Evaluation of pH Levels and Surface Roughness After Bleaching and Abrasion Tests of Eight Commercial Products

Abstract

Objective: This in vitro study evaluated the effect of different bleaching protocols and the variation of pH levels of bleaching gels regarding roughness and wear of bovine enamel, after in-office bleaching protocols and brushing. Materials and methods: Ninety fragments were randomly divided into nine groups: C, control; WHP15, 35% hydrogen peroxide (HP) (Whiteness HP, FGM) three gel applications of 15 min each, three sessions with 1 week intervals; WHP45, 35% HP (Whiteness HP) one application/45 min, three sessions with 1 week intervals; LPS, 35% HP (Lase Peroxide, DMC) plus hybrid light (HL) [light-emitting diode (LED)/diode laser], four applications/7 min 30 sec (6 min of HL activation), one session; LPSII, 25% HP (Lase Peroxide II, DMC) plus HL, four applications/7 min 30 sec (6 min of HL activation), one session; LPL, 15% HP (Lase Peroxide Light, DMC) four applications/7 min 30 sec (6 min of HL activation), one session; WO, 35% HP (Whitegold Office, Dentsply) three applications/15 min, three sessions with 1 week intervals; WBC40, 35% HP (Whiteness HP Blue Calcium, FGM) one application/40 min, three sessions with 1 week intervals; and WBC50, 20% HP (Whiteness HP Blue Calcium) one application/50 min, three sessions with 1 week intervals. The median pH values were determined utilizing a pH meter during the initial and final gel applications. A rugosimeter was utilized to evaluate the surface roughness (R_a) before and after bleaching and brushing (100,000 strokes), and the surface wear after brushing. Results: For the results of the pH values, there was a decrease in the pH levels from the initial to the final bleaching time, except for the WBC50. The WO and WBC40 groups exhibited higher pH values. For the results of roughness and wear, there was an increase in surface roughness and wear among the groups. Conclusions: The pH values tended to decrease from the initial to the final bleaching. After tooth brushing, bleaching procedures with lower pH products provided a significant increase in enamel wear and surface roughness.

Introduction

With the development of new bleaching techniques, agents, light sources, and methods of activation, a variety of bleaching agents have been available in concentrations of 10–38% hydrogen peroxide (HP) gels, at low and high concentrations, with different consistencies, activated by light sources or not, with specific characteristics and indications. These photosensitive agents can be activated by different light sources, such as halogen, plasma arc, light-emitting diode (LED), and laser equipment.¹ Some authors have questioned whether light sources actually catalyze the oxidation reaction and contribute to produce better results than home bleaching.²

Despite all the developments in bleaching materials, there is still concern over the effects on the enamel surface after the bleaching treatment. The effects usually noted when home bleaching agents are used in low concentrations include shallow depressions, increased porosity, and mild erosion, whereas higher concentrations of whitening agents may cause morphological alterations of the enamel surface.^3,4

However, when using high-concentration whitening agents, some authors did not notice morphological changes to the enamel surface but saw greater susceptibility of the enamel surface to wear, and coarsening of the teeth through abrasion.^3,4 The pH levels of the bleaching gels are related to the changes in the surface morphology of the enamel.

Recent studies pointed out the relation to the time application of bleaching gels on the tooth surface roughness and wear. Matis and others⁵ demonstrated that three 15 min applications were more effective than a single 40 min application; on the other hand, Mondelli and others⁴ showed that roughness was greater for the at-home bleaching group than for the group that received in-office bleaching with a hybrid light source (that is, the group with reduced exposure to the bleaching gel, which produced less damage to the tooth surface).

