Evaluation of the effect of tooth polishing on color stability after in-office bleaching treatment

Polimento dentário; estética; clareamento dental. Abstract Introduction: One of the most commonly used corrective methods for staining teeth is tooth bleaching. However, subclinical alterations may occur in the micromorphology of tissues during the cola group. The difference was significant in the red wine group (p<0.022), but not in the coffee or cola group. Conclusion: Polishing after in-office bleaching using felt discs and polishing paste can reduce the degree of pigment impregnation in patient with red wine rich diet. Descriptors: Dental polishing; esthetics; tooth bleaching. The present study demonstrated the influence of pigmented substances on the color of bleached teeth. To Nogueira et al. 22 , the consumption of pigment drinks during the bleaching treatment can influence the speed of treatment. This surface is likely to be more susceptible to staining 15 . Côrtes et al. 4 also observed the staining of teeth during and after bleaching treatment when teeth are immersed in coffe and red wine. During the treatment, the remineralization of the enamel with artificial saliva were effective in preventing enamel staining 4 . However after the bleaching procedures, both stain therapies (coffee and red wine) caused enamel color changes. These drinks also have the ability to staining composite resins color changes after bleaching and immersion of bovine teeth in pigmented and observed less discoloration in the groups immersed in coffee compared to those with Coca Cola. findings agree with the present study in which the level of tooth discoloration (lower ∆L) was lower for samples immersed in coffee . This result may be related to the fact that acid beverages (soft drinks) have a greater demineralization potential and cause alterations in enamel morphology 9,15 . On the other hand, Liporoni et al. 8 , who evaluated the post-bleaching staining of teeth immersed in coffee and red wine 30 or 150 minutes after the bleaching procedure, found no difference between waiting times for either beverage. However, the bleached enamel was susceptible to staining with red wine at both time points, while it did not influence the bleaching process. The presence of ethanol in alcoholic beverages such as red wine may facilitate the binding of pigments to enamel group which polishing ∆L of 0.54. The group coffee without ∆L of 3.27. Color varia tion values greater than or equal to 3.3 show unacceptable clinical color changes results show that polishing after


INTRODUCTION
The desire for a harmonious smile appears to be guiding dentistry today. Stain-free, white, well-contoured and properly aligned teeth set the standard for beauty, health, social status, and success 1 . One of the most commonly used corrective methods for staining teeth is tooth bleaching 2 . However, subclinical alterations may occur in the micromorphology of dental tissues during the bleaching procedure, such as increases in porosity and surface roughness 3 . Consequently, dental enamel may become more permeable and susceptible to staining 4 .
The main bleaching agents used are hydrogen peroxide and carbamide peroxide 5 . Peroxides are compounds with a low molecular weight, which permits their penetration through the porosities of dental tissues 3 . Bleaching agents promote the oxidation of macromolecules in dentin stains and their subsequent breakage into smaller structures and diffusion towards the surface, causing bleaching of the tooth 6 .
An increase in the number of pores on the enamel surface is observed after bleaching treatment, which is accompanied by morphological modifications that are caused mainly by the temporal loss of calcium and phosphorus 7 . Changes in the protein layer of the enamel surface may also occur, a fact that renders this structure even more permeable to the penetration of fluids and consequently more susceptible to staining 8 . Within this context, instructions of not consuming chromogenic foods have been discussed in the literature in an attempt to evaluate their influence on the degree of tooth staining during and/or immediately after treatment 9,10 .
One possibility to reduce the problems caused by the use of peroxides in tooth bleaching techniques is to perform post-bleaching polishing 11 . Tooth staining after bleaching treatment are closely related to microstructural changes of the enamel surface 7 . Some studies report that the consumption of colored foods immediately after bleaching procedures can increase the staining of teeth because of the greater porosity and permeability of the tooth surface to the penetration of fluids 9,10 . However, few studies have investigated the use of polishing as tool to reduce the risk of discoloration post tooth bleaching. Therefore, the objective of the present study was to evaluate the influence of post-bleaching tooth polishing on color stability. The null hypotheses are: 1) that no significant color differences exist between teeth immersed in different pigmented substances, and 2) that tooth polishing does not significantly affect color changes after bleaching.

Study Design
This in vitro laboratory study was submitted and approved by the Ethics Committee on Research Involving Humans of the University of Pernambuco (Approval No. 2.504.102).

Sample Preparation
The sample size was determined based on previous similar studies 12 . Thirty-three human molars with healthy enamel extracted due to therapeutic reasons, which were previously stored in distilled water and frozen, were used. The teeth were cut longitudinally in the mesiodistal direction with a diamond disc in order to obtained two fragments of each tooth (total of 66). The roots were removed at approximately 4 mm from the cementoenamel junction, leaving only the coronary portion of each fragment. Before and after cuts, the teeth/samples were examined with a stereoscopic magnifying glass at 40x magnification to verify their integrity. Teeth/samples with enamel defects, cracks and/or fractures were excluded from the study. The samples were fixed in PVC cylinders with acrylic resin, with the enamel surface facing upward. To facilitate sample handling, seal dentin and prevent dentin staining.

