PEDIATRIC DENTISTRY/Copyright@ 1982 byThe American Academy of Pedodontics/Vol.4, No. 2

Effects of acid etch parameters on enamel topographyand composite resin -- enamel bond strength

Deborah Bates, MS D. Hugo Retief, MSc, BDS, PhD (Dent)Homer C. Jamison, DDS, DrPH F.R. Denys, MS

AbstractThe objective of this study was to evaluate the

effects of methods of acid application, wash times, andsaliva contamination on the tensile bond strength of acomposite resin to enamel Ninety human maxillarycentral incisors were used and the tensile bondstrength was determined for the composite resin toenamel surfaces subjected to the following acid etchparameters. In the control group (A) the acid wasapplied by dabbing followed by a 30-secand wash. Theacid was similarly applied for 30 seconds (B) and 120seconds (C), rubbed for 60 seconds (D), not agitated 60 seconds (E) followed by 30 seconds washrespectively. Wash times of 5 seconds (F) and seconds (G) were used after 60 seconds of acidapplication by dabbing. Control procedures werefollowed by 30 seconds of saliva contamination and 5seconds wash (H) or 5 seconds wash, 10 seconds re-etchand 30 seconds wash (I) were finally used. Incisorstreated with one o.f each of the procedures were viewedin the SEM. Method of acid application by dabbing,rubbing, or no agil:ation resulted in bond strengthsthat were not significantly different, a 5-second washafter acid etching was sufficient to maintain bondstrength and saliva contamination was eliminated by a5-second wash. The effect of acid etch parameters onenamel topography was difficult to evaluate because ofvariable etch patterns.

The acid etch procedure is essential for the

establishment of the mechanical bond between com-posite dental resins and enamel.1 It consists ofseveral steps which may affect the bond strength ofthe composite resin to etched enamel.

The first step of the acid etch technique involvesthe application of the orthophosphoric acid (H3PO4)etching solution to the enamel surface. Rubbing theenamel during conditioning adversely affects thebonding of composite resins to these surfaces.2 Hor-mati and coworkers3 reported, however, that theshear bond strength of a composite resin to enamelsurfaces which were etched by a dabbing or a rubbing

action were not significantly different despite themarked differences in etch patterns as observed byscanning electron microscopy (SEM).

The duration of etch and subsequent wash timesare also important factors which may affect the bondstrength of composite resins to etched enamel. It hasbeen reported that with a short etch time of 10seconds, the bond strengths increased as the washtime increased? Other studies showed that the bondstrengths of a composite resin to enamel surfacesetched with 50% H3PO4 for five seconds or 60seconds were not significantly different if the enamelwas washed with at least two ml of water afteretching?.~ Washing with quantities of water greaterthan this volume provided no additional benefit. Onthe other hand, it was reported that the tensile bondstrength increased with a thorough 60-second washtime as opposed to a 15-second washy

Bond strength may also be adversely affected bysaliva contamination of the etched enamel surface.Hormati and coworkers3 reported that etched enamelsurfaces which were air dried after a 60-second salivacontamination significantly reduced the shear bondstrength of a composite resin when compared withbond strengths obtained on enamel surfaces whichwere re-etched, rewashed, and redried after similarsaliva contamination.

The objective of this investigation was to studyfurther several of the parameters which mayinfluence the bond strength of composite resins toetched enamel. Scanning electron microscopy (SEM)was used to determine the effects of the various pro-cedures on the etching pattern of enamel.

Methods and MaterialsNinety noncarious, extracted human maxillary cen-

tral incisors were used to study the tensile bondstrength of a composite resin to etched enamel sur-faces. The test method developed by Kemper andKilian s was used in this investigation. Subsurface

106 EFFECTS OF ACID ETCH PARAMETERS: Bates, et al.

Group Etch Application Wash Contamination Wash Re-etch Rewash(sec) Method (sec) Saliva {sec) (sec) (sec) (sec)

Table 1. Acid etchprocedures.

