Diode Laser Effect on Enamel Microhardness After Dental Bleaching Associated with Fluoride

Abstract

Objective: The aim of this in vitro study was to evaluate the effects of diode laser irradiation on human dental enamel treated by bleaching and acidulated phosphate fluoride gels (APF) according to microhardness analysis. Background Data: The high interest of patients for aesthetics in cosmetic dentistry has led to new bleaching materials and the development of new techniques. Materials and Methods: Sixty embedded, flattened human third molar enamel fragments were randomly assigned to three groups (n = 20). Group 1 received topical application of 1.23% APF photoactivated using a diode laser. Group 2 received three applications of 35% hydrogen peroxide for 5 min and was photoactivated with laser for 30 s in each application. Group 3 treated the same as group 2 plus a 1.23% APF application for 1 min after bleaching followed by 30 s of laser irradiation. Microhardness analyses were performed before and after all the treatments. Results: Analysis of variance followed by Tukey and Student statistical t-tests (p < 0.05) showed significantly higher microhardness values for group 1 after treatment. No significant differences were shown before and after treatments for groups 2 and 3. Conclusion: No change was observed in enamel microhardness after treatment with hydrogen peroxide gel photoactivated using diode laser with or without APF. There was an increase in microhardness when enamel was treated only with APF photoactivated using a laser.

Introduction

T he high interest of patients in aesthetics in cosmetic dentistry is contributing to growth in the development of new bleaching materials and techniques. Bleaching agents–(hydrogen peroxide and carbamide peroxide) are used, in combination with different light sources, as chemical agents to remove intrinsic or extrinsic stains, thus breaking the carbon strain in te organic portion of enamel and dentin.^1,2

Of several bleaching agents, 30% and 35% hydrogen peroxide are optimized to be used in in-office dental bleaching techniques. Because of its high concentration, hydrogen peroxide can be used with or without light or heat stimulation; when faster results are needed, diode or argon lasers are associated in safety protocols.^3,4 Many effects have been analyzed in vital teeth after 35% hydrogen peroxide bleaching treatment.^4,5 An increase in roughness,^5,6 a decrease in superficial enamel microhardness values,^7,8 and loss of calcium have been noted using atomic force microscopy observations^9. Bleaching occurs by liberating free oxygen after degradation of hydrogen peroxide. Free oxygen (oxidizing agents) penetrates through the enamel structure through the protein space (amelogenin and enamelin) around the hydroxyapatite crystals.⁹ Mineral changes have been observed in enamel when a low pH (4.0–6.0) is reached, which may happen after 5 min of contact with dentin and 15 min of contact with enamel.^3,10

Decreases in enamel microhardness can be minimized with the use of APF after bleaching, aiming for a potential remineralization provided by calcium fluoride on the dental structure.^7,10,11 The enamel structure can become hypermineralized after diode laser irradiation with fluoride in caries prevention.⁸ Fluoride ions incorporated into or absorbed by hydroxiapatite crystals after laser irradiation provided enamel acid resistance.¹¹

To study the effect of a high-power diode laser irradiation on enamel using microhardness analysis, human teeth were submitted to a 35% hydrogen peroxide bleaching treatment activated by an 830-nm diode laser with or without 1.23% APF.

Materials and Methods

The procedures in this experimental research were conducted in agreement with Resolution # 196/96 from the National Health Department Committee (Brazil).

Thirty extracted intact third human molars, non-erupted, immersed in a 0.1% thymol water solution at a pH of 7.0 were scraped and then polished with a rubber drill, paste, and distilled water at low speed. First, a transverse section was made to separate the crown from the root, followed by a longitudinal slice cut at the central portion of the crown teeth. Ninety 4.0- × 4.0-mm (16.0 mm²) enamel fragments were obtained from the crown portion (three from each tooth) and embedded in polystyrene colorless resin in polyvinyl chloride molds with a 2.0-cm diameter. The enamel external surface was exposed, polished with sandpaper (400, 1000, and 1200) and then with diamond paste (6, 3, 0.5, and 0.25 μm) and felt discs, aiming for a smooth surface to obtain microhardness measurements.^12,13 Fragments with cracks and stains were discarded.

