Influence of light and laser activation of tooth bleaching systems on enamel microhardness and surface roughness.

OBJECTIVE: The objective of this study was to compare the effects of light and laser activation of in-office tooth bleaching systems on enamel microhardness and surface roughness.
MATERIALS AND METHODS: Twenty-five enamel slabs were divided into three treatment groups: light-activated bleaching, laser-activated bleaching, and control. The baseline data were recorded for enamel microhardness (Vickers microhardness [VMH]) and surface roughness (Roughness average, Ra). The specimens were cured for 10 min upon hydrogen peroxide application for the light-activated bleaching group and activated with a laser source, 8 cycles, 10 s per cycle for the laser-activated group. The changes in VMH and Ra at days 1, 7, and 28 were evaluated. Kruskal-Wallis, Friedman, Wilcoxon, and Mann-Whitney tests were used to analyze both VMH and Ra between the treatment groups at different time intervals.
RESULTS: There were a significant reduction in VMH values and significant differences between days 1, 7, and 28 against the baseline in the light-activated bleaching group (P = 0.001). The Ra values revealed significant differences in both light- (P = 0.001) and laser-activated (P = 0.033) groups.
CONCLUSION: Light activation of a bleaching agent caused a reduction in enamel microhardness and an increase in surface roughness when compared to laser activation. Copyright:

INTRODUCTION

Tooth bleaching is a common sought-after dental treatment to enhance esthetics by improving tooth color to a brighter shade.[1234] A study showed that 28% of adults in the United Kingdom were not satisfied with the appearance of their teeth and 34% of adults in the United States of America were not satisfied with their existing tooth color.[5] Tooth bleaching is not considered a new technology. In 1848, the first nonvital tooth bleaching with chloride of lime was practiced. Many different bleaching agents were also successfully used on nonvital teeth including aluminum chloride, oxalic acid, pyroxene, hydrogen peroxide, sodium peroxide, sodium hypophosphate, sulfurous acid, and potassium cyanide.[6] All mentioned are oxidizing agents that worked directly or indirectly on the organic portion of a stain.[7] By the 1860s, vital teeth were bleached externally using oxalic acid and later using hydrogen peroxide or pyroxene. In the early 1900s, there was an addition of heated instruments or a light source to accelerate the bleaching process.[16]

In-office bleaching uses a relatively high concentration of bleaching agent or gel for a shorter period, while home-applied bleaching uses a low concentration of this agent. The concentration of an in-office bleaching agent that is normally used is between 25% and 35% hydrogen peroxide or 35% carbamide peroxide.[89] These are often used together with activating agents such as light or laser.[610] The in-office bleaching treatment used can result in significant whitening after only one visit but may require multiple treatments for optimum result or can act as a boost therapy by initiating the process and will be continued later by home-bleaching procedures.[31112] Peroxide bleaching systems are suspected of producing tooth whitening through the oxidation of intrinsic or extrinsic chromogens on or within the tooth enamel.[21113] To do this, hydrogen peroxide must be generated and/or diffuse to chromogenic materials on or within teeth.[413] The delivery of effective peroxide bleaching requires stabilized gel systems which are often formulated at low pH.[814]

Bleaching is a relatively safe procedure, but this treatment, through its mechanism of action, produces some structural changes to the enamel that has raised concerns to general dentists and dental researchers including mineral loss, loss of fluoride, increased surface roughness, reduced enamel hardness, reduced fracture stability, and increased susceptibility to erosion or caries.[1516171819] Moreover, patients are often concerned about the unfavorable consequences of bleaching treatment including tooth sensitivity and gingival irritation.[2920] There are limited studies available looking at the two tooth bleaching activation methods, light and laser, and simultaneously compare the changes in the enamel microhardness and surface roughness with time.

Therefore, this study aimed to compare the effects of light and laser activation of in-office tooth bleaching systems on enamel microhardness and surface roughness at different time intervals. The null hypotheses were a laser-activated in-office bleaching system producing the same amount of reduction in enamel microhardness and a similar increase in surface roughness to a light-activated bleaching system.

