Restorative Dentistry

Effects of repeated use on bond strengthsof one-bottle adhesives

Jorge Pertjigäo, DMD, MS. PhDVEdward J. Swift, Jr. DMD, MS"/Guilherme C. Lopes. DDS***

Objective: Mosf one-bottle adhesives contain organic solvents ¡acetone or ethanol) that displace thewater entrapped within the network of collagen fibers in efched dentin. Acetone and ethanol are volatilesubstances that could easily evaporate from botties during use. The objective ot this in vitro study was toassess the effects of repeated opening ot bottles on dentin shear bond strengths of 4 adhesives. Methodand materials: Eighty freshly extracted bovine incisors were mounted, polished to 600 grit and randomlyassigned to 8 groups ¡n = W). After the application of the materials to 40 specimens at baseline, adhesiveuse was simulated by opening bottles for 1 minute. 2 times a day for 3 weeks, except during weekends,when the bottles were kept refrigerated. After 3 weei<s of simulated use. the same adhesives were appiiedto 40 additionai specimens. Resin composite was appiied to dentin in a No. 5 gelatin capsule and waslight-cured. After 500 thermal cycles (5°C to 55''C). shear bond strength testing was performed wiih a uni-versal testing machine af 0.5 cm/mm. Results: Adhesives containing ethanoi or water had similar meanbond strengths af baseline and at 3 weeks. The acetone-based materiai had a significantiy tower meanbond strength at 3 weeks than at baseline. Conclusion: Acetone-based adhesives may have a shorteruseful life than ethanol-and water-based adhesives. {Qij\n\essence Int 1999:30:819-823]

Key words: acetone, one-bottle adhesive, shear bond strength

CLINICAL RELEVANCE: Evaporation of acetone duringclinical use might affect the performance of acelone-based adhesives.

The introduction of the total-etch technique' andrecent developments in the chemistry of dentin

adhesives have converted these materials into virtuallymicroleakage-resistant materials, with hond strengthsapproaching those of enamei bonding.-"' Nevetiheless,bonding to dentin remains a challenge, even thoughan enamel-like micromechanical interlocking betweenresins and dentin has been hypothesized.^' Several

'Associate Professor and Director, Division of Operative Dentistry,Department of Restorative Sciences, University of Minnesota, Scliool ot

Dentistry, Minneapolis, Minnesota.

"Professor and Graduate Program Director, Department of OperativeDentistry, School of Dentistry, University of North Carolina at Chapel

Hill, Ctiapel tHill. North Carolina.

'•• Clinical Instructor, Department cf Operative Dentistry, Federal Universityof Santa Catarina, Flcnanopolis, Brazil

Reprinl requests: Dr Jorge Perdigäo, Division of Operative Dentistry,Departrrent of Restorative Sciences, University cf Minnescta School ofDentistry, 8-450 Moos Tower, 515 Delaware Street SE, Minneapolis,rWinnesota 554S5. Fax: 612-B25-7440.

Presented at ttie Annual Meeting of the International Association for DentalResearch, Vancouver, Britisti Columbia, Canada, March 10-t3, 1999

factors account for this difference between enameland dentin bonding; their dissimilar composition isthe primary factor. While enamel is mainly composedof hydroxyapatite (mineral), dentin contains a greaterpercentage of water and organic material, mainly typeI collagen.

Hybridization of enatnei and dentin with a blend ofmonomers is considered a fundamental mechanismfor retention of tooth-colored restorations.^-^ Severalsimplified dentin-enamel adhesives that rely on thathybridization mechanism have been introducedrecently. After etching of dentin and enamel with anacidic gel, the adhesive is applied to a moist substratein 1 or 2 applications and is air dried and light cured.Thus, the adhesive must be capable of wetting andspreading over the demineralized dentin surface. Toaccomplish this prerequisite, most new simplifiedadhesives are dissolved in an organic solvent (acetoneor ethanol) that replaces the water within the networkof collagen fibers.

Acetone and ethanol are volatile substances thatcould easily evaporate from bottles during use of adhe-sive systems in the clinical environment. Accordingly,the objective of this in vitro study was to evaluate theeffects of repeated opening of bottles of 4 simplifiedadhesives on their dentin shear bond strengths (SBSs),The null hypothesis tested was that the bond strengthswould not vary from baseline to 3 weeks.

