INTRODUCTION

Orthodontic treatments with fixed appliances increase the probability of enamel demineralization and white spot formation caused by the accumulation of dental plaque around brackets.12 A clinical study showed that the incidence of white spot formation during treatment with fixed orthodontic appliances was 46.9% when no preventive fluoride program was used.1 Another study demonstrated that visible white spots appeared around fixed orthodontic appliances within only 1 month after the start of treatment; some preventive measures with a fluoride, therefore, were required.2

As a solution to cariogenic problems that occur during orthodontic treatment, many types of orthodontic adhesive containing fluoride have been developed, such as glass-ionomer cements, resin-modified glass-ionomer cements, and composite resins containing a fluoride.34 The fluoride contained in adhesives releases into the oral environment fluoride ions, which are absorbed by teeth at the adhesive/enamel interface. However, previous studies reported that most fluoride ions were released within 24 hours after placement of brackets,5 and that fluoride released from fluoridated materials might not last long.6 Fluoride recharging into fluoridated adhesives is needed for continuation of release of fluoride ions. Recently, a fluoride-releasing and -recharging adhesive resin that keeps releasing fluoride ions for a considerably long time has been developed.7 This resin contains surface pre-reacted glass-ionomer (S-PRG) fillers that can release and recharge fluoride,7 which has anticariogenic properties. So, it reduces demineralization, enhances remineralization, checks plaque formation, and inhibits microbial growth and metabolism.8 Scougall Vilchis et al9 reported that Beauty Ortho Bond, which contains resin-based S-PRG fillers, had a significantly lower mean bond strength than Transbond XT and Plus adhesive systems.

An acid-etching adhesive system for orthodontic brackets mainly involves acid-etching, rinsing, priming, and bonding steps. A self-etching primer combines acid-etching, rinsing, and priming in a single step, resulting in simplification of the clinical handling of adhesive systems and savings in chair time.10 Some researchers reported no significant differences in mean bond strength between self-etching and acid-etching adhesive systems.1112 Others showed that the self-etching adhesive system had a significantly lower bond strength than the acid-etching adhesive system.1314

Bracket failure due to uncontrollable impact forces or the inevitable removal of brackets to replace them in an ideal position is not a rare experience for clinicians during orthodontic treatment. Findings regarding bond strength of rebonded brackets have been contradictory. Some investigators have reported that rebond strength was lower,1516 while others have reported that it was comparable with,17 or higher than,18 original bond strength.

The purpose of this study was to ascertain the effects of repeated bonding on the shear bond strength of orthodontic brackets bonded with a fluoride-releasing and -recharging adhesive system with a self-etching primer vs two other types of adhesive system.

MATERIALS AND METHODS

The protocol was approved by the Local Committee of Ethics. Informed consent was obtained from all participants.

A total of 48 freshly extracted permanent teeth were collected and stored in a solution of 0.1% thymol at 4°C prior to testing. The criteria for tooth selection included buccal enamel unaffected by any pretreatment chemical agents, no cracks incidental to extraction, no hypoplastic enamel, and no caries.

The 48 teeth were divided equally into three groups of 16. Each group was made up of seven maxillary first premolars, three maxillary second premolars, four mandibular first premolars, and two mandibular second premolars.

Premolar stainless steel brackets with a 0.018-inch slot (Victory series; 3M Unitek, Monrovia, Calif) were used, and average bracket base area was 9.94 mm². Brackets were bonded to teeth according to one of the following three protocols, in accordance with the manufacturer's instructions.

• Bonding protocol with Transbond XT adhesive system: The buccal surface of each tooth crown was cleansed with a mixture of water and fluoride-free pumice in a rubber prophylactic cup for 10 seconds. Each tooth then was rinsed with a water spray for 10 seconds and dried with an oil-free air drier. The buccal enamel surface was etched with 35% phosphoric acid gel (3M Unitek) for 15 seconds, and the tooth was thoroughly rinsed and dried. A frosted appearance was considered an indication that etching was a success. A thin uniform layer of Transbond XT primer (3M Unitek) was applied to the etched enamel surface, and Transbond XT adhesive (3M Unitek) was applied to the bracket base. The bracket was placed onto the buccal surface of the tooth and was pressed firmly into place to express adhesive from the margins of the bracket base. Excess adhesive was removed with an explorer before curing. Then, the bracket was light-cured with an ortholux light-emitting diode (LED) curing light (3M Unitek) for 10 seconds: 5 seconds mesially and 5 seconds distally.