MATERIALS AND METHODS

Sound human premolar teeth, extracted for an orthodontic reason, were embedded in epoxy resins to ease anchorage in preparing enamel slabs. Enamel slabs were prepared using a high-precision cutting machine (IsoMet™ 2000, Buehler, Illinois, USA) to separate the buccal and lingual surfaces. The two surfaces were further sectioned, separating the mesial and distal parts. Twenty-five enamel slabs were then randomly selected and embedded in epoxy resins measuring 1 cm × 1 cm × 1 cm. The enamel surfaces were then polished using 1200-grit silicon carbide paper in a polishing unit (Metaserv 2000 Twin Grinder/Polisher, Buehler, Illinois, USA) with water irrigation to get a flat surface, and these specimens were then randomly divided into three treatment groups: light-activated bleaching, laser-activated bleaching, and control groups. They were stored in distilled water at 37°C. Baseline data were collected for enamel microhardness and surface roughness.

The specimens were then applied with 35% hydrogen peroxide activated with a light source (Power Whitening, Beyond™, Texas, USA) for the light-activated group and 35% hydrogen peroxide activated with a laser source (LaserSmile™, Biolase, California, USA) for the laser-activated group. Both the specimen groups were tested again for microhardness and surface roughness. This step was repeated at 7 days and 28 days after the first bleaching.

Bleaching technique

For the light-activated bleaching group, after the application of hydrogen peroxide gel of about 2–3 mm thickness onto each specimen, they were cured with a halogen light source at a wavelength of 480 nm for 10 min. The specimens were then cleaned with tissue paper without washing before the same step was repeated.

For the laser-activated bleaching group, after the application of hydrogen peroxide gel of about 2–3 mm thickness onto each specimen, the agent was activated with a diode laser in the range of 815 nm, 8 cycles, 10 s for each cycle. The specimens were then washed out of HP before reapplication.

Measurement of enamel microhardness

Vickers hardness testing machine (HMV-FA, Shimadzu, Kyoto, Japan) was used to determine the changes in enamel microhardness (Vickers microhardness [VMH]) after tooth bleaching. First, the pattern of indentation was set using the HMV software, three straight-line indentations 0.5 mm apart. The right lens was positioned into place before the specimen was being focused. Upon focusing, three indentations were made with a load of 500 g for 15 s each. The depth of indentation measures the hardness of the enamel.

Measurement of enamel surface roughness

After hardness testing, the surface texture was evaluated using an optical three-dimensional measurement system (InfiniteFocus, Alicona, Graz, Austria). The surface roughness was measured as Sa and in the unit of the micrometer (μm). Using the software, the specimens were put into focus under ×10 magnifications and a vertical resolution was set at 195 nm. The surface was then evaluated, and three areas with measurement of 200 × 200 nm at the flat surface and a mean value per specimen were calculated for baseline and after bleaching treatment.

Statistical analysis

The results of the enamel microhardness were statistically analyzed using Kruskal–Wallis and, subsequently, Wilcoxon and Mann–Whitney tests. The mean of the surface roughness values was analyzed using Kruskal–Wallis and Friedman tests, followed by the Wilcoxon test. SPSS® 10.1 software SPSS software (IBM SPSS Inc, Chicago, IL) was used for this purpose.

RESULTS

The means of enamel surface microhardness values showed that there was a significant difference for the light-activated bleaching group (P = 0.001) [Table 1]. When comparing different time intervals, it was found that there were no significant differences between the groups except at day 28 (P = 0.005) [Table 1]. At day 28, there was a significant difference between the light- and laser-activated bleaching groups (P = 0.001) [Table 2].

Table 1

Means of enamel surface microhardness values (Vickers microhardness±standard deviation) at different time intervals for a 1-month treatment period

Activation	n	Baseline	Day 1	Day 7	Day 28	Pa
Light	10	303.9±81.8	313.3±93.1	312.8±75.1	238.7±71.1	0.001
Laser	10	331.0±46.4	314.1±51.6	323.0±64.1	337.7±44.3	0.356
Control	5	333.2±82.8	337.0±46.2	306.0±25.5	300.2±43.0	0.215
Pb		0.721	0.663	0.862	0.005

aFriedman test, bKruskal–Wallis test. VMH: Vickers microhardness

Table 2

Results of Mann–Whitney test on enamel microhardness at day 28 within different bleaching activation groups

Time interval	Activation	Pa
Day 28	Laser and light	0.001
	Laser and control	0.206
	Light and control	0.129

aMann–Whitney test

The means of enamel surface roughness showed that there were significant differences in the laser- (P = 0.033) and light-activated (P = 0.001) bleaching groups from baseline to day 28 [Figure 1]. When comparing time intervals, it was found that there was no significant difference between the three treatment groups [Table 3].