Sixty enamel fragments were then randomly assigned to three groups (n = 20). The samples were cleaned with water and pumice and stored at 37 ± 2°C in deionized distilled water until treatment. Group 1 received a topical application of 1.23% APF (Fluor Gel, DFL, São Paulo, Brazil), applied in a 3-mm-thick layer with a plastic spatula to all the buccal surfaces of the teeth and photoactivated using a diode laser (Opus Dent 10, Sharplan, Tel-Aviv, Israel). A red APF was used for its high interaction with the diode laser because pigments absorb it better.^2,4 Group 2 received three applications of 35% hydrogen peroxide (Whiteness HP/FGM, Santa Catarina, Brazil), applied in a 3-mm-thick layer with a plastic spatula to all the buccal surfaces of the teeth, for 5 min and photoactivated with laser for 30 s in each application. Group 3 was treated the same as group 2 plus a 1.23% APF application for 1 min after bleaching, followed by 30 s of laser irradiation (Table 1).

Table 1.

Experimental Group Distribution

Groups		Treatment	N
1		1.23% acidulated phosphate fluoride + diode laser	20
2	HP + L	35% hydrogen peroxide + diode laser	20
3		35% hydrogen peroxide + diode laser + 1.23% acidulated phosphate fluoride + diode laser	20

The bleaching agent (Whiteness HP/FGM) was applied following the manufacturer's instructions. For all three groups, the high-power diode laser irradiation (gallium-aluminum-arsenide (GaAlAs), 830 nm, Opus Dent 10) was performed after application of the bleaching gel on the enamel surfaces. The peak power was 1.4 W over an area of 16 mm² for 30 s 3 mm from the bleaching gel (without contact), at a fluency of 262.5 J/cm², considering the irradiated area and not the spot size area.² The laser was irradiated in a non-contact mode 2 mm from the bleaching gel. Three different movements were performed for 10 s each (10 s in a vertical mode, 10 s in a horizontal mode, and 10 s in circular movements) to prevent heat concentration at a specific point (Table 2). The operator had been previously trained to keep the same distance between the laser handpiece thand the bleaching gel. White et al.² proposed this protocol with a safe standard temperature to a high-power diode laser equipment at a parameter of 2.0 W for 30 s three times to prevent heat accumulation. The bleaching gel remained in contact with the enamel surfaces after laser irradiation for 5 min.

Table 2.

Laser Parameters

Target tissue	Emission mode	Power (W)	Time (S)	Area (Cm²)	Fluency (J/Cm²)	Spot size (μM)
Red bleaching gel	Q-switched	1.4	30	0.16	262.50	600
Red acidulated phosphate fluoride	Q-switched	1.0	30	0.16	187.5	600

After laser activation, the specimens were rinsed with water and air sprayed, and the protocols for the bleaching gel application were repeated two times, following the same methodology. A solution of 7% bicarbonate water was applied to the enamel surfaces to neutralize excessive oxidation effects of gel, following the manufacturer's recommendations.

For group 3, in addition to the treatment steps described above, 0.02 mL of 1.23% red APF (Fluor Gel, DFL) was used after the dental bleaching procedure and activated using the high-power diode laser (Opus Dent 10), with power setting of 1 W at an interrupted mode with circular movements to prevent heat concentration in a specific area. The red fluoride gel was activated using the high-power diode laser for 30 s at fluency of 187.5 J/cm² and maintained for 1 min. For group 1, only the procedures with fluoride gel associated with the high-power diode laser were used in the same manner as for group 3 (Table 2).

Microhardness tests were performed before (baseline) and after the bleaching treatments with three indentations of 25 g for a period of 5 s using a Knoop indentator. The baseline microhardness of the samples was used for allocation of the samples.

Analysis of variance, using Tukey and Student t-tests (p < 0.05) for multiple comparison of the averages, was used to evaluate the mean microhardness values for each treatment at different times.