Sample Allocation
The teeth were divided into six groups (n=11) by simple randomization. The samples were treated by only one operator and assessed by a blinded evaluator (Table 1).  Table 2 lists the materials used in the bleaching, polishing and staining procedures.

Color Measurement
The difference in the color of the samples before and after the bleaching treatment is expressed as the change in lightness, which is the optical property of teeth along the black-white axis in the CIELAB system. The following formula was applied to evaluate the presence of tooth bleaching or staining: ∆L = L* 1 -L* 0 (reading after bleaching minus reading before bleaching).
Objective color measurement was performed using the VITA Easy Shade digital spectrophotometer (Wilcos, Rio de Janeiro, Brazil) according to manufacturer's instructions. Two measurements were obtained per tooth at each time point and the mean was used for analysis. To standardize the position for analysis before and after bleaching, a matrix was vacuum fabricated with a 1-mm acetate plate and a hole corresponding to the tip diameter of the digital spectrophotometer (Easy Shade) was made on the buccal side.

Bleaching Procedures
Bleaching was performed with the same agent (Whiteness HP, FGM, Santa Catarina, Brazil) in all groups. The gel was applied to the surface of the samples according to manufacturer's recommendations (three 15-minute applications per session). Bleaching was performed in three sessions at intervals of 7 days between sessions. During the intervals, the samples were stored in artificial saliva (0.625% potassium chloride, 0.865% sodium chloride, 0.0558% magnesium chloride, 0.1662% calcium chloride, and 100ml distilled water). The saliva was changed weekly.

Polishing
In the coffee 13 , red wine and cola groups submitted to polishing, the samples were polished with felt discs (Diamond Flex, FGM, Santa Catarina, Brazil) and polishing paste (Diamond Excel, FGM, Santa Catarina, Brazil) after each bleaching session. The felt discs received a small amount (0.1g) of polishing paste and were pressed against the sample surface for 20 s under low rotation.

Immersion in Pigmented Substances
To simulate the oral condition during the consumption of colored beverages, the specimens were submitted to alternating cycles of immersion in colored beverages and artificial saliva. A complete immersion cycle consisted of the following steps: 1) 5 minutes in 3 mL of the potential staining solution (coffee, red wine, and cola-based soft drink); 2) rinsing with distilled water (3 mL), and 3) storage in artificial saliva for 4 hours. The cycle was repeated four times per day for 21 days.
Each sample was immersed in the solutions in individual glass tubes under gentle shaking at room temperature. The solution was changed at each cycle. During the intervals, the specimens were stored in artificial saliva.

Statistical Analysis
The data were entered into a Microsoft Excel spreadsheet, which was exported to the SPSS 18 software for analysis. To evaluate the degree of bleaching of the materials after the experiment, the mean change in lightness before and after treatment was calculated. The Shapiro-Wilk test was used to verify the normality of the data. Since the changes in lightness showed a normal distribution, the Student t-test was applied for comparison between the groups with and without polishing. The tested products were compared by ANOVA in the groups with or without polishing. All conclusions were drawn considering a level of significance of 5%. Table 3 shows the mean lightness change of the samples according to the tested product and use of polishing. In the groups submitted to polishing, a greater reduction in lightness (staining) was observed in the soft drink group (mean = -12.38), followed by the red wine (mean = -5.82) and coffee group (mean = -0.54). In the groups without polishing, the use of soft drink resulted in an even greater reduction of mean lightness than in the group submitted to polishing (mean = -16.46), followed by red wine (mean = -10.31) and coffee (mean = -3.27). ANOVA showed a significant difference between the groups submitted or not to polishing (p < 0.05 for both), indicating a relevant difference in the level of staining between the products evaluated. Comparison of the use or not of polishing revealed an increase in the lightness change of the samples for all tested products but the difference was only significant for the red wine group (p = 0.022), in which a higher value was observed for the group without polishing (mean = -10.31) compared to the group with polishing (mean = -5.82).

