High-concentrated carbamide peroxide bleaching agents effects on enamel surface

Concern has been expressed regarding the adverse effects of peroxide-containing tooth bleaching agents on enamel surface. This study examined enamel average surface roughness before (baseline) and after an in-office bleaching protocol and investigated the influence of high concentrations of carbamide peroxide gels on its surface staining and morphology. Flat enamel surfaces were submitted to 35 and 37% carbamide peroxide or to no bleaching treatment (n = 10) and evaluated with a profilometer. Eight specimens from each group were randomly selected and immersed in a 2% methylene blue solution. Afterwards, specimens were ground into powder and prepared for the spectrophotometric analysis. Two remained specimens of each group were examined using a scanning electron microscope. Data were subjected to analysis of variance and Tukey test (P > 0.05). Baseline roughness average was statistically similar for all groups, however, 35% carbamide peroxide produced the roughest enamel surfaces. Different concentrations of carbamide peroxide produced similar staining means and enamel surface morphological alterations.     

Microcomputerised tomography evaluation of 10% carbamide peroxide applied to enamel

OBJECTIVES: There is still some controversy in the dental literature whether carbamide peroxide bleaching causes demineralization of teeth. One of the reasons for this controversy is that there is as yet no reliable, non-destructive in vitro method for assessing mineral loss in bleached teeth. The objective of this study was to investigate the possible demineralization effect of 10% carbamide peroxide bleaching agent on enamel and dentine non-destructively. METHODS: microCT images were obtained of 12 human molar tooth sections. These sections had 10% carbamide peroxide applied for eight hours a day over a period of 15 days. Further tomographic images were obtained and the mineral content prior to and post bleaching assessed. RESULTS: A total of 144 regions were evaluated using the image processing language available in the work station. The application of 10% carbamide peroxide was found to cause demineralization of the enamel extended to a depth of 50 microm below the enamel surface (Paired t-test, p<0.05). CONCLUSIONS: This study confirmed that microCT was indeed a highly suitable method for assessing mineral content of dental enamel after bleach application. It is recommended that application of bleaching agents should be carefully considered in patients susceptible to caries and tooth wear.     

Thirty-five percent carbamide peroxide application causes in vitro demineralization of enamel.

OBJECTIVES: The objective of this in vitro study was to investigate whether a high concentration 'in-office' bleaching agent affected the mineral content of enamel and dentin. METHODS: A commercially available 35% carbamide peroxide bleaching agent was applied for 2h to sectioned teeth (n=11). Specimens were then immersed in artificial saliva at 37 degrees C for a further 24h to simulate the oral environment. Tomographic images of these sections were obtained (micro-CT 80, Scanco, Switzerland) prior to and post-bleach application. Eight three-dimensional regions of interest (ROI), starting from the enamel surface extending to the dentinoenamel junction, were selected for each section. The hydroxyapatite equivalent mineral concentrations (g/cm(3)) of the ROIs were calculated. Any changes in mineral content as a consequence of the bleaching procedure were calculated in relation to each ROI. RESULTS: There was a significant reduction in the mineral content of enamel specimens post-bleach application extending to a depth of 250microm (paired t-test, p<0.05); this reduction in mineral content was greatest in the ROI's closest to the tooth surface. There was, however, no significant difference in the mineral content of dentin as a consequence of bleaching. SIGNIFICANCE: This in vitro study has shown that significant demineralization of enamel occurred following bleaching with 35% carbamide peroxide. The concept that 'in-office' bleaching is a non-destructive cosmetic procedure should be reconsidered.     

