31 The purpose of this study was to determine the effects at home and in officebleaching on the shearbond strengths of metal, composite and ceramicbrackets bonded with light cure composite material to human enamel and to compare the shearbond strengths of metal, ceramic and compositebrackets after at-home and in-officebleaching. Descriptive statistics, including the mean, standard deviation, and minimum and maximum values, were calculated for each group. The data is expressed in MEAN ± SD. Statistical Package for Social Sciences (SPSS 16.0) version was used for statistical analysis. One way ANOVA was applied for analysis. Post Hoc followed by Dunnet t test was used to find statistical significance between and within the groups. P value less than 0.05 (P<0.05) considered statistically significant at 95% confidence interval.
Vital tooth bleaching which is a routine treatment in modern dental practice is accomplished by either an at-home technique or in-office procedures with high-concentration bleaching agents [1,2]. Although the clinical effectiveness of tooth bleaching has been demonstrated extensively , there are some concerns about potential complications. Whitening agents have adverse effects on the dental pulp [4,5] may decrease micro-hardness of the bleached substrate , and have deleterious effect on bondstrength of the resin materials [7,8]. One of the theories regarding the deleterious effect of bleaching on the bondstrength of resin materials is related to the decrease in bondstrength with the free radicals from oxygen that remain in the dental tissues released by the bleaching agents. After an adhesive system application, the oxygen responds to the closures of the forming polymeric chains, finishing the polymeric extension, lessening the level of transformation of the adhesive system and resin composites and declining the bond quality . The use of bleaching techniques modified by a remineralizing agent called CPP-ACP (casein phosphopeptide amorphous calcium phosphate) has been suggested in an attempt to recover minerals that are lost during bleaching . A recent study revealed that the associative utilization of CPP- ACP and high concentration hydrogen peroxide may be a fruitful strategy for diminishing tooth sensitivity and restricting changes in the enamel morphology during in-officebleaching . Another modification is using fluoride with bleaching agents to prevent either hypersensitivity or demineralization accompanying tooth-whitening therapy. The addition of sodium fluoride to the bleaching agent was found to generate fluoridated hydroxyapatite and calcium fluoride crystals on the enamel surfaces, which potentially accelerated the remineralization of the bleached enamel . But there is inadequate evidence on the influence of hydrogen peroxide and CPP-ACP on composite-enamel bonding. Additionally, despite profound scientific suggestions which support the remineralization capability of fluoride , the impact of fluoride on resin–enamel holding is dubious. Decreased resin bondstrength has been reported for fluoride-treated enamel, particularly
One study reported the positive effects of 10% sodium ascorbate gel as an antioxidant on the bonding capacity to enamel surface bleached with 10% carbamide peroxide;10% sodium ascorbate was applied for 10, 60, 120, 240, and 480 minutes on bleached enamel surface. This study reported that as the application time of the antioxidant increased, SBS of resin composite to enamel increased, too. For greater effectiveness, antioxidant gel should be applied to enamel surface for at least 60 minutes. Consistent with the results of the present study, no significant increase in shearbondstrength was observed in the group that received the antioxidant for 10 minutes . In the present study, in group A2, 15% carbamide peroxide was used, producing more oxygen reactive molecules than when 10% carbamide peroxide was used; the antioxidant was applied for 10 minutes. Therefore, based on the results of the current study, it seems that with increasing the concentration of bleaching agent, the application time of antioxidant should be increased.
Recent clinical results for Zirconia all-ceramic restorations revealed that the fracture rate of the Zirconia framework is so low and Zirconia core has high stability [8,9]. However, chipping-off fractures of porcelain are the most common reason for failure of Zirconia in FPDs [8,9]. The rate of porcelain fracture in Zirconia in FPDs is extremely high in comparison with that in PFM . It seems that the weakness in layered Zirconia-based porcelain is caused by the gap of bonding between the veneer material and the core of Zirconia. For longevity of restorations, this weakness has to be critically considered [9-10]. Long-term studies indicate that the fracture rates of porcelain in PFMs are 2.7-5.5% for a follow-up period of 10 to 15 years [11,12].