Results

The Knoop microhardness mean values for all groups and the results of Tukey and Student t-tests (p < 0.05) are shown in Table 3. Three indentations were performed in each sample using a Knoop penetrator with a static load of 25 g for 5 s. The baseline microhardness of the samples was used for allocation of the samples.

Table 3.

Knoop Microhardness Values for All Groups at Different Times

Group	Baseline	Final	P-Value
		Mean ± Standard Deviation
1.23% acidulated phosphate fluoride + diode laser	315,73 ± 29.42^A	347.00 ± 17.60^B	0.03
35% hydrogen peroxide + diode laser	286.85 ± 22.40^A	288.52 ± 18.78^A	0.81
35% hydrogen peroxide + diode laser + 1.23%acidulated phosphate fluoride + diode laser	302.50 ± 18.22^A	310.55 ± 16.27^A	0.34

Values followed by the same letters do not differ between themselves according to Tukey statistical test (p < 0.05).

Significant differences between baseline and final mean microhardness were found for group 1 according to the Tukey test. The Student t-test showed that there was not a decrease in enamel microhardness exposed to a 35% hydrogen peroxide bleaching agent with or without fluoride treatment (p < 0.05). There were no statistical significance differences for groups 2 and 3 after treatment (final).

Discussion

There are several different ways to perform a bleaching procedure. These are separated into external techniques (where the bleaching agent is placed outside the tooth on the enamel surface) and internal techniques (where the bleaching agent is placed inside the tooth, in the pulp chamber). Adequate etiologic evaluation of color shades may accurately indicate different techniques to remove the stains with bleaching agents using at-home or in-office techniques. In in-office bleaching protocols, the agent can be activated using light, laser, or light-emitting diodes (LEDs) to reduce the length of the procedure and minimize sensitivity after bleaching.^4,14

Thus, different energy generators can be used, such as light or heat (halogen lamps; LEDs; xenon lamps; and argon, neodymium-doped yttrium aluminium garnet (Nd:YAG), and diode lasers with high intensities) to accelerate the oxidizing oxygen reaction with faster dental bleaching procedures.^15
–17 However, Nd:YAG and diode lasers promote the redox effect when associated with hydrogen peroxide bleaching agents to activate the reaction and achieve free oxygen liberation.¹⁸ This association also promotes greater enamel resistance when exposed to acidogenic challenge and promotes a reduction in demineralization.^18,19 When the bleaching agent interacts with the laser beam, the latter is absorbed, which promotes ionic molecular dissociation of peroxide particles.² Free water molecules, carbon gas, and free oxygen are liberated. Free oxygen is able to react with intrinsic and extrinsic stain chains of the dental structure, leading to a lighter shade.²⁰

Stern et al.²⁶ described the diode laser prevention action in the enamel as a positive combination between laser irradiation and enamel fluoride treatment promoting phosphate and hydroxyl ion reorganization and formation of pyrophosphate to hydrogen-phosphate, protein decomposition that can increase the mineral content. This is done by the increasing the temperature and the coalition of the crystals reorganizing them, thus providing a vitrified aspect.¹¹ Recently, this vitrified structure has not been clearly defined, and it is believed that this “resolidification” phenomenon occurs as a change of crystal phase due to the fusion of liquid phase to amorphous solid or melting phase.¹¹

The bleaching procedures may always follow safe protocols, maintaining the pulpal temperature profile lower than 5.5°C. The diode laser used in the present study was found to be safe for this procedure.^2,8,17,27,28

Three different movements were used to activate the bleaching agent in order to minimize thermal damage by not producing heat in a specific spot or area. The parameters of energy pulse, real output power, irradiation time, fluency, thermal relaxation time, beam diameter (spot size), and laser wavelength specific to the bleaching gel were randomized and controlled. Therefore, the Opus Dent 10 diode laser is safe, because these parameters were followed to 2.0 W of power setting and not exposed for more than 30 s.²⁷

