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ORIGINAL ARTICLE
Year : 2013  |  Volume : 7  |  Issue : 2  |  Page : 48-53

The effect of power bleaching on enamel microhardness activated by three different light sources: An in vitro study


1 Department of Conservative Dentistry and Endodontics, Rajah Muthiah Dental College and Hospital, Chidambaram, Tamil Nadu, India
2 Department of Conservsative Dentistry and Endodontics, Rajah Muthiah Dental College, Annamalai University, Chidambaram, Tamil Nadu, India

Date of Web Publication3-Jan-2014

Correspondence Address:
Vinoth Kumar Deepa
Department of Conservative Dentistry and Endodontics, Rajah Muthiah Dental College, Annamalai University, Annamalainagar - 608 002, Chidambaram
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0976-2868.124262

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  Abstract 

Aims and Objectives: The aim of this study was to evaluate and compare the effect of power bleaching on human enamel microhardness activated by three different light sources, namely the light emitting diode (LED), diode laser, and neodymium: Yttrium-aluminium-garnet (Nd: YAG) laser. Materials and Methods: Fifty enamel samples were randomly divided into five test groups each containing 10 samples (n = 10). The enamel surfaces of samples were polished and baseline Vickers microhardness values were evaluated. Control group samples were untreated and stored in artificial saliva. Experimental groups were subdivided into four groups based on bleaching treatment carried out. Enamel samples were treated with 40% hydrogen peroxide gel for 2 cycles of 20 min per bleach cycle (with and without light activation). Microhardness was reassessed on 1 st , 7 th , and 21 st day after storage in artificial saliva. Results were subjected to statistical analysis using one-way analysis of variance (ANOVA) and t-test. Results: There was a statistically significant difference present in enamel microhardness values compared to the unbleached and bleached group. On comparing 1 st and 7 th day to the baseline values there was a statistically significant difference in all the experimental groups. No statistically significant difference existed between the baseline values and 21 st day. Conclusion: Activation with various light sources (LED/diode laser/Nd: YAG laser) do not have additional deleterious effects on enamel microhardness and thus can be used safely to hasten the bleaching procedure.

Keywords: Diode laser, light emitting diode, microhardness, neodymium: yttrium-aluminium-garnet laser, power bleach


How to cite this article:
Ahamed S, Thayyil S, Sathyanarayana SS, Kulandaivelu A, Satheesh Kumar KS, Deepa VK. The effect of power bleaching on enamel microhardness activated by three different light sources: An in vitro study. J Dent Lasers 2013;7:48-53

How to cite this URL:
Ahamed S, Thayyil S, Sathyanarayana SS, Kulandaivelu A, Satheesh Kumar KS, Deepa VK. The effect of power bleaching on enamel microhardness activated by three different light sources: An in vitro study. J Dent Lasers [serial online] 2013 [cited 2022 Sep 26];7:48-53. Available from: https://www.jdentlasers.org/text.asp?2013/7/2/48/124262


  Introduction Top


Over the past few decades there has been a growing trend towards seeking esthetic improvements. The various procedures performed by dentists to cater the esthetic needs of patients having discolored teeth are bleaching, veneers, laminates with direct and indirect composites, or ceramics and full coverage ceramic restorations. The esthetic dental treatment bleaching is one of the most conservative and essential step in management of discolored teeth. Bleaching provides the advantage of no tooth preparation and is cost effective over the other conservative procedures such as laminates and veneers or more radical procedures such as full coverage ceramic restorations.

History of power bleaching dates back from 1918, by Abbot. [1] It is known that bleaching treatment may cause morphological alterations in mineralized structure. [2],[3] Studies using hydrogen peroxide at concentrations ranging from 30 to 37% promoted superficial alterations and a reduction in the calcium-phosphorous ratio. [4],[5],[6],[7]

Demineralization, the loss of mineral content at the tooth surface, alters enamel microhardness. [8] Microhardness tests are sensitive and capable of revealing small changes that occur during a short demineralization period. Literature reports on the effect of bleaching and power bleaching on enamel microhardness is very conflicting and inconclusive.