A 60 dabbingB 30 dabbingC 120 dabbingD 60 rubbingE 60 no agitationF 60 dabbingG 60 dabbingH 60 dabbingI 60 dabbing

30 ....30 ....30 ....

30

30 30 5 -- --30 3O 5 10 3O

enamel was exposed by lapping the facial surfacesof the crowns of the teeth on 600 grit silicon carbidepaper in a polishing machinea under a continuousstream of water. The crowns were embedded in epoxyresin in the tooth specimen cups with the flat sur-faces parallel to and projecting just above the lip ofthe cup. The epoxy resin was allowed to cure at roomtemperature for 24 hours and the embeddedspecimens were then stored in water at 37°C for atleast 24 hours. Immediately prior to the preparationof the bonded test specimens the crown surfaces werelightly polished by placing the tooth cup in apolishing block on 600 grit silcon carbide paper in thepolishing machine. This provided a planar enamelsurface perpendicular to the direction of force applica-tion during the bond strength testing procedure. Theembedded teeth were subjected to one of the pro-cedures outlined in Table 1. A random distributiontable was computer processed to determine the orderin which the test specimens were to be prepared.

The tooth surfaces were acid etched with 37%H3PO4, washed and dried with an air-water syringe.Water was collected in a graduated cylinder to deter-mine the volume used in each washing procedure.Each timed collection was repeated 10 times and theaverage volume of water determined. A two pastecomposite resinb was mixed as directed, placed in thecomposite cup and assembled in the bonding align-ment block. A low viscosity bonding resin was notapplied to the etched enamel surfaces. The compositecup was lowered to contact the flat enamel surface.The composite resin was allowed to cure against theenamel surface under a load of one pound for 15minutes at room temperature, after which the bondedspecimens were carefully removed and stored inwater at 37°C for 24 hours. Test specimens weremounted in the measurement alignment block andsuspended in the jaws of an Instron c testingmachine. The tensile load was applied at a rate of 0.02

aBuehler Ltd., 2120 Greenwood St., Evanston, I L.bConcise, 3 M Co., St. Paul, MN.

inch/min until the bonded specimens failed. Tensilebond strengths were calculated and expressed inMN.m.-~ Statistical comparisons of the data weremade by an analysis of variance at the 0.05 level ofsignificance.

Etch patterns for each treatment group were deter-mined by subjecting two maxillary central incisorcrowns to one of the procedures listed in Table 1. Theetched crowns were coated with gold/palladium in ahigh vacuum evaporator d and examined in a Cam-bridge Mark 2Ae scanning electron microscopeoperated at 20 kV.

Results

The mean (_ SD) of the volumes of water used each wash regimen were as follows: five seconds, 16.3_+ 1.0 ml; 10 seconds, 33.8 _ 2.0 ml; 30 seconds, 95.3_+ 1.6 ml.

The Mean (_ SD) of the tensile bond strengths the composite resin to enamel subjected to the dif-ferent etching procedures are given in Table 2.Evaluation of the data by an analysis of varianceshowed that there were no statistically significantdifferences among the tensile bond strengths.

When the acid was applied with a dabbing actionfor 60 seconds, typical Type 1 (Figure 1) Type (Figure 2) and Type 3 (Figure 3) etching patterns observed on the same tooth surface with the Type3 pattern predominant in the cervical third of thecrown. At a lower magnification all three etching pat-terns were frequently observed in the same field ofvision (Figure 4). The etching produced by the ap-plication of the acid for 30 seconds could not bedistinguished from that of a 60-second etch while amore pronounced etching effect was produced by a120-second etch. When the acid was applied with arubbing action, the prism peripheries were obscuredresulting in a more uniform etching (Figure 5). The

¢Instron Engineering Corp., 2500 Washington St., Canton, MA.dHitachi Ltd., Tokyo, Japan.eCamb~idge Scientific Instruments Ltd., Cambridge, England.

PEDIATRIC DENTISTRY: Volume 4, Number 2 107

Figure 1. Etching solution was appliedfor 60 seconds with a dabbing action.Type 1 etching pattern (2,500x).

