Microstructural effect of a laser-activated bleaching agent containing titanium dioxide on human enamel.

AIM: This study aimed to evaluate the effect of a laser-activated bleaching agent containing titanium dioxide (TiO2) nanoparticles on enamel roughness and hardness.
MATERIALS AND METHODS: Twenty human premolars were randomized into two groups according to the bleaching treatments performed: HP - 35% hydrogen peroxide and HP + TiO2 - 30% hydrogen peroxide containing TiO2 light-activated by diode laser (980 nm). It was performed two bleaching sessions with an interval of 7 days. Microhardness and roughness of the enamel were assessed at three times: T0 - Before 1st appointment, T1 - after 2nd appointment, and T2-7 days after 2nd appointment.
RESULTS: The HP + TiO2 did not cause changes on enamel roughness and hardness and presented the same effects of the HP.
CONCLUSIONS: Both bleaching agents showed no difference between them. Then, it is possible to conclude that both are viable for clinical use during in-office dental bleaching technique regarding the microstructural changes that they might cause. Copyright:

INTRODUCTION

Smile esthetics' and tooth color are considered of great importance for patients.[1] Therefore, the desire for white teeth among the population is one of the responsible factors for the increase in patients' number in dental offices looking for dental bleaching procedures.[23]

Under properly conditions, dental bleaching is a low-cost and effective procedure for the treatment of discolored teeth and it is also possible to apply it for a short period of time.[45] Bleaching agents are mostly based on hydrogen peroxide (H 2O2) and can be used in different techniques by professionals and/or by patients, at dental offices and/or at home.[1]

Considering the technological advance, a new in-office bleaching agent, containing titanium dioxide nanoparticles (TiO2), have been introduced into the market. Its use has the premise of enhance H2O2 action, reduce the operative time, and the number of bleaching sessions. These features may decrease the dental structure damage and the occurrence of tooth sensitivity during and after bleaching procedures.[67] It is important to note that the use of a light source is necessary to activate the TiO2 photocatalytic oxidation reaction. Diode laser (light amplification by stimulated emission of radiation), represents an example of light source that can be used.[89] Despite the advantages, there still are hesitations in the use of H2O2 containing TiO2 regarding the structural changes of the dental tissue.[7] Moreover, the use of light units and their consequences is not clearly elucidated.[10]

Thus, this study aimed to evaluate the effects of a laser-activated bleaching agent containing TiO2 nanoparticles on hardness and surface roughness of the human enamel. The null hypothesis investigated was that both bleaching agents tested do not provide changes on the human enamel.

MATERIALS AND METHODS

Experimental design

This study was single-blind (statistician). The teeth were randomized into two groups, with a similar allocation rate, according to the type of enamel bleaching treatments [Table 1].

Table 1

Experimental design

Group	Treatment	Product (manufacturer)	n	Tests
HP	Dental bleaching with 35% H2O2	Whiteness HP (FGM Produtos Odontológicos Ltda, Joinville, SC, Brazil)	10	Roughness and hardness
HP+TiO2	Dental bleaching with 30% H2O2 containing TiO2 and light activated by diode laser (980 nm wavelength)	Pearly White Smile (Heydent GmbH, Kaufering, Germany)	10	Roughness and hardness

H2O2: Hydrogen peroxide, TiO2: Titanium dioxide

Samples’ preparation

Twenty (n = 20) human premolars were used. These teeth did not have enamel defects or cracks visible in a stereoscopic magnifying lens (×10). The premolars were extracted because of periodontal disease or orthodontic treatment. This study received approval from the Research Ethics Committee in Involving Human Beings of State University of Londrina (CEP UEL 1.730.528).

Premolars were cleaned using periodontal curettes (Trinity Indústria e Comércio Ltda., Jaraguá, São Paulo, Brazil) and disinfected by 0.1% thymol solution (Dinâmica, Piracicaba, São Paulo, Brazil). The teeth were sectioned at 1–2 mm below the enamel/cementum junction using flexible diamond disks (7020, KG Sorensen, São Paulo, Brazil). In sequence, pulp chamber of each tooth was cleaned with dentin excavators (N° 5, Duflex, S.S. White, Rio de Janeiro, Brazil) and endodontics files (K, Maillefer instruments AS-CH 1338, Ballaigues, Switzerland). After this, a pulp chamber irrigation was performed with 1% sodium hypochlorite solution (Ciclo Farma Indústria Química, São Paulo, Brazil) and 0,9% sodium chloride (Fresenius Kabi Brasil Ltda., Aquiraz, Ceará, Brazil) using an irrigation syringe (Ultradent Products Inc., UT, USA).

