title: Peroxide bleaching agent effects on enamel surface microhardness, roughness and morphology. The Influence of Post Bleaching Treatments in Stain Absorption and Microhardness Combined Bleaching Technique Using Low and High Hydrogen Peroxide In-Office Bleaching Gel. Effect of a carbamide peroxide bleaching gel containing calcium or fluoride on human enamel surface microhardness. Microhardness of demineralized enamel following home bleaching and laser-assisted in office bleaching Effects of new formulas of bleaching gel and fluoride application on enamel microhardness: an in vitro study. In vitro Comparison of Impact of Different Bleaching Agents on the Microhardness of Enamel. Effect of 10% Sodium Ascorbate on Bleached Bovine Enamel Surface Morphology and Microhardness Effects of 10% carbamide peroxide on the enamel surface morphology: a scanning electron microscopy study. In Vitro Comparative Study of Two Different Bleaching Agents on Micro-hardness Dental Enamel. Evaluation and comparison of the microhardness of enamel after bleaching with fluoride free and fluoride containing carbamide peroxide bleaching agents and post bleaching anticay application: An in vitro study Effects of 10% carbamide peroxide treatment and sodium fluoride therapies on human enamel surface microhardness. In situ effect of 10% carbamide peroxide on microhardness of human enamel: function of time. In-Vitro Comparative Study of In-office and Home Bleaching Agents on Surface Micro-morphology of Enamel. Effect on enamel microhardness of two consumer-available bleaching solutions when compared with a dentist-prescribed, home-applied bleaching solution and a control. Effect of tray-based and trayless tooth whitening systems on microhardness of enamel surface and subsurface. The effect of 10% carbamide peroxide bleaching material on microhardness of sound and demineralized enamel and dentin in situ. Effect of carbamide peroxide bleaching agents on tensile strength of human enamel.
abstract: The aim of this study was to evaluate the surface roughness , microhardness and morphology of human enamel exposed to six bleaching agents ( at baseline and post-treatment ) . Human dental enamel sample s were obtained from human third molars and r and omly divided into seven groups ( n = 11 ) : control , Whiteness Perfect--10 % carbamide peroxide ( 10 % CP ) , Colgate Platinum--10 % CP , Day White 2Z--7.5 % hydrogen peroxide ( 7.5 % HP ) , Whiteness Super--3 % CP , Opalescence Quick--35 % CP and Whiteness HP--35 % HP . Bleaching agents were applied according to manufacturers ' instructions . The control group remained not treated and stored in artificial saliva . Microhardness testing was performed with a Knoop indentor and surface roughness was analyzed with a profilometer . Morphologic observations were carried out with scanning electron microscopy ( SEM ) . Results were statistically analyzed by two-way analysis of variance and Tukey 's test ( 5 % ) , and revealed a significant decrease in microhardness values and a significant increase in surface roughness post-bleaching . Changes in enamel morphology after bleaching were observed under SEM . It was concluded that bleaching agents can alter the microhardness , roughness and morphology of dental enamel surface Objectives : This study investigated the effects of post bleaching treatments to prevent restaining and the change of enamel surface microhardness after dental bleaching in vitro . Methods : Sixty intact human incisor teeth were stained in tea solution and r and omly assigned into four groups ( n=15 ) . Then sample s were bleached for two weeks ( 8 hours daily ) by 15 % carbamide peroxide . Tooth color was determined both with a spectrophotometer and visually before bleaching ( T1 ) and immediately after bleaching ( T2 ) . Next , it was applied in group 1 fluoride ( Naf 2 % ) gel for 2 minutes , and in group 2 a fractional CO2 laser ( 10 mJ , 200 Hz , 10 s ) , and in group 3 , nanohydroxyapatite gel for 2 minutes . The bleached teeth in group 4 remained untreated ( control group ) . Then teeth placed in tea solution again . Color examinations were repeated after various post bleaching treatments ( T3 ) and restaining with tea ( T4 ) and color change values recorded . The microhardness was measured at the enamel surface of sample s. Data was analyzed using ANOVA , Tukey HSD test and Dunnett T3 ( α = 0.05 ) . Results : Directly after bleaching ( ΔE T3-T2 ) , the treatment with nanohydroxyapatite showed significantly the least color lapse in colorimetric evaluation . In experimental groups , the color change between T3 and T4 stages ( ΔE T4-T3 ) was significantly lower than control group ( P < 0.05 ) . Different methods of enamel treatment caused a significant increase in surface microhardness compared to control group ( P < 0.05 ) . Significance : Application of fluoride , fractional CO2 laser and nanohydroxyapatite as post bleaching treatments are suggested for prevention of stain absorption and increasing the hardening of bleached enamel OBJECTIVES The aim of this study was to evaluate the efficacy , color stability , risk , and intensity of tooth sensitivity ( TS ) of combined bleaching techniques performed with 20 % or 35 % hydrogen peroxide for an in-office protocol . METHODS Thirty patients were r and omly divided into two groups and su bmi tted to a single 45-minute in-office bleaching session with 35 % hydrogen peroxide or 20 % hydrogen peroxide . At-home bleaching was performed with 10 % carbamide peroxide for two hours daily over the course of two weeks . The color was evaluated with the value-oriented shade guide Vita Classical at different periods up to 12 months after bleaching . Patients recorded the intensity of TS using a five-point verbal scale . Color change data were su bmi tted to a two-way repeated- measures analysis of variance and Tukey test ( α=0.05 ) . The absolute risk and intensity of TS were compared with the Fisher exact test and Mann-Whitney test , respectively ( α=0.05 ) . RESULTS On average , an effective and similar whitening of three units in shade guide was observed for both groups , which remained stable for 12 months . When both protocol s were compared , the one with hydrogen peroxide 35 % showed a higher risk ( p=0.02 ) and intensity of TS ( p=0.04 ) . In regard to the TS intensity , no significant difference was observed up to 48 hours after in-office bleaching ( p=0.09 ) and during the at-home bleaching phase of the study ( p=0.71 ) . CONCLUSION The combined bleaching technique using at-home bleaching associated with in-office bleaching was effective and stable over the course of 12 months , regardless of the concentration of the hydrogen peroxide used for in-office bleaching . However , the protocol with 20 % hydrogen peroxide produced lower risk and intensity of TS This in vitro study evaluated the surface microhardness of human enamel su bmi tted to bleaching with 10 % carbamide peroxide ( CP ) containing calcium or fluoride . Ninety-eight dental blocks ( 5 x 5 mm2 ) with polished enamel surfaces were r and omly assigned to 7 treatment groups ( n=14 ) , as follows : without bleaching and storage in artificial saliva ( control ) ; 10 % CP ; 10 % CP + 0.05 % calcium ; 10 % CP + 0.1 % calcium ; 10 % CP + 0.2 % calcium ; 10 % CP + 0.2 % fluoride ; and 10 % CP + 0.5 % fluoride . During 14 days , enamel surfaces were daily exposed to a 6-h bleaching regimen followed by storage in artificial saliva .  Surface microhardness was measured before ( baseline ) , during ( 7th day ) , immediately after bleaching ( 14th day ) and 1 week post bleaching . Data were analyzed by two-way ANOVA and Tukey 's test ( p<0.05 ) . All treatments reduced SM significantly during the bleaching cycle ( 7th day ) , immediately after bleaching ( 14th day ) and 1 week post bleaching , compared to baseline and to the unbleached control group . In conclusion , in spite of the addition of calcium and fluoride , all bleaching treatments affected the enamel surface microhardness Background There is little data regarding the effect of tooth whitening on microhardness of white spot lesions . This study was conducted to investigate the effect of home-bleaching and laser-assisted in-office bleaching on microhardness of demineralized enamel . Material and Methods Forty bovine incisors were selected and immersed in a demineralizing solution for 12 weeks to induce white spot lesions . Enamel blocks were prepared and r and omly assigned to two groups of 20 each . The first group underwent home bleaching with 15 % carbamide peroxide which was applied for 8 hours a day over a period of 15 days . In the second group , in-office bleaching was performed by 40 % hydrogen peroxide and powered by irradiation from an 810 nm gallium-aluminum-arsenide ( GaAlAs ) diode laser ( CW , 2W ) . This process was performed for 3 sessions every seven days , in 15 days . The specimens were stored in Fusayama Meyer artificial saliva during the experiment . Surface microhardness was assessed before and after the bleaching therapies in both groups . Results Microhardness decreased significantly following both home bleaching and laser-assisted in-office bleaching ( p<0.05 ) . There were no significant