INTRODUCTION

In recent years there has been an increase in the consumption of soft drinks among children and adolescents.1 Soft drinks are damaging not only because of the high levels of sugar they contain but also because most of them have pH levels below the critical limit for enamel demineralization (pH 5.5). Moreover, frequently consumed soft drinks have been shown to cause extreme dental erosion.2–5 Investigators have demonstrated that the erosive potential of soft drinks depends on the initial pH and buffering capacity of the drink. Carbonated soft drinks are potentially more erosive than noncarbonated beverages because of the additional carbonic acid present.6

Dental erosion is defined as the acidinduced loss of hard tissue, a chemical process in which bacteria play no part for this reason, it is not associated with dental plaque.7–9 In an in vivo study, Jensdottir et al.7 found that the prevalence of dental erosion increased as pH levels of the studied drinks decreased and as consumption increased. Other studies using scanning electron microscopy (SEM) have shown that soft drinks produce large areas of enamel decalcification.10–12 Decalcification of enamel around bonded orthodontic brackets has long been a concern for orthodontists. Investigators agree that decalcification is the first step in the breakdown of enamel. Decalcification is defined as loss of calcified tooth substance; it occurs when the pH of the oral environment favors diffusion of calcium and phosphate ions out of the enamel.13,14

A new approach in treating incipient caries lesions by an infiltration technique was introduced recently.15 Infiltration of caries lesions with lowviscosity lightcuring resins is a treatment option for noncavitated lesions not expected to arrest or remineralize. In contrast to the conventional sealing concept, wherein a resin layer is glued onto the lesion surface, caries infiltrants penetrate the porous body of initial caries lesions.16 Caries infiltrants are optimized for rapid capillary penetration and exhibit very low viscosity and high surface tension.6 Thus, laboratory experiments showed significantly deeper penetration of infiltrants into the lesion body than is seen with conventional adhesives.16–18 Clinical followup studies have proved this concept to be more effective than fluoridation measures in stopping progression of a carious lesion within 1.5years of observation.19 A recent study confirmed that the microinvasive therapy of caries by resin infiltration facilitates an early, virtually painless esthetic treatment and masking of postorthodontic white spot lesions on the enamel surface.20–22

Clearly, the subject of decalcification is complex and of great importance to the orthodontic specialty. Based on the fact that many of our patients routinely drink acidic soft drinks, the following study was conducted to evaluate the in vivo effects of two acidic soft drinks (Coca-Cola and Sprite) on the shear bond strength of metal orthodontic brackets with and without the application of resin infiltration technique and to evaluate the enamel surface after debonding using SEM.

MATERIALS AND METHODS

The present study was approved by the research ethical committee of Mansoura University. Sixty noncarious maxillary premolar teeth, scheduled for extraction in 30 orthodontic patients (12 to 17 years old), were used. All subjects were provided with verbal and written information concerning the study, and informed consents were signed. Patients were randomly divided into two groups according to the soft drink tested (Coca-Cola and Sprite, Atlanta, GA, USA; pH2.44 and 2.90 respectively). In each group, application of resin infiltration (Icon, DMG, Hamburg, Germany) was done on one side only, according to the manufacturers instructions, before bonding of brackets.23,24 Brackets were placed only on the teeth to be extracted and not on any other teeth in the mouth. Stainless steel premolar brackets (Victory series, 3M Unitek, Monrovia, Calif) with an average base area of 9.94mm² were identically bonded on all 60 teeth. Patients in the two groups were told to rinse their mouth with their respective room temperature drink for 5minutes, three times a day, for 3months. They were told not to drink any acidic soft drinks apart from these. All volunteers who participated in this study brushed their teeth three times a day for 3 minutes. At the end of 3 months, the premolar teeth were extracted without damaging brackets. Accordingly, four subgroups of 15 premolars each were obtained group 1 (Icon, brackets, and Coca-Cola), group 2 Icon, brackets, and Sprite, group 3 (brackets and Coca-Cola), and group 4 (brackets and Sprite).

RESULTS

Means and standard deviations obtained from the shear test are shown in Table 1. The Coca-Cola group without Icon showed the lowest mean bond strength (7.605 MPa), and the Sprite group with Icon showed the highest (11.602 MPa). The Coca-Cola treated specimens showed a significantly reduced bond strength compared with that of Spritetreated specimens (P < .0001). There was also a significant difference in bond strength between specimens from the Coca-Cola group without Icon and those infiltrated with Icon (P < .0001), wherein the Icon-infiltrated specimens showed higher values. Similarly, a significant difference was observed between specimens of the Sprite group without Icon compared with those infiltrated with Icon (P < .0001).

Table 1. Shear Bond Strength Results (in MPa)

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SEM

The enamel surfaces and adhesiveenamel borders of the teeth in the four groups were analyzed using SEM. In Figure 1A, areas of enamel defects caused by erosion are seen on the samples taken from the Coca-Cola group (2000× magnification). Areas of enamel defects of the Sprite group were not as extensive as those of the Coca-Cola group (2000× magnification) (Figure 1C). Figures1B and 1D show healthier enamel surfaces to which Icon resin infiltration was applied before exposure to Coca-Cola and Sprite, respectively.