Aim

The aim of this study was to evaluate and compare the effect of power bleaching on human enamel microhardness activated by three different light sources, namely the light emitting diode (LED), diode laser, and neodymium: Yttrium-aluminium-garnet (Nd: YAG) laser.

The following objectives are formulated for the present research work. They are:

  • To compare the significant difference in Vickers microhardness values (VHN) of enamels samples between unbleached and bleached groups at different time intervals
  • To compare the significant difference in VHN of enamels samples between unbleached and bleaching with various light sources at different time intervals
  • To compare the significant difference in VHN of enamels samples between bleached and bleaching with various light sources at different time intervals.



  Materials and Methods Top


Preparation of the samples

Twenty-five human premolars freshly extracted for orthodontic purposes were used for the study. The teeth were decoronated at cementoenamel junction using a slow speed diamond disc under copious water irrigation. The buccal and lingual surface was separated and one single block of enamel supported by dentin measuring 2 mm × 2 mm was made from each buccal and lingual surface, respectively.

The enamel was flattened, completely finished, and smoothened with Shofu finishing and polishing kit. A cylindrical shaped plastic pipe was cut into 25 pieces each measuring 18 mm × 18 mm in diameter. Self-cure epoxy resin was mixed and poured into each pipes. Two enamel blocks were mounted on epoxy resin in each pipe. The enamel surfaces were further polished using polishing paste and rubber wheel.

The 50 enamel samples were randomly divided into five test groups each containing 10 samples (n = 10).

The groups were broadly categorized into:

  • Control group unbleached (Group I)
  • Experimental groups (Group II, III, IV, and V).


Opalescence extra boost (40% hydrogen peroxide gel) was the bleaching agent used in this study and bleaching procedure was carried out by strictly following manufacturer's instructions.

Group II (n = 10) - Bleaching only group (two cycles of 20 min each)

Group III (n = 10) - Bleaching + LED activation (BT cool LED) wavelength = 430-490 nm, 100-240 V, 50/60 Hz, and 0.6 A. Each test sample was subjected to two cycles of bleaching procedure, each cycle lasting for 20 min. LED activation was done for 90 s in each cycle (18 s after every 2 min).

Group IV (n = 10) - Bleaching + diode laser activation (Zolar photon plus) wavelength = 980 nm, power = 6 W. Each test sample was subjected to two cycles of bleaching procedure, each cycle lasting for 20 min. Diode laser activation was done for 90 s in each cycle (18 s after every 2 min).

Group V (n = 10) - Bleaching + Nd: YAG laser activation (Fotona Laser) wavelength = 1,064 nm, power = 6 W. Each test sample was subjected to two cycles of bleaching procedure, each cycle lasting for 20 min. Nd: YAG activation was done for 90 s in each cycle (18 s after every 2 min).

Enamel microhardness testing of samples

Enamel microhardness of samples was tested on microhardness tester (Zwick 3212 and 3212.01) using Vickers diamond pyramid indenter point which was ground in the form of squared pyramid with an angle of 136° between faces. Each sample was first kept at the specific area on the microhardness tester. Then the lens of the tester was focused at magnification in ×200 on the sample to determine the polish and flatness of the enamel surface. After finding the appropriate area for testing, the Vickers diamond indenter was focused on the same area and the testing was loaded with load = 100 g and time = 15 s [Figure 1].
Figure 1: Schematic representation of Vickers pyramid diamond indenter indentations

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The Vickers hardness value/number (VHN) was displayed, which uses formula

Where F = load in Kgf, d = arithmetic mean of two diagonals (d 1 , d 2 ), HV = Vickers hardness value = VHN

Group I (control) - For the unbleached group VHN were measured at baseline that is before immersion in artificial saliva. Then the samples were immersed in artificial saliva at room temperature and microhardness values were taken on 1 st , 7 th , and 21 st day.