• Bonding protocol with Transbond Plus adhesive system: The buccal surface of each tooth crown was cleansed and rinsed. Excess water was blotted with cotton pellets to ensure the formation of a thin uniform layer of water. Transbond Plus self-etching primer (3M Unitek) was rubbed on the buccal enamel surface for 5 seconds and was blown off gently with the oil-free air drier. Brackets then were bonded with Transbond XT and light-cured with the ortholux LED curing light.

• Bonding protocol with Beauty Ortho Bond adhesive system: The buccal surface of each tooth crown was cleansed and rinsed and thoroughly dried. Beauty Ortho Bond self-etching primer (Shofu, Kyoto, Japan) was rubbed onto the buccal enamel surface for 5 seconds and was blown off gently with the oil-free air drier. Brackets then were bonded with Beauty Ortho Bond paste (Shofu) and were light-cured with the ortholux LED curing light.

The root of each tooth bonded to the bracket was cut off with a separating disk. The tooth crown was embedded in a specimen holder ring with a self-curing acrylic resin and was oriented so that the buccal enamel surface was parallel to, and projected above, the brim of the cylindrical specimen holder ring. All specimen holder rings with embedded teeth were stored in distilled water at 37°C for 24 hours.

Bracket debonding was performed in a universal testing machine (ET test; Shimadzu, Kyoto, Japan) for determination of shear bond strengths. Specimen holder rings were arranged in this machine so that a load was applied to the bracket wings with a force in the occlusogingival direction parallel to the buccal enamel surface. The force required to shear off the bracket was recorded in Newtons at a cross-head speed of 1.0 mm per minute. Shear bond strength (MPa) then was calculated by dividing shear force by bracket base area.

After debonding, all visible residual adhesive was removed with a pair of adhesive-removing pliers (3M Unitek). The appearance of a smooth enamel surface with no trace of composite was regarded as a sign that removal of all residual adhesive had been completed. Bonding and debonding procedures were repeated two more times on the same tooth surface with the use of a new bracket each time. The same order of the teeth was maintained so that shear bond strengths of teeth could be compared in their proper sequence.

After each shear bond strength was tested, bracket bases and enamel surfaces were examined by a single investigator, who used a stereomicroscope with 8 times magnification to evaluate the adhesive remnant index (ARI).19 ARI scores ranged from 0 to 3, with 0 indicating that no adhesive remained on the tooth surface, 1 showing that less than half the adhesive remained on the tooth surface, 2 indicating that more than half of the adhesive remained on the tooth surface, and 3 meaning that all adhesive remnants on the tooth surface had a distinct impression of the bracket base.

RESULTS

Table 1 shows that overall mean shear bond strengths of three adhesive systems and the shear bond strength of each specimen at every debonding reached 6 MPa, which is a minimum requirement for clinical use.20

Table 1.  Descriptive Statistics of Shear Bond Strengths for Three Debondings in Three Adhesive Systems

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As is shown in Table 2, two-way ANOVA demonstrated a significant difference in mean shear bond strength between adhesive systems, no significant differences between bonding/debonding sequences, and no significant interaction between these two variables. The Scheffe post-hoc test showed that Transbond XT and Transbond Plus had significantly higher mean shear bond strengths than Beauty Ortho Bond, although no significant differences were found between Transbond XT and Transbond Plus.

Table 2.  Two-Way ANOVA of Shear Bond Strength

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One-way ANOVA and the Scheffe post-hoc test showed that Transbond XT and Transbond Plus had significantly higher mean shear bond strengths when compared with Beauty Ortho Bond after each debonding; the mean shear bond strength was not significantly different between Transbond XT and Transbond Plus. Analyses also revealed no significant differences in mean bond strength between three debondings with each adhesive system.

Table 3 shows the distribution of ARI scores for three adhesive systems after each debonding. Kruskal-Wallis and Bonferroni tests demonstrated significant differences in the distribution of ARI scores between Transbond XT and Beauty Ortho Bond after the first debonding, and between Transbond Plus and Beauty Ortho Bond after the third debonding. These results showed that bond failure occurred more frequently at the enamel/adhesive interface when Beauty Ortho Bond was used than when either Transbond XT or Transbond Plus was used. These tests also showed no significant differences in the distribution of ARI scores between three debondings in each adhesive system.

