as the demand for adult orthodontic treatment increases and the popularity of esthetic dentistry expands , orthodontists are more likely faced with the problem of placing orthodontic appliances on teeth restored with resin and porcelain fixed prostheses or veneer laminates . when esthetics is a concern , orthodontists will have to depend on the direct bonding technique . numerous methods and materials have been suggested for bonding to ceramic restorations [ 17 ] . the conventional method for optimal bonding of brackets and retainer wires to ceramic surfaces is application of hydrofluoric ( hf ) acid for etching the ceramic surface , because phosphoric acid etching , which is used for the enamel , is ineffective in the preparation of ceramic surfaces for mechanical retention of orthodontic attachments [ 810 ] . however , the in vivo use of hf acid is too hazardous . mucosal or skin contact with hf acid can cause erythema and burning associated with loss of tissue along with intense pain for several days . the purpose of this study was to evaluate tensile bond strength between metal brackets and non - glazed ceramic surfaces with three different surface treatments to evaluate application of phosphoric acid instead of hf acid . we evaluated the non - glazed ceramic surfaces because clinicians sometimes remove the glaze layer of porcelain restorations by a low speed bur in the oral cavity and then apply phosphoric acid instead of hf . forty - two non - glazed , feldspathic porcelain disks , 2.0 mm thick and 8.0 mm in diameter , were used . the disks were fabricated from super porcelain ex-3 body ( noritake , japan ) using a handmade mold . the specimens were washed with water , dried with air , and then divided into three equal groups randomly . in groups i , and ii ; 9.5% hf acid gel ( ultradent , usa ) was applied for 2 minutes , then rinsed with water and dried with oil - free air . subsequently , in group i , a layer of silane ( scotchbond ceramic primer , 3 m unitek , usa ) was applied and dried with light oil - free air spray . then a layer of scotchbond multi purpose plus adhesive ( 3 m unitek , monrovia , calif . , usa ) was applied , thinned with air spray and cured with light ( 500 ) for 10 seconds . in group ii , after etching with 9.5% hf acid , only a layer of bonding agent ( unfilled resin , 3 m unitek , usa ) was used . in group iii , ceramic surfaces were treated with 35% phosphoric acid ( 3 m unitek , usa ) and then silane and adhesive were applied similar to group i. metal brackets for maxillary central incisors ( dyna lock , std . edgewise 018 , 3m - unitek , usa ) were bonded to each conditioned ceramic surface with light - curing composite resin ( transbond xt , 3m - unitek , usa ) . excess composite was removed and then light cured for 40 seconds ( coltolux 75 , swiss ) with an intensity of 500 . the surface areas for the bases of brackets was 16.52 mm which were obtained bfrom the manufacturer . after bonding , all specimens were stored in a water bath in room temperature for 24 hours , then thermocycled 500 times between 5c and 55c with a dwelling time of 60 seconds . all specimens were partially embedded in acrylic resin using a hand - made jig , especially designed for this purpose ( fig 1 and 2 ) . zwick universal testing machine ( z/100 , germany ) was used to measure the tensile bond strength . the machine had an upper jaw that was mounted to a movable crosshead and a lower jaw mounted on the base . the tensile force at a crosshead speed of 0.5 mm / minute was transmitted to the bracket by a steel wire that was placed under the bracket wings . the force required for debonding was recorded and the tensile bond strength in megapascal ( mpa ) was determined from the load at failure and the area of bracket base . after debonding , the specimen surface was examined under magnification 2 to determine how much residual adhesive remained on the ceramic according to the following scale : 1=all the composite remained on the ceramic , 2=more than 90% of the composite remained on the ceramic , 3=more than 10% but less than 90% remained on the ceramic , 4=less than 10% remained on the ceramic , and 5=no composite remained on the ceramic . descriptive statistics including the mean , standard deviation , and minimum and maximum values were calculated for the three groups . then tensile bond strength data were subjected to one - way anova and tukey s hsd post hoc tests . the nonparametric kruskal - wallis test was used to compare the adhesive remnant index ( ari ) score for the three groups . four specimens in group i , three specimens in group ii , and two specimens in group iii did not have a measurement of tensile bond strength because of the operator s mistake . the difference of tensile bond strength between groups i , and iii was not significant ( p=0.999 ) . however , the tensile bond strength of group ii was significantly lower than groups i , and iii ( p<0.001 ) . the ari scores show that in group i specimens , all the composite remained on the ceramic . in group ii , 63.7% of the specimens had an ari higher than 4 , meaning that less than 10% of the composite remained on the ceramic . in group iii , more than half of the specimens had an ari lower than 2 ( i.e. more than 90% of the composite remained on the ceramic ) . the direct bonding of orthodontic attachments has revolutionized and improved the clinical practice of orthodontics . more recently bonding materials have been introduced for bonding to ceramic . however , before clinical experiments , these materials should be evaluated in vitro to find which products and materials seem most valuable to include in supplementary clinical evaluation . although both shear and tensile loading modes are valid tests for studying bond strengths of orthodontic materials , choosing the tensile bond strength test as the more frequently used shear bond strength test needs some explanation . in order to allow the calculation of the true interfacial failure stress , the experiment should be designed so that a uniform stress distribution is created across the interface . production of complex stress distribution in the shear bond strength test may start fracture at sites with high concentration of local stress ; therefore , the adhesive characteristics of the bonded interface are not expressed . the tensile bond strength test used in this study provides a specimen design with a stress distribution across the interface as near to uniform as possible . we used long thin wires under the bracket wings , as suggested by katone and chan . the relatively slow crosshead speed used for bond strength in this study moreover , viscoelastic behavior that is absent in vivo may occur at low strain rates for the adhesive which may be an important area for future investigation . newman   stated that 14 kg / cm2 ( 1.5 mpa ) was the maximum force that could be applied to a tooth by an orthodontic appliance . the tensile bond strength in our study exceeded this value greatly , but were still less than reynold s value   ( 50 kg / cm5 mpa ) recommended for in vivo success . group i and iii had the closest results to this value with a mean of 3.91 mpa and 3.90 mpa , respectively . olsen et al   evaluated shear bond strength between metal brackets and enamel using scotch bond multipurpose adhesive , 37% phosphoric acid and 10% maleic acid . their results were 13.1 mpa ( sd=4.8 ) for phosphoric acid and 10.3 mpa ( sd=3.1 ) for maleic acid . moreover , it is proved in orthodontics that shear bond strength is higher than tensile bond strength [ 2022 ] . thurmond et al   found significantly lower shear bond strength with application of phosphoric acid plus silane on ceramic than with hf acid plus silane ( p<0.001 ) , but saygili and sahmali   reported that acid etching of the surfaces with hf acid had a weak tendency to improve bond strength . aida et al   showed that acid etching of porcelain with hf acid could be eliminated . our finding showed no statistical difference between application of hf acid with scotch bond multipurpose plus adhesive and phosphoric acid with scotch bond multipurpose plus adhesive on non - glazed ceramic ( p=0.999 ) . major et al   showed that the site of failure at low bond strength tended to be at the porcelain - adhesive interface . when the bond strengths became greater , the chemical retention was equal or exceeded the mechanical retention provided by the bracket base . therefore , increased bond strengths resulted in failure at the bracket - adhesive interface or a cohesive failure within the composite resin in a way that some composite was left on both the bracket and the porcelain surfaces . in our study , in group i , 100% of the specimens and in group iii , 60% of the specimens had more than 90% composite remaining on the ceramic . however , in group ii , which had a lower bond strength ( 2.70 mpa ) , about 60% of the specimens had less than 10% composite remaining on the ceramic . the tensile bond strength to ceramic surface reported by cochran et al , kocadereli et al   and harari et al   using the same method were higher than our results . but it should be noted that they did not thermocycle their specimens and they also used different material . eustaquio et al   reported a tensile bond strength of bonding brackets to deglazed ceramic using conocise / scotch prime ( 3 m ) comparable with groups i and iii of our study . overall , these differences may be due to variables such as the type of bond strength test , ceramic , adhesive , brackets , storage environment and presence or absence of thermocycling . in some researches , the specimens were stored in 37 degrees and in others ; they were kept in room temperature . we used the second method . regarding the effect of phosphoric acid on porcelain , the results of studies are mixed and confusing . while some researches show that this acid does not have a considerable effect on the porcelain surface [ 810 ] , others show that this effect is clinically acceptable [ 3335 ] . our results showed that by using scotch bond multipurpose plus adhesive on non - glazed ceramic we can use phosphoric acid for treating the ceramic surface instead of hf acid gaining a statistically not different tensile bond strength and also preventing hazard for the oral tissue , skin and the respiratory system .