still not clear, the reduced bondstrength of bleached enamel has been related to the presence of residual free radicals due to the breakdown of hydrogen peroxide [14, 18] and alter- ations in the enamel composition and structure [6, 19] fol- lowing the bleaching treatment. The residual oxygen in the interprismatic spaces can hamper resin infiltration and in- hibit resin polymerization . Moreover, morphological and compositional changes (e.g., porosity, loss of enamel prismatic form, loss of calcium, and changes in organic sub- stances) in the enamel may weaken the adhesive interface and compromise bondstrength [21, 22]. Therefore, bonding procedures should not be performed immediately after bleaching treatment . A waiting period of 1–3 weeks has been advocated by various researchers [21, 24, 25]. In addition to the delayed bonding procedure, the application of antioxidant agents (e.g., sodium ascorbate, sodium bi- carbonate, and grape seed extract) [18, 26, 27] and laser ir- radiation [28, 29] have been proposed to restore the compromised bondstrength of bleached enamel. By neu- tralizing residual free radicals  and promoting micro-retentions in the enamel surface , antioxidant agents and laser irradiation have been shown to reverse the reduced bondstrength between the composite resin and bleached enamel. However, it is important to point out that most of the above-mentioned studies measured the bondstrength without thermocycling [24–29]. Ther- mocycling is the in vitro process of subjecting a restor- ation and tooth to temperature limits similar to those experienced in the oral cavity . It would be of interest to investigate the effects of thermocycling on the bondstrength between the composite resins and the bleached enamel.
Page 5 determined whether clinically the two enamel conditioning methods resulted in similar or different failure rates in terms of the number of loose brackets. They developed and adhesive remnant index on the basis of a pilot study on twenty extracted teeth and gave the criteria with four scores. In their in vivo study, theyconcluded that the failure rates were significantly higher after enamel conditioning with dilute sulfuric acid containing sodium sulphate than after conditioning with a solution containing dilute sulfuric acid plus 10% phosphoricacid and after conditioning with this latter solution than after phosphoric acid etching. They also reported that nearly all the brackets became loose during the first two weeks of bonding subsequent to dilute sulfuric acid conditioning. The failure rates occurred at a later point of time when conditioning with the combination solution of dilute sulfuric acid plus 10% phosphoric acid.
Mandibular central incisor brackets (American Orthodontics, California, USA) were bonded to the surface of the discs by the same operator. Transbond XT Light Cure Adhesive (3M Unitek, Monrovia, Cali- fornia, USA) was used for this purpose with 5 N pres- sure with the help of a Correx gauge (Haag Streit, Berne, Switzerland). It was then light cured for 40 se- conds with a light intensity of 650 mW/cm 2 . The discs were then mounted in auto-polymerizing acrylic resin (Pars Dental, Tehran, Iran) such that the bracket slot was parallel to the horizon. The discs were immersed in distilled water for 24 hours and were then subjected to 500 thermal cycles between 5-55°C for 24 hours. Shearbondstrength testing was then performed using an Instron universal testing machine (Z020; Zwick/Roell, Ulm, Germany) with a crosshead speed of 0.5 mm/minute.
strate. In this way, nanometric porosities of intertubular dentin created by the NaOCl treatment were not reached by monomer, leaving an adhesive interface with voids. This may explain the lowering of bondstrength in the using of NaOCl.  Ebrahimi et al.  showed that the decrease of bondstrength as a result of the use of NaOCl can also be attributed to damages to the organic matrix of dentin, especially to collagen fibers. Approx- imately, 22% wt of dentin is composed of organic mate- rials, which predominantly consist of type I collagens that have an important role in the mechanical properties of dentin. NaOCl reacts with amino acids of dentin pro- teins and breaks down peptide chains; therefore, it may change the mechanical properties of dentin by destroy- ing the organic content of dentin. They showed that NaOCl damages the organic component of dentin; therefore, organic monomers do not sufficiently pene- trate into the demineralized dentin, resulting in a lack of proper bondstrength.  Owing to the similarities between the findings of the present study and those of Ebrahimi et al.,  it can be suggested that the applica- tion of 5% NaOCl offers a credible explanation for the decrease of bondstrength in nanofilled and silorane resin composites.
