dentistry, Sctiool ofDentistry of Piracicaba, University of Campinas, Piracicaba, Sao Paule,Bra2il

••Assistant Professor, Deparlment ot Restorative DenWstry, Sohool of Den-tistry of Piracicaba, University of Campinas, Piracicaba, Sao Pajio, Braiii.

Reprint requests; Dr LJIZ Andíé Freiré Pimenta, Deparlamerilo de Odon-lologia Restauradora, Faculdade de Odontología de Piracicaba ¡UNi-WMP), Av. Limeira 901, Caixa PoElal 52, CEP 13414-900, Piracicaba, SâoPaulo, Brazil. E-maiI: IpimentaSfop.uricamp.bí

is slow and releases hydrogen peroxide in low concen-tration, giving a wider margin of safety in relation toother techniques.' -' Another hleaehing agent that mayhe used in the "walking hleach" technique is 37% car-hamide peroxide, which has heen considered to be safefor the practice.

Once the hleaehing process is complete, estheticrestorations may need to be replaced in order toachieve optimal shade matching. Thus, they are alwaysfiiled with esthetic restorative materials such as resincomposite or glass-ionomer cement. The prerequisitesof these materiais are to prevent microieakage and toimprove the esthetic results.'*-"

Some authors have reported detrimental effects onresin-tooth honds as well as an increase of microieak-age for the teeth restored after bleaching treatment.''̂ '̂̂This effect is attributed to the presence of residual per-oxide or oxygen released from bleaching agents andstructural changes on enamel and dentin compositionthat can affect the seal at the resin-tooth interface."'^It is therefore important to know the real effect of thebleaching agents on the dental structure in order toavoid unsuccessful resin restorations.'"

The purpose of this paper was to evaluate themicroieakage of Class V adhesive resin restorations inteeth suhmitted to nonvitai bleaching with two differ-ent bleaching agents.

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• Sfiinohara et ai

Nonuital bleaching: "walking bleacti" technique

Bovine tooth 3-mm tjase ol IRM Gl : Sodium G2: Carbamide G3: Control Ciass V cavityperfórale peroxjde 37% preparation

Thermocyciing:1,500 eye i es

5± reand 55 ± r e

Polishing the Restoration withcomposite surface resin composite

2% méthylène bluesoiution pH 7

4 hours

Sectioning

.M

Microleakage analyses(scores 0 to 4)

Fig 1 Scheme ot the experiment.

METHOD AND MATERIALS

One hundred and twenty freshly extracted bovineteeth were stored in 2% formalin solution, cleaned,selected, and then stored in distilled water at roomtemperature to prevent dehydration. These teeth werehorizontally sectioned approximately 7 mm incisallyand apically to the cementoenamei junction using adouble-faced diamond disc (reference no. 7020, KGSorensen) to produce tooth segments (Fig 1). Tbe con-tents of the pulp chamber and root canal wereremoved with a dental explorer, and the puip chamberwas enlarged with a No. 8 carbide bur using a low-speed handpiece (Kavo).

Three millimeters below the cementoename! junc-tion, a 3-mm thick base of intermediary restorativematerial (IRM) (Dentsply) was placed to prevent api-cal leakage of tbe bleaching material during the"walking bleach" tecbnique. The teeth stored in ahumidor (humidifier) were then randomly assigned tothe following groups:

• Group 1/sodium perborate (SP): 40 bovine teethbleached by a paste of sodium perborate (Pro-derma) and water (2 g of sodium perborate per 1mL of water).

• Group 2/carhamide peroxide (CP): 40 bovine teethbleached by 37% carbamide peroxide gel (White-ness, FGM Produtos Odontológicos).

• Group 3/no bleaching (control): 40 nonbleachedhovine teeth soaked in distilled water beforerestoration.

In groups SP and CP, the respective hieachingmaterial was inserted in the pulp chamber, and a 2-rnmtbick surface seal was made with IRM. The bleachingagents were changed every 7 days for 2 weeks. Tbebleaching treatments were performed for 21 days. Theteeth were stored in a humidor at 37''C during thebleaching period.

After bleaching, the incisai and apical regions weresealed with epoxy resin (Araldite Ciha EspecialidadesQuímica). A standardized cyÜndrical Class V cavity

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TABLE 1 Statistical results

Enamei (P= 0.063)

Groups Median

Sodium perborate 037% Carbamide peroxide 0Control 0

Trie difterence in letters neirt to the sumtical difterences.

n

373537

of rank

Sum ofranks

2104.5 a2200.0 a1690.5 a

Dentin (P<

fvledian

110

n

373537

0.05)

Sum ofrani<s

2313.5 a2416.0 a1265.5 b

cDlumn expresses the sigritrcant statis-

preparation (approximately 2.0 mm in diameter and2.0 mm in deptb) was done in tbe cementoenameljtmction on tbe facial surface. A special diamond bur(reference no. 2294/KG Sorensen) in a high-speedhandpiece (Kavo} witb a constant water-spray coolantwas used to prepare the cavities.