Figure 2. Etching solution was appliedfor 60 seconds with a dabbing action.Type 2 etching pattern (2,500x).

Figure 3. Etching solution was appliedfor 60 seconds with a dabbing action.Type 3 etching pattern (2,500x).

etching effect of nonagitated acid could not bedistinguished from that produced by acid appliedwith a dabbing action. The reaction products of a60-second etching were not completely removed fromthe etched surfaces by a 5-second wash (Figure 6).The etched surfaces obtained after washing for 10seconds were similar to those described for 30-secondwash times. Salivary contamination of 30 secondswas effectively removed by a 5-second wash, and re-etching of these surfaces for 10 seconds produced asimilar etching effect.

DiscussionUnder the experimental conditions defined in this

in vitro study, it would appear that it is not necessaryto adhere strictly to some of the generally acceptedparameters associated with the acid etch techniquein order to obtain comparable bond strengths of com-posite resins to etched enamel surfaces. The tensilebond strengths of test specimens prepared on enamelsurfaces to which the acid was applied by a rubbing

Table 2. Mean and SD of the tensile bond strengths of thecomposite resin to enamel subjected to the various etchingprocedures.

Procedure

Mean and SD ofTensile Bond

Strength

MN.m-2

Coefficient ofVariation

ABCDEFGHI

16.8 ± 5.817.8 ± 8.216.1 ± 6.620.7 ± 7.619.8 ± 8.113.8 ± 5.619.6 ± 5.616.4 ± 5.315.4 ± 6.8

34.546.241.136.841.240.528.532.544.2

Means are not significantly different.

action were not significantly different from those inwhich the acid was applied by dabbing as usuallyrecommended. Similar findings were previouslyreported.3 Although the etching pattern on enamel sur-faces exposed to acid applied by a dabbing actionvaried greatly from one tooth to another and evenin adjacent areas of the same tooth, it resulted inclear delineation of the enamel prisms in the Type 1and Type 2 etching patterns (Figures 1 and 2). Whenthe acid was applied by a rubbing action, the prismperipheries were obscured and the etched surface ap-peared more uniform (Figure 5). One would expectthat this procedure would limit the extent of resinpenetration into the etched enamel surfaces and thusadversely affect bond strength. This was not ob-served in this study however. A direct relationshipbetween resin tag length and bond strength has notyet been demonstrated. Intimate interfacial contactbetween a dental resin and relatively smooth enamelsurface without deep resin penetration may result inadequate bond strengths.9

The various parameters investigated in this studyhad no significant influence on tensile bondstrengths. The high coefficients of variation in thetensile bond strengths ranging from 28% to 46% inthe present study, are usual in this type of test.10 Ifthe number of specimens in each group had been ap-preciably larger, some of the nonsignificant dif-ferences may have been statistically significant. Thetest method used in this investigation reduces theintroduction of forces other than tensile during thetesting procedure. It is, however, impossible toeliminate these forces altogether. A slight malalign-ment of the test specimens in the bonding ormeasurement alignment blocks may introduce ashear component at the resin-enamel interface dur-ing the application of the tensile load. Examinationof fractured test specimens by SEM revealed that themajority of test specimens failed partly within theresin and partly at the resin-enamel interface, andthat air bubbles incorporated into the resin during

108 EFFECTS OF ACID ETCH PARAMETERS: Bates, et al.

Figure 4. Etching solution was appliedfor 60 seconds with a dabbing action.Mixed etching pattern (l,250x).

Figure 5. Etching solution was appliedfor 60 seconds with a rubbing action(2,500x).