Effects of three concentrations of carbamide peroxide on the structure of enamel

Carbamide peroxide bleaching agents have raised important questions on their potential adverse effects on the structure of enamel. The purpose of this study was to examine the effects of three carbamide peroxide bleaching agents in different concentrations (10, 16 and 35%) on the structure of enamel. Forty enamel slabs prepared from human third molars were divided equally among four groups. The specimens in the first and second group were subjected to 10% or 16% carbamide peroxide for 8 h per day for 6 weeks. Thirty-five percent carbamide peroxide was applied to the third group for 30 min a day for a 4 day period according to the manufacturers recommendation. The fourth group served as the control and was kept in artificial saliva during the test period. At the completion of the treatment, all the specimens were examined by infrared absorbtion spectroscopy and X-ray diffraction analysis. The results revealed that 10% or 16% carbamide peroxide did not seem to effect the structure of enamel, whereas 35% carbamide did affect the structure. The use of lower concentrations of carbamide peroxide (10-16%) is recommended over higher concentrations (35%) to avoid changes to the enamel.     

The effect of whitening agents on caries susceptibility of human enamel

This in vitro study evaluated whether the treatment of human enamel with whitening agents containing different concentrations of carbamide or hydrogen peroxide changes the susceptibility of enamel to caries. Twenty-four sound human incisors were selected for this study. For each tooth, the crown was sectioned into two halves in the cervical-incisal direction. One half of the sectioned tooth was treated and the other half was used as a control specimen. Each half was randomly divided into three treatment groups (eight two-halves/group). The whitening agents were 10% carbamide peroxide, 20% carbamide peroxide with fluoride and 35% hydrogen peroxide. Following pretreatment, the specimens were demineralized for four days in an in vitro microbial caries model and then analyzed using a confocal laser scanning microscope (CLSM). Results showed that there were no significant differences between the treated and controlled specimens for teeth treated with 10% carbamide peroxide or 35% hydrogen peroxide. However, specimens treated with 20% carbamide peroxide with FP (0.11% fluoride and potassium nitrate) were less susceptible to caries than their controls at p < or = 0.05. In conclusion, application of bleaching agents does not increase the caries susceptibility of human enamel.     

The influence of two home-applied bleaching agents on enamel microhardness: an in situ study

OBJECTIVE: This in situ study evaluated the influence of two home-applied bleaching agents (10% carbamide peroxide and 7.5% hydrogen peroxide) on enamel microhardness. METHODS: Ninety enamel slabs were obtained from human third molars and baseline Knoop hardness measurements were recorded under a 50 g load for 5s. The specimens were attached to intra-oral devices delivered to 10 volunteers (donor of the teeth) who used the devices 24h/day throughout the experiment. The specimens were divided in three groups, two experimental and one control; in the last group, enamel slabs were not submitted to bleaching agents, but exposed only to the action of saliva. Two custom-made trays were made for each volunteer to recover the teeth and the appliances containing the enamel slabs. The bleaching regimen was 1h/day for 21 consecutive days. Subsequently, additional hardness measurements were recorded. RESULTS: The mean values of the baseline and final microhardness were: 348 and 352.2; 346.6 and 354.5; 342 and 340.8, for control, 10% carbamide peroxide and 7.5% hydrogen peroxide groups, respectively. One-way ANOVA of the microhardness mean values revealed no statistically significant differences among groups, the specimens treated with 7.5% hydrogen peroxide has shown a tendency (p=0.056) to microhardness decrease. CONCLUSIONS: Data suggested that the tested home bleaching agents did not change the superficial enamel microhardness.     

In vitro evaluation of the effect of delaying toothbrushing with toothpaste on enamel microhardness subsequent to bleaching the teeth with 15% carbamide peroxide