As seen, the data relating to changes in the dental tissues are conflicting, depending upon the variety of methodologies used and the diversity of bleaching agents, time of applications, concentrations, light activations, pH levels of the bleaching agents, and trademarks used.⁶

The morphological problems in the enamel surface after bleaching are still controversial; therefore, a large number of studies have investigated the source and maintenance of these changes.⁷ Nevertheless, there are few published articles regarding the effects of pH level values. The aim of this study was to investigate the wear, surface roughness of the enamel, and pH level of different concentrations of bleaching gels, activated physically by hybrid light sources or not (LED/diode laser). The first null hypothesis of the study found that the differences in pH values and bleaching gel concentrations did not influence the roughness, and the second found that the differences in pH values and bleaching gel concentrations did not influence the wear of the enamel.

Materials and Methods

Materials

The materials and equipment used for the present study are outlined in Table 1.

Table 1.

Materials, Equipment and Manufactures Used

Material/manufacturer	Description	Groups
Whiteness HP Maxx (FGM Produtos, Joinville, SC, Brazil)	35% Hydrogen peroxide gel	WHP45 and WHP15
Lase Peroxide Sensy (DMC Equipamentos Ltda., São Carlos, SP, Brazil)	35% Hydrogen peroxide gel	LPS
Lase Peroxide Sensy II (DMC Equipamentos Ltda., São Carlos, SP, Brazil)	25% Hydrogen peroxide gel	LPSII
Lase Peroxide Lite (DMC Equipamentos Ltda., São Carlos, SP, Brazil)	15% Hydrogen peroxide gel	LPL
Whitegold Office (Dentsply Ind. Com., Petrópolis, RJ, Brazil)	35% Hydrogen peroxide gel	WO
Whiteness HP Blue Calcium (FGM Produtos, Joinville, SC, Brazil)	35% Hydrogen peroxide gel	WBC40
Whiteness HP Blue Calcium (FGM Produtos, Joinville, SC, Brazil)	20% Hydrogen peroxide gel	WBC50
Ultrablue IV (hybrid light) (DMC Equipamentos Ltda., São Carlos, SP, Brazil)	19 LEDs: 460–80 nm;	—
	73 mW/cm²;
	Diode laser: 830 nm;
	200 mW/cm²

Specimen preparation

A total of 90 recently extracted bovine mandibular central incisors, stored at 4°C in a solution of 1% chloramine-T, were chosen for the study. They were cut into rectangular shapes, using a low-speed diamond saw under cooling water (South Bay Technology Inc., San Clemente, CA) and embedded in slow-setting composite resin (15×5×4 mm). A flat enamel surface was produced by wet-polishing with 320, 400, 600, and 1200 grit silicon carbide abrasive papers. To control the individual variation in the enamel surface characteristics among the tested teeth, each specimen was divided into two sides. One side was used for treatment and the other as a control.⁴ They were stored in deionized water at 37°C for 1 week prior to testing (Table 2).

Table 2.

Groups and Bleaching Protocols

Groups	Description	Bleaching protocol	Total time
C	Control	Artificial saliva	——
WHP15	35% H₂O₂	3 sessions (3 gel applications×15 min=45 min)	135 min gel application
WHP45	35% H₂O₂	3 sessions (1 gel application×45 min=45 min)	135 min gel application
LPS	35% H₂O₂	1 session (4 gel applications×7 min 3 sec) (30 sec int.+2 min HL+30 sec int.+2 min HL+30 sec int.+2 min HL)	30 min gel application (24 min of light activation)
LPSII	25% H₂O₂	1 session (4 gel applications×7 min 30 sec) (30 sec int.+2 min HL+30 sec int.+2 min HL+30 sec int.+2 min HL)	30 min gel application (24 min of light activation)
LPL	15% H₂O₂	1 session (4 gel applications×7 min 30 sec) (30 sec int.+2 min HL+30 sec int.+2 min HL+30 sec int.+2 min HL)	30 min gel application (24 min of light activation)
WO	35% H₂O₂	3 sessions (1 gel application×45 min=45 min)	135 min gel application
WBC40	35% H₂O₂	3 sessions (1 gel application×40 min=40 min)	120 min gel application
WBC50	20% H₂O₂	3 sessions (1 gel application×50 min=50 min)	150 min gel application

HL, hybrid light.