After that process, all the samples were submitted to surface hardness assessment in order to standardize the samples and select those with 270 ± 5 Vickers hardness number for further experiments. Samples were stored in distilled water at 37°C until the beginning of the experimental procedures.

The samples were covered, except for the enamel surface, by an acrylic resin, inside of polyvinyl chloride tubes. After, the enamel surface was planned with sandpaper of increasing granulation, #400, #600 and #1200 using a politrix machine (Teclago – Tecnologia em Máquinas Metalográficas, Vargem Grande Paulista, São Paulo, Brazil). Surface polishing was performed using diamond pasts and felt discs (Top, Gold, e Ram, Arotec Ind e Com Ltda). The samples were washed for 10 min in an ultrasonic cleaner using distilled water, to remove any residue from the abrasives.

Bleaching treatment

In HP + TiO2 samples, the bleaching agent was applied and activated, for three times, using a diode laser for 15 s (DCLase, DC International LLC, Wellington, FL, USA; 980 nm wavelength). The bleaching agent remained in contact with the tooth surface for 5 min, according to the manufacturer's instructions.

In HP group, only one bleaching agent application was performed, which remained in contact with the tooth surface for 40 min. Each 10 min, a microbrush was used to move the bleaching agent, release any oxygen bubbles, and renew the contact of the gel with the teeth.

In both groups, HP and HP + TiO2, two bleaching sessions were performed, with an interval of 7 days. After each session, the samples were immersed in artificial saliva (Odontofarma Farmácia de Manipulação, Londrina, PR, Brazil) at 37°C, which was changed every 2 days.

Roughness and microhardness tests

On the buccal surface of the samples, the enamel roughness (left half) and the enamel hardness (right half) were evaluated. For each test, three measurements were performed at different times: T0 (before 1st bleaching session– control group), T1 (after 2nd bleaching session), T2 (7 days after 2nd bleaching session). All the measurements were performed by the same calibrated operator.

Roughness test

Samples were submitted for roughness evaluation using a portable profilometer (Mitutoyo Sul América Ltda., Santo Amaro, SP, Brazil), which had been calibrated before the readings. The standard roughness (Ra) was measured using a cut off value of 0.25 mm and a speed of 0.1 mm/s and 1 mm/s. Three occlusal-cervical equidistant readings were taken, in areas visibly more regular, at each time reading.

Microhardness test

Samples were submitted to Vickers microhardness evaluation (Mitutoyo Sul América Ltda., Santo Amaro-SP-Brasil) using a static load of 25 g for 10 s. Three indentations were performed in the right region of the sample surface with 100 μm between each indentation, at each time reading.

Statistical analysis

The normal distribution was verified by Shapiro–Wilk test (P > 0.05). After that, all groups presented normal distribution allowing the use of a parametric analysis. ANOVA two-way followed by Bonferroni test was used. All analyzes were performed at a significance level of 5% using the statistic software SPSS Statistics version 22 (International Business Machines Corp., NY, USA).

RESULTS

The dental bleaching performed did not change roughness [Table 2] and microhardness [Table 3] of the enamel for both the groups (GPHTiO2 and GPH). Moreover, there was no statistical difference between the time readings (P > 0.05), except for microhardness between T0 and T1 (P = 0.028).

Table 2

Mean±standard deviation of Vickers hardness number of the enamel surface before and after the bleaching treatments (n=10)

Bleaching agent	Time
	T0	T1	T2
HP	272.80±26.74Aa	247.30±22.43Ab	258.50±19.29Aa
HP+TiO2	279.30±46.82Aa	267.00±41.56Ab	273.40±32.22Aa

Different lowercase letters on the same line denote the presence of a statistically significant difference (P=0.028), equal capital letters in the same column denote absence of statistically significant difference (P≥0.18). TiO2: Titanium dioxide

Table 3

Mean±standard deviation of roughness (μm) of the enamel surface before and after the bleaching treatments (n=10)

Bleaching agent	Time
	T0	T1	T2
HP	0.21±0.04Aa	0.20±0.06Aa	0.21±0.05Aa
HP+TiO2	0.22±0.04Aa	0.18±0.03Aa	0.20±0.04Aa

Equal capital letters in the column and lowercase letters in the line denote absence of significant statistical difference (P≥0.27). TiO2: Titanium dioxide

DISCUSSION

In this study, the null hypothesis was accepted showing that there were no changes on roughness and microhardness after dental bleaching procedures using both bleaching agents (HP + TiO2 and HP) for T1 and T2 when compared to the control group (T0).