Figure 1

Optical three-dimensional view of surface roughness of a specimen at (a) baseline, (b) day 1, (c) day 7, and (d) day 28

Table 3

Means of enamel surface area roughness values (Sa±standard deviation) (μm) at different time intervals for a 1-month treatment period

Activation	n	Baseline	Day 1	Day 7	Day 28	Pa
Light	10	0.30±0.08	0.35±0.07	0.36±0.04	0.36±0.04	0.001
Laser	10	0.34±0.07	0.40±0.09	0.35±0.06	0.41±0.07	0.033
Control	5	0.38±0.04	0.39±0.03	0.41±0.07	0.40±0.03	0.077
Pb		0.536	0.311	0.117	0.103

aFriedman test, bKruskal–Wallis test

DISCUSSION

The negative impact of tooth bleaching on the integrity of organic enamel components such as protein and collagen creates issues involving short-lasting whitening effect, rough surfaces, and softened enamel.[915] Softened and rough enamel surfaces may predispose the tooth to the development of dental caries and noncarious tooth surface loss such as abrasion and attrition.[9] This study compared how light and laser activation of a bleaching agent caused alterations to enamel microhardness and surface roughness.

The results of this study indicated that the effects of hydrogen peroxide bleaching showed a significant reduction in enamel microhardness at day 28 when the light was used to activate the bleaching agent as compared to laser activation. A significant reduction in hardness was also observed between the time intervals when light activation was used. Hence, the first null hypothesis was rejected. The findings of this study are in agreement with an earlier study that found a significant reduction in enamel hardness by 13% after 5 min of exposure to 35% hydrogen peroxide activated by light, and this was further reduced by 25% after 35 min of exposure.[5] Other studies, however, reported that bleaching with 35% hydrogen peroxide activated by light or laser did not affect the enamel microhardness.[2122] Nevertheless, the hardness testing for these studies was conducted at prebleaching and single postbleaching treatment. Contrary to the findings of the present study, a past study revealed a reduction in enamel hardness when 25% and 35% hydrogen peroxide were irradiated with a laser light source.[16] Another previous study that used 6% hydrogen peroxide illuminated with light-emitting diode (LED) light interestingly saw a reduction in surface hardness by 5.6% after a whitening treatment, however, this finding was insignificant as compared to orange juice treatment which reduced the enamel hardness by 84.4%.[9] The lower concentration of hydrogen peroxide used in their study caused less reduction in hardness as the pH of the orange juice could be more acidic than that of the bleaching agent, leading to more acid dissolution of the enamel surface by the juice.[9] Furthermore, a study that looked at home-applied 3.3%, 5.3%, and 8.75% equivalent of hydrogen peroxide found a significant reduction in enamel microhardness at day 7 and day 14 when 8.75% of the agent was used.[23]

An increase in surface roughness and the subsequent ability of plaque and bacteria to adhere to the enamel surface after a bleaching treatment had also been observed in previous studies.[2425] In this study, the results showed an increase in enamel surface roughness between the time intervals when both light and laser were used to activate the agents. There was, however, no significant difference in the increase in surface roughness between the two activators. Thus, the second null hypothesis was accepted. These findings correspond with the results of a study that observed an increase in roughness after exposure to a similar concentration of hydrogen peroxide activated by light and laser.[1] A study that investigated the changes in enamel surface roughness and adhesion of Streptococcus mutans to the surface concluded that vital bleaching treatment with 35% hydrogen peroxide increased the surface roughness. Moreover, the adhesion of S. mutans to the enamel surface increased with repeated application of the bleaching agent.[17] Histologically, bleaching agents are also found to cause a significant reduction in the calcium/phosphorus (Ca/P) ratio of the enamel following hydrogen peroxide treatment.[26]

