Effect of Bonding Material , Etching Time and Silane on the Bond Strength of Metallic Orthodontic Brackets to Ceramic

In certain clinical conditions, ceramics may serve as substrates for bonding orthodontic brackets. Bonding to ceramic is normally achieved after etching the ceramic surface with hydrofluoric acid. This etching procedure is used to create a rough surface in the ceramic bonding area to enhance the bond between bonding material and ceramic. However, the etching time required for optimal bonding to ceramic is controversial in the literature. Several authors showed decreased bond strengths with Effect of Bonding Material, Etching Time and Silane on the Bond Strength of Metallic Orthodontic Brackets to Ceramic


INTRODUCTION
In certain clinical conditions, ceramics may serve as substrates for bonding orthodontic brackets.Bonding to ceramic is normally achieved after etching the ceramic surface with hydrofluoric acid.This etching procedure is used to create a rough surface in the ceramic bonding area to enhance the bond between bonding material and ceramic.However, the etching time required for optimal bonding to ceramic is controversial in the literature.Several authors showed decreased bond strengths with by a material for bonding.Composite resins are used to attach orthodontic brackets to enamel or ceramic surfaces.The protocol comprises a series of techniquesensitive steps and failures with composite resins have been attributed to moisture contamination (11).Other materials used to attach brackets to enamel or ceramic surfaces are resin-modified glass ionomer cements.They have cariostatic properties due to a slow release of fluoride at low levels over an extended period (12).However, it has been extensively demonstrated that glass ionomer cements are associated with significantly lower bond strength than composite resins (13,14).
The purpose of this study was to evaluate the bond strength of metallic orthodontic brackets to feldspathic ceramic, using different etching times (20 or 60 s) for etching ceramic, different bonding materials (Transbond XT and Fuji Ortho LC) with or without silane application.The hypotheses tested were that there is (1) no significant difference in shear bond strength between the two etching times, (2) no significant difference between with or without silane application, and (3) no significant difference between the bonding materials.

MATERIAL AND METHODS
Eight feldspathic ceramic (Certec Advanced Ceramics, Barueri, SP, Brazil) cylinders (15 mm diameter x 20 mm height) were prepared and had their surfaces cleaned using pumice-water slurry (S.S. White, Petrópolis, RJ, Brazil).Four cylinders were etched using 10% hydrofluoric acid gel (Dentsply Caulk, Milford, DE, USA) for 20 and four for 60 s.After etching, the ceramic surfaces were rinsed with air-water spray for 30 s and dried with an air stream for 30 s.In half of the cylinders for each etching time, two layers of a silane coupling agent (RelyX Ceramic Primer; 3M ESPE, St. Paul, MN, USA) were applied and dried for 60 s.After that, stainless steel standard maxillary incisor metallic orthodontic brackets (Synergy; Rocky Mountain Orthodontics, Denver, CO, USA) were bonded to the specimens using Transbond XT light-cured bonding resin (3M Unitek, Monrovia, CO, USA) or resin-modified glass ionomer Fuji Ortho LC (GC America Inc., Alsip, IL, USA), following the manufacturer's instructions.
The brackets were positioned and seated firmly on the ceramic surface.Excess resin was removed using a microbrush and light-activation was carried out with 10 s exposures on each of the four sides of the bracket.Total exposure time was 40 s using a LED curing unit (UltraLume 5; Ultradent, South Jordan, UT, USA), emitting at 1,200 mW/cm 2 .In total, 20 brackets were bonded to each ceramic cylinder (n=20) for each etching time -bonding resin/resin-modified glass ionomersilane combination, totaling a 160 bonded brackets.As several brackets (n=20) were bonded to the same ceramic cylinder, a punch-holed strip of black adhesive tape was used to avoid light exposure to adjacent brackets, restricting the polymerization light to the specimen being bonded (5).Afterwards, the specimens were stored in distilled water at 37 o C for 24 h.
The shear bond strength test was conducted in a mechanical testing machine (Model 4411; Instron, Canton, MA, USA).A mounting jig was used to align the ceramic-bracket interface parallel to the testing device.The shear load was applied using a knife-edged rod at a crosshead speed of 1.0 mm/min until failure occurred (5,15).The shear bond strength values were calculated in MPa and subjected to three-way ANOVA and Tukey's test (α=0.05).
After debonding, the ceramic and bracket surfaces were examined under ×8 magnification with a stereomicroscope (Olympus Corp., Tokyo, Japan).The Adhesive Remnant Index (ARI) modified after the method of Årtun and Bergland ( 16) was used to classify the failure modes: 0: indicates that no bonding resin or resin-modified glass ionomer remained on the ceramic; 1: indicates that less than half of the bonding resin or resin-modified glass ionomer remained on the ceramic; 2: indicates that more than half of the bonding resin or resin-modified glass ionomer remained on the ceramic; and 3: indicates that all bonding resin or resin-modified glass ionomer remained on the ceramic, along with a distinct impression of the bracket mesh.

RESULTS
The results for shear bond strength of brackets to ceramic are given in Table 1.The statistical analysis showed that regardless of bonding material and etching time, silane application increased bond strength significantly (p<0.05)compared with no silane application.The bonding material Transbond XT promoted a significantly higher (p<0.05)shear bond strength than Fuji Ortho LC, with or without silane application and for both etching times.The specimens etched for 20 s showed significantly lower (p<0.05)shear bond strengths than those etched for 60 s, for both bonding materials.
The results for ARI are shown in Table 2.A predominance of score 0 was observed for specimens etched for 20 s, independent of bonding material or silane application.On the other hand, despite the large number of scores 0 observed for the specimens etched for 60 s, there was an increase in scores 1, 2 and 3 for the longer etching time.