Figure 6. Enamel surface etched bydabbing action for 60 seconds andwashed for 5 seconds (2,500x).

the mixing procedure were distributed within theresin along the fracture lines."Stressconcentrationmay arise at these sites (which develop duringpolymerization of the resin) and fractures may bepropagated along the defects during the applicationof the tensile load. Test specimens may failprematurely as a result of the well-known zippereffect.12

The phosphoric acid solutions used as etchingagents in clinical dentistry contain more than 27%H3PC-4. Hydroxyapatite dissolution duringphosphoric acid application proceeds rapidly ini-tially, but is eventually impeded by the precipitationof monocalcium phosphate monohydrate[Ca(H2PO4>2.H20] on the enamel surf ace.13 This maybe another reason for the recommendation of acid ap-plication by dabbing. It is believed that the agitationresulting from the dabbing action will disrupt theprecipitate and continually expose the underlyingenamel to dissolution. The test specimens which wereprepared on enamel surfaces to which the acid wasapplied but not agitated were expected to yield lowerbond strengths than those specimens prepared onenamel surfaces to which the acid was applied by adabbing action. This however, was not observed inthe present study and may be explained by the factthat nonagitated acid also produced a well-definedetching pattern.

Unlike Soetopo and his colleagues,7 no increaseswere found in bond strengths as wash times afteretching were increased. Beech5 and Beech and Jalaly6

reported that two ml were adequate for washingetched enamel surfaces. The volume of water usedwith a 5-second wash in the present study (16.3 ml)exceeded this volume greatly. Monocalciumphosphate monohydrate is readily soluble in waterand would be completely washed away in the clinicalsituation.13 Although some precipitate may remain inisolated areas as shown in Figure 6, this did not af-fect bond strength adversely.

Contamination of etched enamel surfaces by saliva

can readily occur in the clinical situation — par-ticularly if rubber dam is not used. The results of thepresent study indicated that tensile bond strengthswere maintained by washing salivary contaminatedetched enamel surfaces for 5 seconds with an air-water spray and that it was not necessary to re-etchcontaminated surfaces. It is not possible to ex-trapolate the results of this in vitro study directlyto the clinical situation. The authors believe thatsalivary contamination of etched enamel surfacesprior to the application of composite resins shouldbe avoided at all costs. This can readily be achievedby the use of rubber dam.

The great variation in etching patterns on enamelsurfaces etched with H3PO4 has previously beendemonstrated.'41(i The reason for this is not known butthe Type 1 and Type 2 etching patterns may be dueto variations in crystallite orientation in the enamelprisms.17 The Type 3 etching pattern is produced onprismless enamel, that is enamel surfaces in whichthe enamel prisms do not reach the surface.18

No significant differences were observed in tensilebond strengths of test specimens prepared on enamelsurfaces etched with 37% phosphoric acid for 30, 60,or 120 seconds respectively. The tensile bondstrengths were determined on enamel surfaces inwhich the superficial enamel had been removed bythe polishing procedure prior to the preparation ofthe test specimens. Enamel fluoride concentration isgreatest in superficial enamel.'9Etchingtimeslongerthan 30 seconds may be required in the clinical situa-tion as high fluoride concentrations have an adverseeffect on bond strength20 and decrease the surf ace freeenergy of enamel.21

ConclusionsThe results of this in vitro study indicate that the

method of acid application by dabbing, rubbing, andno agitation had no significant effect on the tensilebond strength of a composite resin to etched enamel.The tensile bond strengths to enamel surfaces etch-

PEDIATRIC DENTISTRY: Volume 4, Number 2 109

ed for 30, 60, and 1.20 seconds were not significantlydifferent. A 5-second wash after acid etching was ade-

quate to maintain bond strength. Saliva contamina-

tion of etched enamel surfaces was effectively

eliminated by a 5-:second wash without resorting to

re-etching of the enamel surface.

The authors wish to thank Mrs. Shirley Nolen for typing themanuscript.

This study was supported by USPHS Grant No. DE 02670 fromNIHoNIDR and The C.ollege Workstudy Program.

Dr. Bates is a senior dental student; Dr. Retief is professor,biomaterials; Dr. Jami~on is professor, community dentistry; andF. R. Denys is research associate, The University of Alabama inBirmingham School of Dentistry/Institute of Dental Research, Bir-mingham, AL 35294. Requests for reprints should be sent to Dr.Retief.