To keep the dental structure intact after a bleaching treatment, research regarding the study of mineral tissue content alteration after the use of hydrogen peroxide as a bleaching agent were conducted.^5,7,21,22 Langsten et al.,²² when using carbamide peroxide, showed a decrease in enamel microhardness with concentrations of 20% and 35%. Lopes et al.²³ compared the use of carbamide peroxide and hydrogen peroxide, showing that hydrogen peroxide provided microhardness changes. Spalding et al.²⁴ used 35% hydrogen peroxide and carbamide peroxide and showed that a deleterious effect on enamel micromorphology may occur and that this effect increases roughness, even after saliva hydration. Haywood et al.,¹ Pinto et al.,⁷ Lewinstein et al.,¹⁰ and Garcia-Godoy²¹ reported mineral loss and a decrease in microhardness after the bleaching procedure.

Microhardness changes can be related to the use of high concentrations of bleaching agents for longer than 20 min, when higher amounts of free reactive oxygen are released in the dental structure,²⁴ in which white precipitation may be observed. This white precipitation layer contains mineral remains, caused by denaturation of interprismatic proteins and structure modification, which is a negative aspect of dental enamel.^23,24

However, the results obtained in this study showed that there were no microhardness alterations when the 35% hydrogen peroxide was used in association with the diode laser (group 2), in accordance with the results obtained by Friedman and Reyto.²⁵ Therefore, it seems that the diode laser plays an important role in the maintenance of the initial microhardness and that enamel mineral content loss can be avoided.

Preventive effects of diode laser associated with fluorides can be interesting, to maximize increase of dental human enamel microhardness maintenance. However, it is believed that fluoride application should not coincide with a dental bleaching session, because molecular weight loss of sodium fluoride can promote high oxidizing reaction, with an increase in sensitivity. In this case, the use of sodium bicarbonate after the bleaching treatment is necessary, because it is a neutralizing antioxidant agent with high molecular weight and smaller reaction power.²⁹

APF has been used to increase the acid resistance of enamel, with preventive effects on dental caries. A red APF was used in this study, the same color as the bleaching agent, to enhance the absorbance of the laser beam energy. It was verified that enamel in group 3 did not present significant microhardness changes. When APF was applied and irradiated using a diode laser for 1 min, a significant increase in microhardness was observed. For group 3, microhardness values were maintained after the treatment.

On the other hand, the enamel surface treatment with APF acttivated with a diode laser increased its microhardness. Furthermore, morphological changes that occur in enamel after laser irradiation²⁷ suggest that hydroxyapatite crystals are able to restructure the surface of the enamel crystals and decrease its solubility. Zhang et al.³⁰ studied the effects of potassium titanyl phosphate (KTP), diode, and LED irradiation on tooth bleaching and observed that neither LED nor the two lasers produced significant differences in enamel microhardness after the treatments. Polydorou et al.³¹ did not notice a significant difference in microhardness after bleaching either.

New bleaching materials such as 3.5% hydrogen peroxide solution containing titanium dioxide proved to have a strong bleaching effect, with no changes in the enamel surface morphology and open dentinal tubules with no smear layer at the pulpal dentin surface.³² Goharkhay et al.³³ also studied several bleaching agents and concluded that the surface effects were not related to the pH of the bleaching agents, and improved changes in brightness of up to 10 steps on the Vitapan classical shade guide were detected.

For clinical applicability, the protocols that use hydrogen peroxide activated using a laser proved to be safe regarding the enamel mineral content because of its microhardness maintenance properties. Furthermore, the use of APF seems to be an efective protocol aiming to increase the enamel microhardness, which may suggest a possible increase of enamel mineral content, indicating the use of APF after the dental bleaching procedure.

Conclusion

Within the limits of this in vitro study, it is possible to conclude that APF associated with diode laser irradiation promoted a significant increase in enamel microhardness. Diode laser irradiation of 35% hydrogen peroxide associated or not with APF did not interfere with enamel microhardness.

Footnotes

Author Disclosure Statement

No competing interests exist.

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