intraradicular posts are commonly used to restore endodontically treated teeth when their remaining coronal tissue can no longer provide adequate support and retention for the restoration11,16 . for decades , endodontically treated teeth have been restored using cast metal posts . despite the high retention and thin cement layer , these conventional posts present high elastic modulus and can lead to root fracture , according to several in vitro studies8,20 , though clinical evaluations have presented high success rates for cast metal posts1,8,17 . the fact that the fiber post , resin cement and dentin have similar elastic modulus has been considered as advantageous for improving the performance of restorations25 . however , the mismatch between fiber post and post space diameters post remains a clinical challenge6 . although the use of size - matched drills supplied by post manufacturers permits good fitting of posts to the canal walls , some canals have an elliptical shape in cross - section while posts have circular shape7 . in addition , flared canals from carious extension , trauma , pulpal pathosis and iatrogenic misadventure also compromise the adaptation of fiber post to canal walls6 . in these two cases , if the post does not fit well , especially at the coronal level , the resin cement layer would be excessively thick , and bubbles are likely to form in it , thus predisposing to de - bonding14 . one solution to overcome this problem is the use of dentin - bonded composite resin18 . this technique increases the internal thickness of dentin of root wall and reduces the resin cement thickness . however , the light - activation of composite resin in more apical regions of the root canal is difficult and this may compromise this technique9,19,23 . this individual anatomic post increases the adaptation of the post to root walls and reduces the resin cement thickness14 . therefore , the aim of this study was to evaluate the effect of post relining with composite resin on the push - out bond strength of fiber posts luted to flared root canals . the null hypothesis tested was that the individual anatomic post has no influence on the push - out bond strength . twenty recently extracted bovine incisors with similar root sizes and lengths were selected for this study . the crowns were removed above the cementoenamel junction with a low - speed diamond saw under water cooling in order to obtain a remaining root length of 14 mm . for the endodontic treatment , after determination of the working length using a size 15 k - file ( dentsply / maillefer , ballaigues , switzerland ) , the canals were enlarged to a size 35 k - file ( dentsply / maillefer ) using a filling action prior to using gates - glidden drills ( dentsply / maillefer ) . in order to flare the coronal and middle third of the canal a # 2 gates glidden drill was inserted several millimeters into the canal , and then followed by sizes 3 and 4 . apical preparation was done with a size 40 k - file ( dentsply / maillefer ) as the final master apical file . the remaining of the canal was instrumented up to a size 70 k - file ( dentsply / maillefer ) . the prepared root canals were obturated with gutta - percha cones using the lateral condensation technique and sealer-26 resin sealer ( dentsply indstria e comrcio ltda . , the filled roots were stored in relative humidity for at least 72 h to allow the resin sealer to set . after the storage period , the coronal gutta - percha was removed with a heated rhein instrument and 5-mm thick layer of gutta - percha was left for apical seal . in order to obtain a standardized flared canal , the root canals were enlarged with # 4138 and # 4137 high - speed diamond burs ( kg sorensen indstria e comrcio ltda . the prepared root canals were allocated to one of the two experimental groups ( n=10 ) : 
 non - relined fiber posts : the fiber post fiberkor # 2 ( penetron corp . , wallingford , ct , usa ) was treated with 37% phosphoric acid ( 3 m scotchbond etchant ; 3 m espe , st paul , mn , usa ) for 1 min , coated with a layer of the silane coupling agent ( 3 m espe ) and then adper single bond 2 adhesive system was applied and light - cured for 20 s. 