Bleaching protocols

The prepared teeth were randomly divided into the following nine groups (n=10) to receive the bleaching procedures: Control group, only artificial saliva; WHP15 group [35% hydrogen peroxide (HP) (Whiteness HP, FGM)], three sessions with 1 week intervals (three gel applications×15 min=45 min) 135 min gel application; WHP45 group [35% hydrogen peroxide (HP) (Whiteness HP, FGM)], three sessions with 1 week intervals (three gel applications×15 min=45 min) 135 min gel application; LPS group [35% HP (Lase Peroxide, DMC)], one session (four gel applications×7 min 3 sec) [30 sec interval+2 min hybrid light (HL)+30 sec interval+2 min HL+30 sec interval+2 min HL] 30 min gel application (24 min of light activation); LPSII group [25% HP (Lase Peroxide II, DMC)], one session (four gel applications×7 min 3 sec) (30 sec interval+2 min HL+30 sec interval+2 min HL+30 sec interval+2 min HL) 30 min gel application (24 min of light activation); LPL group [15% HP (Lase Peroxide Lite, DMC)], one session (four gel applications×7 min 3 sec) (30 sec interval+2 min HL+30 sec interval+2 min HL+30 sec interval+2 min HL) 30 min gel application (24 min of light activation); WO group [35% HP (Whitegold Office, Dentsply)], three sessions with 1 week intervals (one gel application×45 min=45 min) 135 min gel application; WBC40 group [35% HP (Whiteness HP Blue Calcium, FGM)], three sessions with 1 week intervals (one gel application×40 min=40 min) 120 min gel application; and WBC50 group [20% HP (Whiteness HP Blue Calcium)], three sessions with 1 week intervals (one gel application×50 min=50 min) 150 min gel application as shown in Table 2. The bleaching agents were manipulated and applied according to the manufacturer's instructions. For the new bleaching gel applications, the specimens were cleaned with deionized water until the complete removal of the gel and dried with absorbent paper. The groups were submitted to one and three sessions with 7 day intervals. No flouride gel application was applied after the bleaching procedure, to avoid influencing the surface roughness. The specimens were placed in artificial saliva at 37±1°C, specifically formulated for the remineralization of the dental hard tissues, and was changed daily.

Measurement of pH levels of bleaching gels

To analyze the pH levels of the bleaching gels, a portable pH meter was used with a digital display (Model 1001 Sentron, Sentron, WA). This device has an electrode of small dimensions, compatible with the size of the specimen. It is capable of making a reading accuracy of 0.01 in a few seconds and with minimal waste of the bleaching gel. Prior to the beginning of each group of readings, the calibration of the pH meter was performed with two standard solutions (pH 4.0 and 7.0). The initial and final pH levels of the bleaching gels were obtained from the average of the readings taken for each gel application, regardless of the number of applications or the number of sessions in each group assessed. The first reading was performed 30 sec after the contact of the bleaching gel with the bovine enamel and the second reading after 30 sec of the end of the time employed, with or without activation with a hybrid light source.

Baseline surface profile and roughness (Ra) measurements

Each specimen was gently dried with absorbent paper, and the surface roughness was analyzed with a roughness meter (Hommel Tester T1000, Hommel-Etamic, Schwenningen, Germany) and the Turbo Datawin software (Hommel-Etamic, Schwenningen, Germany). Multidirectional readings were made from the center of the surface of each specimen for up to three readings that then provided the mean results before and after bleaching and after brushing. The parameter used for the surface roughness calculation was Ra (μm),⁴ which indicated the arithmetic mean of all absolute distances of the roughness profile.