DISCUSSION
The first hypothesis that no difference exists in the staining potential of the pigmented substances was rejected (p<0.001). Coffee was found to cause less staining than the other colored solutions. The second hypothesis that post-bleaching polishing exerts no effect on the esthetic outcome was also rejected, since a statistically significant difference was observed in the red wine group (p<0.022).
The samples were immersed in highly pigmented substances (red wine, cola-based soft drink, and coffee). In the intervals between treatments, the samples were stored in artificial saliva containing calcium and phosphate ions to simulate the remineralization of bleached teeth 14 . According to Monteiro et al. 15 , saliva is an essential factor in the prevention of tooth staining. The artificial saliva can reverse the minerals loss of enamel and reduce the enamel porosity caused by bleaching treatment 4 . It is important to mention that although bleaching agents can induce alterations on the dental surface 16 , immersion in artificial saliva after bleaching showed a significant effect on enamel roughness reduction and the immersion for 2 h in is effective in preventing bleached enamel staining 4 .
A recent study demonstrated that surface treatments (2% neutral sodium fluoride, CPP-ACPF, PVP-containing rinse, and polishing with feltrum disks impregnated with aluminum oxide) were similar to artificial saliva and did not affect bleached enamel color maintenance 17 . These findings partially corroborate with the present study. In the Coffee and Coca Cola groups, polishing and only immersion in saliva were equivalent in preventing pigmentation. However, in the red wine group, polishing after tooth bleaching was statistically superior to the group just immersed in artificial saliva.
A digital spectrophotometer was used for color measurement since the perception of color by visual analysis is a subjective interpretation and varies among different observers. This variability is due to factors such as the object examined, lighting, metamerism, and fatigue, age and emotional state of the observers 18,19 . According to Liberato et al. 20 , the use of equipment for color measurement is a more accurate, reproducible and reliable method than human visual assessment. Furthermore, the Easy Shade® spectrophotometer has been used to improve the color reproduction of ceramic restorations in clinical practice and in the dental laboratory 21 .
The present study demonstrated the influence of pigmented substances on the color of bleached teeth. To Nogueira et al. 22 , the consumption of pigment drinks during the bleaching treatment can influence the speed of treatment. This surface is likely to be more susceptible to staining 15 . Côrtes et al. 4 also observed the staining of teeth during and after bleaching treatment when teeth are immersed in coffe and red wine. During the treatment, the remineralization of the enamel with artificial saliva were effective in preventing enamel staining 4 . However after the bleaching procedures, both stain therapies (coffee and red wine) caused enamel color changes. These drinks also have the ability to staining composite resins 18 .
Carlos et al. 9 evaluated color changes after bleaching and immersion of bovine teeth in pigmented substances (coffee and Coca Cola) and observed less discoloration in the groups immersed in coffee compared to those treated with Coca Cola. These findings agree with the present study in which the level of tooth discoloration (lower ∆L) was lower for samples immersed in coffee 9 . This result may be related to the fact that acid beverages (soft drinks) have a greater demineralization potential and cause alterations in enamel morphology 9,15 . On the other hand, Liporoni et al. 8 , who evaluated the post-bleaching staining of teeth immersed in coffee and red wine 30 or 150 minutes after the bleaching procedure, found no difference between waiting times for either beverage. However, the bleached enamel was susceptible to staining with red wine at both time points, while it did not influence the bleaching process. The presence of ethanol in alcoholic beverages such as red wine may facilitate the binding of pigments to enamel 23 .
The coffee-immersed group in which polishing was performed had a ∆L of 0.54. The group immersed in coffee without polishing had a ∆L of 3.27. Color variation values greater than or equal to 3.3 show unacceptable clinical color changes 13 . These results show that polishing after tooth bleaching, despite favoring minor changes in luminosity, is not superior to the group just immersed in saliva.
Hwang et al. 24 also observed that acidic and alcoholic beverages in the composition may accentuate the pigmentation of composite resin restorations. The authors state that the color change was enhanced by the presence of ethanol, increasing the pigment penetration in the resin. These results agree with the present study. The groups immersed in coca-cola and red wine showed higher values of decay of luminosity (Table 3).
Regarding the influence of post-bleaching polishing on color stability, the degree of tooth staining was lower (lower ∆L) in the groups submitted to this procedure. However, postbleaching polishing only exerted a significant effect in the red wine group (p<0.022). These findings disagree with those reported by Monteiro et al. 15 who found no statistically significant differences between teeth immersed in red wine for 15 minutes treated only with artificial saliva and those submitted to post-bleaching polishing.
In addition to improving the esthetic outcome, the smooth surface obtained by polishing prevents the formation of a pigmented biofilm and the retention of plaque because of the absence of microrugosities 25 . According to Freitas et al. 26 , polishing at the end of each bleaching session is aimed at reducing structural alterations caused by the bleaching agents, particularly minimizing surface roughness and consequently increasing surface gloss. In a study evaluating the surface roughness index, color and gloss of enamel of human central incisors bleached and polished using three different bleaching protocols, the authors obtained positive results in terms of reduced surface roughness and increased gloss when the teeth were polished.
One of the limitations of the present study is the difficulty in reproducing natural clinical conditions. This limitation is present in all in vitro studies. Randomized clinical trials and systematic reviews could provide further scientific evidence. However, clinical trials evaluating the influence of polishing after tooth whitening would be difficult to control variables and could subject the research participants to the risk of dental staining.

CONCLUSION
Polishing after in-office bleaching using felt discs and polishing paste reduced the degree of pigment impregnation in the groups immersed in red wine. In the Coffee and Coca-cola groups, polishing after tooth bleaching was not efficient in reducing pigmentation. Analyzing the patient's diet can be an important factor in choosing a therapeutic decision.