INTRODUCTION

The direct bonding method of orthodontic brackets has been preferred for use in most clinics for esthetic reasons.1–4 Nowadays, the acid-etching bonding system has been widely accepted by most orthodontists as a routine technique that provides strong adhesion.1,2 On the other hand, some clinical concerns have remained (eg, the occurrence of white spot lesions, the fractures and cracks in the enamel surface after the brackets are debonded). Indeed, several reports have revealed damage to the enamel surface during debonding procedures, and this has raised questions about the safety of various procedures used to remove these attachments.1,5–8

Recently, a self-etching system has been introduced onto the market as a substitute for phosphoric acid–etching systems.1,2,9 The self-etching system combines etching and priming procedures into one, eliminating the need for separate etching, rinsing, and drying, and it can reduce chair time.10 It also demonstrates a more conservative etch pattern of the enamel surface and less demineralization, which prevents loosening of the enamel structure compared with phosphoric acid systems.11 To date, the system has become a favorite alternative to the traditional two-step etching and priming systems.

On the other hand, sufficient strength of enamel bonding was attained when the enamel was treated with a self-etching primer, which varied widely, ranging from 2.8 to 11.55 MPa.10 Although Reynolds12 suggested that the mean bond strength of the system was enough for clinical orthodontics, most previous studies concluded that the strength was significantly less than that of the conventional acid-etching system.1,2,10,13–16 Several reports also showed that brackets placed on the posterior teeth had high failure rates.17,18 Therefore, reinforcement of the bond strength of the self-etching system should be considered for these reasons.

In the present study, we observed enamel surfaces treated with various pretreatment solutions under scanning electron microscopy (SEM); shear bond strengths of orthodontic brackets bonded to enamel with a self-etching adhesive system were compared among subjects treated with different pretreatment solutions, to identify a suitable preparation of the enamel surface for bonding with a self-etching system.

MATERIALS AND METHODS

Enamel Preparation and Surface Morphology

As pretreatment solutions, phosphoric acid (K-Etchant, Kuraray Medical, Tokyo, Japan), polyacrylic acid (Acros Organics, Geel, Belgium), citric acid (Wako Pure Chemical Industries Ltd, Osaka, Japan), and ammonium hexafluorosilicate (SiF; Kanto Chemical Co Inc, Tokyo, Japan) were used in this study (Table 1). With each pretreatment solution, the enamel surface of each two teeth was treated. Treatment duration was 40 seconds for phosphoric acid and 60 seconds for the other pretreatment agents. After washing with water and air-drying, the specimens were placed on aluminum stabs and spattered with gold. A scanning electron microscope (SEM; JSM-5300, Japan Electron Optics Laboratory Co Ltd, Tokyo, Japan) was used to examine the enamel surface structure. The remaining two teeth were not treated with any pretreatment and/or priming procedure as the enamel was intact.

Table 1 Pretreatment Solutions for Enamel Surface Preparation

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RESULTS

Debonding Strength

All etching treatments increased debonding strength. Debonding strength in the SiF-treated group was the greatest (12.204 ± 1.048 MPa) among the five groups, followed by the polyacrylic acid group (12.030 ± 2.130 MPa) (Table 2). The control group with no acid-etching treatment showed significantly (P < .05) lower strength (9.078 ± 1.678 MPa) than the polyacrylic acid, citric acid, and SiF groups. However, all groups showed greater bond strength compared with the minimum requirement for clinical orthodontics.12

Table 2 Shear Bonding Strength Values (MPa) and Descriptive Statistics^a

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ARI

The frequency distribution of the ARI score is shown in Table 3. Chi square comparisons revealed significant (P < .05) differences among the five groups. All pretreatments increased the ARI score, compared with the control group, which underwent no acid-etching procedure.