differences in hardness values among the two groups either before ( p=0.131 ) or after ( p=0.182 ) the bleaching procedures . Conclusions Tooth whitening through home bleaching or laser-assisted in-office bleaching can result in a significant reduction in microhardness of white spot lesions . Therefore , it is suggested to take protective measures on bleached demineralized enamel . Key words : White spot lesion , bleaching , laser , microhardness , demineralized enamel , home bleaching , in-office bleaching This in vitro study evaluated the new formulas of bleaching products and the effect of subsequent applications of fluoride on the hardness of enamel during and after tooth bleaching . The crowns of 60 extracted intact human molars were sectioned longitudinally ; the buccal part was embedded in acrylic resin , the occlusal part was ground flat , exposing enamel and dentin , and then polished . Baseline Knoop microhardness ( KHN ) of enamel was determined . The specimens were then r and omly divided into six groups of 10 specimens , and each group was assigned to a specific 10 % carbamide peroxide ( CP ) bleaching agent . A : Opalescence , B : Opalescence PF ( 3 % potassium nitrate and 0.11 % fluoride ) , C : Nite White Excel 3 ( ACP ) , D : Opalescence + F ( acidulated phosphate fluoride 1.23 % ) , E : Opalescence PF + F , F : Nite White Excel 3 + F. The teeth were bleached for eight hours ; after each procedure , the specimens were stored in artificial saliva at 37 degrees C. Immediately after day 21 of bleaching , the specimens in groups D , E and F received fluoride 1.23 % for five minutes . KHN tests w ere performedbefore ( baseline = control ) , during ( 14 , 21 ) and two weeks ( 35 days ) after the bleaching procedure and were statistically compared using ANOVA/Tukey 's t-test ( alpha < 0.05 ) . The statistical analysis revealed no significant difference among the bleaching material s ( p = 0.123 ) . A significant enamel KHN reduction ( p < 0.001 ) was observed for all bleaching material s , with no difference among them . Two weeks after bleaching , all the groups that received fluoride showed a significant increase in microhardness . For the new bleaching formulas , the enamel was restored to a value similar to baseline BACKGROUND Various agents are used these days for increasing the esthetics . One such procedure is bleaching that offers various advantages , as it is minimal invasive and cheap option to color the teeth and remove stain . The altered enamel after the bleaching process shows surface demineralization and porosities . The present study aim ed to evaluate the effect of different bleaching agents on the microhardness of enamel . MATERIAL S AND METHODS A total of 100 freshly human extracted maxillary premolar teeth were selected for the study . Teeth with sound tooth structure were included for the study . All the specimens were r and omly divided into four groups with 25 specimens in each group depending upon the type of bleaching agent used : Group A , artificial saliva ( Control group ) ; Group B , 35 % hydrogen peroxide ( HP ) ; Group C , 25 % HP ; Group D , 10 % carbamide peroxide ( CP ) .  Knoop Hardness Number ( KHN ) was calculated at 24 , 48-hour , and 7-week interval . RESULTS Results showed no statistical significant differences between the microhardness of enamel of different groups ( p < 0.005 ) . A slight fall in the value of KHN was seen in all the groups , except for the control group , although the results were statistically nonsignificant ( p > 0.005 ) . CONCLUSION Although nonsignificantly , all the bleaching solutions produced some amount of alterations in the microstructure of enamel . More studies with higher study groups and more advanced estimation technologies are required to minimize microstructure alterations and promote for better outcome of bleaching procedures Sodium ascorbate has recently been suggested to compensate decreased bond strength of composite resin to bleached tooth surfaces . The aim of present study was to evaluate the effect of 10 % sodium ascorbate on bleached bovine enamel morphology and microhardness considering the possibility of its effect on enamel surface characteristics . A total of 69 bovine enamel slabs were prepared and mounted in acrylic resin . Subsequent to polishing , they were r and omly divided into 3 groups of 23 specimens each ; 3 for Scanning Electron Microscopy ( SEM ) analysis and 20 for hardness test . In group 1 the specimens were immersed in distilled water as control group ; in group 2 the specimens were bleached with 35 % carbamide peroxide for 30 min a week for 3 