Dental Research

Effects of acid-etching solutions on hnman enamel and dentinMarco Franchi*/Lorenzo Breschi**

Nine noncarioiis human molars were exiracled and stored in saline solution. Three standardocdiisal caviiies with beveled enamel margins were prepared on each tooth andetchedwith theetching solutions of three dminai adhesive systems.- ( 1 ) 37% plmsphoric acid solution. (2)4.3% oxalic add and 2.6% alumium salts solution, and (3) 10% maleic add solution. Scanningelectron microscopic analysis revealed that all the etching .solutions affected the enamelsurface morphology. The solution of oxalic acid and aluminum salts removed primarily theprism core material and partially the periphery of the prisms, but did not affect the nonbeveledenamel surface. Phosphoric and maleic adds removed both prism core tnateriah and prismperiphery',- these specimens also showed areas in which no prism morphology could be detected.These two acids also removed apatite crystals from the prism core of the intact enamel surface.(Quintessence Int 1995.-26:431-435.)

Introduction

The acid-etching technique with phosphoric acid,introduced by Buonocore' in 1955, is still used tocreate an irregular surface of beveled enamel. Thisallows an increase in the cavity surface available for theretention of resin composites and an improvement inmarginal adaptation of restorations.^'* Moreover, thebenefits of acid etciiing include an increased resistanceto recurrent caries both in enamel and cement.'''"

Three types of etching patterns have been describedby Silverstone et al" after exposure of the enamelprisms to etching solutions: type I, preferential remov-al of prism core material, leaving the periphery intact;type II, preferential removal of periphery core material,leaving the prism core relatively unaffected; and typein, a more random etching pattern in which adjacentareas of the tooth surface correspond to types I and II,mixed with regions in which the pattern could not berelated to prism morphology.

" Depajtment of Oral Surgery, University of Bologna, School ofDentistry, Bologna, Italy.

" University of Bologna, School of Dentistry, Bologna, Italy,Reprint requests: Dr Marco Franchi, via del Triumviralo 9, 40132Bologna. Italy.

Many factors influence the effects of acid etching onbeveled enamel: the acid-etching solution, its concen-tration, the technique used, and the time of applicationarc the parameters subject to greatest investgation.'-"'^Different etching solutions have recently been tested:phosphoric acid (solutions between 30% and 50%),flouridated phosphoric acid, pyruvic acid, citric acid,maleic acid, oxalic acid, tannic acid, ethylenediamine-tetraacetic acid, trichloracetic acid, and polyacrylicacid,'-'^'*"^' The physical status of the solution isanother important parameter. The gel-type etchingagent seems easier to apply on the enamel surface thandocs the liquid form, and it is reported to produce awider and deeper penetration of enamel.^ ^ The methodof application of the etching agent has also beeninvestigated to improve clinical success. A continuousbrushing technique may be used to apply an acidetching gel to obtain a more etched enamel wall andtherefore an improvement in the marginal adaptationof the resin composite restoration.'^"''' Another factorthat may influence the effects of acid etching and canreduce microleakage as well is the shape of the beveledenamel, ^ '^^ ^

In this study, the morphologic aspects of the enamelsurface after treatment with three different acid-etchingagents have been investigated by scanning electronmicroscope (SEM).

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Method and materials

Nine noncadous and nonfractured human molars thatwere extracted because of periodontal disease werestored in 0.9% saline solution for 48 hours at 37C.Three standard occlusal cavities, approximately 2 mmwide, were prepared in each tooth with a rounddiamond (Intensive No. 201 ), in a high-speed hand-piece. The enamel margins were then heveled ( 120-to130-degree cavosurface angle) with a diamond butt-joint bur (Intensive No, 206),

All cavity walls and cavosurface margins wereetched with one of the following solutions in accor-dance with each manufacturer's instructions: (I) 37%phosphoric acid solution (Enamel etchant of Scotch-bond 2. 3M Dental) for 30 seconds; (2) 4.3% oxalicacid and 2.6% aluminum salts solution (Conditioningsolution of Gluma 2000, Bayer Dental) for 30seconds, (3) \()% maleic aeid solution (Enameletchant of Scotchbond MP, 3M Dental) for 15seconds.

To obtain comparable results, one of three acid-etching agents was applied to one of three cavities ineach tooth. This procedure prevented factors such asdifferences in tooth age or rates of tissue calcificationfrom affecting results. After the application of theacid-etching solutions, all the cavities were rinsed withwater and dried with compressed air.

All the specimens were air dehydrated, positionedon stubs, and coated (Edwards 150A Sputter Coater)for 120 seconds at 30 mA with a gold-palladiumcoating. The SEM observations (Philips 515) werecarried out at voltages ranging from 5.9 to 10.4 kV. Thepicmres of enamel and dentin selected are representa-tive of the pattern that was most often observed on thespecimens.

Results

All the acid-etching solutions affected both the enameland the dentinal surface: enamel prism morphologyand dentinal tubule orifices were clearly detectable.

The beveled enamel surface etched with the threedifferent acid solutions showed (I) areas with prefe-rential removal of prism core material (type 1), (2)areas with dissolution of the periphery of the prisms(type II), and (3) areas with amorphous appearances(type III). No evident morphologie differences wereobserved in the beveled enamel surfaces treated withthe three different acid-etching solutions (Figs 1 to 3).

