the desire for whiter teeth has motivated an increasing number of patients to seek 
 dental offices for tooth - bleaching treatments . despite the esthetic benefits brought to patients
, it has been 
 demonstrated that bleaching agents might have a negative influence on the integrity of 
 organic enamel structures .
mineral loss , increased surface 
 roughness , decreased microhardness , decreased fracture toughness , and reduced enamel 
 microtensile strength , which can result in decreased bond strength to enamel by up to 
 75% immediately after bleaching , have already been reported .
changes in the amount , length , and 
 morphology of resin tags observed in studies using scanning electron microscopy ( sem ) 
 suggest decreased penetration of resin materials into bleached enamel .
clinically , this decrease is important 
 as bleaching is often performed as a preliminary esthetic treatment on teeth to be 
 reconstructed with adhesive techniques .
enamel surface alterations after tooth bleaching , such as those mentioned above , have 
 been found to be more pronounced under in vitro than in in 
 situ settings .
this finding is attributed to the 
 ability of saliva to prevent such alterations in the dental structure , and suggests the reversibility of 
 bleaching - related changes . by placing bleached teeth under the direct influence of human 
 saliva
hence , in situ 
 methodologies have been used to estimate the time needed to reverse bleaching - related 
 effects on enamel bonding , and to determine how soon after bleaching teeth can be 
 restored .
some authors have 
 postulated a waiting period ranging from 24 hours to 3 weeks before performing bonding 
 procedures after bleaching , while others have suggested a waiting period of 7 days for 
 enamel and 14 days for 
 dentin .
nevertheless , in some 
 clinical situations , patients may not be able to wait to have their teeth rehabilitated . 
 in order to permit immediate restorations after bleaching ,
several researchers have 
 attempted the use of different antioxidant agents prior to performing restorative 
 procedures , among which sodium ascorbate has shown promising results . therefore , the debate on how to more adequately deal with the different clinical demands 
 of patients who undergo teeth - whitening and their need for rehabilitation afterwards is 
 still open . in order to shed some more light on the issue , the objective of the present 
 comparative study was to evaluate the influence of different exposure times to saliva 
 in situ and a treatment with 10% sodium ascorbate gel on composite 
 bond strength to human enamel after bleaching .
this study was approved by the state university of maring research ethics committee 
 ( protocol 609/2009 ) .
fifty sound extracted human teeth ( premolars and molars ) were cleaned with 
 periodontal curettes and ultrasound , submitted to prophylaxis with pumice and 
 water , and stored in 2% 
 formaldehyde solution ( ph 7.0 ) between 30 to 50 days .
an enamel block specimen 
 measuring 5x5 mm was obtained from the flattest portion of each tooth ( proximal face 
 for premolars and vestibular or proximal face for molars ) with a diamond disc 
 ( diamond wheel 012 " x fine , south bay technology inc , ca , usa ) adapted to a 
 sectioning machine ( isomet 1000 , buehler , lake bluff , il , usa ) at low speed , under 
 water cooling . to facilitate their handling during subsequent stages
, specimens were 
 kept with a minimum height of 3.1 mm . after inspection under optical microscopy 20x 
 ( bel microimage analyzer , bel photonics , monza , milano , italy ) ,
10 specimens 
 presenting cracks , fractures or stains were excluded from the study . before their use in the experiment ,
specimens were ultrasound - washed with distilled 
 water for 10 min , dried with jets of air , and randomly allocated into 5 experimental 
 groups with 8 specimens each : gct ( unbleached specimens restored with composite 
 resin ) ; gbl ( specimens restored immediately after bleaching ) ; gsa ( bleached specimens 
 treated with sodium ascorbate gel for 60 min before restoration ) ; g7d ( bleached 
 specimens restored after being exposed to saliva in situ for 7 
 days ) ; and g14d ( bleached specimens restored after being exposed to saliva in 
 situ for 14 days ) .
figure 1 
 illustrates the steps involved in the different experimental procedures described 
 below .