DISCUSSION
In this study, the first hypothesis tested was rejected.An increase in shear bond strength was observed for specimens etched with 10% hydrofluoric acid for 60 s compared with 20 s, regardless of the bonding material.
Efficient bonding to ceramic is determined by the bonding mechanisms that are controlled in part by the specific surface treatment used to promote micromechanical or chemical retention to the ceramic substrate (17).The micromechanical retention of the ceramic surface plays an important role for bonding with resin cement (8).Modification of ceramic surface morphology may be performed to increase bond strength (18).According to Yen et al. (19), hydrofluoric acid starts etching the ceramic by reacting preferentially with the silica phase, creating retentive microchannels.time, the acid reacts with the glassy matrix, partially dissolving it and increasing the formation of retentive channels.The morphological changes produced on surfaces treated with 10% hydrofluoric acid increase the surface area and facilitates the penetration and retention of resin cement into the created micro-retentions (8,18,20).Therefore, longer etching times with 10% hydrofluoric acid on feldspathic ceramic surface may create more irregularities for bonding, explaining the results of the present study.
Reynolds (21) suggested that a bond strength value between 6.0 to 8.0 MPa was adequate for most clinical orthodontic needs in the oral environment.In this study, bond strength values lower than 6.0 MPa were detected only for one group in which the ceramic surfaces were etched for 20 s and bonded with Fuji Ortho LC without previous silane application.Therefore, 20 s etching time with 10% hydrofluoric acid application might be insufficient for creating retention for proper bonding of orthodontic brackets with Fuji Ortho LC to feldspathic ceramic without silane application and resist forces during orthodontic treatment.
Another factor evaluated in this study was the influence of silane on the shear bond strength between orthodontic brackets and bonding materials.Silane coupling agents are usually monomeric species in which silicon is linked to reactive organic radicals and hydrolyzable ester groups.The reactive organic groups become chemically bonded to the resin molecules.Hydrolyzable monovalent groups bond chemically to silicon contained in the glass matrix and lithium disilicate crystals (8,22).Then, a chemical bond is formed between the silane coupling agent and silica layer on the ceramic surface or the bonding materials.*The ARI scoring system has a range between 0 and 3, with 0 indicating that no bonding resin or resin-modified glass ionomer remained on the ceramic; 1: less than half of the bonding resin or resin-modified glass ionomer remained on the ceramic; 2: more than half of the bonding resin or resin-modified glass ionomer remained on the ceramic; and 3: indicates all bonding resin or resin-modified glass ionomer remained on the ceramic, along with a distinct impression of the bracket mesh.
The second hypothesis tested was also rejected.The present study showed that independent of the acid etching time and bonding materials, there was always an increase in the shear bond strength for the groups that received silane application, which is in accordance with previous studies (3,(8)(9)(10).
Transbond XT produced significantly higher shear bond strength than Fuji Ortho LC in all tested conditions, and thus the third hypothesis was also rejected.Despite the brackets having been fixed on ceramic surface, these findings are in agreement with those of previous studies that reported lower shear bond strength for Fuji Ortho LC than Transbond XT, when brackets were bonded to enamel surface using these materials (13,14).However, other studies reported that the shear bond strength for Fuji Ortho LC was not statistically different from Transbond XT when bonded to enamel (22)(23)(24).
The difference between two bonding materials can be related to differences in viscosity and composition.Fuji Ortho LC showed less flow than Transbond XT, probably due to the presence glass particles, which may reduce the effect of wetness for this material on ceramic surface.Bond strength values between 6 to 8 MPa have been recommended (21).Based on this recommendation, bracket bonding to ceramic with Fuji Ortho LC using the protocol of 20-s etching with 10% hydrofluoric acid without previous silanization does not have the potential to resist forces during orthodontic treatment, as a shear bond strength of only 5.75 MPa was obtained.
The ARI scores showed that most failures after debonding were scored 0, with no bonding resin or resin-modified glass ionomer remaining on the ceramic surface.This may be clinically advantageous because there is less adhesive to remove from the ceramic surface after debonding.The increase in ARI scores 1, 2 and 3 for the 60-s etching time probably occurred due to the increased retention created on the ceramic surface.Therefore, the increased mechanical interlocking of the bonding resin or resin-modified glass ionomer to ceramic generates failures involving both bonded materials, and not only adhesive failures.This finding is in agreement with those of a previous study that reported that shorter etching times might affect not only the bond strengths of brackets to ceramic, but also the mode of failure to the bonding substrate (5).
In summary, the present study demonstrated that etching time and silane application is a decisive factor for the bond strength of brackets to ceramic surfaces.Although clinicians follow the manufacturers' instructions for bonding procedures, a 60 s etching time and silane application for bonding to ceramic might be recommended.On the other hand, care should be taken to select the right etching time for a ceramic surface.Some studies have shown negative effects of over-etching ceramics, attributed to difficulties of the bonding materials to penetrate into the irregularities and the potential for cohesive failure of the substrate (1)(2)(3)(4)25).In spite of the stronger shear bond strength found for the Transbond XT, the results of this study suggest that Fuji Ortho LC can be used under the tested conditions, except for the 20 s etching time with 10% hydrofluoric acid without silane application on ceramic surface.Therefore, clinicians need to take the specific properties of bonding materials into consideration and should be careful during clinical application of hydrofluoric acid.