For the simulation of dental crowns or veneers, blocks (30 mm × 15 mm × 12 mm) of inCoris TZI zirconium oxide sinter ceramic (Dentsply Sirona, York, USA) were sintered and prepared according to the technique work steps in a dental laboratory. Subsequently, the ceramic blocks (n = 20) were randomized and divided into two groups and fixation of Marquis 022 Roth brackets (Or- tho Technology, Tampa, USA) was done either by using the bonding agent Monobond S (Ivoclar Vivadent) or Monobond Etch & Prime (Ivoclar Vivadent). Detailed in- formation about the bonding agents is given in Table 2. A total of 240 brackets were used in this study, 120 for the group of Monobond S and 120 for the group of Monobond Etch & Prime. In each group, half of the brackets were positioned simulating an orthodontic lev- eling phase using a 0.14-nickel titanium wire of specified length and rubber ligatures. To ensure that the wire re- mains in situ, both ends were fixed (Fig. 1, activated). The other half of the brackets was also placed simulating an orthodontic leveling phase; however, no wire was ac- tivated (Fig. 1, non-activated). Individual positions of brackets, end positions A and B, as well as the middle position C were considered in the statistical evaluation. The application of the materials (indicated in Table 1) and the bonding procedure were performed following the manufacturers recommendations. Afterwards, all specimens were subjected to thermocycling (5° Celsius
The shearbondstrength also depends on the adhesive materials. Transbond XTTM is one of the most recommended products in current orthodontics. It has been a part of various comparative adhesion studies. In this study, all data were obtained with Transbond XTTM, which strongly associated with previous studies [26-29]. Reynolds and von Fraunhofer  stated that all the retentive designs used in the brackets tested should have an acceptable bond force levels (6– 8 MPa). However, there are various factors related to an oral environment or moisture contamination that may affect the shearbondstrength. The moisture contamination of bracket- bases with water, saliva and blood has been shown to adversely affect the shearbondstrength due to deposits of an organic adhesive layer immediately after exposure that is resistant to washing and subsequently it reduces the shearbondstrength of brackets [17,31-33]. Ahmad Sheibaninia et al.  evaluated the effect of an acidic food simulating environment on shearbondstrength of self-ligating brackets and stated that the margins of bracket-bases allows an acidic food to penetrate, which gradually decreases the shearbondstrength. So care should be taken in predicting the results to those conditions. Arunima Goswami  et al. stated that moisture contamination did not affect the shearbondstrength. It has been suggested that an adverse effect of moisture contamination on orthodontic bonding can be associated with water adsorption, which produce formaldehyde
The sample was randomly divided into three equal groups. Group A (black) was the control group, and the teeth were etched with 37% phosphoric acid gel (Dentsply, York, PA, USA) for 30 seconds. After that, the teeth were rinsed for 20 seconds and then dried with a stream of air for 20 seconds to appear opaque and frosty. For those teeth that did not show a white frosty appearance, the procedure was repeated. Then the pre- adjusted edgewise brackets (Gemini brackets, 3M Unitek, Monorovia, California, USA) of 3.3mm diameter and mesh base were bonded to the etched enamel using light cure composite resin (Megafill MH, Megadenta) as per the manufacturer's instructions. The brackets were seated and positioned firmly in the middle third of the buccal enamel surface.
Fixed appliance therapy in orthodontics involves bond- ing brackets to teeth for a period of approximately 2 years. The adhesive material used to bondbrackets to teeth should neither fail during the treatment period, resulting in treat- ment delays, untoward expenses or patient inconvenience nor should it damage the enamel on debonding at the end of the treatment. Although the effectiveness of a bonding system and any unfavorable effects on the enamel may be studied by conducting in-vivo studies, it is nearly impossible to independently analyze different variables that influence a specific bonding system in the oral environment (1). In-vitro studies, on the other hand, may utilize more standardized protocols for testing different bonding systems and ma- terials available. A systematic review and meta-analysis by Finnema et al. (2) had extensively reported the factors affecting in-vitro orthodontic bondstrength testing and con- cluded that the experimental conditions that considerably influence in-vitrobondstrength were storage of the bonded specimens in water, photopolymerization time and cross- head speed. Furthermore, the authors also reported that the test conditions were not reported properly in many stud- ies, which could have drastically influenced the outcomes. However, studies evaluating the effect of fluorosis of teeth on the shearbondstrength of orthodontic brackets were not
Perhydroxyl radicals are not only associated with high permeability and diffusibility but also split the long chained, dark colored macromolecules of pigments into smaller, less colored and more diffusible molecules which are removed from the structure producing the bleaching effect. Following bleaching, these free radicals react with the organic enamel and can result in morphological alterations, surface irregularities and reduced bondstrength of composite resin to enamel.