consecutive weeks ; and in group 3 the specimens were exposed to 10 % sodium ascorbate for 30 min subsequent to bleaching similar to group 1 . After 3 weeks Vickers hardness of the specimens was measured at 3 points with equal distances from each other under a force of 50 g. The mean of the hardness values of each specimen was calculated and data was analyzed by one-way ANOVA ( P < 0.05 ) . The highest and lowest microhardness values were observed in group 1 and group 2 , respectively . However , there were no statistically significant differences in microhardness between the groups ( P = 0.12 ) . The use of 35 % carbamide peroxide alone or with 10 % sodium ascorbate does not affect bovine enamel hardness . SEM analysis showed a network of sodium ascorbate adsorbed to the bleached enamel surface BACKGROUND Several studies have been published about the effect of carbamide peroxide on enamel surface morphology . However , only a few studies have been carried out under intraoral conditions . The purpose of this study was to investigate the effect of two commercially available 10 % carbamide peroxide bleaching products ( Colgate Platinum and Starbrite ) on the enamel surface morphology under intraoral conditions . MATERIAL S AND METHODS Twenty-four subjects were divided into two groups and assigned a product to use for 2 weeks . Epoxy resin replicas obtained from the maxillary right central incisors of the subjects before , immediately after , and 3 months after the bleaching procedure were examined under a scanning electron microscope . RESULTS Immediately after the bleaching procedure , the surface porosity of enamel had increased in the Colgate Platinum group , whereas erosive surface alterations were observed in the Starbrite group . At 3 months , the surface morphology of enamel in the Colgate Platinum group was similar to the untreated enamel surface . In the Starbrite group , erosive defects on the enamel surface had decreased within 3 months , and the appearance of the enamel surface was almost the same as the untreated enamel . CONCLUSION Even though both bleaching agents caused alterations in the surface morphology of enamel , these alterations were reversed within 3 months following treatment . CLINICAL SIGNIFICANCE The results of this study suggest that 10 % carbamide peroxide bleaching products cause alterations in enamel surface morphology immediately after bleaching , and the degree of these alterations depends on the br and of bleaching agent and duration of application time . However , these alterations revert to almost normal within 3 months OBJECTIVE To evaluate the effect of home-use bleaching agent containing 16 % Carbamide Peroxide ( CP ) and in-office bleaching agent containing 38 % Hydrogen Peroxide ( HP ) on enamel micro-hardness . STUDY DESIGN An in vitroexperimental study . PLACE AND DURATION OF STUDY Department of Operative Dentistry and Science of Dental Material s at Dr. Ishrat-ul-Ebad Khan Institute of Oral Health Sciences , Dow University of Health Sciences and Material Engineering Department of NED University of Engineering and Technology , Karachi , from July to December 2014 . METHODOLOGY Atotal of 90 enamel slabs from 45 sound human 3rd molar were r and omly divided into 3 groups . Each group contained 30 specimens ( n=30 ) . Group 1 was kept in artificial saliva at 37 ° C in incubator during the whole experiment . However , Groups 2 and 3 were treated with power whitening gel and tooth whitening pen respectively . After bleaching session , specimens were thoroughly rinsed with deionized water again for 10 seconds and then stored in artificial saliva at 37 ° C in incubator . Artificial saliva was changed after every 2 days . The Vickers hardness tester ( Wolpert 402 MVD , Germany ) was adjusted to a load of 0.1 kg ( 100 gm ) and dwell time of 5 seconds . Three Vickers were performed on each specimen using a hardness tester according to the ISO 6507 - 3:1998 specification . Micro-hardness measurements were performed before and after bleaching at day 1 , 7 and 14 . RESULTS In the control group , the baseline micro-hardness was 181.1 ±9.3 which was reduced after the storage on day 1 , 7 and 14 ( p = 0.104 ) . In Group 2 , baseline micro-hardness was 180.4 ±10.1 which was reduced to 179.79 ±10.0 units after day 1 . Whereas , on day 7 and 14 , the values of micro-hardness were 179.8 ±10 and 179.7 ±10.29 , respectively ( p=0.091 ) . Furthermore , the baseline micro-hardness in Group 3 was 174.0 ±22.9 units which was reduced to 173 ±23 on day 1 , 170 ±30 on day 7 and 173 ±23 on day 14 ( p = 0.256 ) . The statistically insignificant