The chosen parameters of roughness were: T minimum=0.01 μm, T maximum=8.00 μm (T=Tolerance [extreme values to be considered for readings], trail limit (Lt) (real extension travelled by the tip)=5.0 mm, Measuring limit (Lm) (extension considered for readings)=4.5 mm, cutoff (Lc) (filtering, minimizing the interference of surface ripple)=0.25 mm.

Brushing abrasion

Half of each specimen's top surface was subjected to a toothbrush abrasion test. The other half served as a reference surface and was covered with a polyethylene film to avoid contact with the toothbrush and abrasives.⁴ The test was performed on 10 specimens with a tooth-brushing machine at a controlled temperature of 37±2°C. A load of 300 g with soft nylon bristles (Oral B 35, Gillette do Brasil, Brazil) was applied to the samples. Slurry was prepared by mixing 2:1 of deionized water and Colgate MFP (Colgate Palmolive, SP, Brazil) dentifrice by weight, immediately before testing. One hundred thousand brushing strokes were performed for each specimen. After testing, they were rinsed under deionized water and cleaned in a sonic device in deionized water for 10 min, and then stored in deionized water (37°C).

Final roughness and wear measurements

The final measurements of the roughness after brushing were obtained with the same protocols utilized before. Wear was measured (μm) utilizing the same roughness meter in the perfilometer function. The average depth obtained after three readings was taken for each specimen from the reference surface (control side), across the abraded area. Wear measurements (μm) were obtained from the difference between the control and abraded sides utilizing the Turbo Datawin software (Hommel-Etamic, Schwenningen, Germany).

The parameters used were: T minimum=8.0 μm, T maximum=40.0 μm, Lt=10.0 mm, Lm=9.0 mm, Lc=0.00 mm

Statistical analysis

The results were presented as the mean and standard deviation. One way ANOVA and Tukey's test were used to compare the pH levels and changes among the control group and within the same experimental systems. In relation to wear and surface roughness, the Kruskal–Wallis and Dunn's tests were later utilized for comparisons between the individual groups. In all tests, we used a 5% significance level (p<0.05).

Results

pH values

The averages of the readings were recorded in the initial and final bleaching times of each group by a digital pH meter.

Within the results of the pH values, there was a tendency toward a decrease in the pH levels from the initial to the final bleaching time, with the exception of the WBC50 group, which showed no statistically significant differences. The WO and WBC40 groups exhibited higher values than in the initial means and an increase in the pH level at the end of the bleaching procedure (Table 3).

Table 3.

Means (SD) of the Comparison Between the Initial and Final pH Levels

Groups	Initial pH±SD	Final pH±SD
C	——	——
WHP15	6.251±0.060^{A e}	5.781±0.045^{B f}
WHP45	6.090±0.056^{A f}	5.630±0.105^{B f}
LPS	7.420±0.131^{A d}	6.580±0.286^{B e}
LPSII	7.540±0.051^{A d}	6.920±0.147^{B d}
LPL	8.380±0.122^{A b}	7.990±0.137^{B c}
WO	7.430±0.105^{A d}	7.840±0.126^{B c}
WBC40	8.130±0.125^{A c}	8.400±0.176^{B b}
WBC50	8.920±0.091^{A a}	8.880±0.103^{A a}

Different capital letters indicate significant differences between the bleaching times.

Different lower case letters indicate significant differences among the groups.

Roughness and wear

The data found in the assessment of the surface roughness of the bovine enamel after bleaching and after brushing (Table 4), indicated that there was an increase in surface roughness with statistically significant differences among the groups (initial and after bleaching, F=45.82, p<0.05; between the bleaching and brushing F=53.87, p<0.05).

Table 4.