Quintesse nc 819

• Perdigâo et al

TABLE 1 Materials used in the study

Adhesive system

One-Step(+ Bisfil P)OptiBond SOLO¡+ Prodigy)

Single Bond(+Z100)

Syntac Singie-Component(+Tetric Ceram)

"a = elch enamel and dentin: b =ords; f = gently air dry adtiesive;

Solvent

Acetone

Ethanoi

Ethanoi+ water

Water

rinse off etchant; c

Composition Application steps'

Etchant: 32% phosphoric acid with BACAdhesive: bis-GMA. BPDM, hIEMA, acetoneEtchant: 37.5% phosphoric acidAdhesive: GPDMl, HEMA, bis-GMA, silica.barium g i ass, sodium hexafluorosiiicate, ethanolEtchant: 35.0% phosphoric acidAdhesive: bis-GMA, HEMA, dimeth aery lates.polyalkenoic acid copolymer, water, ethanolEtohant; 37.0% phosphoric acidAdhesive: maleic acid, HEMA, methacrylate-modified poiyacryiic acid, tiuoride, initiators.stabilizers, water

= blot dry with paper tissue: d = apply adhesive lo dertirg = tigW cure adtiesive.

BAC = beníaikonium ctitoride; BPDtul = bipheryl aimetliacrylBte; t^EMA = hydroxyethyl mettiacrylate; GPDM

abcddfg

abceg

abcddfg

abcdtgdfg

. ' • • • I - '

Manufacturer

Biscc Dental

Kerr/Sybron

3M Dental

IvoolarWivadent

e = brush adtiesive continuously (or 20 sec-

- glycerophosphone acid dimethacrylate.

METHOD AND MATERIALS

Eighty bovine teetii were obtained at a local abattoirand were refrigerated in a soiution of O.S^ chloraminefor up to 1 weeii untii use. After the roots were re-moved, the crowns were cleaned of debris and weremounted in piicnohc rings {Buehler) with Trayresincoid-cure acrylic resin (Dentspiy Trubyte), The speci-mens were randomly assigned to 8 groups (n = 10)(Table 1), The labial surfaces of 40 teeth were groundwith a mechanical grinder to expose middle dentin,Dentin was polished using wet 240-, 400-, and 600-grit silicon carbide abrasive paper for 30 seconds each.Each adhesive system was applied according to itsmanufacturer's instructions,

1, Group One-Step (OS), Dentin was etched with 52o/ophosphoric acid with benzali<onium chloride (Uni-Etch, Bisco Dental) for 15 seconds and rinsed withwater for 5 seconds. Excess water was removed byhlotting with a tissue paper, so that the surface wasleft visibly moist, One-Step, an acetone-based adhe-sive, was applied in 2 consecutive coats, gentlydried with oii-free compressed air from an air sy-ringe for 5 seconds to evaporate the solvent [the airsyringe was kept 2 cm from the surface), and lightcured for 10 seconds.

2, Group OptiBond SOLO (OP). Dentin was etchedwith 37,5"/o phosphoric acid (Kerr Gel Etchant,Kerr/Sybron) for 15 seconds and rinsed with waterfor 10 seconds. Excess water was removed by blot-ting with a tissue paper, so that the surface was ieftvisibly moist. OptiBond SOLO, an ethanol-based ad-hesive, was brushed continuously on the dentin sur-face for 15 seconds and light cured for 20 seconds.

3, Group Singie Bond (SB). Dentin was etched for 15seconds with 35% phosphoric acid (ScotchbondEtching Gel. 3M Dental) and rinsed with water for10 seconds. Excess water was removed by blottingwith a tissue paper, so that the surface was left visi-bly moist. Single Bond, an ethanol- and water-based adhesive, was applied in 2 consecutive coats,gently air dried with oil-free compressed air froman air syringe for 5 seconds to evaporate the sol-vent (the air syringe was kept 2 cm from the sur-face), and iight cured for 10 seconds,

4. Group Syntac Single-Component (SY), Dentin wasetched for 15 seconds with 37% phosphoric acidgel (Email Preparator GS, Ivoclar/Vivadent) andrinsed with water for 10 seconds. Excess water wasremoved by blotting with a tissue paper, so that thesurface was left visibly moist. Syntac Single-Component, a water-based adhesive, was brushedon dentin for 10 seconds and then left undisturbedfor 20 seconds. The material was air dried, verygently at the beginning and with gradually increas-ing pressure. The adhesive was light cured for 20seconds, A second application was made and curedin the same manner.

Resin composite (Table 1) was condensed into aNo, 5 geiatin capsule (Torpac) to fill two thirds of thecapsule and was light cured for 80 seconds in a Triad2000 (Dentsply Trubyte) visible light-curing unit,Eollowing the application of the adhesive, a finalincrement of composite was inserted in the gelatincapsule and the capsule was seated securely againstthe flattened dentin surface. Excess composite wascarefully removed from the periphery of the capsule,and the composite was light cured for a total of 80

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Perdigào et ai

TABLE 2 Shear bond strengths and type of fracture

Adhesive system

One-Step -t- Bisfii P

OptiBond SOLO -i- Prodigy

Single Bond + Z100

Syntac Single-GomponenI +Telric Ce ram

A = adhesive; C = cohesive.Means with the same superscriptsigniticartiy a iff e renttjndefiined pairs of means (base!

