after introduction of the acid etch bonding method by buonocore in 1955 , direct bonding of orthodontic brackets was first performed by newman in 1965 [ 1 , 2 ] . this method expanded so rapidly that nowadays , utilization of the acid etch technique along with light cure composites is the most common bonding system in orthodontics . since re - bonding of brackets could be a time consuming and challenging process , achieving an appropriate bond strength is an important clinical objective . while various types of composites such as microfilled , microhybrid and flowable are available , the latest development in this field has been the introduction of nano - filled composites that are claimed to achieve higher wear resistance and appropriate mechanical properties . they also enhance the hybrid layer , increase marginal seal and reduce polymerization shrinkage due to their higher filler content . furthermore , nano - filler bondings have shown satisfactory bond strength to enamel and dentin , and can be utilized for direct and indirect restorations [ 6 , 7 , 8 , 9 ] . therefore , it is likely that nano - filled composites may replace other types of composites in the near future . despite the extensive applications of nano - composites in restorative dentistry , there is inadequate data regarding the possibility of using them for bonding orthodontic brackets . the aim of this study was to evaluate the shear bond strength and failure sites of two types of nano - composites in comparison to the conventional orthodontic composite adhesive system , and evaluate the possibility of their clinical application in bonding orthodontic brackets . sixty human first premolars were randomly divided into three groups . in order to prevent dehydration and inhibit bacterial growth , the teeth were preserved in 0.1% aqueous thymol solutionimmediately after extraction . only teeth with intact buccal surfaces were selected , and any teeth with evidence of cracks , caries , hypoplastic areas or other enamel abnormalities were excluded . after thorough rinsing with water , the buccal surfaces were etched via 35% phosphoric acid gel ( ultradent , usa ) for 30 seconds and then dried until a chalky appearance was visible . standard metal premolar brackets ( dentarum , inspringon , germany ) with surface area of 10.23 mm2 were bonded in this study . three groups of brackets were bonded using the following systems : the first group was bonded using a conventional orthodontic composite :  transbond xt ( 3 m unitek , usa)and transbond xt primer  . for the second group , a nano - filled bonding and composite ; adpertm single bond and filtektm supreme xt ( 3 m espe , usa ) was used . in the third group , bonding was performed via another nano - filled composite bonding system ; aelite aesthetic enamel ( bisco , usa)and one - step plus . the bonding process was initiated by placing a thin layer of bonding on the enamel surface and light curing for 10 seconds . after that , the main bulk of composite was placed onto the bracket bases and pressed with a force of 300 g using a force gauge . after that , the samples were light cured for 40 seconds ( 10 seconds on each side ) using a standard halogen light - curing unit ( optilux , 3 m unitek ) . the samples were then placed in 37c distilled water for 24 hours , followed by thermocycling for 2000 times at temperatures between 5c and 55c . a mounting jig was used to facilitate mounting each tooth onto an acrylic block . in order to prevent deformation of brackets during the debonding process , a stainless steel wire ( 0.0180.025 mm ) was inserted into the brackets ` slots during mounting . after completion of acrylic setting , the samples were placed in a universal testing machine ( zwick gmbh 8 co , germany ) in a manner that the labial surfaces of the teeth were parallel to the shearing force . a wire loop was connected to the machine and an occlusogingival force with a crosshead speed of 1mm / min was exerted onto each sample . the sbs value for each sample was determined by dividing the maximum force by the surface area of the brackets . in order to investigate the sites of bond failure , adhesive remnant index ( ari ) was determined for each sample by observation of the tooth surface under a stereomicroscope at 10 magnification and based on the following scale : 
 0 : no adhesive is remained on the tooth surface1 : less than 50% of the adhesive is remained on the tooth surface2 : more than 50% of the adhesive is remained on the tooth surface3 : 100% of adhesive is remained on the tooth surface 0 : no adhesive is remained on the tooth surface 1 : less than 50% of the adhesive is remained on the tooth surface 2 : more than 50% of the adhesive is remained on the tooth surface 3 : 100% of adhesive is remained on the tooth surface mean and standard deviation ( sd ) values for sbs in each group were determined . one way analysis of variance and tukey hsd multiple comparisons was used for statistical analysis of sbs . descriptive values of the shear bond strength for the three groups are shown in table 1 . the results of this study revealed that aelite aesthetic enamel had the highest sbs value ( 8.442.1 ) , whilst transbond xt ( 6.922.13 ) and filtek tm supreme xt ( 6.052.02 ) were in the next ranks . one - way anova test showed statistically significant differences between the shear bond strengths of the three groups(p= 0.019 ) . multiple comparisons revealed that the shear bond strength of aelite aesthetic enamel was significantly higher than the other two groups ( p<0.001 ) , whilst there was not a statistically significant difference between transbond xt and filtek tm supreme xt ( p=0.453 ) . there was not any statistically significant difference between ari indices of the three groups(p=0.142 ) . the results of the present study revealed that aelite aesthetic enamel nano - composite showed the highest sbs value , followed by transbond xt and filtek supreme xt respectively ; however , these differences were not statistically significant ( p>0.05 ) . therefore , it can be concluded that there is no significant difference in the shear bond strengths of transbond xt and the nano - composites . according to a study by reynolds et al . , an appropriate adhesive for orthodontic purposes should meet a sbs value of at least 5.97.8 mpa . therefore , in addition to transbond xt , both supreme xt and aelite aesthetic enamel are appropriate for orthodontic purposes and could be utilized for bonding , despite not displaying any additional advantages compared to transbond xt . since various studies regarding bond strength of composites have utilized adhesives with different size / concentration of filler , it is difficult to compare their results accurately . the results of this study are consistent with that of bishara et al . , whereby there was no significant difference between the sbs value of transbond xt and a restorative nano - composite , and both materials were considered applicable in orthodontics . on the other hand , , whereby comparison of sbs of a nano - composite ( filtek supreme plus universal ) and a nano - ionomer ( ketac  n100 light curing nano - ionomer ) with transbond xt revealed a significantly higher sbs value of transbond xt . however , such different findings may be due to a number of factors ; in the study by uysal et al . , the teeth were polished with nano - fluoridated pumice , which may have interfered with the entrance of nano - fillers into the etched enamel surface , and led to decreased bond strength . a further significant difference in addition to the use of a qth light source was their use of porcelain based brackets compared to the use of metal brackets in our study . the penetrance of the bonding agent in all of the groups was not matched , as transbond xt paste was applied onto uncured bonding while in the case of the nano - composites ; the bonding was cured prior to application of the paste . for purpose of testing orthodontic shear bond strength , a previous study has shown that the wire loop method has been found to be superior to the shear blade method that was used in the study by uysal et al . . the results of the present study revealed that in the majority of samples of transbond xt and supreme xt , bond failure occurred in a manner that less than 50% of the original composite had remained on the enamel surfaces ( grade 1 ) . meanwhile , in aelite aesthetic enamel group ( displaying highest sbs value ) , more than 50% of the original composite had remained on the enamel surfaces ( grade 2 ) . . however , uysal et al . revealed that the ari was grade 1 in all of the groups , which is consistent with our results for transbond xtand supreme xt . on the other hand , in our study , the majority of the samples in aelite aesthetic group were in grade 2 , which was the second rank amongst the composites in our study . overall , it can be concluded that the failure sites of transbond xt and nano - composites is mainly at the tooth - adhesive interface . considering the results of this study and bishara`s study , successful application of nano - composites
objectives : the purpose of this study was to evaluate the shear bond strength ( sbs ) of orthodontic brackets bonded with two types of nano - composites in comparison to a conventional orthodontic composite.materials and methods : sixty extracted human first premolars were randomly divided into 3 groups each containing 20 teeth . in group 
 i , a conventional orthodontic composite ( transbond xt ) was used to bond the brackets , while two nano - composites ( filtek tm supreme xt and aelite aesthetic enamel ) were used in groups ii and iii respectively . 
 the teeth were stored in distilled water at 37c for 24 hours , thermocycled in distilled water and debonded with a universal testing machine at a crosshead speed of 1 mm / min . the adhesive remnant index ( ari ) 
 was also evaluated using a stereomicroscope.results:aelite aesthetic enamel nano - composite revealed a sbs value of 8.442.09 mpa , which was higher than transbond xt ( 6.912.13 ) and filtek tm supreme xt ( 6.042.01 ) . 
 statistical analysis revealed a significant difference between groups ii and iii ( p < 0.05 ) . 
 no significant difference was found between groups i and iii , and between groups i and ii ( p > 0.05 ) . 
 evaluation of ari showed that transbond xt left fewer adhesive remains on teeth after debonding.conclusion:results of this study indicate that the aforementioned nano - composites can be successfully used for bonding orthodontic brackets .