illustration showing the steps involved in the different experimental 
 procedures : a , dental specimens ; b , restoration with composite resin ; c , 
 bleaching for 40 min ; d , application of sodium ascorbate gel for 60 min ; e , 
 placement of bleached dental specimens in intraoral devices to provide contact 
 with saliva in situ for 7 days ; f , placement of bleached 
 dental specimens in intraoral devices to provide contact with saliva in 
 situ for 14 days ; g , specimens are serially cut for the fabrication 
 of sticks ; h , sticks are attached to a metal grip for microtensile testing specimens belonging to gbl , gsa , g7d and g14d were fixed to a plate of pink dental 
 wax ( cera utilidade rosa epoxiglass , epoxiglass ind . com .
diadema , sp , brazil ) placed on a glass sheet in such a way as to expose the enamel 
 surface . a thin layer ( 1 mm ) of 37.5% hydrogen peroxide - based bleaching gel 
 ( pollaoffice + , sdi limited , bayswater , victoria , australia ) was applied over the 
 enamel surface . after 8 minutes
, the gel was removed with a fine tipped suction 
 device ( sug - plast , dfl indstria e comrcio s.a .
specimens 
 were in contact with the product for a total of 40 min , after which they were rinsed 
 with jets of water for 30 seconds , and dried with compressed air .
still fixed to the plate of pink wax , bleached specimens assigned to gsa were treated 
 with 10% sodium ascorbate gel ( ph 7 ) manipulated according to kimyai and 
 valizadeh ( 2006 ) shortly 
 before its application , and placed into a chamber with 100% humidity for 60 minutes . 
 after this period specimens were rinsed with jets of water for 30 seconds , and dried 
 with compressed air . to evaluate the effect of exposure time to human saliva on composite bond strength to 
 bleached enamel , 3 volunteers ( two female and one male ) were selected .
the inclusion 
 criteria were normal salivary flow , absence of caries or periodontal diseases , 
 non - pregnant women , and absence of systemic disorders ; while the exclusion criteria 
 were smokers and current use of orthodontic appliances or fixed or removable 
 dentures .
alginate impressions ( jeltrate dustless , dentsply , york , pa , usa ) of the 
 maxillary arches were taken , and plaster models ( durone iv , dentsply , york , pa , usa ) 
 were prepared for each volunteer .
polyvinyl siloxane blocks ( elite hd+ , zhermack 
 clinical , badia polesine , rovigo , italy ) measuring 5.5x5.5x5.5 mm were bonded to the 
 palatal area of the plaster models with a cyanoacrylate - based glue ( super bonder , 
 loctite , so paulo , sp , brazil ) .
the plaster models were then isolated and intraoral 
 palatal devices were fabricated in acrylic resin ( jet , clssico artigos odontolgicos 
 ltda . , campo limpo paulista , sp , brazil ) .
the 16 specimens to be assessed in 
 situ ( g7d and g14d ) were fixed with sticky wax ( cera utilidade asfer , 
 asfer - indstria qumica ltda .
, so caetano do sul , sp , brazil ) to the cavities left 
 by the polyvinyl siloxane blocks in the intraoral palatal devices ( 5 in two models 
 and 6 in one ) . to make sure that bleached specimens would be in clear contact with 
 the oral cavity , they were fixed in such a way that the enamel surfaces would 
 coincide with that of the acrylic device .
each volunteer received their palatal device and were instructed to use it throughout 
 the day and night .
it could only be removed during meals , when ingesting liquids 
 ( except water ) , or when cleaning their teeth . when removed from the mouth ,
volunteers 
 were instructed to clean their palatal devices under running water only , and to avoid 
 any contact with fluoride - containing solutions . on the morning of the eighth day , 
 eight specimens ( g7d )
were randomly removed from the palatal devices ( 2 , 3 and 3 ) , 
 and the cavities filled with sticky wax .
the devices were then returned to the 
 volunteers , who wore the palatal devices until the morning of the 15 
 day , when they were returned and the remaining specimens ( g14d ) removed .
after 
 exposure to saliva , specimens were rinsed with jets of water for 30 seconds , and 
 dried with compressed air . before being restored
, all specimens were measured with a thickness gauge 
 ( espessmetro otto inox , arminger & cia ltda , so leopoldo , rs , brazil ) .
the 
 enamel surface of specimens was treated with 37% phosphoric acid etching for 30 sec 
 and rinsed under running water for 30 sec .