Before restoration, all cavities were rinsed withwater in order to loosen all sediment left duringpreparation. Then, they were gently air-dried for 2seconds to avoid dentin dehydration. The adhesivesystem Single Bond (3M Dental) was placed accord-ing to the manufacturer's recommendations. The cav-ity was etched for 15 seconds with a 35% phosphoricacid gel, and the etchant was rinsed for 10 secondswith water from an air-water syringe and briefly driedwith compressed air for 2 seconds. Two consecutivecoats of adbesive were applied using a saturatedbrush tip. After gently air drying for 5 seconds, thematerial was light cured for 10 seconds. The resincomposite ZIOO (3M Dental) was inserted in a bulkincrement. Tbe restored teetb were stored in a humi-dor at 37 ± l^C for 24 bours, and tbe composite sur-face was polished with a graded series of Sof-Lexdiscs (3M Dental).

The teetb were placed in separate mesh bags andthermocycled in a thermocyeling machine (MCT2,Instrumental Instrumentos de Precisào) for 1,500cycles in water between 5 ± l''C and 55 ± TC with adwell time of 60 seconds in each bath. After thermo-cyeling, tJie external surface of eacb tooth was coatedwith 2 layers of nail varnish, leaving a 1-mm widemargin around the restoration that was free of varnish.The teeth were placed in a 2% méthylène blue solu-tion (pH 7) for 4 bours at room temperature andrinsed under tap water. The teeth were sectioned lon-gitudinally tbrough the center of each restoration witha slow-speed double-faced diamond disc. The righthalf of each sectioned tooth was evaluated blindly andindependently by 3 examiners with a stereomicro-scope (Meiji 2000Tecbno, Meiji Techno) at x35 mag-

nification to determine the extent of microleakage atincisai and gingival margins. The following criteriawere used to score dye penetration:

0 = No dye penetration1 = Dye penetration to 'ô of the incisa! or gingival wall2 = Dye penetration to % of the incisai or gingival wall3 = Dye penetration to full length of tbe incisai or gin-

gival wall4 = Dye penetration including axial floor

Data were analyzed by Kruskal-Wallis and Mann-Whitney tests [a — 0.05). Tbese tests were chosen dueto tbe nature of the qualitative random variable, wbicbemploys scores to evaluate tbe pbenomenon understudy (microleakage). The statistic calculations wereperformed by STATA software (Computing ResourceCenter Stata Reference Manual).

RESULTS

Tbe microleakage median scores, tbe sum of ranks perbieacbing treatment, and pair-wise comparisons forthe restorative systems are presented in Table 1. TbeKj-uskal-Wallis test sbowed no statistically significantdifferences in microleakage among treatment groups(P = 0.063, x̂ = 5.507; a < 0.05) in enamel margins;bowever, statistically significant differences inmicroleakage were seen among the treatment groups(P = 0.0001, x̂ = 25.009; a < 0.05) in dentin margins.

Comparisons of tbe sum of ranks on enamel mar-gins showed no statistically significant differencesamong control and experimental groups. On dentinmargins, comparisons of the sum of ranks in thebleacbed groups were statistically significantly differ-ent from tbe unbleached control group (P < 0.05). Anincrease in microleakage was found in botb bleacbinggroups (SP and CP), sbowing the adverse potential ofsodium perborate and 37% carbamide peroxide.

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DISCUSSION

Bleacbing procedures are often considered to be tbefirst step in improving the appearance of discoloredteeth.^" Nonvital bleaching is often followed hy place-ment of esthetic restorations. One of the prerequisitesof such treatment is that the esthetic restoration pre-vents microleakage.^ Several studies have shown thathydrogen peroxide released from bleacbing agentsadversely affects tbe bond strength of adhesive systemsand resin cotnposites to enamel.*'*'̂ ''̂ -'''

This work evaluated the influence nonvital bleach-ing agents might have on the microleakage of Class Vadhesive restorations. The control group showed lessmicroleakage in enamel and dentin margins than thebleaching groups (SP and CP). An increase in resin/dentin microleakage was found in groups SP and CP,showing the adverse potential of sodium perborateand 37"/o carbamide peroxide.