For the experimental groups, VHN were measured at baseline and the samples were subjected to bleaching procedures according to various groups. Then the samples were immersed in artificial saliva at room temperature and microhardness values were taken on 1 st , 7 th , and 21 st day.

Statistical analysis

The microhardness values obtained for different groups at different time intervals were subjected to statistical test to determine the level of significance. Intra-and intergroup comparisons for baseline and at 1 st , 7 th , and 21 st day were analyzed using independent t-test and F-test. Probability value (P < 0.05) is considered to be statistically significant.


  Result Top


The results of microhardness values between Groups I and II showed statistically significant values for 1 st , 7 th , and 21 st day. The baseline values were not statistically significant on comparison between groups [Table 1] and [Figure 2].
Figue 2: Vickers microhardness values of enamel samples between unbleached and bleached at different time intervals

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Table 1: Comparison of mean microhardness values between unbleached (Group I) and bleached (Group II) at different time intervals

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On comparing Group I with experimental groups (III, IV, and V) there was a statistically significant difference existing for the 1 st , 7 th , and 21 st day. The baseline values were not statistically significant on intercomparison between groups (III, IV, and V) and with Group I [Table 2] and [Figure 3].
Figure 3: Vickers microhardness values of enamel samples between unbleached and bleaching with various light sources at different time intervals

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Table 2: Comparison of mean microhardness values between unbleached (Group I) and bleaching with various light sources (Groups III, IV, and V) at different time intervals

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Intercomparison between experimental groups (II, III, IV, and V) revealed no statistically significant difference for baseline values. On comparison of the 1 st , 7 th , and 21 st day within groups and with the baseline values there was no statistically significant difference existing only for the 21 st day [Table 3] and [Table 4] and [Figure 4].
Figure 4: Vickers microhardness values of enamels samples between bleached and bleaching with various light sources at different time intervals

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Table 3: Comparison of mean microhardness values between bleached (Group II) and bleaching with various light sources (Groups III, IV, and V) at different time intervals

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Table 4: Comparison of mean microhardness values between the baseline and at different time intervals using paired -test

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  Discussion Top


The common side effect of bleaching agents is weakening of enamel structure by oxidation of organic or inorganic elements. [9] Changes in microhardness are related to a loss or gain of mineral (demineralization or remineralization) of dental structure. [10] It has been shown that the microhardness test is suitable for determining small changes in surface that demonstrated the effect of bleaching products on enamel. [11] The two types of indenters which can be used for microhardness testing of hard surfaces are Knoops and Vickers diamond indenters.

In the present study, microhardness testing of the samples was done using Vickers diamond indenter, all the samples were tested at a load of 100 g for 15 s, which is the lowest load for enamel because they created Vickers diagonals longer than 20 μm, which was recommended to prevent errors in optical measurement. [12]

In order to simulate the clinical situation and to standardize the experimental conditions, enamel microhardness testing of the samples was done at baseline, that is, before subjecting to bleaching procedure to establish mean baseline VHN for each group. Then the samples were stored in artificial saliva which was chosen in order to minimize the influence presumed to be exerted by the variation in the composition of human saliva collected from different subjects. The microhardness values were taken on 1 st , 7 th , and 21 st day for all the groups. A period of 21 days was taken in accordance to study conducted by Kabbach et al., [13] who confirmed that the saliva was able to increase the mineral content of bleached enamel because it contains calcium and phosphate ions. [14],[15]

In the present study, composition of calcium and phosphate ions in the artificial saliva are 1.5 and 3.0 mM, respectively, pH of saliva was 7.1 which is above the critical pH 5.5 for enamel demineralization. Light curing sources such as halogen, laser, plasma arc, and LED have been used widely in dentistry. The purpose of light is to minimize the time required for tooth bleaching by activating or accelerating the effect of bleaching agents. The objective of laser bleaching is to achieve the ultimate power bleaching process using most efficient energy source, while avoiding any adverse effect. Currently, the laser has been proven to be the most valuable energy source for power bleaching with simple and short in office application. [16]