after that , two layers of an adhesive 
 system ( adper single bond 2 , 3m - espe , 3 m do brasil , campinas , sp , brazil ) were 
 applied on the enamel surface , which was light activated for 20 seconds after excess 
 had been removed with the application of jets of air for 5 seconds .
composite resin 
 ( filtek z350 , 3m - espe , 3 m do brasil , campinas , sp , brazil ) was then placed in 
 increments of 1 mm and light activated ( 600 mw / cm ) with an halogen - curing 
 device ( optilux 501 - demetron , kerr , west collins , ca , usa ) for 20 seconds .
the 
 restoration was considered concluded when the height of the restoration coincided 
 with the initial height of the specimen .
after restoration , specimens were stored in distilled water at 37c for 24 h. each individual specimen was removed from distilled water , jet dried and fixed onto 
 the acrylic brackets of a precision sectioning machine ( isomet 1000 , buehler , lake 
 bluff , il , usa ) with sticky wax .
each slice was then individually fixed to the cutting machine to be further sectioned 
 as to obtain tooth / restoration " sticks " with a cross - sectional area of approximately 
 0.8 mm .
measurements were performed with a digital caliper ( zaas 
 precision , amatools , piracicaba , sp , brazil ) .
after sectioning , sticks were stored at 
 100% relative humidity to avoid desiccation before testing .
each stick was placed in a special microtensile jig ( odeme biotechnology , joaaba , 
 sc , brazil ) and the extremities were completely covered with cyanoacrylate glue 
 ( adhesive zap and zip kicker accelerator , colas , pacer technology , usa ) , leaving just 
 the enamel / resin interface exposed .
this was done to make sure the stick was 
 correctly centralized on the device and tension distribution was concentrated at the 
 interface .
sticks were tensile - tested at 0.5 mm / min in a universal testing machine 
 ( emic dl 2000 , so jos dos pinhais , pr , brazil ) until failure . tensile force in n 
 was divided by the cross - sectional area in mm and bond strength values 
 were expressed in mpa . to determine whether a specimen would be included in the 
 statistical analysis ,
fragments were observed under an optical microscope 40x ( bel 
 microimage analyzer , bel photonics , monza , italy ) , and fracture types were classified 
 as follows : adhesive fracture - failure in adhesion , with fracture at the interface ; cohesive fracture - enamel substrate failure only ; cohesive fracture - restorative material failure only ; or mixed fracture - enamel substrate and resin materials fractures ( adhesive or resin 
 composite ) in the same test specimen .
sticks that presented pretest failures , i.e. , broke during preparation for the test , 
 were discarded from the analysis . 
 moreover , in order to confirm the findings from the optical microscopy , 30% of each 
 sample was randomly selected for interface analysis under scanning electron 
 microscopy ( sem ) .
sticks were placed in aluminum stubs , sputter - coated with 
 gold - palladium alloy ( ion coater , ic-50 , shimadzu biotech ) , and observed under sem 
 ( ss-550 superscan , shimadzu biotech , japan ) . only sticks with " adhesive " or " mixed " 
 interface fractures , in which the cohesive part represented 10% of the interface 
 area ,
to guarantee independence of data , and at the same time reduce the variability , 
 average microtensile strength was calculated for the sticks originating from each 
 dental specimen . as the results 
 presented normal distribution , they were statistically analyzed with one - way analysis 
 of variance ( anova ) and tukey 's post - hoc test , at 5% global 
 significance .
fifty sound extracted human teeth ( premolars and molars ) were cleaned with 
 periodontal curettes and ultrasound , submitted to prophylaxis with pumice and 
 water , and stored in 2% 
 formaldehyde solution ( ph 7.0 ) between 30 to 50 days .
an enamel block specimen 
 measuring 5x5 mm was obtained from the flattest portion of each tooth ( proximal face 
 for premolars and vestibular or proximal face for molars ) with a diamond disc 
 ( diamond wheel 012 " x fine , south bay technology inc , ca , usa ) adapted to a 
 sectioning machine ( isomet 1000 , buehler , lake bluff , il , usa ) at low speed , under 
 water cooling . to facilitate their handling during subsequent stages ,
after inspection under optical microscopy 20x 
 ( bel microimage analyzer , bel photonics , monza , milano , italy ) , 10 specimens 
 presenting cracks , fractures or stains were excluded from the study . before their use in the experiment , specimens were ultrasound - washed with distilled 
 water for 10 min , dried with jets of air , and randomly allocated into 5 experimental 
 groups with 8 specimens each : gct ( unbleached specimens restored with composite 
 resin ) ; gbl ( specimens restored immediately after bleaching ) ; gsa ( bleached specimens 
 treated with sodium ascorbate gel for 60 min before restoration ) ; g7d ( bleached 
 specimens restored after being exposed to saliva in situ for 7 
 days ) ; and g14d ( bleached specimens restored after being exposed to saliva in 
 situ for 14 days ) .