Table 3.  Distribution of Adhesive Remnant Index (ARI) Scores for Three Adhesive Systems After Each Debonding

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DISCUSSION

Our results show no significant differences in mean shear bond strength between Transbond XT and Transbond Plus at each debonding; findings were consistent with those reported by earlier investigators.1112 The pH of Transbond Plus self-etching primer worked out at 0.4 and increased little within 10 minutes (pH 0.4 to 0.5); that of Beauty Ortho Bond self-etching primer was 1.5 and did not increase at all. Although self-etching primers generally produce less demineralization of enamel than is produced by phosphoric acid etchants, 14 the pH of Transbond Plus self-etching primer might be too acidic to yield a satisfactory bond strength for attaching brackets to enamel. It might be speculated that the high bond strength noted with Transbond Plus was due to nanoretensive interlocking between enamel crystallites and adhesives. Other researchers have showed that self-etching adhesive systems had significantly lower bond strengths than acid-etching adhesive systems.1314

Our findings that Transbond Plus had a significantly higher mean shear bond strength at each debonding than Beauty Ortho Bond could be explained by differences in the pH of self-etching primers. Beauty Ortho Bond had a higher pH value and a milder self-etch ability than Transbond Plus, thus creating a shallow etching pattern and insufficient penetration of adhesive resins into the enamel surface. This deficient adhesive resin penetration may cause a lower bond strength in Beauty Ortho Bond than in Transbond Plus and Transbond XT.

Figure 1, which was obtained from the scanning electron microscope (SEM), shows that the smooth surface of the untreated enamel became very porous, and numerous enamel crystallites and honeycomb structures were observed after phosphoric acid etching. This figure also shows that a greater number of porosities were observed on the enamel surface self-etched with Transbond Plus than with Beauty Ortho Bond, although the enamel surface treated with both self-etching primers was not very demineralized, as was observed in previous studies.914 These SEM findings could verify our results that Beauty Ortho Bond had significantly lower mean shear bond strength than Transbond Plus or Transbond XT.

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The bond strength of brackets rebonded to enamel surfaces depends on several factors, including the type of adhesive system15–1721–24 and the type of bracket used,162125 reconditioning of enamel surfaces and brackets,172223 the use of recycled or new brackets,1623 and the frequency of bonding/debonding sequences.1524 Egan,17 Mui,23 Montasser,24 and Harris25 and colleagues showed no significant differences in mean bond strength between initial bonding and rebonding, as was confirmed in this study, but the materials and methods that they used were different from the ones we used. The teams of Egan,17 Mui,23 and Harris25 measured initial and rebond bond strengths only twice, while those of Montasser24 measured values three times, as was done in this study. Egan et al17 used the same brackets with phase 2 paste/paste adhesive system. Mui et al23 prepared enamel surfaces with a tungsten carbide bar. Harris et al25 rebonded the same brackets with the Ortho-Concise adhesive system but used ceramic brackets. Montasser et al24 showed that after three debondings, the shear bond strength of the Transbond XT adhesive system did not change, but that of the Rely-a-Bond (Reliance Orthodontic Products Inc, Itasca, Illinois) and M-bond adhesive systems (Tokuyama Dental Corporation, Tokyo, Japan) increased significantly from the first to the third debonding.

In our study, significant differences in mean shear bond strength were reported between adhesive systems, and no significant differences were shown between bonding/debonding sequences. These results were consistent with the findings of Faust et al,21 who reported that differences in bond strength among adhesives were more dramatic than differences between bond and rebond strengths. In this study, regardless of adhesive systems and bonding/debonding sequences used, all shear bond strengths of three adhesive systems at every debonding reached 6 MPa, which is a minimum requirement for clinical use.20 From a clinical perspective, it is important to obtain adequate bond strength for performance of orthodontic treatments without enamel fracture or crack at debonding, rather than to achieve the greatest possible bond strength.

To determine whether adhesive was left or was not left on the enamel surface that was cleansed after debonding with SEM, enamel surfaces were etched with 35% phosphoric acid gel for 20 seconds. Figure 2 shows that the residual adhesive was observed in Transbond XT but not in Transbond Plus or Beauty Ortho Bond. These SEM findings might suggest no association between residual adhesive and shear bond strength at bonding/debonding sequences.

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In this study, bond failure at the enamel/adhesive interface occurred more frequently in the group that used Beauty Ortho Bond than in those that used Transbond XT and Transbond Plus, although no significant differences in the mode of bracket/adhesive failure were noted between three debondings with each adhesive system. These results might have reflected deficient penetration of adhesive resins into the self-etched enamel surface produced by the Beauty Ortho Bond with a high pH value. Awareness that more frequent bond failure occurs at the enamel/adhesive interface with Beauty Ortho Bond with anticariogenic properties might help orthodontists to decrease the risk of enamel fracture and to remove residual adhesive from the enamel surface easily at debonding.

CONCLUSION

• Awareness that the fluoride-releasing and -recharging adhesive system with a self-etching primer (Beauty Ortho Bond) had clinically sufficient shear bond strength in repeated bracket bonding may help orthodontists to decrease the risk of damage to enamel.