Morphological Effect of the Type, Concentration and Etching Time of Acid Solutions on Enamel and Dentin Surfaces
Mario Fernando de GOES1
Mário Alexandre Coelho SINHORETI1
Simonides CONSANI1
Marcelo A.P. da SILVA2
 
1Área de Materiais Dentários, Faculdade de Odontologia de Piracicaba, UNICAMP, Piracicaba, SP, Brasil
2Instituto de Física e Química, Universidade de São Paulo, São Carlos, SP, Brasil


Braz Dent J (1998) 9(1): 3-10 ISSN 0103-6440

| Introduction | Material and Methods | Results | Discussion | Conclusion | References |


Thirty-six noncarious human molars were extracted and stored in saline. The buccal surfaces were ground wet on 400-grit silicon carbide paper to create a flat surface. The enamel and dentin surfaces were treated with 35% phosphoric acid gel, 10% phosphoric acid gel and 10% maleic acid gel for 15 and 60 seconds. Scanning electron microscope analysis showed that all the acid etchants modified the micromorphological appearance of enamel and dentin surfaces independent of the type of acid, the etching time and the concentration. On the enamel surface, the 35% and 10% phosphoric acid gels and the 10% maleic acid gel applied for 15 or 60 seconds preferentially removed the prism core material. In the specimens treated for 15 seconds with 10% maleic acid the prism core material was partially removed. On the dentin surface, the phosphoric acid gels (35% and 10%) and the 10% maleic acid gel applied for 15 and 60 seconds removed the smear layer and opened the dentinal tubule orifices; however, the dentinal surface etched for 15 or 60 seconds with 10% maleic acid gel showed residues of the smear layer.


Key Words: enamel and dentin surfaces, acid solutions, etching time.
 




Introduction

Traditionally, etching enamel surfaces with orthophosphoric acid, a concept first proposed by Buonocore (1955), has been a usual clinical procedure to increase the bond strength between the composite resin and etched enamel. Normally, phosphoric acid at a concentration between 30-40% provided retentive enamel surfaces (Silverstone, 1974). Concentrations greater than 50% resulted in the formation of a monocalcium phosphate monohydrate that inhibits further dissolution, but in concentrations not lower than 10% the phosphoric acid solution did not produce adverse effects on the bond strength (Chow and Brown, 1973; Gottlieb et al., 1982).
Originally, standard treatment time for enamel conditioning was 60 seconds. However, several studies have indicated that a 15-second etching time provides similar surface morphology and bond strength values (Barkmeier et al., 1986). The process involves discrete etching of the enamel in order to provide selective dissolution of prism cores or peripheries, with resultant microporosity into which resin can flow and can be polymerized to form a mechanical bond to the enamel. This procedure practically eliminated microleakage at the tooth/restoration interface (Retief et al., 1982).
The success obtained with enamel etching inspired its use on dentin surfaces also. However, with the use of early hydrophobic resins, acid treatment of dentin did not produce a bond strength similar to that obtained on enamel surfaces (Rider et al., 1978). At that time, Fusayama (1979) described a bonding system that employed 40% phosphoric acid for 60 seconds on enamel and dentin simultaneously. He demonstrated that the adhesion of the bonding system on the tissue was improved. Kanca (1991) reported on another bonding system that also made use of phosphoric acid to simultaneously etch enamel and dentin and Gwinnett and Kanca (1992) reported that conditioning dentin with 37% phosphoric acid for 15 seconds followed by a unique combination of hydrophilic primers and an ambiphilic bonding agent resulted in a gap-free interface between resin and tissue in vivo and in vitro.
Thus, the recent generation of adhesive systems use different kinds of acids to simultaneously etch enamel and dentin. Among these are 10% and 35% phosphoric acid and 10% maleic acid. In addition, the application time of the acid was reduced to 15-30 seconds to simplify bonding procedures. The application of these conditioning agents to dentin surfaces results in partial or total removal of the smear layer, opens the dentin tubules and causes the demineralization of the peritubular and intertubular dentin (Van Meerbeeck et al., 1992). Thus, the microporosity of the intertubular dentin and subsequent shear strength values have increased and are within the normal range recorded for enamel (Barkmeier and Cooley, 1992).
Nevertheless, the degree of surface etching and demineralization of enamel and dentin depend on the type of acid, the etching time, and the concentration of the etchant (Silverstone, 1974; Zidam and Hill, 1986; Legler et al., 1990; Van Meerbeeck et al., 1992). The purpose of this study was to compare micromorphological changes in enamel and dentin surfaces after etching for 15 or 60 seconds by 35% and 10% phosphoric acid, and 10% maleic acid gels.