figure 1 
 illustrates the steps involved in the different experimental procedures described 
 below .
illustration showing the steps involved in the different experimental 
 procedures : a , dental specimens ; b , restoration with composite resin ; c , 
 bleaching for 40 min ; d , application of sodium ascorbate gel for 60 min ; e , 
 placement of bleached dental specimens in intraoral devices to provide contact 
 with saliva in situ for 7 days ; f , placement of bleached 
 dental specimens in intraoral devices to provide contact with saliva in 
 situ for 14 days ; g , specimens are serially cut for the fabrication 
 of sticks ; h , sticks are attached to a metal grip for microtensile testing
specimens belonging to gbl , gsa , g7d and g14d were fixed to a plate of pink dental 
 wax ( cera utilidade rosa epoxiglass , epoxiglass ind . com .
diadema , sp , brazil ) placed on a glass sheet in such a way as to expose the enamel 
 surface . a thin layer ( 1 mm ) of 37.5% hydrogen peroxide - based bleaching gel 
 ( pollaoffice + , sdi limited , bayswater , victoria , australia )
, the gel was removed with a fine tipped suction 
 device ( sug - plast , dfl indstria e comrcio s.a .
specimens 
 were in contact with the product for a total of 40 min , after which they were rinsed 
 with jets of water for 30 seconds , and dried with compressed air .
still fixed to the plate of pink wax , bleached specimens assigned to gsa were treated 
 with 10% sodium ascorbate gel ( ph 7 ) manipulated according to kimyai and 
 valizadeh ( 2006 ) shortly 
 before its application , and placed into a chamber with 100% humidity for 60 minutes . 
 after this period specimens were rinsed with jets of water for 30 seconds , and dried 
 with compressed air .
to evaluate the effect of exposure time to human saliva on composite bond strength to 
 bleached enamel , 3 volunteers ( two female and one male ) were selected .
the inclusion 
 criteria were normal salivary flow , absence of caries or periodontal diseases , 
 non - pregnant women , and absence of systemic disorders ; while the exclusion criteria 
 were smokers and current use of orthodontic appliances or fixed or removable 
 dentures .
alginate impressions ( jeltrate dustless , dentsply , york , pa , usa ) of the 
 maxillary arches were taken , and plaster models ( durone iv , dentsply , york , pa , usa ) 
 were prepared for each volunteer . polyvinyl siloxane blocks ( elite hd+ , zhermack 
 clinical , badia polesine , rovigo , italy ) measuring 5.5x5.5x5.5 mm were bonded to the 
 palatal area of the plaster models with a cyanoacrylate - based glue ( super bonder , 
 loctite , so paulo , sp , brazil ) .
the plaster models were then isolated and intraoral 
 palatal devices were fabricated in acrylic resin ( jet , clssico artigos odontolgicos 
 ltda . , campo limpo paulista , sp , brazil ) .
the 16 specimens to be assessed in 
 situ ( g7d and g14d ) were fixed with sticky wax ( cera utilidade asfer , 
 asfer - indstria qumica ltda .