These results may have been favored by the fact that all the samples were immersed in artificial saliva after the dental bleaching sessions, with systematic refreshing at 2-day intervals to simulate the oral conditions, the human saliva buffering and remineralization capacity. As a result, a neutralization of the bleaching agents' adverse effects could be provided by the saliva.[1112131415]

No significant differences were observed in surface roughness of the enamel after dental bleaching sessions, regardless of the bleaching agent used, according to other findings in the literature.[121617] Therefore, it may be inferred that the proper use of bleaching agents, according to the manufacturer's guidelines, do not cause deleterious effects to the enamel surface micromorphology[12] and make them safe for clinical practice.

Regarding the enamel microhardness, no differences were observed before and after dental bleaching procedures, because both evaluated agents were not able to provide a significant mineral loss to provide structural changes. These results corroborate with other studies, in which the application of 35% H2O2 also did not decreased the enamel surface microhardness.[31618]

Currently, the modernization of procedures has been aimed to simplify techniques. Therefore, semiconductors such as TiO2 have been added to the hydrogen peroxide bleaching agents to accelerate the bleaching process, favoring the biocompatibility of the final product, avoiding or reducing the side effects, such as sensitivity after dental bleaching and damages to the dental structure. Thus, these new bleaching agents may be more effective and safer than traditional formulations.[6919] Despite the evidence of the effectiveness of 35% H2O2 as bleaching agents in clinical and laboratory studies, there is still a lack of studies on bleaching agents containing TiO2 nanoparticles.

The use of a light source is necessary for the photocatalytic oxidation reaction of TiO2.[9] Usually, TiO2 has been known as the most important semiconductor photocatalyst reacting to ultraviolet light.[20] However, according to a preview study, TiO2 nanoparticles (3–30 nm) can give scatter but also absorption radiation of approximately 800 nm to 1100 nm, with the effect that the laser energy remains within the gel and is minimally transmitted.[21] Therefore, in this study, the bleaching agent containing TiO2 was light-activated by a diode laser (980 nm), according to the manufacturer's instructions.

HP + TiO2 samples showed no structural alterations after dental bleaching. Therefore, it is possible to show that the bleaching agents containing TiO2 and the use of the diode laser as a photocatalytic light source are safe options regarding the enamel microhardness and enamel surface roughness properties and can decrease the clinical time. In our study, all the HP + TiO2 bleaching procedures were around 15 min time-consuming, proving to be faster than HP (40 min). However, further studies are needed to better understand the behavior of this light source and to evaluate the influence of its use on the dental bleaching efficacy and the temperature increase of the pulp chamber.

The results of the present study diverge from other findings, in which the reduction of microhardness values and increase of surface roughness after bleaching treatments are reported.[141522] These alterations may be associated with the pH of the bleaching agent used due to acidic agents are more likely to cause enamel demineralization;[152324] can be altered because of the exposure time of the agent in contact with the dental structure[15] and to the morphological characteristics related to the structure itself, because the surface variations and enamel permeability can occur between different teeth and individuals. However, these adverse effects can be eliminated by human saliva.[23]

This study also demonstrated the importance of the experimental design in assessing the adverse effects of bleaching agents[25] and in the clinical extrapolation of the results. Therefore, in situ and in vivo studies, which reliably simulate oral conditions, are ideal and necessary. Moreover, the long-term assessment is very important to provide information on the longevity of the treatments and their adverse effects.

According to the results and considering the limitations of this study, both bleaching treatments (HP and HP + TiO2) were similar in their effect on enamel microstructural changes. Nevertheless, the required devices (light source) to enable the clinical application of the bleaching agents containing TiO2, ends up increasing its cost in the market and consequently the cost paid by patients. Thus, conventional bleaching with 35% H2O2, which exhibited the same effect compared to HP + TiO2, is a safe option, established by the frequent use and low cost.

CONCLUSIONS

Within the limitations of this study, the bleaching agents did not provide significant changes on the roughness and microhardness of the dental enamel surface, regardless of the analysis time. Moreover, both bleaching agents showed no difference between them. Then, it was possible to conclude that both bleaching agents are viable for clinical use in in-office dental bleaching technique.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.