Lasers are utilized in tooth bleaching to catalyze the oxidation reaction of the bleaching gel. There are numerous laser wavelengths available, and one of the most commonly used is the diode laser.[1] The use of lasers provides much faster and effective bleaching than other conventional methods, and the ability to control the generated heat ensures that the temperature is almost always optimal and ultimately prevents pulpal overheating.[127] In addition, it has been shown that patients undergoing bleaching treatment with a higher laser wavelength exhibited less intensity of tooth sensitivity than those treated with a lower wavelength.[2829] Light sources used for light-activated bleaching including quartz-tungsten-halogen lamps, plasma arc lamps (used synonymously for xenon gas discharge or xenon short-arc lamps), and laser sources (laser = light amplification by stimulated emission of radiation) of a variety of different wavelengths as well as LED have been proposed for light activation of bleaching products.[1212] Although light sources have been advocated for use with a bleaching product, a recent systematic review suggested no added advantage of any light-activation protocol, even when compared with no light activation for in-office bleaching.[30]

Differences in the methodology may have affected the outcome of this study from the others including the type of storage medium used, either stored in distilled water, artificial saliva, or human saliva. In an earlier study, it was shown that the use of human saliva was less associated with microhardness reduction as compared to artificial saliva.[15] Attin et al. found the use of 5% equivalent of hydrogen peroxide to be associated with a reduction in the enamel fracture toughness after 10 days, which contradicts the finding of another study using the same concentration of the bleaching agent.[31] The study found the fracture toughness changes to be insignificant after 6 weeks. It is predicted that the remineralization process has taken place upon storage of the specimens in the artificial saliva in the latter study.[32] The present study used distilled water as a storage medium that may not simulate the clinical condition, however, the outcome from this study provides a basis on the degree of acid dissolution of the enamel surface upon bleaching agent application irradiated by two different activators, light and laser. For this reason, dental practitioners should practice the use of remineralizing agents immediately after tooth bleaching procedures to promote remineralization.

Fluoridated hydrogen peroxide bleaching gels can reduce microhardness and accelerate microhardness recovery in the posttreatment phase to a better extent than nonfluoridated bleaching gels.[315] This may be attributable to the fluoride component of bleaching gels that support the repair of the microstructure defects by absorption and precipitation of salivary components such as calcium and phosphate.[15] The use of a high-concentration fluoride dentifrice (5000 parts per million) postbleaching has also been shown to increase the enamel microhardness and decrease the surface roughness.[33] Although fluoride therapy could enhance the remineralization process, the effect would not occur if the mineral contents of the enamel are lost as there would be no mineral substrate upon which the fluoride ions can act on.[9]

The type of microhardness testing could also play a factor in the outcome of a study.[34] In a past study, it was proven that the usage of human saliva and the Vickers hardness test instead of the Knoop hardness test was associated with a less frequency of microhardness reductions as the indentation shape produced by the two varies.[15] Moreover, in some studies, the microhardness was measured directly after the bleaching episodes and others were measured after the posttreatment period, where the samples had been stored in remineralizing solutions before testing, affecting the outcome.[15]

One of the strengths of this study is a comparison of the effect of vital bleaching on the enamel surface between different time intervals were made. In a clinical situation, a bleaching procedure may need to be repeated to achieve the intended results.[35] Although this study revealed that the reduction in microhardness and the increase in surface roughness became more prominent with time, the storage medium used limits the knowledge on the duration required for the remineralization process to take place. Therefore, our recommendations for future studies would include increasing the duration from one bleaching treatment to another to better understand the length of time required before the next bleaching procedure and the use of human saliva as a storage medium to best mimic a clinical condition.

CONCLUSION

Within the limitations of the present study, it can be concluded that the activation of a bleaching agent with light caused a significant reduction in enamel microhardness and an increase in surface roughness after a month when compared to activation with lasers that caused minimal or no significant alterations. This study provides a foundation that the use of lasers as an activator in a bleaching treatment is better in maintaining the integrity of the enamel structures.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.