difference was found among micro-hardness values of different bleaching agents ( p = 0.118 ) . CONCLUSION Bleaching with 38 % Hydrogen Peroxide ( HP ) and 16 % Carbamide Peroxide ( CP ) result ed in insignificant effect on surface micro-hardness of enamel Aims and Objectives : The purpose of the study was to evaluate and compare the microhardness of enamel after the application of anticay on bleached enamel with fluoride containing and fluoride free bleaching agent . Material s and Methods : Twenty freshly extracted teeth decoronated and divided mesiodistally into two halves were r and omly divided into five groups with 10 sample s in each group . The enamel surface was treated as follows : Group 1 - no treatment , Group 2 - fluoride free bleaching agent , Group 3 - fluoride containing bleaching agent , and Group 4 - fluoride free bleaching agent followed by anticay application . The sample s were subjected to indentation to test the microhardness using Vicker 's hardness analyzer . Conclusion : Enamel microhardness significantly increased in sample s where anticay was used after the application of bleaching agent This study sought to evaluate the microhardness of enamel su bmi tted to 10 % carbamide peroxide treatment  and different methods of remineralization involving sodium fluoride ( NaF ) .  Non-erupted third molars were used and 75 enamel blocks with st and ardized dimensions ( 4.0 x 4.0 x 2.0 mm ) were obtained . Enamel blocks were r and omly divided into five groups ( n = 15 ) : one control ( no bleaching and no fluoride treatment ) , one receiving a 10 % carbamide peroxide treatment , one receiving 10 % carbamide peroxide plus acidulated phosphate fluoride , one receiving 10 % carbamide peroxide with 0.2 % NaF , and one receiving 10 % carbamide peroxide and 0.05 % NaF. There were no differences among four of the groups ( p > 0.05 ) ; the group receiving 10 % carbamide peroxide only was different from all other groups ( p < 0.05 ) . Based on these results , NaF therapies are recommended during carbamide peroxide bleaching treatments PURPOSE This study evaluated the effect of two home bleaching regimens ( 1 h/day versus 7 h/day for a total of 21 days ) on the microhardness of superficial enamel treated with 10 % carbamide peroxide gel . MATERIAL S AND METHODS Rectangular enamel specimens ( 2.5 x 2.5 x 1 mm ) were obtained from human third molars and mounted on the palatal aspect of removable orthodontic retainers . Nine enamel specimens were mounted in each of 10 retainers , used by 10 volunteers during the active phase of the study . Modified 0.089 cm thick bleaching trays were fabricated for each participant . Each bleaching tray fit a participant 's arch and the retainer containing the enamel specimens . Enamel specimens were treated with a 10 % carbamide peroxide bleaching gel ( Nite White Excel 2Z , Discus Dental , Culver City , CA , USA ) for either 1 h/day ( n = 30 ) or 7 h/day ( n = 30 ) , both for 21 days . Three enamel specimens in each retainer were never treated and served as controls . The retainers were worn by the participants whether or not they underwent bleaching . This simulated the natural action of the saliva/oral fluids on the enamel specimens . Enamel specimens were subjected to pre- and post-treatment microhardness measurements . Post-treatment Knoop hardness numbers were compared with baseline values and analyzed statistically . RESULTS Bleaching treatment result ed in a nonsignificant reduction in microhardness values when compared with values of the control nontreated specimens ( 1.7 % and 2.5 % reduction in hardness for 1 h/day and 7 h/day , respectively ) . There was no difference in microhardness between the different treatment regimens . CONCLUSION Home bleaching with 10 % carbamide peroxide for up to 7 h/day for 21 days does not significantly affect enamel microhardness . CLINICAL SIGNIFICANCE Bleaching with 10 % carbamide peroxide for 1 h/day or 7 h/day for a total of 21 days is safe for human enamel OBJECTIVE To evaluate the effect of home-use bleaching agent containing 16 % Carbamide Peroxide ( CP ) and in-office bleaching agent with 38 % Hydrogen Peroxide ( HP ) on surface micro-morphology of enamel . STUDY DESIGN An experimental study . PLACE AND DURATION OF STUDY The discs were prepared at Material Engineering Department of NED University of Engineering and Technology , Karachi , and surface morphology was analyzed at Central ized Science Laboratory of Karachi University , Pakistan . Duration of study was one year from January to December 2012 . METHODOLOGY Forty five sound human third molar crowns , extracted for periodontal reason , were included in the study .  