Some evaluations bave demonstrated tbat teetbsubjected to 10% carbamide peroxide bave greatermicroleakage compared to nonbleacbed teeth.̂ -^ '̂̂ "'̂A previous study proved that bleacbing witb 10%carbamide peroxide increased microleakage and inter-fered in tbe adbesion of resin restorations to dentalstructures." Anotber investigation showed no measur-able leakage along tbe enamel margins of Class Vresin restorations, but the gingival dentin marginsexhibited leakage, although the difference was not sta-tistically significant.̂

Barkhordar et al stated that bleaching had a mini-mum effect on the marginal seal of the resin restorationwithin the first 2 days and a significant effect after 4 to7 days of bleaching, and concluded that the microleaii-age increased with the extent of bleaching time.̂

Shear hond strength measurements and electronmicroscopy scanning examinations have shownchanges in the bonds of resin composites añer hleach-¡jig is-20 Xensile and shear tests of hleached teethrevealed a significant reduction in bond strength thatwas caused by an alteration in the adhesion mecha-nism and changes in resin quality at the enamel-resininterface. '̂'"''̂

The loss of adhesive strength may be due to bleach-ing reactions.*'̂ '̂ '̂ °- '̂ Altbougb tbese reactions are nottotally known, the hypothesis is that as the oxidizingagent hydrogen peroxide diffuses through the dentinand enamel, the highly pigmented carbon ring com-pounds are opened and then converted into chains,which are lighter in color. As this process continues,the bleached tissue continually lightens with furtherdecomposition of organic and inorganic matrix. Dur-ing this process, water and oxygen are released.̂ ''̂ ^

Some authors have suggested that the adverseeffects of bleacbing on resin-tootb bonds are causedby residual peroxides and oxygen that could inhihitthe polymerization process of tbe adhesive systems."''*'In an electron microscopic scanning evaluation, non-bleached teeth presented numerous and clearlydefined resin tags, in contrast with the teeth treatedwith 35% hydrogen peroxide for 30 minutes, wherethe resin tags were sparse, shorter, poorly defined, andstructurally incomplete.'^

However, a recent research study did not detectoxygen on the surface of bleached enamel, thus reject-ing the hypothesis that residual oxygen leached frombleaching agents may interfere in the adhesive polymer-ization process.^^ tMoreover, bleaching can inducechanges in the ultra-morphology of enamel-resinbonded interfaces, changing the organic and inorganiccomponent ratios, increasing the solubility of dentalstructures, and affecting dentin more tban enamel.̂ '̂ '

Jn this study, the microleakage of dentin margins ingroups SP and CP was significantly higher than in thecontrol group. However, such a difference did notoccur among the enamel margins in groups SP, CP, andcontrol (P= 0.063). The reason could be the differencein the composition of enamel and dentin. Dentin con-tains less mineral and more organic matrix and caneasily be affected hy hydrogen peroxide-hascd materi-als. These materials are strong oxidizing agents thatmay cause denaturation of proteins in the organic com-ponents, producing morphologic changes that couldreduce the performance of resin bond restorations.

Another consideration is the presence of dentintubules that may enhance the rate of penetration ofthe bleacbing agents and residual oxygen diffusionfrom tbe puip chamber through dentin. The conse-quence may be a higher concentration of residualoxygen in the more porous dentin margins than onenamel margins, thereby increasing microleakage.

It should be noted that the teeth used in thisinvestigation were bovine, not human teeth, and themanner in which the hleachlng agents were used wasnot the same as when bleaching is clinically per-formed.^ Moreover, in the oral environment, theinteraction with saliva may repair the tooth hy min-eral precipitation, and the action of enzymes (such asperoxidase and catalase) may leach out all residualperoxides and oxygen from the tooth, over anextended period of time, and improve the quality ofresin-tooth bonds."-"

It is necessary to know tbe time that has elapsedfrom the bleaching treatment to the restoration proce-dure to achieve an optimal seal, as well as to reducethe risk of microleakage in adhesive restorations.

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CONCLUSION

1. Under the experimental circumstances used in thisstudy, the Class V restorations in teeth submitted tononvital bleaching with 37% carhamide peroxidegel or sodiutn perborate showed a significantincrease in microleakage on dentin margins.

2. On enamel margins, the microleakage was not sta-tistically different from the control group for hothh leach in g agents.

3. Even with regard to distance, the bleaching agentssodium perborate and 370/0 carhamide peroxide gelcan interfere in the resin-tooth interface.

It is necessary to know the time that has elapsedfrom the bleaching treatment to the restoration proce-dure to achieve an optimal seal as well as to reducethe risk of microleakage in adhesive restorations.

ACKNOWLEDGMENT

The authors would like to acknowledge ihe support received fromFundaçâo de Amparo à Pesquisa do Estado de Sao Pauto (FAPESP).grant#99/008lü-3.

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