Material and Methods

Thirty-six extracted, noncarious human molars with sound buccal surfaces were obtained, thoroughly cleaned with a water slurry of pumice powder with a soft rubber prophylaxis cup rotating at low speed and stored in saline at room temperature. The roots of the teeth were removed and each crown was then mounted horizontally with the buccal surface exposed, in phenolic rings with self-cured acrylic resin. The specimens were then randomly assigned to two groups (group I - enamel and group II - dentin ) with 18 teeth in each group. The groups were further divided into 3 subgroups of 6 teeth each, based on the type and concentration of the etchant used and application times.
In groups I (enamel) and II (dentin), the buccal surface was ground wet in a polishing machine (P.F. Dujardin & Co., Dusseldorf, Germany) on 180, followed by 400 grit silicon carbide paper to create a flat surface on enamel and dentin. The enamel and dentin surface preparations received the following treatments: 35% phosphoric acid gel (Scotchbond Etching Gel, 3M Dental Products Division, St. Paul, MN, USA), 10% phosphoric acid gel (All-Etch, Bisco, Inc., Itasca, IL, USA), or 10% maleic acid gel (Scotchbond Multi-Purpose Etchant, 3M Dental Products Division, St. Paul, MN, USA). The etchants were applied for 15 and 60 seconds. After etching, the specimens were washed with distilled water for 15 seconds and dried with compressed air for another 15 seconds.
The specimens were coated with gold/palladium in an SCD 050 sputter coater (Balzers, Munich, Germany) and viewed with a scanning electron microscope (DSM 960, Carl Zeiss, Munich, Germany) operated at 20 kV. Photomicrographs were taken of representative areas that were most often observed.




Results

All etchants changed the micromorphological appearance of enamel and dentin surfaces independent of the type of acid, the etching time and the concentration.
The enamel surface etched with 35% and 10% phosphoric acid gels for 15 and 60 seconds showed similar etching patterns. The prism core was preferentially removed leaving the prism peripheries relatively intact (Figure 1).
No evident morphological differences were observed in the type of enamel etching patterns when treated with 10% maleic acid gel for 15 and 60 seconds (Figures 2 and 3). However, etching with 10% maleic acid gel for 15 seconds resulted in a partial removal of the prism core and after an etching time of 60 seconds the micromorphological characteristics of the enamel surface approached those produced by 35% and 10% phosphoric acid gels.
Etching with 35% and 10% phosphoric acid gels and 10% maleic acid gel for 15 and 60 seconds had a similar effect on the micromorphology of the dentin surface. All the acid-etchants removed the smear layer and opened the dentinal tubules (Figures 4-6). However, the 35% phosphoric acid gel, at both etching times, left a precipitate on dentinal surfaces (Figure 4). The dentinal surface revealed residues of the smear layer with the 10% maleic acid gel and 15 or 60 seconds etching time (Figure 6).


Figure 1 - Enamel surface etched with 35% and 10% phosphoric acid gels for 15 and 60 seconds. Areas with preferential removal of prism core material and the prism peripheries relatively intact (SEM X2000).

Figure 2 - Enamel surface etched with 10% maleic acid gel for 15 seconds. The apatite crystals were partially removed from the prism core, but the type I etching pattern may be observed (SEM X2000).

Figure 3 - Enamel surface etched with 10% maleic acid gel for 60 seconds. The topographical view of the prism core structure is similar to the etching patterns produced by 35% and 10% phosphoric acid gels (SEM X2000).

Figure 4 - Dentinal surface etched with 35% phosphoric acid gel for 15 and 60 seconds. The acid has removed the smear layer and opened the dentinal tubule orifices. Note the silica that is used to thicken the etching gel (SEM X2000).

Figure 5 - Dentinal surface etched with 10% phosphoric acid gel for 15 and 60 seconds. The smear layer was removed and the dentinal tubules orifices were opened. Note that the etchant did not leave any debris on the dentin surface (SEM X2000).

Figure 6 - Dentinal surface etched with 10% maleic acid gel for 15 or 60 seconds. Note residues of the smear layer on the dentin surface (SEM X2000). 