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DISCUSSION

In recent decades, soft drink consumption has steadily increased among children and adolescents in both Western and developing countries.1 Enamel white spot lesions not only develop during orthodontic treatment but might also be present even at the start of orthodontic treatment, especially in high softdrink consumers. Tufekci et al.21 and Gorelick et al.22 found that 11% and 24% of their orthodontic patients, respectively, had existing white spot lesions at the time of bracket fixation. Thus, preventive strategies are needed in orthodontics to remineralize previously demineralized enamel to allow for bracket fixation and improving shear bond strength.23

As a result of the sealing effect on sound enamel and the stabilization of demineralized enamel, it is conceivable that the caries infiltration technique may be beneficial as a form of pretreatment before bracket fixation.24 So the aim of this study was to evaluate the effects of Coca-Cola and Sprite on shear bond strength of orthodontic brackets with and without the application of resin infiltration and to evaluate the effects on enamel surface mineralization using SEM. These particular soft drinks were chosen for several reasons first, because of their high levels of consumption in Egypt. In developed countries, Coca-Cola has the largest segment within the carbonated sector, with a share approaching 50, followed by lemonflavor drinks (22%).25 Second, they have a pH level below the critical limit for demineralization of tooth enamel (pH < 5.5).4 Finally, Coca-Cola contains phosphoric acid and Sprite contains citric acid, both of which are used in acid etching for placement of orthodontic brackets. The study was conducted in vivo to confirm previous in vitro studies3,24 and to consider the defense mechanism of saliva against erosion26 and the bacterial functions of the oral environment. Steffen14 stated that the presence of bacteria in the mouth along with acidic soft drinks accelerates erosion.

The shear test was performed with a universal testing apparatus, similar to the procedure of others.27–29 Gillis and Redlich29 and Mascia and Chan30 also applied shearing to their samples with a stable speed of 0.5mmmin using the universal test apparatus. Results of this study indicated that the Coca-Cola group without resin infiltration application showed the lowest mean resistance to shearing forces, which agrees with a previous study.3 Significant statistical differences were observed between the Coca-Cola and Sprite groups with and without application of resin infiltration. On the other hand, resin infiltration has resulted in significant improvement in bond strength, regardless of the type of soft drink consumed. We believe the erosive defects caused by Coca-Cola and Sprite on enamel, as shown by SEM, has a negative effect on bracket retention.3 When the Coca-Cola and Sprite groups without resin infiltration were compared using SEM, the enamel defects in the Coca-Cola group were more extensive and more noticeable than those in the Sprite group. This may be due to the more extensive enamelerosive effect of the phosphoric acid (pH 2.44) in Coca-Cola.

Rugg–Gunn et al.31 compared the erosive capabilities of a citric acidbased orange juice drink and a phosphoric acidbased diet cola drink. They determined that the phosphoric acidbased diet cola had more erosive potential than the citric acidbased juice drink. This supports our results in the present study. Our findings, evaluated using SEM, were parallel to the results of Dinçer et al.4 Steffen,14 Gedalia,32 and Grando et al.33 The erosive effect may be attributed to the low pH value of the acidic soft drinks , which lowers pH of the oral cavity.2 The erosive potential depends on the acidic properties, which is the amount of acid available (titratable acidity) and the amount of the acid actually present (concentration of H⁺ ions—pKa). In addition, complex interactions between solid and soluble components of a beverage, such as the acid/ hydroxyapatite reaction, affect the erosive potential.31

The increase in bond strength of brackets with application of the resin coincides with the results of previous studies23,24 and was most likely the result of deeper penetration of the resin infiltrant into the body of the lesion compared with the primer of the orthodontic cements. Monomer formulations with an increased TEGDMA (triethyleneglycol dimethacrylate) content have a high penetration capability,33 which probably allows a chemical connection of the resin infiltrant to the monomers of the primer. Conventional adhesives, like Transbond XT, are able to penetrate carious lesions to some extent thus, it is assumed that its primer might also partly penetrate demineralized enamel and strengthen the outermost part of the infiltrated enamel when applied after Icon preconditioning.24

Scanning electron micrographs of both groups after resin application showed a significant improvement compared with groups without resin use, as enamel appears smoother and less eroded. With the resin infiltration technique, the unique lowviscosity resin is drawn deep into the pore system of a lesion, like a sponge draws up liquids. The resin completely fills the pores within the tooth, replacing lost tooth structure and stopping caries and erosion progression by blocking further introduction of any nutrients into the pore system.23,4

Although the results of this study are promising as a confirmation of the sealing effect of resin infiltrant and the stabilization of demineralized enamel in high consumers of soft drinks, further studies are needed to evaluate the longterm effect of this technique with orthodontics.