Mean Values (μm) and Standard Deviation (SD) of Roughness and Wear (μm)

Groups	Initial roughness, mean±SD (T₁)	Roughness after bleaching, mean±SD (T₂)	Roughness difference, mean±SD (T₂ – T₁)	Roughness after brushing, mean±SD (T₃)	Roughness difference, mean±SD (T₃ – T₂)	Wear, mean±SD
C	0.304±0.082	0.256±0.061	0.048±0.042^A	0.400±0.072	−0.143±0.030^AB	9.150±0.669^ab
WHP15	0.245±0.052	0.539±0.228	−0,296±0.213^CD	1.152±0.143	−0.612±0.254^BC	14.790±1.971^c
WHP45	0.294±0.109	0.382±0,182	−0.085±0.091^ABCD	1.262±0.123	−0.879±0.212^B	11.780±1.675^bc
LPS	0.307±0.086	0.787±0.256	−0.479±0.314^CD	0.723±0.159	0.064±0.237^A	10.420±2.292^abc
LPSII	0.391±0.056	0.581±0.111	−0.190±0.103^BCD	0.623±0.185	−0.030±0.145^A	9.090±0.738^ab
LPL	0.363±0.044	0.707±0.203	−0.343±0.217^C	0.714±0.147	−0.007±0.157^A	9.301±1.200^ab
WO	0.268±0.056	0.302±0.076	−0.033±0.046^ABD	0.409±0.061	−0.107±0.067^AC	8.661±0.589^a
WBC40	0.252±0.117	0.414±0.235	−0.162±0.277^ABCDE	0.519±0.130	−0.105±0.123^A	9.078±0.847^ab
WBC50	0.314±0130	0.344±0.221	−0.020±0.107^ABE	0.513±0.127	−0.179±0.115^ABC	9.872±1.475^abc

Different capital letters indicate significant differences between initial and after-bleaching roughness (F=45.82, p<0.05); and between the bleaching and brushing (F=53.87, p<0.05).

Different lower case letters indicate significant differences in wear among the groups evaluated (F=39.63, p<0.05).

With regard to the wear, an increase was seen after brushing for all groups evaluated, with statistically significant differences between them (F=39.63, p<0.05) (Table 4).

Discussion

Many bleaching products are formulated at lower pH values to ensure the stability of the HP.⁸ Therefore, the pH value of the bleaching agents is an important factor in the reactions of the bleaching process.⁸ Verifying the results obtained in this study through analyses of the pH level of bleaching agents, it was observed that there was a trend toward a decrease in pH levels from the initial times to the end. These results are in accordance with the study of Price et al.⁹ who claimed that HP products can display a low pH value. These findings are in accordance with the results of the present study with the exception of the WBC50 group, which did not provide statistical differences proving that the pH levels were not affected. In the WO and WBC40 groups, the final averages exhibited higher values than the initial values, with an increase from the initial pH levels to the end.

It has been reported that the greater the HP concentration, the more acidic the pH level of the bleaching product.^9,10 Cadenaro et al.,⁷ and Alexandrino et al.,¹¹ reported that the use of an HP-based gel capable of 7.0 pH maintenance did not cause morphological changes over the specimen's surface. Unfortunately, there are no studies that relate the change in the pH level in direct contact with the substrate. According to Cadenaro and others,⁷ the results found in this study are associated with the pH level of HP, which generally displays values pH of 7.0–7.5. However, the WHP15 and WHP45 groups had the lowest pH values from start to end when compared with the other groups studied. According to Sun and others,¹² 30% neutral HP (pH≈7.0) resulted in less deleterious effects than acidic HP (pH≈3.6), and also had the same efficiency of tooth bleaching.

When comparing the surface roughness of different groups after the bleaching process, it was noted that groups bleached with 35% HP (WHP15 and WHP45) showed statistically larger roughness values when compared with the other groups studied, because of the low pH values found in 35% HP. These results were found by Mondelli and others,⁴ who related greater changes to the enamel surface treated with the bleaching agent at 35%.