 relined fiber posts : the fiber post fiberkor # 2 was treated in the same manner as described for the other group . after lubricating the canal walls with glycerin gel , the fiber post was covered with composite resin ( filtek z-250 ; 3 m espe ) and inserted into the canal . the composite resin was light - cured for 20 s. the relined fiber post was removed and the composite resin was light - cured for additional 20 s. copious rinsing was done to remove lubricant gel from the root canal . in both groups , the canal walls were etched with 35% phosphoric acid for 15 s , water - rinsed for 15 s and gently air dried . two coats of adper single bond 2 adhesive system were consecutively applied to the root canal with microbrush tips ( microbrush corp . , adhesive excess was removed with an absorbent paper point , and the remaining material was gently air dried and light - cured for 40 s. next , the dual - cured resin cement relyx arc ( 3 m espe ) was mixed and inserted into the root canal with # 40 lentulo spiral ( dentsply / maillefer ) . the posts were cemented into the root canal with light pressure , and excess luting material was removed . light activation was performed through the cervical portion of the root for 40 s on the buccal and lingual surfaces , totalizing 80 s of light exposure . the resin cement and adhesive resin were light activated with a halogen light source ( optilux 501 ; demetron kerr , orange , ca , usa ) with output intensity of approximately 650 mw / cm constantly measured during the experiment . the root / cemented post sets were stored in distilled water for 1 week at 37c . three 1.5-mm thick slabs were obtained per root and identified as cervical , medium and apical specimens . each slab was positioned on the push - out jig , and load was applied at a crosshead speed of 0.5 mm / min until the post was dislodged . data were analyzed statistically by split - plot anova and post - hoc tukey 's test at a pre - set alpha of 0.05 . after debonding , the tested specimens were mounted on aluminum stubs , gold sputter - coated and observed by scanning electron microscopy ( sem ) for failure mode determination . the failure mode was classified into four categories : adhesive between resin cement and fiber post ; between the composite resin and resin cement ; between resin cement and root dentin ; or mixed . however , the factor depth ( p=0.169 ) and the interaction between factors ( p=0.475 ) were not significant . the relined fiber posts presented higher retention than non - relined fiber posts in all root canal thirds . there was no difference among thirds ( p>0.05 ) , irrespective of the post type . the sem images showed that all failures occurred at the interface between the resin cement and root dentin . figures 1 and 2 illustrate the failures for non - relined and relined fiber posts , respectively . in the present study , fiber post relining improved fiber post retention at all evaluated root canal thirds . the main goal of fiber post relining is to reduce the thickness of the resin cement layer . thin layers of cement present fewer bubbles and other defects than thick ones , and voids within the material may act as crack raisers and decrease post retention6,14 . relining may reduce the possibility of cohesive failures . all failures in the present study occurred at the interface between the resin cement and root dentin , eliminating the effect of reducing defects and increasing the cement layer strength . thus , the possible explanation for the improvement in retention with fiber post relining may be associated with the interaction between the adhesive and cement . the adhesive system used in this study was the adper single bond 2 , which is material indicated by the manufacturer of relyx arc luting system . this is a two - step etch - and - rinse adhesive that presents hydrophilic and hydrophobic monomers in the same bottle and is incompatible with dual - cured resin cement used in the absence of light - activation21,28 . this incompatibility may occur because of the adverse chemical interaction between the unpolymerized acidic monomers of the adhesive and the basic tertiary amine in the cement24 . this interaction prevents the adequate polymerization of the resin cement and reduces the bond strength to the adhesive . however , the main factor contributing to the resistance to dislocation of the bonded fiber posts seems to be sliding friction12 . this occurs due to the complexity of adhesive procedure in the root canal because of both the difficulty of humidity control and adhesive light - activation22 . in addition to the high c - factor ( ratio between bonded and non - bonded surfaces ) of the post hole , the bond strength of the adhesive system to root canal dentin is low27 . since the friction occurs by contact , it is reasonable to assume that closer contact between resin cement and root dentin improves fiber post retention5 . the simplified adhesives , such as adper single bond 2 , behave as permeable membranes that allow the fluids to cross the adhesive layer after polymerization29 . the water may migrate to the composite - adhesive interface where it is trapped as water blisters , which might result in debonding at the resin - dentin interface . despite the use of extracted and endodontically treated teeth , the intrinsic water that contributes to the stability of the collagen matrix in mineralized tissue can not be