Changes in enamel surface microhardness as a result of bleaching with carbamide peroxide in various in vitro conditions have been reported. The present study evaluated the effect of oral hygiene procedures on enamel microhardness at three time intervals following bleaching with 15% carbamide peroxide. Although this was an in vitro study, the purpose was to address whether or not a patient's toothbrushing following at-home bleaching might affect surface changes in tooth enamel. Eighty enamel slabs were prepared from impacted human third molars that had been extracted surgically. Subsequent to placing the specimens in acrylic resin, their surfaces were smoothed, and they were randomly divided into four equal groups. The specimens were initially evaluated for microhardness by Vickers test. The bleaching procedure was carried out for 21 days for 6 hours daily. In each group, the surfaces of specimens were brushed with toothpaste immediately, 1 hour, and 2 hours after bleaching except for the control group. The specimens were stored in artificial saliva. Enamel microhardness was again measured at the end of the bleaching period. Then the differences in enamel microhardness between the two periods were calculated. Data were analyzed with a nonparametric Kruskal-Wallis test at a significance level of p<0.05. The differences in the microhardness values before and after intervention between the groups were not significant (p=0.59). Daily oral hygiene procedures either immediately or 1 or 2 hours after daily bleaching procedures and exposing the specimens to artificial saliva during the study period produced no significant differences in enamel microhardness values.     

Effect of peroxide bleaches on resin-enamel bonds.

Human third molar teeth were divided into three groups: untreated control; enamel treated with 35% hydrogen peroxide for 2 hours; and enamel treated with 10% carbamide peroxide gel for 14 days. All teeth were ground to present a flat enamel surface, to which cylinders of light-cured composite resin were bonded. Shear bond strengths were determined for each specimen. The mean shear bond strengths of resin-enamel bonds after pretreatment with both 35% hydrogen peroxide and 10% carbamide peroxide were significantly lower than those for untreated controls. External bleaching with these materials prior to resin bonding procedures may reduce the quality of resin-enamel bonds.     

Shear bond strength after dentin bleaching with 10% carbamide peroxide agents

This in vitro study evaluated the shear bond strength (SBS) of dentin treated with two 10% carbamide peroxide bleaching agents 15 days after bleaching and storage in artificial saliva. Dentin fragments were randomly divided into 3 groups (n = 20) for the treatment with the two different bleaching agents (Rembrandt 10% or Opalescence 10%) or with a placebo agent, applied to the tooth surface for 8 hours a day. During the remaining time, the specimens were stored in artificial saliva. After 42 days, the fragments were stored in artificial saliva for 14 days. Another group (n = 20) was exposed to distilled and deionized water for 56 days. An adhesive system and microhybrid composite resin were used to prepare specimens for the SBS test. SBS tests were performed and the fractured surfaces were visually examined using a stereoscope at 30 x magnification. The analysis of variance (ANOVA) and SIDAK tests showed higher SBS values for dentin treated with Opalescence 10% than for dentin treated with Rembrandt 10% or placebo. Groups treated with Rembrandt 10%, Opalescence 10% or placebo did not differ from the group treated with distilled and deionized water. Ten percent carbamide peroxide agents or a placebo agent caused no differences in SBS of dentin after 15 days of storage in artificial saliva.     

Surface changes and acid dissolution of enamel after carbamide peroxide bleach treatment

OBJECTIVES: To evaluate the effects of home bleaching (carbamide peroxide) on enamel surface morphology and the degree of acid dissolution. METHODS: Buccal surfaces of 15 caries-free human premolars were used in the study. The 15 teeth were cut in half in a buccal-lingual direction at midline; in total, 15 pairs of specimens were obtained. Group A consisted of five pairs that studied surface morphology change and Group B consisted of 10 pairs that studied the susceptibility of bleached enamel to acid dissolution. Tooth halves were prepared following a fluoride-free prophylaxis paste cleaning. One half of the tooth was untreated (control), and the other half was bleached (experimental) for eight hours daily for 10 days using 10% carbamide peroxide. Tooth samples were then stored in distilled water for seven days, after which scanning electron microscopy (SEM) was performed on Group A. Only halves from the same tooth were compared to rule out natural variations between teeth. Group B was etched with 37% phosphoric acid before being examined by SEM. The severity of the acid attack was graded from I to V, and the grades of each pair were compared. RESULTS: Results of the SEM observation showed that surface porosity slightly increased after bleaching, and more surface dissolution by phosphoric acid was seen with bleached, compared to unbleached, enamel.     

