these materials provide good esthetic results , better stress distribution and adhesion to tooth structure . an important problem in effective bonding of post surface to composite resin core and luting cement is insufficient functional groups of surface structure . , application of hydrogen peroxide in different concentrations improves adhesion between fiber post and composite core . elseka , yenisey , and vano reported that chemical surface treatment of post surface with hydrogen peroxide increases bond strength of quartz fiber post to composite resin core . however , studies have shown that fiber post - composite resin core interface is yet the weakest area . it appears that release of oxygen - free radicals from the entrapped hydrogen peroxide in microporosities of the post - surface is responsible for this problem . application of antioxidants might inhibit the process of trapping residual oxygen - free radicals in the rough post surface . ascorbic acid ( vitamin c ) is a water - soluble antioxidant present in citrus fruits , potatoes , tomatoes and green leafy vegetables . one important property is its ability to act as a reducing agent ( electron donor ) . ascorbic acid is a reducing agent with a hydrogen potential making it capable of reducing some compounds into molecular oxygen . this antioxidant is routinely applied in operative dentistry , especially for increasing bond strength after tooth bleaching . many studies have indicated that application of ascorbic acid and related salts can reverse the compromised bond strength between composite resin and tooth structures after bleaching . therefore , it is possible that free radicals on the post - surface are removed by ascorbic acid . due to the lack of sufficient data concerning the role of antioxidants in adhesion of fiber post to composite core , this study was carried out to evaluate the effect of ascorbic acid on bond strength between the hydrogen peroxide - treated fiber post and composite resin core . the null hypothesis was that ascorbic acid has no effect on bond strength of hydrogen peroxide - treated fiber posts to composite resin core . in order to prepare 10% w / v ascorbic acid solution , 10 g l - ascorbic acid powder with purity of 99.0% , ( product number a4403 , sigma  aldrich , st . louis , mo , usa ) were mixed and diluted with 100 ml distilled water in a standard flask . all the posts were immersed in 24% hydrogen peroxide solution for 10 minutes at room temperature , rinsed with distilled water for two minutes and gently air - dried . then , the treated posts were divided into 4 groups ( n = 6 ) based upon the duration of ascorbic acid used as follows : group 1 : control group ( no application of ascorbic acid ) ; groups 2 to 4 : immersion in 10% ascorbic acid solution for 10 , 30 and 60 minutes , respectively . post surfaces were rinsed under running water for 1 minute and then gently air - dried . each post was positioned upright on a glass slide in the center of a cylindrical polyglass matrix with a diameter of 10 mm and a height of 6 mm . it was fixed with sticky wax . a light - cured flowable composite resin ( lite flow , bisco inc . , shaumburg , il , usa ) was applied around the post in 1-mm - thick increments and each layer was separately cured using a halogen light - curing unit ( degulux , degussa dental , hanau , germany ) for 40 seconds with an intensity of 450 mw / cm . the mold was separated from the glass slide and an additional 40-second curing was performed from the bottom of the cylinder to ensure optimal polymerization of the core material . then , all the prepared samples were stored in deionized water at 37c for 24 hours . each sample was sectioned by a water - cooled blade of a sectioning machine ( isomet , buehler ltd . , first , two sections were made parallel to the long axis of the post , followed by sections perpendicular to the long axis of the post . the final sections contained the posts in the center and composite resin cores on both ends and had a diameter of 1  0.1 mm , measured by a digital caliper ( mitutoyo cd15 , mitutoyo co. , kawasaki , japan ) . each stick was glued to the two free - sliding components of a jig mounted on the microtensile tester ( ez test , shimadzu co. , kyoto , japan ) . the sticks were loaded at a crosshead speed of 0.5 mm / min until failure . bond strength was expressed in mpa , by dividing the load at failure point ( n ) by the bonding surface area ( mm ) . please consult a statistician using spss 16.0 version ( statistical package , spss inc . , failure modes of the samples were observed under a stereomicroscope ( nikon eclipse e600 , tokyo , japan ) at 20 magnification and scored as cohesive failure ( failure in the post or the core material ) , adhesive failure ( failure at the interface of post and core material ) , or mixed failure ( adhesive - cohesive failure ) . two fractured sticks of each group were randomly selected for evaluation of the morphology of fractured surfaces under a scanning electron microscope ( sem ) . each specimen was gold sputter - coated ( polaron range sc7620 , quorum technology , newhaven , uk ) and placed in a scanning electron microscope unit ( jsm-5310 , jeol , tokyo , japan ) and assessed under 300 , 500 and 2000 magnifications . means and standard deviations of microtensile bond strengths of the tested groups are summarized in table 1 . one - way anova demonstrated significant differences between the study groups ( p < 0.05 ) [ table 1 ] . paired comparisons of groups showed significant differences between groups 1 and 2 , 2 and 3 , 3 and 4 ( p < 0.05 ) . stereomicroscopic evaluation revealed that all the failures in groups 1 , 2 and 3 were adhesive failures , but 3 samples in group 4 showed mixed failures [ figure 1 : c1 ] . means and standard deviation of microtensile bond strength and the results of one - way anova microscopic views of the fractured surfaces showing adhesive failure on post surface in g1 [ a1 : ( 500 ) , a2 : ( 2000 ) ] and g4 [ b1 : ( 80 ) , b2 : ( 300)]and mixed failure in g4 [ c1 : ( 20 ) , c2 : ( 2000 ) ] . exposure of quartz fibers ( 1 ) and