completely removed , and water can remain mainly inside the dentin tubules3 . acid etching of the dentin surface with phosphoric acid removes the smear layer and the smear plugs , increasing tubule diameter and dentin permeability . rinsing with water probably results in the retention of a substantial volume of water within the widened tubule entrances . as such water may not be completely removed by absorbent paper points ; it may contribute to blister growth at the adhesive / resin cement interface . considering the evidence that sliding friction is the main factor responsible for fiber post retention , these blisters may have reduced the contact between the resin cements and root canal walls , resulting in low push - out bond strength10 . blister growth is related to the availability of water and the polymerization rate of the resin cement , which in a slow polymerization reaction results in more water blisters29 . thus , the effect of the adhesive permeability on the post retention is expected to be more accentuated in the more apical areas of the root canal where the light - activation of the resin cement is compromised . however , blister formation might also compromise the fiber post retention in the cervical third , where light - activation of the resin cement is effective . the adhesive resin flows in the apical direction and the solvent evaporation reduces the thickness of the adhesive layer . as the adhesive layer is thin , blisters can be formed due to rapid water movement across the adhesive , even when the resin cement is light activated29 . in addition , thin adhesive layers are not adequately polymerized due to the inhibition caused by oxygen30 . relined fiber posts present more intimate contact with the root canal walls than non - relined posts . good adaptation of the post increases the pressure on the resin cement , which is transmitted to the cement - adhesive interface . pressure application suppresses water sorption and blister formation , resulting in a better contact between the cement / post set and dentin4 . this results in higher sliding frictional retention compared to when the fiber post is not relined , and consequently in higher push - out bond strength . based on these results , it seems that relining increases fiber post retention by improving the contact between the cement and the adhesive , rather than by reducing the defects observed in the thin resin cement layers . several different mechanical testing methods have been used to measure the bond strength of fiber post to intraradicular dentin , to include : microtensile , pull - out and push - out tests . the push - out test provides a better estimation of the bond strength than the other tests because the fracture occurs parallel to the dentin - adhesive interface26 . in addition , the push - out test , such as pull - out , allows for evaluating the contribution of frictional resistance that is relevant to post retention to root canal13 . the main drawback of the pull - out test is that it does not permit the comparison between the bonding to different depths of the root canal . in relation to microtensile test , the specimen preparation generates stress on the post - root interface , resulting in premature fails and compromising the results13,26 . it is important to emphasize that only the retention of the posts was evaluated in this study . thus , further investigations , mainly clinical trials , are necessary to support the indication of fiber post relining procedure in the clinical practice . although the weakest link in post cementation is the interface between resin cement and root dentin , relining the fiber post with composite resin improved its retention at all thirds of the root canal .
one of the clinically relevant problems dentists face when restoring endodontically treated teeth is the mismatch between fiber post and post space diameters , which results in an excessively thick resin cement layer . 
 fiber post relining appears as a solution for this problem.objectives:the aim of this study was to evaluate the effect of fiber post relining with composite resin on push - out bond strength.material and methods : twenty bovine incisors were selected to assess post retention . 
 the crowns were removed below the cementoenamel junction and the root canals were treated endodontically and flared with diamond burs . 
 the roots were allocated into two groups ( n=10 ) : g1 : fiber posts without relining and g2 : fiber posts relined with composite resin . 
 the posts were cemented with a dual - cured resin cement and the specimens were sectioned transversally . 
 three 1.5-mm thick slabs were obtained per root and identified as cervical , medium and apical thirds . 
 the push - out test was performed at a crosshead speed of 0.5 mm / min until post dislodgement occurred . 
 the failure mode of fractured specimens was analyzed under scanning electron microscopy . 
 data were analyzed by split - plot anova and post - hoc tukey 's test at a pre - set alpha of 0.05.results:relined fiber posts presented higher retention values than non - relined post in all thirds . 
 no statistically significant differences ( p>0.05 ) were found among thirds for relined posts . 
 all failures occurred at the interface between resin cement and root dentin.conclusions:relining with composite resin seems to be an effective method to improve the retention of fiber posts to flared root canals .