Type I and type il patterns were observed in areaswhere the prisms were accidentally cut longitudinally(Figs 4 to 6).

The intact enamel surface etched with the three acidsolutions showed different results. The 37% phos-phoric acid removed the apatite crystals from the prismcore thoroughly, leaving a very rough enamel surface(type I pattern) (Fig 7). The 4.3% oxalic acid and 2.6%aluminum salts solution did not affect the normalprism morphology (Fig 8). The 10% maleic acidresulted in only a partial removal of the enamel apatitecrystals (Fig 9).

Each of the three acid-etching solutions had adifferent effect on the dentinal walls. In particular, the31% phosphoric acid solution removed the smear layerand opened the dentinal tubule orifices, which ap-peared empty (Fig 10). With the 4.3% oxalic acid and2.6% aluminum salts solution, about 50% of thedentinal tubule orifices were filled with smear layer,while the rest were partially empty (Fig 11). The dentintreated with the 10% maleic acid solution showedopened dentin tubule orifices, and coliagen fibrilswere visible inside the tubules (Fig 12).

DiscussionAll three acid-etching solutions modified the beveledenamel surface, forming pores by selective dcalcifica-tion. The new porous enamel surface enhances mecha-nical interlocking by tag penetration of bonding resininto the enamel.-""'* Researchers have measured theinteraction between different adhesive systems andresin composites with etched enamel by shear bondstrength tests.'^-'^'^''^'-* The present morphologicstudy did not reveal the qualitative differences describ-ed by Silverstone et al" among the effects on thebeveled enamel of the three etching solutions. Type I,type II, and type III etching patterns were observed inthe different specimens. Confirming the present obser-vations, a clinical trial showed maleic acid andphosphoric acid to have similar results in Class IV andClass V restorations.^'

Areas where the prisms were longitudinally cut andtreated with the three acid-etching solutions showedbotli type I and type II enamel etching patterns. Thiseffect demonstrates that, even when an incorrectenamel bevel is prepared, the modified enamel surfacemight offer retention for penetration of the bondingresin.

The 37% phosphoric acid solution appiied on intactenamel promoted enamel dissolution with a type Ipattern, the 4.3% oxalic add and 2.6% aluminum saltssolution did not affect the intact enamel surface, andthe 10% maleic acid partially removed prism corematerial. Further clinical studies might demonstrate

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Fig 1 Beveled enamel surface etched with 37% phos- Fig 4 Enamel prisms longitudinally cut and etched withphoric acid solution. Silverstone's etcfiing type II pattern is 37% phosphoric acid solution. Bar " 0,1 mm,)evident. (Bar = 10 jim.)

Fig 2 Beveled enamel surface elched with 4.3% oxalicacid and 2.6% aluminum salts solution Silverstone's type IIIetching pattern is detectable, (Bar - 10 \im.)

Fig 5 Enamel prisms longitudinally cut and etched with4.3% oxalic acid and 2,6% aluminum salts solution.(Bar = 0.1 mm.)

Fig 3 Beveled enamel surtace etched witii 10% maieioacid solution Silverstone's type 1 etching pattern may berecognized, (Bar = 10 \im.)

Fig 6 Enamel prisms longitudinally cut and etched vi/ith10% maleic acid solution. (Bar = 0,1 mm)

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Fig 7 intact enamel surface etched with 37% phosphoricacid solution. Highly rough enamel surface (type i patlerni isvisible. (Bar = 10 [irn )

Fig 10 Dentinal surface treated with 37% phosphoric acidsolution. The acid has removed the smear layer and openedthe dentinal tubule orifices, which appear empty.{Bar= 10 |im.)

Fig 8 Intact enamel surface etched with 4.3% oxalic acidand 2.6% aluminum salts solution. Tliis etching soiution hasnot affected the normal prism morphology. (Bar = ID |im.

Fig 11 Dentinai surface treated with 4.3% oxalic acid and2.6% aluminum salts solution. About 50% of the dentinaitubule orifices appear to be fiiied with smear layer, while therest are partially empty. (Bar = 10 |im.)

Fig 9 Intact enamel surface etched with 10% maleic acidsolution. Only a partial removal of the enamel apatitecrystals is evident. {Bar - 10 [.im.]

Fig 12 Dentinal surface treated with 10% maleic acidsolution. Denfinal tubule orifices are opened, and collagenfibriis are visible inside the tubules. (Bar = 10 um.)

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that use of these three acid-etching solutions on intactenamel (as when orthodontic brackets are applied)could provide different clinical results.

This study also investigated the effect of applying thethree acid-etching solutions on the dentinal cavitywalls, because clinical rpons'*^ have demonstratedthat patients experience no postoperative sensitivityand no pain after 18 months. The best removal ofdentinal smear layer resulted from the 37% phosphoricacid treatment; all the dentin tubules orifices appearedto be completely opened. With the 4.3% oxalic acidand 2.6% aluminum salts solution, only 50% of thedentinal tubule orifices were partially free; tlie restwere covered by the smear layer. Opened dentin tubuleoriftces, with collagen fibrils inside the tubules, werepresent in dentinal surface treated with the 10% maleicacid solution.

AckuowledgmentsThe authors thanks Mr Gianfranco Filippini for technical assistance.

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