, so caetano do sul , sp , brazil ) to the cavities left 
 by the polyvinyl siloxane blocks in the intraoral palatal devices ( 5 in two models 
 and 6 in one ) . to make sure that bleached specimens would be in clear contact with 
 the oral cavity , they were fixed in such a way that the enamel surfaces would 
 coincide with that of the acrylic device .
each volunteer received their palatal device and were instructed to use it throughout 
 the day and night .
it could only be removed during meals , when ingesting liquids 
 ( except water ) , or when cleaning their teeth . when removed from the mouth ,
volunteers 
 were instructed to clean their palatal devices under running water only , and to avoid 
 any contact with fluoride - containing solutions . on the morning of the eighth day , 
 eight specimens ( g7d ) were randomly removed from the palatal devices ( 2 , 3 and 3 ) , 
 and the cavities filled with sticky wax .
the devices were then returned to the 
 volunteers , who wore the palatal devices until the morning of the 15 
 day , when they were returned and the remaining specimens ( g14d ) removed .
after 
 exposure to saliva , specimens were rinsed with jets of water for 30 seconds , and 
 dried with compressed air .
before being restored , all specimens were measured with a thickness gauge 
 ( espessmetro otto inox , arminger & cia ltda , so leopoldo , rs , brazil ) .
the 
 enamel surface of specimens was treated with 37% phosphoric acid etching for 30 sec 
 and rinsed under running water for 30 sec .
after that , two layers of an adhesive 
 system ( adper single bond 2 , 3m - espe , 3 m do brasil , campinas , sp , brazil ) were 
 applied on the enamel surface , which was light activated for 20 seconds after excess 
 had been removed with the application of jets of air for 5 seconds .
composite resin 
 ( filtek z350 , 3m - espe , 3 m do brasil , campinas , sp , brazil ) was then placed in 
 increments of 1 mm and light activated ( 600 mw / cm ) with an halogen - curing 
 device ( optilux 501 - demetron , kerr , west collins , ca , usa ) for 20 seconds .
the 
 restoration was considered concluded when the height of the restoration coincided 
 with the initial height of the specimen .
each individual specimen was removed from distilled water , jet dried and fixed onto 
 the acrylic brackets of a precision sectioning machine ( isomet 1000 , buehler , lake 
 bluff , il , usa ) with sticky wax .
each slice was then individually fixed to the cutting machine to be further sectioned 
 as to obtain tooth / restoration " sticks " with a cross - sectional area of approximately 
 0.8 mm .
measurements were performed with a digital caliper ( zaas 
 precision , amatools , piracicaba , sp , brazil ) .
after sectioning , sticks were stored at 
 100% relative humidity to avoid desiccation before testing .
each stick was placed in a special microtensile jig ( odeme biotechnology , joaaba , 
 sc , brazil ) and the extremities were completely covered with cyanoacrylate glue 
 ( adhesive zap and zip kicker accelerator , colas , pacer technology , usa ) , leaving just 
 the enamel / resin interface exposed .
this was done to make sure the stick was 
 correctly centralized on the device and tension distribution was concentrated at the 
 interface .
sticks were tensile - tested at 0.5 mm / min in a universal testing machine 
 ( emic dl 2000 , so jos dos pinhais , pr , brazil ) until failure . tensile force in n 

was divided by the cross - sectional area in mm and bond strength values 
 were expressed in mpa . to determine whether a specimen would be included in the 
 statistical analysis ,
fragments were observed under an optical microscope 40x ( bel 
 microimage analyzer , bel photonics , monza , italy ) , and fracture types were classified 
 as follows : adhesive fracture - failure in adhesion , with fracture at the interface ; cohesive fracture - enamel substrate failure only ; cohesive fracture - restorative material failure only ; or mixed fracture - enamel substrate and resin materials fractures ( adhesive or resin 
 composite ) in the same test specimen .
sticks that presented pretest failures , i.e. , broke during preparation for the test , 
 were discarded from the analysis . 
 moreover , in order to confirm the findings from the optical microscopy , 30% of each 
 sample was randomly selected for interface analysis under scanning electron 
 microscopy ( sem ) .
sticks were placed in aluminum stubs , sputter - coated with 
 gold - palladium alloy ( ion coater , ic-50 , shimadzu biotech ) , and observed under sem 
 ( ss-550 superscan , shimadzu biotech , japan ) . only sticks with " adhesive " or " mixed " 
 interface fractures , in which the cohesive part represented 10% of the interface 
 area ,
to guarantee independence of data , and at the same time reduce the variability , 
 average microtensile strength was calculated for the sticks originating from each 
 dental specimen . as the results 
 presented normal distribution , they were statistically analyzed with one - way analysis 
 of variance ( anova ) and tukey 's post - hoc test , at 5% global 
 significance .