presence of epoxy resins ( 2 ) between fibers are observed the sem evaluation demonstrated the exposed fibers in adhesively failed samples [ figure 1 : a1 , 2 , b1 , 2 ] and different values of epoxy resin on fiber posts in mixed failures [ figure 1 : c2 ] . adhesion of post to root canal dentin and also composite resin core play an important role in the longevity of restoration . many studies have evaluated the effects of surface treatments on adhesion of fiber - reinforced posts to cores . applying hydrogen peroxide on quartz fiber post previous studies have shown several advantages of this procedure for treatment of quartz fiber - post surfaces . the presence of oxygen - free radicals could compromise bond of post  core interface . thus , the present study was carried out to evaluate the effect of ascorbic acid as an antioxidant agent for improving the bond strength between the hydrogen peroxide- treated fiber post and composite resin core . the findings of this study indicated that use of ascorbic acid enhanced microtensile bond strength of quartz fiber post to composite resin core compared with control group , when the post surface was treated with hydrogen peroxide . ascorbic acid is an accessible non - enzymatic antioxidant agent , which has exogenous source . as mentioned in the introduction , the main property of ascorbic acid is its ability for transmission of electron to oxygen - free radicals and providing a stable form of the material . therefore , it acts as a reducing agent and has an inhibitory effect on hydrogen peroxide - derived oxygen - free radicals . the results of the present study are consistent with those of a study conducted by khamverdi et al . , which showed that antioxidant agents increase adhesion of post and core . in addition , weston et al . , showed that the considerable reduction in resin - dentin bond strength , which resulted from the use of 5.25% naocl irrigations , can be reversed by 10% sodium ascorbate treatment . another finding of this study showed significant differences between bond strength values of the ascorbic acid - treated groups and the increase was directly proportional to the duration of ascorbic acid application , which might be due to greater effects of ascorbic acid regarding its antioxidant properties , resulting from an increase in fiber post surface exposure time to this agent . since , there were no studies to determine the minimum application time of ascorbic acid on fiber post surface ; duration of 10 minutes was selected based on study results of trkn et al . , and kaya et al . kaya et al . , have used longer times of 60 , 120 , 240 and 480 minutes . selection of a 60-minute duration in the present study was based on the study done by kaya . however , a 10-minute application of the antioxidant in this study resulted in a significant increase in bond strength ; this duration is applicable in clinical situations . in the present study , evaluation of sample fracture modes in groups revealed that the majority of fractures were adhesive . observation of 3 mixed fracture modes within group 4 might be an indication of bond strength reinforcement between the post and core . previous studied reported a 10% ascorbic acid salt that can compensate the reduced bond strength of dentin to composite resin after bleaching . ten percent solution was applied in the present study , as it is suggested to be the minimum effective concentration . the microtensile bond strength test was performed according to previous studies , which have proved as a standard test for bond strength testing in small sections . flowable composite resins have a good penetration property into surface porosities after application of hydrogen peroxide . the solution form accelerates onset of action and is recommended for shorter clinical time of application . it is recommended that other forms of this material , for example , hydrogel form , which are clinically more acceptable , be prepared and compared with the solution form . in the present study , thermocycling was not carried out and this was another limitation of the study . for more similarities to the oral conditions , future studies with thermocycling are recommended . in addition , one antioxidant was used . application and comparison of other new antioxidants in future investigations appears to be useful . within the limits of this study , it can be concluded that : 
 application of ascorbic acid can increase microtensile bond strength between the hydrogen peroxide - treated fiber posts and composite resin cores.the increase is dependent on the duration of exposure to the antioxidant . application of ascorbic acid can increase microtensile bond strength between the hydrogen peroxide - treated fiber posts and composite resin cores .
aim : this study evaluated the effect of 10% ascorbic acid on the bond strength between fiber post and composite resin core after applying 24% hydrogen peroxide.materials and methods : twenty - four hydrogen peroxide - treated fiber posts were divided into 4 groups ( n = 6 ) . 
 group 1 was the control group with no treatment . in groups 2 - 4 , 
 post surfaces were treated with 10% v ascorbic acid solution for 10 , 30 and 60 minutes , respectively . 
 cores were built up using flowable composite resin . 
 two sticks were prepared from each specimen . 
 microtensile bond strength test was performed for each stick . 
 failure modes of sticks were evaluated under a stereomicroscope ( 20 ) . 
 surface morphologies of two fractured sticks from each group were assessed by sem.statistical analysis : data were analyzed using one - way anova and tukey hsd tests (  = 0.05).results : the highest microtensile bond strength was observed in group 4 ( 20.55  2.09 ) and the lowest in group 1 ( 10.10  0.55 ) . 
 there were significant differences in microtensile bond strength between all the groups ( p < 0.05).conclusion : it is concluded that ascorbic acid application increased the microtensile bond strength between the hydrogen peroxide treated fiber post and composite resin core . 
 the increase is dependent on the duration of exposure to the antioxidant .