MeanBaseline

± SD Fracture'

21 3 ±4.4' 5A50

17.0 + 4.68 7A3C

20.7 + 3 4 ' 5A5C

10.5 ±2.3= IDAOC

uppercase to

-IS us 3 weeiis

Mean <3wk

SD Fracltjre*

17.1 ±3,7» 9A

153±

22.1 +

9.7 +

baseiine, lowercase for 3 weeks) a

a'e stalistrcaily significantiy diHerer

1C4.0» 9A

1C3.7" 4A

6C1.3'= 10A

OC

e not statistically

ta{P<0.05.

seconds (40 seconds from 2 opposite directions) witha Demetron 401 curing light (Demetron/Kerr). The in-tensity of the hght was monitored mth a curing radio-meter (Demetron/Kerr) and was in excess of 500raW/cm- throughout the study.

After the application of materials to the specimensat basehne, adhesive use was simulated hy openingbottles for 1 minute, twice a day (hetween 10 and 11AM, and hetween 3 and 4 PM), for 3 consectitive weeks,except during weekends, when the adhesives were keptrefrigerated. The hottles were squeezed so that the ad-hesive was evident at the opening during the l-minuteperiod. After 3 weeks of simulated use, the adhesiveswere applied to 40 additional speeimens using thesame methodology used for the haseline specimens.

After 24 hours in distilled water at 37°C, the speci-mens were thermocycled for 500 cycles between waterhaths held at 5°C and 55°C. Dwell time in each bathwas 30 seconds and a transfer time was 10 seconds.Shear bond strengths were measured with a universaltesting maehine (Instron), using the MTS Testworkssoftware (MTS Systems) to record the data. A kniie-edged shearing rod with a crosshead speed of 0.5cm/min was used to load the specimens until fracture atthe interface between the composite rod and dentin sur-face. The distance from the rod to the dentin surfacewas monitored with a spacer of 2 celluloid matrixes.After faiiure, the specimens were examined with a dis-secting microscope at x20 to evaluate the type of failure.

The data were suhjected to one-way analysis ofvariance (independent variable: adhesive; outcomevariable: SBS) and a paired t test for each adhesive(mean SBS at baseline vs mean SBS at 3 weeks). Thestatistical analysis was performed with the SPSS S.Osoftware package (SPSS).

RESULTS

Mean SBSs are summarized in Table 2. At baseline,the mean SBSs ranged from 10.5 + 2.3 MPa for SYto 21.8 + 4.4 MPa for OS. At 3 weeks, the meanSBSs ranged from 9.7 ± 1.3 MPa for SY to 22.1 ± 3.7MPa for SB. One-way analysis of variance revealed astatistically significant difference between tiiaterialsat P < 0.0001 for both testing periods. Duncan's testwas used to rank the differences at a confidence levelof 95%.

At baseline, OS and SB had significantly higherSBS than did the other 2 adhesives. Optihond SOLOhad significantly higher bond strengths than did SY. At3 weeks, SB had a significantly higher SBS than didthe other 3 adhesives. The OP and OS specimens wereranked in the intermediate Duncan's subset, while SYhad the lowest SBS-

When a paired t test was used to compare each ad-hesive at baseline and at 3 weeks, the acetone-basedmaterial (OS) had a significantly lower SBS at 3 weeksthan it did at baseline (P < 0.01). All other adhesiveshad similar mean SBSs at baseline and 3 weeks.

DISCUSSION

The application of adhesives on moist dentin is madepossible hy the incorporation of organic solvents, suchas acetone or ethanol, in the composition of most ad-hesive materials. The wet-bonding technique'" allowsresin monomers to penetrate the labyrinth ofnanochannels formed by dissolution of hydroxyapatitecrystals between collagen fibers.'" The high vaporpressure of the solvent can displace water from the

821

Perd i gao et ai

moist collagen network, thus promoting the infiltra-tion of resin monomers through the nanospaces of thecollagen web. This wet-bonding technique has beenrepeatedly shown to enhance bond strengths,'"'^

One-Step is an acetone-based, one-componentbonding agent that relies on dentin and enatnel etch-ing with 32°.'o phosphoric acid. It contains biphenylditnethacrylatc, which was first used in Primer B ofAll-Bond 2 (Bisco Dental), plus bis-GMA andhydroxyethyl methacrylate (HEMA), The mean bondstrengths obtained at baseline in the present study arecomparable to those obtained in other studies withsimilar surface conditions without any air drying,'̂ '-*When applied in vivo, One-Step reportedly provides awell-deflned rcsin-dentin interdiffusion zone.'' Someareas of debonding may occur at the interface and al-ternate with areas without debonding between the hy-brid layer and the adhesive,'= Although BPDM is avery reactive molecule, its concentration may not behigh enough to allow the formation of a unifonn fllmon dentin surface,"-' This phenomenon, along with dif-ferent interpretations of the concept of moist dentinby different researchers, might explain the wide varia-tions in bond strengths found in the literature.