The effect of 10% carbamide peroxide, carbopol and/or glycerin on enamel and dentin microhardness

This study evaluated the effects of 10% carbamide peroxide, carbopol and glycerin and their associations on microhardness over time on enamel and dentin. Eight treatment agents were evaluated: a commercial bleaching agent containing 10% carbamide peroxide (Opalescence 10% Ultradent), 10% carbamide peroxide, carbopol, glycerin, 10% carbamide peroxide + carbopol, 10% carbamide peroxide + glycerin, carbopol + glycerin and 10% carbamide peroxide + carbopol + glycerin. Three hundred and twenty human dental fragments, 80 sound enamel fragments (SE), 80 demineralized enamel fragments (DE), 80 sound dentin fragments (SD) and 80 demineralized dentin (DD) fragments, were exposed to the treatment agents (n=10). These agents were applied onto the surface of the fragments eight hours a day for 42 days. After eight hours, they were washed from the dental fragment surfaces after five back-and-forth movements with a soft bristle toothbrush under distilled and deionized running water. During the remaining time (16 hours per day), the fragments were kept in individual vials in artificial saliva. After the 42-day treatment period, the specimens were kept individually in artificial saliva for 14 days. Knoop microhardness measurements were performed at baseline, after eight hours, and 7, 14, 21, 28, 35 and 42 days, and 7 and 14 days post-treatment (corresponding to 49 and 56 days after the initial treatment agent applications). The non-parametric Kruskal-Wallis analysis showed significant differences among the agents at each time interval, except at baseline for sound and demineralized enamel and dentin. For SE, SD and DD, there was a decrease in microhardness values during treatment with all agents. There was a tendency towards lower microhardness values after treatment with carbopol and its associations for sound tissues. DD showed low microhardness values during and after treatment with CP and its associations. For DE, there was an increase in microhardness values during treatment with all agents and in the post-treatment phase. The baseline microhardness values were not recovered during the 14-day post-treatment phase. Opalescence 10%, carbamide peroxide, carbopol, glycerin and their associations may change the microhardness of sound and demineralized dental tissues, even in the presence of artificial saliva.     

Brushing effect of abrasive dentifrices during at-home bleaching with 10% carbamide peroxide on enamel surface roughness

During tooth bleaching abrasive dentifrices might change the outer superficial enamel. The aim of this in vitro study was to evaluate the roughness of human enamel exposed to a 10% carbamide peroxide bleaching agent at different times and submitted to different superficial cleaning treatments. The study consisted of 60 sound human enamel slabs, randomly assigned to different treatment groups: G1--not brushed; G2--brushed with a fluoride abrasive dentifrice; G3--brushed with a non-fluoride abrasive dentifrice; and G4--brushed without a dentifrice. There were 15 enamel slabs per group. Slabs of molar teeth were obtained and sequentially polished with sandpaper and abrasive pastes. A perfilometer was used to obtain the mean of Ra value on the surface of each specimen to initial and experimental times. Bleaching was performed on the enamel surface for six hours daily. After that, each slab received a cleaning surface treatment and was stored in artificial saliva. Analysis of variance (ANOVA) and Tukey's HSD hoc analysis (alpha =0.05) revealed significant differences in roughness values over time for enamel bleached and treated with different superficial cleaning methods. G1 and G4 showed no significant differences in roughness over time, G2 and G3 showed a significant increase in the surface roughness values. This in vitro investigation showed the sole use of 10% carbamide peroxide did not alter the enamel surface roughness, but the cleaning treatments that employed the use of brushing with abrasive dentifrices resulted in a significant increase of enamel surface roughness.     

Effect of different bleaching techniques on enamel surface microhardness.