the cross - sectional areas of sticks were statistically homogenous ( 0.70 - 0.92 
 mm , p=0.52 ) , indicating that differences in bond strength among groups 
 would not be due to differences in the cross - sectional area .
mean ( sd ) bond strength found were : gct 39.61 ( 13.71 ) , gbl 25.89 ( 11.38 ) , gsa 36.27 
 ( 16.04 ) , g7d 43.60 ( 15.24 ) , and g14d 49.26 ( 12.97 ) mpa ( figure 2 ) .
results found for gbl in comparison to gct indicate that 
 bleaching significantly reduced enamel adhesiveness ( p<0.01 ) .
however , no 
 statistically significant differences were found between gct , gsa and g7d ( p>0.05 ) , 
 demonstrating that the treatment with sodium ascorbate and exposure to saliva for 7 days 
 recovered bond strength to enamel to the same level of unbleached specimens . although 
 bond strength found for g14d was statistically the same as for gct and g7d , it was 
 significantly higher than that found for gsa ( p<0.01 ) . mean bond strength ( mpa ) and standard deviation found in the microtensile testing . 

gct : control group ; gbl : group restored immediately after bleaching ; gsa : group 
 restored after bleaching and treatment with sodium ascorbate ; g7d : group restored 
 after 7 days in human saliva ; and g14d : group restored after 14 days in human 
 saliva .
similar letters indicate no significant differences between groups 
 ( tukey 's test ; p>0.05 ) 
 table 1 illustrates premature failures and 
 fracture mode observed in the 
 different experimental groups .
qualitative microscopic analysis ( optical and sem ) 
 revealed that fractures modes ( figures 3 - 4 ) were predominantly of a mixed type ( 52% ) , followed 
 by adhesive ( 28% ) and cohesive ( 20% ) .
the frequency of premature failures was highest in 
 the gbl group ( 78% ) , followed by g7d ( 23% ) , gct and g14d ( both 18% ) , and gsa ( 11% ) .
premature failures and fracture modes observed in the experimental groups , n 
 ( % ) interface of a test specimen from g7d seen with scanning electron microscopy . 

arrow 
 points to the fracture region in enamel ( bar=100 m ) interface of the same test specimen as shown in figure 3 , at higher magnification .
to the best of our knowledge , this is the first report to evaluate the influence of 
 exposure time to saliva in situ ( natural antioxidant effect ) in 
 comparison to a treatment with 10% sodium ascorbate gel ( artificial antioxidant effect ) 
 on composite resin bond strength to human enamel after bleaching .
the results found for gbl in comparison with gct confirmed that the treatment with 
 hydrogen peroxide reduced enamel adhesiveness in approximately 35% , which is consistent 
 with previous results .
this finding 
 may be explained by the delayed release of oxygen due to the presence of peroxides and 
 their subproducts , oxygen and water , which compete with free radicals produced during 
 the photoactivation , affecting the 
 polymerization of resin components . in the present study ,
the treatment with sodium ascorbate increased composite resin 
 bonding to enamel immediately after bleaching .
this finding is in agreement with 
 previous studies that have shown antioxidants to be able to immediately restore bond 
 strength to bleached enamel .
however , kaya , et al.12 
 ( 2008 ) suggested that to obtain effective results the exposure time to 
 the antioxidant agent should be of at least 60 min .
a similar protocol was used in the 
 present study ( gsa ) , with satisfactory results . in another study ,
( 2006 ) observed that the 
 treatment of dental specimens with a sodium ascorbate solution for 20 minutes was not 
 sufficient to restore bond strength to bleached enamel .
the authors ' findings may have 
 not only been due to insufficient exposition to the antioxidant agent , but also to the 
 way it was conducted . in the present study , the sodium ascorbate agent used was in a gel 
 form , which is easy to apply and , according to some studies , more 
 effective than solutions , as the active compounds in the gel are released more 
 slowly . although an in vitro study has demonstrated that the amount of sodium 
 ascorbate required to reduce hydrogen peroxide is directly related to the concentration 
 of the latter , and the reaction kinetics between oxidant and antioxidant showed that 5 
 minutes was sufficiently long for the antioxidant to exert an effect , the exact time required for the 
 antioxidant to reestablish bond capacity to bleached enamel in vivo has 
 yet to be established .
