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 Table of Contents  
Year : 2020  |  Volume : 17  |  Issue : 3  |  Page : 106-113

Effect of frequent bleaching modalities on color stability of enamel

1 Department of Dentistry, Egyptian Ministry of Health, Tanta University, Tanta, Egypt
2 Department of Restorative Dentistry, Faculty of Dentistry, Tanta University, Tanta, Egypt

Date of Submission15-Dec-2019
Date of Acceptance01-Jun-2020
Date of Web Publication30-Oct-2020

Correspondence Address:
Sara H Mostafa
2 Al Hewary Street, Cairo Alexandria Highway, Qism 1, Tanta
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/tdj.tdj_58_19

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The purpose of this study was to evaluate effect of frequent bleaching regimens on the color stability of enamel.
Materials and methods
Thirty sound bovine incisor teeth were used representing three main groups. Group 1 (control group) was not subjected to any bleaching procedure. Group 2 was bleached with 15% carbamide peroxide (CP) gel; Opalescence PF. Group 3 was bleached with chemical-activated hydrogen peroxide bleaching gel (Opalescence Boost PF 40%). All groups were subdivided into two subgroups A and B according to the storage solution either artificial saliva or staining cycle containing three staining solutions. This was performed for 30 days, with an immersion time 6 h/day resembling a period of 24-month clinical use. All the procedures were performed twice with 2 months interval where the teeth were stored in daily renewable artificial saliva at room temperature (23 ± 1°C). Spectrophotometer was used to measure the color change of the specimens. One-way analysis of variance test and pairwise test were used, at a 95% level of significance.
A statistical significant rebound was recorded after first bleaching regimen (P = 0.009 and 0.032) and after second bleaching regimen (P = 0.016 and 0.013) for groups 2 and 3, respectively. A statistical significant difference was recorded between the (ΔE) after staining for first vs second time in the tested groups (1, 2, and 3) (P = 0.000, 0.047, and 0.002), respectively. Regarding specimen preserved in saliva, no statistical significant difference between the tested groups (P = 0.074, 0.793, and 0.756), respectively.
The hypothesis of this study was rejected as the color was proved to be unstable after frequent bleaching regimens. There was an increase in color change due to exposure to staining cycles and/or artificial saliva.

Keywords: dental bleaching, spectrophotometer, staining

How to cite this article:
Mostafa SH, Etman WM, Salama MM. Effect of frequent bleaching modalities on color stability of enamel. Tanta Dent J 2020;17:106-13

How to cite this URL:
Mostafa SH, Etman WM, Salama MM. Effect of frequent bleaching modalities on color stability of enamel. Tanta Dent J [serial online] 2020 [cited 2020 Nov 27];17:106-13. Available from: http://www.tmj.eg.net/text.asp?2020/17/3/106/299634

  Introduction Top

Tooth discolorations vary in etiology, appearance, localization, severity and adherence to tooth structure, and they are categorized as intrinsic, extrinsic, or internalization[1]. Although extrinsic discoloration can be removed with a prophylactic cleaning procedure, intrinsic staining necessitates chemical bleaching[2].

Tooth-whitening products are commonly divided into three categories known as dentist-supervised home bleaching products, in-office bleaching products and over-the-counter whitening products. Hydrogen peroxide (HP) or its precursor carbamide peroxide (CP) is the most common active ingredients in tooth-whitening products which might also contain one of a few other products such as oxalic acid, chlorine and muriatic acid[3]. Lower concentrations of CP or HP are used for home bleaching whereas higher peroxide concentrations are used for in-office procedures[4].

The fact that diffusion of peroxide into dental tissue is related to the concentration and application time[5] lightened up the idea that the whitening products with different concentrations and application methods will vary in efficiency[6]. This was estimated by Kielbassa et al.[7].

Coloring agents are present in most foods. Beverages such as cola-based soft drinks, black tea, coffee and red wine produce a greater influence on the staining of teeth[8], and the consumption of such beverages is highly popular worldwide.

Color change following bleaching treatment is most commonly determined by visual comparison of standard shade guides against teeth. However, color determination using a shade guide is highly subjective and influenced by a number of factors such as lighting conditions, the human eye and brain[9] while computer-based instruments are more objective and reliable[7]. In addition, the lack of consensus about how the efficiency of tooth-whitening should be evaluated makes it difficult to compare the results of studies using different products and methods.

Therefore, the purpose of this study was to measure the color stability of teeth after two bleaching and rebleaching regimens following 2 years simulating period under a controlled in vitro environment. However, the hypothesis of this study was to assume that color could be stable after frequent bleaching regimens even if teeth were exposed to staining as coloring stuff.

