INTRODUCTION

Despite the advances in orthodontic material and treatment mechanics, the placement of fixed appliances is still linked with a high risk of developing white spot lesions.1–3 The prevalence of new decalcifications among orthodontic patients with fixed appliances is reported to range from 13% to 75%.1–3 White spot lesions are caused by the increase of Streptococcus mutans. Because of the presence of fixed orthodontic appliances, oral hygiene is impeded and plaque accumulation at the bracket base is prolonged.45 In the plaque around the bracket, a low resting pH value is present.45 White spot lesions are characterized by an extremely rapid development and by a preferred localization on the vestibular sides of lateral maxillary incisors, mandibular canines, and premolars.3 Most demineralization lesions are irreversible and, therefore, become a long-term concern for the patient and the orthodontist.6

Approaches to inhibit the development of carious lesion in patients with fixed appliances have focused on the control of the bacterial biofilm around the brackets and on the maintenance of a constant fluoride level in the oral cavity.78 Fluoride-releasing bonding material for bonding brackets showed almost no demineralization-inhibiting effect.8 Therefore, it has been suggested that the combined use of antimicrobials and fluoride enhances the cariostatic effect of fluoride.9

In restorative dentistry, a new antimicrobial and fluoride-releasing bonding system, Clearfil Protect Bond™ (Kuraray Medical Inc, Okayama, Japan), has been introduced. Clearfil Protect Bond™ consists of a newly developed antibacterial monomer 12-methacryloyloxydodecylpyridinium bromide (MDPB). MDPB shows antibacterial activity before and after curing against oral bacteria. MDPB is useful for incorporation into dental resin-based materials. Primers containing MDPB have been demonstrated to kill S. mutans within a short time of contact and to exhibit an inhibitory effect on the growth of bacteria on its surface.10–14 From the same manufacturer, a direct-bonding orthodontic adhesive Kurasper F (Kuraray Medical Inc) was obtained. This adhesive continuously releases small amounts of fluoride ions to the tooth surface around the bracket.

The purpose of this study is to evaluate the shear bond strengths (SBSs) of the new antimicrobial bonding system in combination with the fluoride-releasing adhesive. In the event of acceptable bond strength, clinical studies can then be used to assess the effectiveness in preventing demineralization in patients with fixed appliances.

MATERIALS AND METHODS

Bonding procedures

The teeth were randomly divided into three groups of 40 teeth each (20 incisors, 20 premolars). Detailed information on the products used is shown in Table 1. In all groups, the bonding procedure was performed according to the manufacturer's instructions as follows.

Table 1.  Materials Used in this Study^a

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Group 1 (Transbond XT)

The enamel surfaces were etched for 20 seconds with a 37% phosphoric acid (Gel Etch®, 3M Unitek, Monrovia, Calif), then rinsed with water for 10 seconds, and air-dried. Sealant (Transbond XT®, 3M Unitek) was applied to the etched surface, and after cleaning the bracket base with alcohol, the brackets were coated with Transbond XT®. The brackets were polymerized with a halogen light for 20 seconds.

Group 2 (Kurasper F)

Kurasper™ F (Kuraray Medical Inc), a direct-bonding adhesive that continuously releases small amounts of fluoride ions, was applied according to manufacturer's recommendations. Phosphoric acid etching was performed with K-etchant™ gel for 40 seconds. The gel was rinsed off with water, followed by drying of the enamel surface with compressed air for 5 seconds. Kurasper™ F-Bond was brushed on. Care was taken to have a thin film thickness. Then the F-Bond was light activated for 10 seconds. The brackets were coated with Kurasper F paste and light cured from both the mesial and distal sides for 20 seconds each.

Group 3 (Clearfil Protect Bond instead of K-etchant, Kurasper F)

Instead of etching with K-etchant™, the self-etching primer Clearfil Protect Bond™ was applied as suggested by the manufacturer. After a gentle application for 20 seconds, the primer was dried with a mild air flow. Then Clearfil Protect Bond was applied, gently air flowed, and light cured for 10 seconds. After these steps, the bonding procedure similar to that in group 2 was performed.