( 2002 ) recommended the use of 10% sodium ascorbate for at least a third of the bleaching 
 agent application time . following this recommendation ,
kimyai and valizadeh ( 2006 ) left the antioxidant in contact 
 with bleached teeth for 3 hours , which was considered too long from a clinical 
 perspective .
studies have also reported differences in the effective application time 
 between carbamide peroxide and hydrogen peroxide .
studies using 10% carbamide peroxide 
 showed that bonding strength to enamel was restored within 10 to 180 minutes or 10 to 480 minutes after the application of a sodium 
 ascorbate gel and solution , respectively .
whereas , for enamel bleached with 35% hydrogen 
 peroxide , the effective reversal of reduced bonding was achieved after 10 - 20 
 minutes of exposure to a 10% sodium ascorbate solution .
( 2009 ) attempted to 
 simulate home - bleaching using a tray and artificial saliva .
bond strength to bleached 
 enamel was not restored by the treatment with 10% sodium ascorbate solution , perhaps due 
 to the fact that the antioxidant was diluted by the artificial saliva in which it was 
 incubated for 2 hours . in the present study ,
dental specimens with the gel were kept in 
 an environment at 100% humidity without any contact with fluid . this scenario was used 
 to reproduce the clinical application of the antioxidant in a dental office , which is 
 performed under relative isolation .
thus , the sodium ascorbate gel used was not 
 dissolved or diluted , which may have improved its ability to restore bond strength to 
 enamel more effectively .
therefore , in the present study , the treatment of bleached enamel surface with 10% 
 sodium ascorbate antioxidant gel for 60 min was found to be an effective clinical option 
 for patients in need of immediate esthetic restorations following bleaching . in a 
 follow - up study performed by garcia , et al .
( 2012 ) , the authors reported on restorations carried out immediately 
 after bleaching with the use of 10% sodium ascorbate gel for one hour .
the study 
 demonstrated that no alterations in restoration shape or color were observed after one 
 year , and considered the method efficient , user - friendly and little time - consuming .
one 
 of the limitations of using sodium ascorbate as an antioxidant , however , is that it 
 gradually oxidizes with time , becoming less reductive . to circumvent this problem in the present study ,
no statistical differences between gct , g7d , and g14d groups were found , indicating that 
 bond strength to enamel was fully reestablished after exposure to saliva for 7 days 
 in situ ( figure 2 ) .
although 
 several authors have recommended that restorative procedures should wait between 7 to 14 
 days after bleaching , the findings in this study suggest that exposition to 
 saliva for at least one week is a satisfactory waiting period for restorative procedures 
 after in - office dental bleaching treatments .
however , different hydrogen peroxide 
 concentrations can also influence the waiting time required between the end of the 
 bleaching treatment and the performance of bonding procedures .
studies using hydrogen peroxide have suggested a waiting 
 period of 14 days after bleaching before starting with the restorative procedure .
the 
 results of the present study support this assumption , as bond strength to bleached 
 enamel after being exposed for 14 days ( g14d ) to human saliva was significantly higher 
 than that found for sodium ascorbate - treated enamel ( gsa ) .
results obtained in laboratory studies may differ from the clinical situation , because 
 they commonly use artificial saliva to store the specimens . in situ studies have the advantage of 
 being nearest to the clinical situation , as bleached dental specimens are placed under 
 the direct effect of human saliva . 

human saliva acts on the oxidative stress mediated by free radicals , and presents 
 alternative antioxidants , including ascorbic acid and vitamin e. a previous in situ and 
 in vitro study has shown that although bleaching agents are capable 
 of altering dental enamel surface 's microhardness , roughness , and morphology , natural 
 saliva allows for enamel mineral reposition .
this process helps to reestablish enamel surface microhardness , 
 which is an important component in the adhesive strength of enamel . in an attempt to simulate the clinical 
 situation , as well as to permit that only the natural antioxidant effect of saliva could 
 be assessed in the reestablishment of adhesive bond strength after bleaching
, volunteers 
 in the in situ experiment were asked to avoid the use of any type of 
 fluoride - containing products .
these products have been shown to alter salivary fluoride 
 concentration , which could have 
 had an influence on enamel 's remineralization process . in this study ,
the frequency of 
 premature failures in the gbl group was much higher ( 78% ) than that observed in other 
 groups . during the specimen preparation procedures ,
microcracks might have been 
 inadvertently produced mainly by the vibrations of the cutting instruments .
such 
 defects , coupled with the brittle nature of enamel , are responsible for the premature 
 failure of microtensile bond specimens .
the high number of pretest failures seen in the gbl group emphasizes 
 the fragility of bonding immediately after bleaching . in order to standardize procedures 
 and avoid variability as much as possible
, we followed the recommendations set out by 
 roulet and van meerbeek ( 2007 ) . 