The current study showed that there was a statistically significant difference existing for microhardness values when the unbleached (control) group was compared to bleaching and bleaching with different light sources at 1 st , 7 th , and 21 st day. Microhardness value of unbleached enamel (Group I) did not differ significantly from 1 st day enamel microhardness value of the same group (Group I) (P > 0.05). The results of 7 th and 21 st day after immersion in artificial saliva was significantly greater than baseline enamel microhardness value of the same group (Group I) (P < 0.05). This was in accordance with the results of the study conducted by Kabbach et al., [13] he showed that microhardness values were significantly lower in the groups irradiated with light when compared with the nonirradiated groups. Furthermore, the nonirradiated groups showed that saliva was able to enhance the microhardness during measurement times. Since no bleaching treatment was given to Group I and the pH of artificial saliva in which the samples were immersed was 7.1, which was well above critical pH (5.5) of enamel; [17],[18] we can expect no loss of calcium and phosphate ions from enamel surface. Also artificial saliva was able to increase enamel microhardness by providing calcium and phosphate ions.

The result of our study showed that baseline enamel microhardness values of experimental group (Group II-V) was statistically significant with that of 1 st day after bleaching (P < 0.05). The above result is in accordance to studies by Lopes et al., [19] Justino et al., [20] Rodrigues et al., [21] Borges et al., [22] Gomes et al., [23] Nagy et al., [24] Araujo et al., [25] and De Abreu et al. [26] This decrease in enamel microhardness may be due to morphological alterations in mineralized structures caused by bleaching agents [27] which considerably reduces the amount of calcium and phosphate ions, in addition to modifying the morphology of a large quantity of crystals in the superficial layer, when compared with non-treated enamel. [28],[29] It has been noted that alterations in the mineral content of dental enamel are directly related to its microhardness. Remineralization increases and demineralization decreases enamel microhardness. [30] The result of our study showed that 7 th day enamel microhardness values of experimental group (Group II-V) was significant to that of baseline enamel microhardness values (P < 0.05). This is in accordance to the study by Araujo et al., [25] and De Abreu et al. [26]

The result of our study showed that 21 st day enamel microhardness values of experimental group (Group II-V) did not differ significantly from baseline values. The above result is in accordance with studies by Justino et al., [20] Mielczarek et al., [14] and De Abreu et al. [26] This increase in microhardness is due to remineralizing solution (artificial saliva), suggesting their role in reversing the deleterious effects of bleaching agent.

The differences in enamel microhardness values of samples from Group II to V after complete bleaching procedure on 1 st day is not statistically significant implying that reduction in microhardness is due to bleaching agent and not by different light source activation (LED/diode/Nd: YAG laser).


  Conclusion Top


Within limitations of this study, the following conclusions are obtained

  • On comparing Groups I and II there is a statistically significant difference in the microhardness values existing at different time intervals
  • On comparing Group I with Groups III, IV, and V there is a statistically significant difference in the microhardness values existing at different time intervals
  • Intercomparison of the experimental groups (II, III, IV, and V) reveals:

    • Statistically significant difference in the microhardness values of baseline to that of different time intervals in each group
    • No statistically significant difference in the microhardness values of baseline within experimental groups
    • Statistically significant difference present in the microhardness values of baseline with 1 st day and baseline with 7 th day within experimental groups
    • No statistically significant difference present in the microhardness values of baseline with 21 st day within experimental groups


  • Groups III, IV, and V using different light sources (LED, diode laser, and Nd: YAG laser) had no significant difference in the VHN after bleaching for 1 st day when compared to only bleaching (Group II) for the 1 st day.


Activation with various light sources (LED/diode laser/Nd:YAG laser) do not have additional deleterious effects on enamel microhardness and thus can be used safely to hasten the bleaching procedure. Light source can play a major role on parameters like the time taken for bleaching, postoperative sensitivity, and effectiveness in shade change of the bleached teeth.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]


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