All teeth were bonded with Mini Diamond® brackets (Ormco, Orange, Calif). Upper lateral incisor brackets (order no 351-0271, Ormco, Amersfoort, The Netherlands) were used for the incisors and premolar brackets (order no 350-0514, Ormco, Amersfoort, The Netherlands) for the premolars. The average surface of the bracket base was 8.75 mm² for the upper lateral incisor brackets and 11.25 mm² for the upper premolar brackets.

After placing the brackets on each tooth at room temperature, a 250-g force was applied for 5 seconds to ensure a uniform adhesive thickness. Excess adhesive was removed before light polymerization with a halogen light (Ortholux™ XT, 3M Unitek). The bonding procedure was conducted by a trained orthodontist. Before bond strength testing, the prepared specimens were stored in distilled water at 37°C for 48 hours.

RESULTS

Figure 1 and Table 2 show the results of the Kaplan-Meier survival analysis. The log rank test revealed statistically significant differences (P = .0035) in SBS. When Mann-Whitney test was applied, those differences were found between the premolars of group 3 when compared with premolars of group 1 (P = .001). No further statistically significant differences were found.

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Table 2.  Shear Bond Strength (MPa) (Means, Standard Deviations, Medians, Standard Errors, 95% Confidence Intervals)^a

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No enamel fractures were found in any of the specimens. The distribution of the ARI results and the median ARI score are shown in Table 3. The Kruskal-Wallis test indicated that there were significant differences among both groups (premolars ℵ² = 32.560; P = .000 and incisors ℵ² = 31.344; P = .000). The Mann-Whitney test showed that the ARI score for the incisors and the premolars in group 3 were significantly lower than those for the incisors and premolars in groups 1 and 2. With all adhesive remaining on the tooth after debonding in group 2, the ARI scores for the incisors and premolars of groups 1 and 2 were significantly different as well.

Table 3.  Frequency Distribution of ARI scores for group 1 (acid etch + Transbond XT), group 2 (K-etchant + F-Bond + Kurasper F paste), group 3 (Clearfil Protect Bond + F-Bond + Kurasper F paste)^a

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DISCUSSION

White spot lesions have been reported in 50% of teeth treated with brackets and in up to 50% of the orthodontic patients.316 Therefore, carious prevention in orthodontic patients is a serious concern in daily practice.417

The high prevalence of postorthodontic treatment decalcifications is caused by the increase of S. mutans because of the impeded oral hygiene and pH values lower than 4.5 in the plaque around the brackets. The application of fluoride is one approach to reduce the caries risk. However, because of the acidic environment around the bracket, the remineralization progress is hampered and more fluoride will not necessarily give a cariostatic effect.17 As proven by Øgaard et al,4 the combination of antimicrobial and fluoride varnishes more effectively reduces the increments of new lesions when compared with the application of fluoride varnish alone. It was speculated that this is due to an inhibiting effect of the antimicrobial varnish in an area with low oral clearance and partly to an inhibiting effect of the varnish on S. mutans.18

In this in vitro study, the fluoride-releasing Kurasper F-Bond was combined with the antimicrobial self-etching primer Clearfil Protect Bond. Both products are offered by one manufacturer. As demonstrated by previous studies, earlier, because of the incorporated MDPB, Clearfil Protect Bond has effective antimicrobial properties.11–14 In addition to its antibacterial characteristics, Clearfil Protect Bond is claimed to release fluoride. However, the results from Kawashima et al.11 showed that 450 μg/g was released within 1 month in vitro. Considering the clinical situation with a small part of the material being exposed to the oral cavity, this is a very small amount. According to the manufacturer, Kurasper F has the potential to release four times more fluoride than Clearfil Protect Bond. Therefore, the etched enamel was precoated with Kurasper F-Bond because only this component will release sufficient fluoride within a shorter time. The combined application of Clearfil Protect Bond and Kurasper F-Bond is claimed to release sufficient fluoride and to have an antimicrobial effect in the microenvironment around the bracket—the region where demineralization lesions are primarily located.