sem was used to 
 analyze in detail the type of failure , and only those sticks presenting adhesive or 
 predominantly adhesive failures were considered , while cohesive failures were discarded 
 as well .
this was done to circumvent the inherent problems with the microtensile tests 
 mentioned by el zohairy , et al.7 
 ( 2010 ) , concerning the high presence of cohesive fractures . moreover , to 
 minimize the differences in adhesion that could occur in different parts of the 
 specimen , we calculated the average microtensile bond strength for the sticks 
 originating from the same dental specimen . in conclusion , based on the results found in this study
, the use of sodium ascorbate 
 after in - office bleaching may be indicated in clinical situations in which the patient 
 can not wait for adhesive techniques to be performed .
however , whenever possible , bonding 
 should be postponed for at least 7 days after tooth bleaching . from a chemical - molecular 
 perspective , the alterations in the bleached enamel caused by the antioxidant treatment 
 and exposure to saliva remain unknown and warrant further investigation .
when 10% sodium ascorbate gel was applied to the bleached enamel for 60 minutes , bond 
 strength was reverted to the same level found for unbleached enamel .
when bleached enamel was exposed to human saliva for 7 days , bond strength was 
 reestablished to the same level of enamel treated with 10% sodium ascorbate gel and 
 unbleached enamel .
when bleached enamel was exposed to human saliva in situ for 14 
 days , bond strength was significantly better than that found for specimens treated with 
 10% sodium ascorbate gel .
 objectivesthis study evaluated the influence of different exposure times to saliva 
 in situ in comparison with an antioxidant treatment on 
 composite resin bond strength to human enamel restored after tooth bleaching . 
 material and methodsforty human teeth specimens measuring 5x5 mm were prepared and randomly allocated 
 into 5 groups with 8 specimens each : gct ( control group , restored on unbleached 
 enamel ) ; gbl ( restored immediately after bleaching ) ; gsa ( bleached , treated with 
 10% sodium ascorbate gel for 60 min and restored ) ; g7d ( bleached , exposed to 
 saliva in situ for 7 days and restored ) ; and g14d ( bleached , 
 exposed to saliva in situ for 14 days and restored ) . restored 
 samples were cut into 0.8 mm2 sticks that were tested in microtensile . 
 
 specimens were microscopically analyzed and failure modes were classified as 
 adhesive , cohesive , or mixed . 
 pretest and cohesive failures were not considered in 
 the statistical analysis , which was performed with one - way anova and tukey 's 
 post - hoc test ( =0.05 ) , with the dental specimen considered as 
 the experimental unit . 
 resultsmean bond strength results found for gbl in comparison with gct indicated that 
 bleaching significantly reduced enamel adhesiveness ( p<0.01 ) . 
 however , no 
 statistically significant differences were found between gct , gsa and g7d 
 ( p>0.05 ) . 
 bond strength found for g14d was significantly higher than for gsa 
 ( p<0.01 ) . 
 fractures modes were predominantly of a mixed type . conclusionsbonding strength to bleached enamel 
 was immediately restored with the application 
 of sodium ascorbate and exposure to human saliva in situ for at 
 least 7 days . 
 best results were obtained with exposure to human saliva in 
 situ for 14 days . 
 treatment with sodium ascorbate gel for 60 min may 
 be recommended in cases patients can not wait for at least 7 days for adhesive 
 techniques to be performed . 
INTRODUCTION
MATERIAL AND METHODS
Dental specimen preparation
Bleaching procedure
Treatment with sodium ascorbate
Exposure to saliva in situ
Restoration with composite resin
Microtensile testing
Statistical analysis
RESULTS
DISCUSSION
CONCLUSIONS