Conservative techniques for treatment of discolored human enamel include in-office bleaching with heat-activated 35% hydrogen peroxide, Nightguard vital bleaching with 10% carbamide peroxide, and enamel microabrasion with 18% hydrochloric acid. OBJECTIVES: In this study, these bleaching techniques were performed on 30 extracted teeth to evaluate their effects on microhardness of enamel surfaces. METHODS: The enamel surface microhardness measurements were performed 0, 24, 48, and 72 hours and 1 week after treatment. Paired t tests were performed in the statistical analyses. RESULTS: No changes were found on specimens treated with carbamide peroxide. There was a significant decrease in the surface microhardness of enamel after 0 and 24 hours when the specimens were treated with 18% hydrochloric acid (softening 85.7 and rehardening 99.4) or with 35% hydrogen peroxide (softening 85.7). However, after 72 hours, significant rehardening was observed in these groups (P <.001). CONCLUSION: According to the results, except Nightguard vital bleaching, all other techniques softened the enamel surface.     

Peroxide bleaching agent effects on enamel surface microhardness, roughness and morphology

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 samples were obtained from human third molars and randomly 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.     

牙齿漂白剂对正畸托槽粘接强度的影响

目的:评价三种牙齿漂白剂对自酸蚀粘接系统粘接正畸托槽粘接强度的影响。方法:将100颗人前磨牙均分为四组:空白对照组(组1)、38%过氧化氢漂白组(组2)、10%过氧化脲漂白组(组3)和20%过氧化脲漂白组(组4),漂白牙齿后即刻使用自酸蚀正畸粘接系统Transbond Plus SEP将托槽粘接在牙面上,测量各组的抗剪切粘接强度,评价其剩余粘接剂指数,对结果进行统计学分析。结果:空白对照组的抗剪切粘接强度高于各漂白组,组2的抗剪切粘接强度高于组3和组4,而组4的抗剪切粘接强度最低,其差异均具有统计学意义(P<0.05)。各组的剩余粘接剂指数差异均有统计学意义(P<0.05),剩余粘接剂指数最低者为空白对照组,最高者为20%过氧化脲组。结论:在使用自酸蚀粘接系统粘接正畸托槽前应禁止使用过氧化物漂白牙齿,特别是高浓度的过氧化脲。     

Effects of 10% carbamide peroxide on the enamel surface morphology: a scanning electron microscopy study

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.
Materials 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 brand of bleaching agent and duration of application time. However, these alterations revert to almost normal within 3 months.     

不同过氧化物对牙釉质结构和硬度的影响

目的:探究30%过氧化氢(hydrogen peroxide,HP)、22%过氧化脲(carbamide peroxide,CP)、3%过硼酸钠(sodium perborate,SP)对离体牙漂白后牙齿釉质表面结构和硬度的差异。方法:筛选正常人离体牙标本并随机分为A、B、C、D 4组,分别用生理盐水、30%过氧化氢、22%过氧化脲、3%过硼酸钠对4组牙齿进行漂白处理,最后对离体牙进行电镜扫描(×2000)和维氏硬度测定并进行统计分析。结果:实验组与对照组相比,扫描电镜显示: 30%过氧化氢和3%过硼酸钠组,其釉质表面呈散在的孔状,蜂窝状破坏,说明牙齿出现了不同程度的脱矿、溶解;22%过氧化脲组牙齿釉质表面结构基本完整,无明显脱矿和溶解现象;硬度测定结果显示:组间两两比较存在差异性(P＜0.05),漂白后4个组牙齿硬度大小是:对照组＞22%过氧化脲组＞3%过硼酸钠＞30%过氧化氢。结论:3种过氧化物漂白试剂中22%过氧化脲对牙釉质结构和硬度的影响最小。     

An in vitro comparison of the bleaching efficacy of 35% carbamide peroxide with established intracoronal bleaching agents