1. Retief, D. H. The raechanical bond. Int Dent J 28:18, 1978.2. ADA Council on Dental Materials and Devices. Status report

on acid etching procedures. JADA 97:505, 1978.3. Hormati, A. A., Fuller, J. L., and Denehy, G. E. Effects of con-

tamination and mechanical disturbance on the quality of acid-etched enamel. JADA 100:34, 1980.

4.Williams, B. and Von Fraunhofer, J. A. The influence of thetime of etching and washing on the bond strength of fissuresealants applied to enamel. J Oral Rehabil 4:139, 1977.

5. Beech, D. Adhesion in the oral environment: biophysical andbiochemical considerations. Int Dent J 28:338, 1978.

6. Beech, D. R. and Jalaly, T. Bonding of polymers to enamel: in-fluence of deposits formed during etching, etching time, andperiod of water immersion. J Dent Res 59:1156, 1980.

7. Soetopo, Beech, D. R., and Hardwick, J. L. Mechanism of adhe-sion of polymers to acid-etched enamel. J Oral Rehabil 5:69,1978.

8. Kemper, R. and Kilian, R. New test system for tensile bondstrength testing. J Dent Res 55 B:138, Abstract #308, 1976.

9. Retief, D. H. The use of 50 percent phosphoric acid as an etch-ing agent in orthodontics: a rational approach. Am J Orthod68:165, 1975.

10.Brauer, G. M. and Huget, E. F. Dental adhesives, in TheChemistry of Biosurfaces, ed. Hair, M. L. New York, MarcelDekker, 1972, pp 731-799.

ll.Retief, D. H. and Mallory, W. P. Evaluation of two pit andfissure sealants: an £n vitro study. Pediatr Dent 3:12, 1981.

12. Zisman, W. A. Improving the performance of reinforced plastics.Ind Eng Chem 57:26, 1965.

13.Chow, L. C. and Brown, W. E. Phosphoric acid conditioningof teeth for pit and fissure sealants. J Dent Res 52:1158, 1973.

14.Gwinnett, A. J. Histological changes in human enamel follow-ing treatment with acidic adhesive conditioning agents. ArchOral Biol 16:731, 1971.

15.Retief, D. H. Effect of conditioning the enamel surface withphosphoric acid. J Dent Res 52:338, 1973.

16.Silverstone, L. M., Saxton, C. A., Dogon, I. L., and Fejerskov,O. Variations in the pattern of acid etching of human dentalenamel examined by scanning electron microscopy. Caries Res9:373, 1975.

17.Simmelink, J. W., Nygaard, V. K., and Scott, D. B. Theory forthe sequence of human and rat enamel dissolution by acid andby EDTA: a correlated scanning and transmission electronmicroscopic study. Arch Oral Biol !9:183, 1974.

18.Gwinnett, A. J. The ultrastructure of the "prismless" enamelof permanent human teeth. Arch Oral Biol 12:381, 1967.

19.Brudevold, F., Gardner, D. E., and Smith, F. A. The distribu-tion of fluoride in human enamel. J Dent Res 35:420, 1956.

20.Sheykholeslam, Z., Buonocore, M. G., and Gwinnett, A. J. Ef-fects of fluorides on the bonding resins to phosphoric acid-etchedbovine enamel. Arch Oral Biol 17:1037, 1972.

21.Glantz, P. On wettability and adhesiveness. Odont Revy 20(Suppl 17) :1, 1969.

Quotable QuoteSometimes give your services for nothing, calling to mind a previous benefaction or present satisfaction.

And if there be an opportunity of serving one who is a stranger in financial straits, give full assistance to allsuch. For where there is love of man, there is also love of the art. For some patients, though conscious thattheir condition is perilous, recover their health simply through their contentment with the goodness of thephysician. And it is well to superintend the sick to make them well, to care for the healthy to keep them well,but also to care for one’s own self, so as to observe what is seemly.

From: Hippocratic Collection, Precepts,Chapter 6.

1 10 EFFECTS OF ACID ETCH PARAMETERS: Bates, et al.