The LPS, LPSII, and LPL groups, bleached with 35%, 25%, and 15% HP, respectively, did not show significant changes after the bleaching procedure. This fact is explained by the LED/laser-based systems that heat up the whitening agent and not the tooth structure, thereby avoiding deleterious effects on the enamel surface.¹³ In addition, there was reduced contact with the enamel with hybrid light activation, for a total of 30 min, against the 135 min application in groups WHP15 and WHP45. This observation is in agreement with the results of Mondelli et al.,¹⁴ who claimed that the hybrid light-assisted photoactivation bleaching provided a procedure with reduced clinical time and lower sensitivity levels. A light source is useful to activate the bleaching procedure for low-HP bleaching gels, because the increase in the formation of hydroxyl radicals compensates for the low concentration of the gel.¹⁵ Other studies also observed that some whitening protocols are safe in the clinical situation with regard to pulpal temperature, as well as being efficient with LEDs, diode lasers, and halogen lamps as irradiation sources. There was no evidence of deleterious effects of a light source on the integrity of the enamel after the bleaching process.¹⁶ Thus, a low concentration of hydrogen peroxide solution containing titanium dioxide nanoparticles proves to be safe and has a strong bleaching effect.¹⁷ Light activation with bleaching procedures have also been studied, with positive results. ^10,18 The first null hypothesis was rejected because the differences in pH values and bleaching gel concentrations did not influence the superficial roughness.

During bleaching, enamel demineralization is associated with the mechanism of oxidation, the pH of the peroxides, and some components of the whitening agents.¹⁹ The formation of free radicals occurs during the oxidation of peroxide. These free radicals act nonspecifically and are able to degrade both the organic and inorganic matrix of the substrate, causing enamel loss and affecting the ionic balance that would enable the deposition of calcium on the enamel surface.¹⁹

The roughness values after treatment and bleaching in this study simulating brushing showed that the abrasive procedure altered the roughness of all groups, including the control group that did not receive any bleaching treatment. All groups presented greater roughness values than the control group, and the groups treated with 35% HP (WHP15 and WHP45) exhibited the greatest roughness values when compared with the other groups. These results are in accordance with Mondelli and others,⁴ who related changes to the enamel surface treated with a 35% HP bleaching agent activated with HL and halogen light sources. The second null hypothesis was accepted because the differences in pH values and bleaching gel concentrations did not influence the superficial wear of the enamel.

A limitation of most of the commercial bleaching agents is that they are manufactured in gel form, which may contain some added ingredients such as carbomer, glycerin, fluoride, metal salts, and flavors to aid in the manipulation of the gel, and it is necessary to involve these factors in future studies to investigate the effects of HP bleaching agents on the superficial enamel.¹² Therefore, the association between bleaching and acid exposure and abrasive challenges should be further studied in order to better understand the mechanisms involved. However, in a clinical situation, in addition to the bleaching procedure, other challenges might be presented in the oral cavity, such as erosion, abrasion, and cariogenesis, which might play an important role together with the bleaching procedure, leading to enamel surface alterations.²⁰

Conclusions

Within the limitations of this in vitro study, the following conclusions can be drawn:

• Bleaching gels have a tendency to decrease the pH values from the initial time to the final treatment time, except for the groups treated with 35% Whitegold Office (WO) and 35% Whiteness HP Blue Calcium (WBC40).

• Bleaching gels that exhibited acidic pH values lead to changes in roughness and surface wear of the bovine enamel, especially in groups treated with Whiteness HP Maxx (WHP15 and WHP45) after a toothbrush abrasion test.

Footnotes

Acknowledgments

The authors thank the funding agency Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for the financial support (Protocol no. 2009/04574-6).

Author Disclosure Statement

No competing financial interests exist.

References

Buchalla

, Attin

. External bleaching therapy with activation by heat, light or laser – a systematic review. Dent Mater, 2007; 23:586–596.

Briso

ALF

, Fonseca

MSM

, Almeida

LCAG

, Mauro

, Santos

. Color alteration in teeth subjected to different bleaching techniques. L Physics, 2010; 20:2066–2069.