Because the presence of acetone is crucial for thepenetration of adhesives on moist dentin,^ the evapo-ration of the acetone solvent from OS during the 3weeks of simulated use may have decreased the con-centration of acetone, therefore decreasing thereactivity of the adhesive on moist dentin.

For Single Bond, the presence of water in its cornpo-sition might be beneflcial, ît has been suggested that thewater present in the composition of some adhesiveswould be able to reopen the collapsed network of coUa-getT fibers on dry spots inadvertently left on the surfaceand prevent the formation of "ghost" hybrid layers,''-'̂Because water has a plasticizing effect on collagen fib-rils/^ the shrunken collagen coagulate would reexpandand allow monomers to infiltrate the filigree of fibers.

Nonetheless, one study found that the 3% to 8%water concentration in SB (information supplied bythe manufacturer) might not be sufficient to expandthe collapsed collagen fibrils and allow a completepenetration of the adhesive. However, that water con-centration may be sufficient to result in bond strengthsto dried dentin that are statistically similar to those ofdentin that was dried and rewetted,^' Some criticalconcentration of water may be needed to decrease themodulus of elasticity of the collagen fibrils to allow thereexpansion of the collapsed network to its originallevel." That concentration tnight be somewhere be-tween 90/0 (because the probable 80/0 water in SB doesnot seem to work) and 50%," Interestingly, a recent re-search report suggested that the highest concentrationof water permissible in a HEMA-based primer is 9%,̂ "

The polyalkenoate salt formed by SB has beenclaimed to provide water stability to the adhesive sys-tem by a dynamic potential of breaking and renewingthe bonding between the carboxyl groups and calcium,forming a stress-relaxation zone at the bonded inter-face.2' The formation of an electron-dense layer on thedentin surface after treatment with polyalkenoicacid-based adhesives reacting with residual calciumhas been described elsewhere,̂ ^

For OP, most studies have reported boniä strengthswithin the same range found in the present study-''*OptiBond SOLO is a relatively viscous, fllled adhesiveand forms a thick layer at the resin-dentin interface,"Thick layers may be able to absorb the shock pro-duced by occlusal loads and thermocycling,̂ ^ workingas a stress-dissipating element. When applied to driedand rewetted dentin, OP forms electron-dense agglom-erates that could correspond to inorganic particlesforced into the hybrid layer by vigorous brushing,"The possibility of incorporation of porosity during theuninterrupted 15-second brushing of OP on the dentinsurface cannot be excluded. Electron-lucent small is-lands resemblitig bubbles were observed in the hybridlayer of OptiBond and tnight have been formed bythat mechanism,^" The porosity may have some impor-tance as a stress-breaking component.^*

The mean SBSs obtained for SY are in agreementwith those reported in previous studies.^' SyntacSingle-Component contains neither acetone norethanol. It has been demonstrated that the behavior ofthese water-based materials may improve when themonomers are combined with acetone or when thepritning time is increased.'- In these systems, the con-version of the bonding resin and the resultant bondstrengths are markedly reduced when water is addedto the resin, because failure to remove water could re-sult in the dilution of water-soluble resin compo-nents'^ and a risk for incomplete polymerization in-side the zone of demineralized dentin,-" Additionally,there tnust be a balance between surface tension andviscosity and sufficient capillary pressure to ensurepenetration of the fluid resin into the dentin surface ir-regularities,"'" Because SY is a water-based material,the capillary pressure may not be sufficient to providethorough penetration into the moist filigree of exposedcollagen fibers. These factors might explain widespaces within the hybrid layer,̂ " suggesting incompletesaturation of the collagen scaffold.

CONCLUSION

1, Water-based one-bottle adhesives may result inlower bond strengths than acetone- or alcohol-based one-bottle adhesives.

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Perdigao et al

2, The evaporation of the acetone solvent from ace-tone-based adhesives over the 3 weeks of simulateduse may decrease the reactivity of the adhesive onmoist dentin.

3. The concentration of organic solvents varies widelyamong adhesives.^' Therefore, further studiesshould be directed to the behavior of other ace-tone-based dentin adhesives, both at 3 weeks andat extended periods of simulated use.

ACKNOWLEDGMENTS

The materials used in ihis project were donated by the respectivemanufacturers. Special thanks to Dr Bruno Rosa for helping withopemiig the adheiiie boltlei on the schedule times.

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