Table 3 Frequency Distribution of ARI Score^*a

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DISCUSSION

A self-etching primer consists of acidic adhesive monomer, deionized water, activator, and stabilizer.10 The bonding performance of an adhesive monomer can be mainly influenced by its hydrophilic acid moieties (ie, carboxylic acid, phosphoric acid, and phosphonic acid moieties). When enamel is treated with these acidic monomers, the hydroxyapatite of enamel is demineralized, and the pH of the monomer is neutralized.22 During the etching phase, the adhesive monomer penetrates to the etched tooth surface; then the hydrogen ions released by hydroxyapatite crystals are chelated in the primer, resulting in microlinkage to the hydroxyapatite.10,22–25 The etching performance of self-etching primer is weaker than that of 37% phosphoric acid etching. As a result, the self-etching primer shows a more conservative etch pattern but has fewer adhesive penetrations, leading to lower bond strength.2,17–19 Thus, we hypothesized that the bond strength of a self-etching system should be reinforced by increasing the chemical reaction between the self-etching primer and hydroxyapatite if the enamel surface was dissolved with minimal damage.

In this study, enamel surfaces were pretreated, with four etching or conditioning solutions used as possible candidates. Polyacrylic acid has been used as a surface conditioner for glass ionomer cement or resin modified glass ionomer adhesive. Because glass ionomer cement chemically attaches to the hydroxyapatite of teeth, a less adhesive penetration to enamel is suitable for conditioning.26–28 Citric acid was tested as an etching solution for dental adhesive materials and showed a more conservative etching pattern and lower bond strength than phosphoric acid.29,30 SiF (ammonium hexafluorosilicate), in which the silver of diamine silver fluoride (AgF) is replaced with silicon, was also employed. Applying fluoride solution to the tooth leads to dissolution of the hydroxyapatite, resulting in synthesis of CaF (calcium fluoride) and fluoroapatite.31 Silicon is known to induce apatite formation, which may be useful in a remineralization process.32 Furthermore, it is suggested that a larger amount of fluoroapatite was formed after SiF treatment compared with sodium fluoride, APF (acidulated phosphate fluoride), or AgF treatment.33 These findings show that SiF treatment has the potential ability to prevent dental caries during orthodontic treatment.

Self-etching primers offer the advantage of minimal damage to the enamel surface.19 Enamel surfaces treated with self-etching primers are almost the same as that of the untreated tooth, and enamel surfaces treated with phosphoric acid show dissolution of the prism cores and prism peripheries and areas of incomplete demineralization. Meanwhile, two pretreatment agents, polyacrylic acid and SiF, as used in this study, had a more conservative action compared with phosphoric acid and citric acid. In SEM findings, enamel surfaces treated with these agents showed similar surface conditions compared with intact enamel. However, all pretreatments significantly increased bond strength. In particular, bond strength after SiF or polyacrylic acid treatment was increased by 1.3-fold. As the results of the ARI score show, bracket failures were almost induced in the tooth-adhesive interface of the control group. Meanwhile, the adhesives remained on the tooth surface in the treated groups. These results may indicate that an increase in dissolution and in the etch pattern of the enamel surface can enhance micromechanical bondings.18

The different etched enamel pattern seen with each different pretreatment agent is caused by complex factors29 such as difference in size and electrical charge of radicals of the etching agent,34 reprecipitation of dissolution products,35,36 and residual organic matter in the enamel.37 In addition, treatment time of the enamel surface is an important factor in the surface morphology of treated enamel. In this investigation, pretreatment time was determined to be 40 seconds for phosphoric acid and 60 seconds for the remaining pretreatment agents. All pretreatment durations were relatively long compared with clinical use or recommended time, except for phosphoric acid.28,29,38,39 Nevertheless, the most severely etched surface was observed with phosphoric acid treatment. Several studies reported that phosphoric acid pretreatment improved the bond strength of enamel-adhesive interfaces40–42; however, severe etching will spoil the benefits of mild etching if it is used before a self-etching primer. Our results suggest the usefulness of a mild etching agent such as polyacrylic acid or SiF for the pretreatment of an enamel surface to enhance the bond strength of a self-etching system.

In particular, SiF might have a suitable property as the pretreatment agent because of its strong improvement of bond strength with minimal morphologic changes to the enamel surface and its ability to provide caries protection with fluoroapatite formation. However, we did not measure fluoroapatite formation on the enamel surface after treatment. Additional investigations are needed to verify the effects of SiF treatment on remineralization and on caries protection of enamel surfaces and bonded orthodontic brackets.

CONCLUSIONS

• Surface conditioning before bracket adhesion could reinforce the bond strength of the self-etching system with minimal enamel damage, resulting in a more reliable bonding system.

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