Discussion

Independent of the type and concentration of the acid used, and the etching time, all the acid etchants used in this study affected the enamel surface morphology, creating microporosity by selective removal of the prism material.
According to Busscher et al. (1987) and Legler et al. (1990) the increase in the surface free energy of the enamel surface and the increase in the porosity of already porous tissue produced by the acid conditioning are probably more important than the depth of resin penetration into the etched enamel surface. Zidam and Hill (1986) reported that the amount of enamel surface lost was dependent on the concentration of acid and that, to a certain point, the surface loss increased as the acid concentration increased. Nevertheless, a significant difference in surface roughness of tooth enamel did not result in a significant difference in the shear bond strengths (Legler et al., 1989). Gottlieb et al. (1982) also used concentrations of phosphoric acid ranging from 10% to 60% for 60 seconds and found no significant differences in resin-to-enamel tensile bond strengths. However, they did find a significantly lower tensile bond strength when a 70% phosphoric acid solution was used. It has also been suggested that reduction of etching time may produce effective retention characteristics and may be less destructive to enamel (Barkmeier et al., 1986).
The present micromorphological study did not reveal any difference in the etching patterns between 35% and 10% phosphoric acid after 15 or 60 second application on enamel. At these etching times and concentrations, the pattern most often observed was Silverstone’s type I etching (Silverstone et al., 1975), in which the prism core material was preferentially removed leaving the periphery intact. Previous studies also confirm that these etching times did not have a significant effect on bond strength (Silverstone et al., 1975; Barkmeier et al., 1986; Triolo et al., 1993; Guba et al., 1994). On the other hand, etching with 10% maleic acid gel for 15 seconds partially removed the apatite crystals from the prism core and an etching time of 60 seconds resulted in more dissolution of prism core material. Although there was no micromorphological difference in the type of etching pattern produced in enamel with either etching time, after 60 seconds of etching the topographical view of the prism core structure approached that produced by 35% and 10% phosphoric acid gels.
The retentive characteristics of acid-conditioned enamel surfaces depend on the type of acid, etching time, and chemical composition of the enamel (Silverstone, 1974; Zidam and Hill, 1986; Legler et al., 1989, 1990; Carstensen, 1992). Furthermore, the choice of the acid etchant must be based on its dissociation constants (Bertolotti, 1992). Acids with lower pKa’s (maleic acid pKa = 1.8) tend to be used in a more dilute solution than those with higher pKa’s (phosphoric acid pKa = 2.1). Although the values that measure the dissociation constants are thermodynamic and the time factor is not considered, maleic acid is an organic acid and has higher molecular weight than phosphoric acid. Thus, it seems difficult to link the carboxylic groups on the enamel surface because the kinetic reaction seems to be slow. For that reason the application time of the 10% maleic acid gel for 60 seconds improved the retentive characteristics on the enamel surface and left micromorphological features similar to those produced by 35% and 10% phosphoric acid gels. This finding probably clarifies other studies in which the bond strengths of specimens etched with 10% maleic acid gel for 15 seconds were just over half of the bond strength of phosphoric acid-etched enamel, but with a 60-second application of 10% maleic acid gel, the bond strengths were not statistically different from those etched with 35% and 10% phosphoric acid gels for 15 and 60 seconds (Triolo et al., 1993).
This study has also shown that the acid conditioning of a dentin surface with 35% and 10% phosphoric acid gels and 10% maleic acid gel for 15 and 60 seconds produced similar micromorphological effects on dentin surface. With both concentrations, application times and types of acids, the etching removed the smear layer and opened the dentinal tubule orifices, as previously shown (Perdigão and Swift, 1994). However, the dentin surfaces treated with 35% phosphoric acid gel for 15 and 60 seconds showed a particulate residue left after rinsing. According to Perdigão et al. (1994), the surface residue is silica used to thicken the etching gel and is not entirely removed by rinsing with water; however the presence of a silica residue did not interfere with bonding of resin to the tooth structure. When the dentin surface was etched with 10% phosphoric acid gel for 15 or 60 seconds it left no debris on the dentin surface because this etchant is thickened with a polymer instead of silica (Perdigão et al., 1994). With the 10% maleic acid gel and 15 or 60 seconds of etching time the dentinal surface revealed smear layer residues. This occurs because maleic acid is organic and has a higher molecular weight which requires a greater time for acid reaction on a dentin surface (Pashley et al., 1992). Furthermore, the interaction of the etchant with the dentin surface is limited by the buffering effect of hydroxyapatite and other dentin components (Wang and Hume, 1988). Thickening the maleic acid with polyvinyl alcohol instead of silica did not influence the etching process of the dentin surface, which was similar to the surface topography etched by 10% phosphoric acid gel.




Conclusion

Phosphoric acid gel used at concentrations of 35% and 10% produced similar micromorphological etching patterns on enamel and dentin surfaces after 15 or 60 seconds of application. Longer etching times with 10% maleic acid gel improved the retentive features on enamel surfaces.
 


 
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Correspondence: Dr. Mario Fernando de Goes, Av. Limeira, 901, 13414-900 Piracicaba, SP, Brazil.


Accepted October 17, 1997
Electronic publication: October, 1998


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