our study but we found air abrasion to be the most effective method and more efficient than Er:YAG laser. Sandblasting procedure was done differently in the two studies (2.5 bar, 15 seconds versus 20 Psi, 5 seconds); moreover, Ahrari et al.  used a luting cement containing 10-methacryloxydecyl dihydrogen phosphate (MDP), which is advocated to produce a chemical bond . This additional chemical bond had a positive effect on the bondstrength of all samples. Also, this study concluded that application of MDP containing primer can significantly increase the SBS compared to that of control and air abraded samples; we also found higher SBS in silane group samples compared to the control group but it was not significant. This is probably because our applied silane was not a MDP-based primer to form a chemical bond, and the small increase in SBS was probably due to the wetting effect of the primer. It should be noted that the samples in the study by Ahrari et al.  were not aged by the thermocycling process, which can cause great differences in the results. Kasraei et al,  also assessed the effect of CO 2 laser as a
regarding orthodontic bondstrength use ‘shear’ bondstrength rather than peel, tension, torsion, or cleavage because it is the most reproducible. It is important to note, however, that the shearbondstrength can be very significantly affected based on the location of the blade applying the force during debond. Ideally, the blade of the debonding instrument should be placed at the bracket base where it meets the tooth enamel. In this way, the entire layer of the bracket based is being evenly shifted laterally in relation to the enamel surface. As Klocke et al. demonstrated, a shift of the blade toward the ligature groove of the bracket wings can generate significant peel forces and decrease the bond failure point by over 50%. In the in-vitrostudy, when the blade of the universal testing machine was placed at the bracket base in contact with enamel, the peak debonding force 22.70±4.23 MPa, when the blade was tested again in the ligature groove the peak debonding force fell to 11.52±2.74 MPa and fell again to 9.44±2.96 MPa when the blade was moved out to the tie-wings. 61 Klocke also showed that the direction of force against the bracket can significantly affect the shearbond strengths measurements and called for
Shear stress is considered to be more representative of the clinical situation. Bondstrength is the force per unit area that is required to break a bonded assembly with failure occurring in or near the adhesive/ adherend interface. For Shearbondstrength evaluation, a total of 60 non carious premolars extracted due to orthodontic reasons were selected and stored in distilled water before and during the study period, since the distilled water does not affect the dentin permeability and bondstrength compared to saline. The buccal surfaces of all the 60 teeth were considered in this study, because this surface allowed the shearing force to be exactly perpendicular to the bonded specimen (Shinohara, 2004).The dentin surface was etched with 35% Phosphoric acid for 15 seconds. The single bond multi-purpose etchant is a versatile system which is recommended for bonding all classes of restorations. “Single Bond”, a fifth generation (type 2) adhesive, was used having both primer and adhesive in one bottle. The main reasons to use an adhesive were to facilitate the penetration of Composite into the etched dentin surface to provide a better bond to tooth structure. Single bond
Similar to Er:YAG laser, Er,Cr:YSGG laser works selectively and does not cause damage to the base unlike the sandblasting method . In the present study, the SBS of this group was 6.48 Mpa which can be due to some degrees of damage to the bracket base. Ahrari et al,  in 2013 concluded that this laser has the ability to recycle debonded ceramicbrackets with some degrees of damage to the bracket base with bondstrength comparable to that of new brackets. In the study conducted by Ahrari et al,  to investigate the efficacy of this laser for composite removal, similar to our study, no significant difference was found between the bondstrength of this laser compared to other groups. Although these lasers may be effective for composite removal, they are not used in common clinical work. They may even cause damage to the teeth. Previous authors showed the ability of this laser for removing restorative materials and roughening the surface of old composite restorations [34-36].
Treatment of bleached teeth is challenging for dentists because they cannot immediately perform a resin restoration on bleached teeth due to the presence of oxygen or peroxide residues on the surface, since they prevent complete polymerization of adhesive resin . However, by postponing the composite restoration for two weeks following bleaching, no reduction in bondstrength would occur [5,15,17]. But, sometimes it is not possible for the patient to wait that long. Therefore, use of antioxidants like ascorbic acid or sodium ascorbate is one method to immediately increase the bondstrength of composite to bleached enamel [8,10]. It has been proven that application of sodium ascorbate can result in accumulation of Streptococcus mutants on bleached surfaces . Also, pH of sodium ascorbate is 1.8, which has adverse effects on tooth structure in clinical application. Short shelf life of sodium ascorbate solution or gel is another disadvantage of using it [26,27]. It has been demonstrated that use of herbal antioxidants such as green tea and grape seed is an effective alternative strategy for this purpose.
Bishara S. Trulove T 13,14 (AJO 1990) mentioned that the ultrasonic debonding technique has been used to create a purchase point within the adhesive between the bracket base and the enamel surface. In this technique, the brackets are debonded with KJS ultrasonic tips and the Cavitron 2002 ultrasonic unit (Dentsply International). The advantages of the ultrasonic debonding approach include a decreased chance of enamel damage, a decreased likelihood of bracket failure and the ability for the removal of the residual adhesive with the same instrument after debracketing. Many authors found bond failures at the enamel-adhesive interface with this approach However, there are a number of disadvantages associated with the ultrasonic technique, including a significantly increased debonding time, excessive wear of the expensive ultrasonic tips, the need to apply moderate force levels, which could create some discomfort to sensitive teeth, the potential for soft tissue injury by a careless operator, and the need for a water spray to reduce the heat build-up and to minimize any possibility of pulpal damage. Since the ultrasonic method is effective but time consuming, its use might be indicated when a ceramic bracket fractures while the conventional method is being used and part of it remains attached to the tooth.