The risk of decalcification is mostly determined by the patient's oral hygiene status.16 Preventive approaches such as the application of fluoride, chlorhexidine, and sealants attempt to reduce the risk of developing new lesions. A study on the use of fluoride mouth rinsing detected a compliance of less than 15% on behalf the patients.19 Approaches to minimize the risk of caries that are independent of the patient's compliance are of special value.

To provide a caries preventive attempt that is independent of the patient's compliance, antimicrobial agents were mixed with orthodontic adhesives. In vitro investigations showed that the combined application of chlorhexidine varnish and adhesive still produced clinically acceptable SBSs.20–22 However, SBS values were significantly reduced when the enamel surface was treated with the same primer and other forms of antimicrobial varnish EC40.20 Indirect bonding of brackets after the application of an antimicrobial varnish showed significantly reduced SBSs.23 The differences in SBSs after the application of antimicrobial agents were reflected in the site of failure.20–22 The bond failure shifted significantly toward the composite-varnish interface.20–24 Therefore, it was concluded that the different chemical properties or compositions of varnishes may have some effects on SBS and on the site of failure.20

The results of this in vitro study showed that the combination of the antimicrobial self-etching primer plus a fluoride-releasing adhesive did not reduce the SBSs. All observed SBSs were clinically acceptable.2425 Although in other studies the application of antimicrobial agents led to a reduction of SBSs, SBSs of premolars in group 3 were even statistically significantly higher than those in groups 1 and 2, that are comparable with those of previous findings of other working groups.2627 Bond strengths between 8 and 9 MPa have been proposed to be clinically acceptable.2425 The Kaplan-Meier graph shows that at this level of stress the survival probability was higher than 75% in all groups, whereas the highest survival probability was found for the incisors and premolars of group 3.

Self-etching primers from different manufacturers have been tested previously. The results are contradictory, and some studies reported sufficient bond strength. Here, the authors underlined the advantage of a moderate etching pattern on enamel after application of self-etching primers resulting in less enamel damage.2628–30 Other studies detected significantly lower SBS when compared with total-etch systems.31–33 The lower SBSs became evident when self-etching primers were bonded to unground enamel.34 The thin and less-uniform resin tags observed in the SEM photographs of acidic primer-treated enamel were taken as an explanation for the poor adhesion.32 Recently it was shown that thermal cycling and water storage led to a decrease in SBSs of brackets bonded with self-etching primers.31 In the absence of these procedures before shear bond testing, the study presented here provides only initial bond strength data. However, in contrast to recently published studies,31–34 the achieved SBSs of teeth bonded with the used self-etching primer were higher than those of the control teeth bonded with a total-etch system.

Despite the fact that Clearfil Protect Bond combined with Kurasper F produced the statistically highest SBS, it also resulted in less composite resin remaining on the tooth. The combination of a bonding regime that produces high SBS and minimal composite removal from the tooth after debonding with the additional fluoride-releasing and antibacterial characteristics would be attractive for the majority of clinical orthodontist.

Although the testing procedure was based on recommendations in the literature,35–37 mean bond strength measurements might be of limited value for interpreting clinical performance. In clinical settings bonding systems are exposed to a number of different intraoral factors.38 Nevertheless, in vitro testing still remains a necessity for the initial evaluation of bonding systems.

Further clinical studies focusing on clinical efficiency in reducing carious lesions in patients with fixed appliances are needed.

CONCLUSIONS

• The combination of an antimicrobial self-etching primer with a fluoride-releasing bonding system does not negatively affect the SBSs. Premolars bonded with this combination showed statistically higher SBSs when compared with teeth bonded with Transbond XT or Kurasper F.

• Teeth bonded with Clearfil Protect and Kurasper F retained significantly less adhesive on the tooth surface when compared with teeth precoated with Transbond XT or Kurasper F after conventional etching.