Longitudinal sections were made using diamond disks ( 0.2 mm ) under water lubrication to obtain enamel slabs measuring ( 3 mm x 3 mm ) . The slabs were embedded in polystyrene resin by using 2.0 cm diameter PVC molds , leaving the outer enamel surface uncovered by the resin . Ninety dental enamel slabs were prepared . The slabs were then r and omly divided into 3 groups . Each group contained thirty specimens ( n=30 ) . Group 1 was kept in artificial saliva at 37 ° C in incubator ( Memart , Germany ) during whole experiment . Group 2 was treated with power whitening gel ( White Smile 2011 , Germany ) . Group 3 was treated with tooth whitening pen ( White Smile 2011 , Germany ) . The most central region or the region that was most representative of the entire surface area was used . The SEM ( Jeol-Japan-JSM6380A , JAPAN ) micrographs were examined to determine the type of surface presented . The enamel changes were classified as no or mild alteration , moderate alteration and severe altered surface . RESULTS Regarding micro-morphology , the enamel surface of control groups showed smooth surface in general with some scattered clear scratches due to the polishing procedure . The specimens bleached in group 2 and group 3 , represented areas of mild erosion . CONCLUSION Bleaching with 38 % Hydrogen Peroxide ( HP ) and 16 % Carbamide Peroxide ( CP ) result ed in mild changes in surface micro-morphology of enamel BACKGROUND There exists limited data in the literature regarding the efficacy and safety of consumer-available , paint-on bleaching solutions . PURPOSE The purpose of this in vitro study was to evaluate the effect of two consumer-available , paint-on bleaching products on enamel microhardness against a control and a dentist-prescribed , home-applied ( DPHA ) bleaching product . MATERIAL S AND METHODS Eighty enamel slabs were obtained from extracted human teeth and r and omly divided into four treatment groups : ( 1 ) control ; ( 2 ) Opalescence ( Ultradent Products , Inc. , South Jordan , UT , USA ) ; ( 3 ) Crest Night Effects ( Procter & Gamble , Cincinnati , OH , USA ) ; and ( 4 ) Colgate Simply White Night ( Colgate-Palmolive Co. , Piscataway , NJ , USA ) . Opalescence is a carbamide peroxide DPHA product , whereas Crest Night Effects and Colgate Simply White Night are consumer-available products . The specimens in groups 2 to 4 underwent 2 weeks of treatment for 8 h/d . Specimens were maintained in artificial saliva at 37 degrees C between treatments . Subsequently , one-half of the specimens in groups 2 to 4 ( n=10 ) underwent an additional seven treatments for 8 h/d , while the other half were stored in artificial saliva , receiving no further treatment . Microhardness was measured as Knoop hardness numbers ( KHNs ) at baseline and after 1 , 7 , 14 , and 21 treatment days . The results were analyzed for statistical significance both intra- and intergroups using analysis of variance ( p=.05 ) . RESULTS A statistically significant reduction in mean KHN was observed compared with baseline at 1 , 7 , 14 , and 21 treatment days for group 4 and at 7 treatment days for group 3 . When compared with the control or DPHA product , group 4 was the only treatment that result ed in significantly lower mean KHNs at 7 , 14 , and 21 treatment days . CONCLUSION When evaluating enamel microhardness , consumer available , paint-on bleaching solutions may adversely affect enamel microhardness compared to a control and 10 % carbamide peroxide DPHA bleaching solution . CLINICAL SIGNIFICANCE With numerous consumer-available bleaching products on the market , it is crucial to be judicious in their selection and use . The results of this in vitro study showed that the consumer-available , paint-on bleaching solutions adversely affected enamel microhardness at some time during the study . Consumers should be made aware of this effect on enamel PURPOSE To evaluate the effect of tray-based and trayless tooth whitening systems on  surface and subsurface microhardness of human enamel . METHODS Enamel slabs were obtained from recently extracted human third molars . Specimens were r and omly assigned to six groups according to tooth whitening treatment ( n = 10 ) : 6.0 % hydrogen peroxide ( HP ) ( Crest Whitestrips ) , 6.5 % HP ( Crest Professional Whitestrips ) , 7.5 % HP ( Day White Excel 3 ) , 9.5 % HP ( Day White Excel 3 ) , 10 % carbamide peroxide ( Opalescence ) , and a control group ( untreated ) . Specimens were treated for 14 days following manufacturers ' recommended protocol s , and were immersed in