AIM: To evaluate the bleaching efficacy of 35% carbamide peroxide, 35% hydrogen peroxide and sodium perborate for intracoronal bleaching of root filled discoloured teeth. METHODOLOGY: Extracted premolars were artificially stained using whole blood then root canal treatment was performed. After obturation, a 2 mm intermediate base was placed 1 mm below the buccal amelo-cemental junction. Intracoronal bleaching was performed in 11 teeth per group, using either 35% carbamide peroxide gel (group CP), 35% hydrogen peroxide gel (group HP) or sodium perborate mixed with distilled water (group SP). The bleaching agents were replaced after 7 days. The shade of the teeth was evaluated at day 0, 7 and 14. The results were analysed using Kruskal-Wallis one-way analysis of variance and Mann-Whitney U-test. RESULTS: At the end of 7 days, both groups CP and HP lightened by 8 +/- 3 Vita tab positions, respectively, whereas group SP lightened by 5 +/- 3 tab positions (P < 0.05). At the end of the second bleaching period at day 14, group CP and HP lightened by a further 2 +/- 2 and 2 +/- 3 tab positions, respectively, whereas group SP lightened by a further 3 +/- 4 tab positions. There were no statistical differences between groups at day 14. CONCLUSIONS: Thirty-five per cent carbamide peroxide and 35% hydrogen peroxide were equally effective for intracoronal bleaching, and significantly better than sodium perborate after 7 days. After 14 days, there were no significant differences between the groups. Thirty-five per cent carbamide peroxide can be recommended as an equally effective alternative to hydrogen peroxide for intracoronal bleaching.     

Evaluation of the bleached human enamel by Scanning Electron Microscopy

Since bleaching has become a popular procedure, the effect of peroxides on dental hard tissues is of great interest in research. Purpose: The aim of this in vitro study was to perform a qualitative analysis of the human enamel after the application of in-office bleaching agents, using Scanning Electron Microscopy (SEM). Materials and Methods: Twenty intact human third molars extracted for orthodontic reasons were randomly divided into four groups (n=5) treated as follows: G1- storage in artificial saliva (control group); G2- four 30-minute applications of 35% carbamide peroxide (total exposure: 2h); G3- four 2-hour exposures to 35% carbamide peroxide (total exposure: 8h); G4- two applications of 35% hydrogen peroxide, which was light-activated with halogen lamp at 700mW/cm2 during 7min and remained in contact with enamel for 20min (total exposure: 40min). All bleaching treatments adopted in this study followed the application protocols advised by manufacturers. Evaluation of groups submitted to 35% carbamide peroxide was carried out after two time intervals (30 minutes and 2 hours per session), following the extreme situations recommended by the manufacturer. Specimens were prepared for SEM analysis performing gold sputter coating under vacuum and were examined using 15kV at 500x and 2000x magnification. Results: Morphological alterations on the enamel surface were similarly detected after bleaching with either 35% carbamide peroxide or 35% hydrogen peroxide. Surface porosities were characteristic of an erosive process that took place on human enamel. Depression areas, including the formation of craters, and exposure of enamel rods could also be detected. Conclusion: Bleaching effects on enamel morphology were randomly distributed throughout enamel surface and various degrees of enamel damage could be noticed. Clinical significance: In-office bleaching materials may adversely affect enamel morphology and therefore should be used with caution.     

Analysis of photoreflectance and microhardness of the enamel in primary teeth submitted to different bleaching agents

Treatment of darkened teeth in children is of great importance from an esthetic-functional point of view and for the psychoemotional development of the child. The objective of the present study was to determine the in vitro efficacy of three bleaching agents for whitening of artificially stained primary teeth. Fifty anterior primary teeth were artificially stained and then divided into three experimental groups (n = 15) submitted to bleaching treatment with 35% hydrogen peroxide gel, 35% carbamide peroxide gel, and 35% carbamide peroxide gel mixed with sodium perborate powder. The control group (n = 5) was not submitted to any bleaching treatment. Color changes were evaluated with a reflectance spectrophotometer and possible alterations in the enamel surface after bleaching were measured by Vickers microhardness testing. The data were assessed using the Student's t test. The results confirmed the bleaching action of the three agents tested. No significant difference in mean microhardness was observed between the three bleaching agents when compared to the control group.