Leonard

Jr , Eagle

, Garland

, Matthews

, Rudd

, Phillips

. Nightguard vital bleaching and its effect on enamel surface morphology. J Esthet Rest Dent, 2001; 13:132–139.

Mondelli

RFL

, Azevedo

JFDG

, Francisconi

PAS

, Ishikiriama

, Mondelli

. Wear and surface roughness of bovine enamel submitted to bleaching. Eur J Esthet Dent, 2009; 4:396–403.

Matis

, Yousef

, Cochran

, Eckert

. Degradation of bleaching gels in vivo as a function of tray design and carbamide peroxide concentration. Oper Dent, 2002; 27:12–18.

Spalding

, Taveira

, Assis

. Scanning electron microscopy study of dental enamel surface exposed to 35% hydrogen peroxide: alone, with saliva, and with 10% carbamide peroxide. J Esthet Restor Dent, 2003; 15:154–164.

Cadenaro

, Breschi

, Nucci

, et al. Effect of two in-office whitening agents on the enamel surface In Vivo: a morphological and non-contact profilometric study. Oper Dent, 2008; 33:127–134.

, Li

, Wang

. Effects of pH values of hydrogen peroxide bleaching agents on enamel surface properties. Oper Dent, 2011; 36:554–562.

Price

RBT

, Sedarous

, Hiltz

. The pH of tooth-whitening products. J Can Dent Assoc, 2000; 66:421–426.

10.

Mondelli

RFL

, Garrido Gabriel

TRC

, Piola Rizzante

, Magalhães

, Soares Bombonatti

, Ishikiriama

. Do diferent bleaching protocols affect the enamel microhardness?. Eur J Dent, 2015; 9:25–30.

11.

Alexandrino

, Gomes

, Alves

, Costi

, Rogez

, Silva

. Effects of a bleaching agent with calcium on bovine enamel. Eur J Dent, 2014; 8:320–325.

12.

Sun

, Liang

, Sa

, et al. Surface alteration of human tooth enamel subjected to acidic and neutral 30% hydrogen peroxide. J Dent, 2011; 39:686–692.

13.

Garber

. Dentist-monitored bleaching: a discussion of combination and laser bleaching. J Am Dent Assoc, 1997; 128:26–30.

14.

Mondelli

, Azevedo

, Francisconi

, Almeida

, Ishikiriama

. Comparative clinical study of the effectiveness of different dental bleaching methods - two year follow-up. J Appl Oral Sci, 2012; 20:435–43.

15.

Kossatz

, Dalanhol

, Cunha

, Loguercio

, Reis

. Effect of light activation on tooth sensitivity after in-office bleaching. Oper Dent, 2011; 36:251–257.

16.

Domínguez

, García

, Costela

, Gómez

. Influence of the light source and bleaching gel on the efficacy of the tooth whitening process. Photomed Laser Surg, 2011; 29:53–59.

17.

Suemori

, Kato

, Nakazawa

, et al. Effects of light irradiation on bleaching by a 3.5% hydrogen peroxide solution containing titanium dioxide. Laser Phys Letters, 2008; 5:379–383.

18.

Wang

, Zhu

, Li

, Liao

, Ai

. Efficacy of cold light bleaching using diferente bleaching times and their effects on human enamel. Dent Mater J, 2013; 32:761–766.

19.

Borges

, Pinheiro

, Feitosa

, et al. Preliminary study of a novel in-office bleaching therapy modified with a casein phosphopeptide-amorphous calcium phosphate. Microsc Res Tech, 2012; 75:1571–1575.

20.

Salomão

, Santos

, Nogueira

, Palma–Dibb

, Geraldo–Martins

. Acid demineralization susceptiblity of dental enamel submitted to diferente bleaching techniques and fluoridation regimes. Oper Dent, 2014; 39:E178–E185.