artificial saliva between treatments . Enamel surface Knoop microhardness ( KHN ) was measured immediately before treatment , and at days 1 , 7 , and 14 of treatment . After treatment , subsurface microhardness was measured at depths of 50 - 500 microm . Data were analyzed for statistical significance using analysis of variance . RESULTS Differences in microhardness for treated vs. untreated enamel surface were not statistically significant at any time interval . For 6.5 % and 9.5 % HP , there was a decrease in surface microhardness values during treatment , but at the end of treatment the microhardness values were not statistically different from the baseline values . For the enamel subsurface values , no differences were observed between treated vs. untreated specimens at each depth . Trayless and tray-based tooth whitening treatments do not significantly affect surface or subsurface enamel microhardness This in situ study evaluated the microhardness of sound and demineralized enamel and dentin su bmi tted to treatment with 10 % carbamide peroxide for three weeks . A 10 % carbamide peroxide bleaching agent -- Opalescence/Ultradent (OPA)--was evaluated against a placebo agent ( PLA ) .  Two hundred and forty dental fragments--60 sound enamel fragments ( SE ) , 60 demineralized enamel fragments ( DE ) , 60 sound dentin fragments ( SD ) and 60 demineralized dentin fragments (DD)--were r and omly fixed on the vestibular surface of the first superior molars and second superior premolars of 30 volunteers . The volunteers were divided into two groups that received bleaching or the placebo agent at different sequences and periods at a double blind 2 x 2 crossover study with a wash-out period of two weeks . Microhardness tests were performed on the enamel and dentin surface . The SE and DE su bmi tted to treatment with OPA showed lower microhardness values than the SE and DE su bmi tted to treatment with PLA . There were no statistical differences in microhardness values for SD and DD su bmi tted to the treatment with OPA and PLA . The results suggest that treatment with 10 % carbamide peroxide bleaching material for three weeks alters the enamel microhardness , although it does not seem to alter the dentin microhardness OBJECTIVES The aim of this study was to evaluate the effects of low concentrations of carbamide peroxide bleaching agents ( CP ) on the ultimate tensile strength of enamel ( UTS ) . METHODS Resin composite structure was built-up on the bonded occlusal surface of 12 sound third molars to facilitate specimen preparation for microtensile testing . The bonded teeth were sectioned in a bucco-lingual direction in slices approximately 0.7 mm thick . Each slice was trimmed with a fine diamond bur to reduce the buccal , internal slope enamel of the cusps to a dumb-bell shape with a cross-sectional area at the ' neck ' of less than 1 mm2 . The sample was r and omly divided into six groups of 10 specimens each : Control ( C-untreated ) ; Opalescence 10 % ( O10 ) , Opalescence 15 % ( O15 ) , Opalescence 20 % ( O20 ) , Whiteness 10 % ( W10 ) and Whiteness 16 % ( W16 ) . Specimens in each group received the application of the CP for 6 h/day , at 37 degrees C , during 14 days and were stored in artificial saliva in between each application . After the bleaching regimen , specimens were tested with the microtensile method at 0.5 mm/min . Data were analyzed by ANOVA and Tukey test ( 5 % ) . RESULTS Mean tensile strengths were ( MPa ) : C = 47.5 + /- 6.2a ; O10 = 37.6 + /- 5.8b ; O15 = 33.2 + /- 6.0b ; O20 = 31.2 + /- 3.5b ; W10 = 32.5 + /- 6.1b ; W16 = 30.6 + /- 7.7b . Specimens subjected to the bleaching regimen presented significantly lower UTS than the control group . No differences were found among the type and concentration of the bleaching agents . SIGNIFICANCE Results suggested that bleaching enamel with CP could significantly reduce its UTS within a 14 days treatment
target: The results of this meta- analysis showed no significant changes in enamel microhardness when using the 10 % carbamide peroxide bleaching gel over periods of 7 , 14 and 21 days
background: Several studies have investigated the effect of bleaching on dental tissues .
  The evaluation of the effect of home bleaching with 10 % carbamide peroxide is important for assessing alterations in enamel microhardness that may affect dental health in terms of resistance to masticatory forces .
  This meta- analysis was performed in order to determine scientific evidence regarding the effects of home vital bleaching with 10 % carbamide peroxide gel on the microhardness of human dental enamel .