  Materials And Methods Top

Thirty sound bovine incisor teeth obtained from the slaughterhouse were used in this study. Approval of this research was obtained from the Ethical committee of Faculty of Dentistry, Tanta University. Teeth were inspected under atomic force microscope to exclude those with stains, cracks, fractures, dental fluorosis, or enamel defects as attrition, abrasion or erosion. The selected tooth shade was between A3 and A3.5 vita classical shade guide. The teeth were cleaned from all debris and calculus using tap water and manual scalers, disinfected and stored from 1 week to 1 month maximum in 0.1% thymol solution until using[6].

For each tooth, the root was sectioned using a water-cooled diamond saw (Edental Golden S.A.W., Bern, Switzerland) and the pulp tissue was removed using endodontic broach. Crowns of all the specimens, with the labial surface faced upward, were individually embedded in chemically cured acrylic resin (Acrostone) (Acrostone Manufacturing and Import Co., Cairo, Egypt.), then polished with polishing brush and washed. Throughout the study all specimens were stored in artificial saliva that were renewed daily[10]. Specimens were divided randomly into three groups (10 each group) [Table 1]:
Table 1: The experimental design of the study

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  1. Group 1: (the control group) was not subjected to any bleaching procedures.
  2. Group 2: (home bleaching regimen) specimens were bleached with 15%[11] CP gel; Opalescence PF [Table 2] according to manufacturer's instructions. About 1 mm thick layer of the gel was applied on the labial surface at room temperature, and after 4 h[11] it was removed using wet cotton pellets, washed under running water then dried with cotton pellets and re-immersed in artificial saliva[12]. This was repeated for 14 days[11]. The storage media was renewed daily[10].
  3. Group 3: (in-office bleaching regimen) chemical-activated bleaching gel (Opalescence Boost PF 40%, Ultradent Inc, Ontario, Canada) [Table 2]. The bleaching material was freshly mixed according to the manufacturer's instructions then was applied as done in group 2. Two applications (of 20 min each). Another application was performed after 3 days and the teeth were immersed in artificial saliva (renewed every day) between the two sessions. After the bleaching procedure, and washing with water, the specimens were dried and re-immersed in artificial renewed saliva.
Table 2: Bleaching agents and storage media used in this study

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All groups were subdivided into two subgroups A and B according to the storage solution.

Subgroups A: were not subjected to any staining solutions but just stored in artificial saliva[13] at room temperature (23 ± 1°C)[14] [Table 1].

Subgroups B: specimens were subjected to a staining cycle containing three staining solutions (coffee, tea, and hibiscus tea) respectively for 2 h daily at each solution for 30 consecutive days [Table 1]. After daily staining cycles they were stored in the artificial saliva at room temperature (23 ± 1°C)[14] for the rest of the day.

According to Karadas and Seven[13], it was concluded that 30 days immersion time for 6 h/day resembles a period of 24 months of clinical use.

Tea solution was prepared by adding one premeasured red tea bag (2 g) (Lipton tea) into distilled water (150 ml) and boiled for 5 min[15]. The coffee was prepared by dissolving 3.5 g of coffee powder (Nescafe original mix 2 in 1) in 300 ml of boiling distilled water, then was stirred for 10 min[16]. The hibiscus solution was prepared by adding (150 ml) freshly boiled water to one premeasured hibiscus tea bag and brew for 3 min. The solutions were cooled to room temperature before used and they were freshly prepared and were changed daily[15]. The specimens were rinsed with distilled water for 5 min during change over from one solution to another and air dried[16].

The specimens of group 2 and 3 were subjected to bleaching procedures, then were stored in artificial saliva for 2 h. Then all specimens of subgroups B were subjected to staining cycles after completion of the bleaching procedures followed by 2 months[12] storage of both subgroups A and B in artificial saliva. Then bleaching procedures performed previously were repeated for group 2 and 3 which were considered as a second-time bleaching followed by repetition of the previous staining cycles for group 1, 2, and 3 which were also considered as a second staining cycles.

All the bleaching procedures and the staining cycles were repeated and carried at room temperature (23 ± 1°C)[14].

The shade was repeatedly determined for all teeth at the base line before any treatment, 2 h. after bleaching whether first or second bleaching regimens and after immersion in staining solution (30 days) whether the first or second staining cycle using spectrophotometer (Vita EasyshadeV) (Ultradent, South Jordan, Utah, USA). This instrument was calibrated.

Base shade determination symbol was selected on the measurement style menu to measure the base shade of specimens. Easyshade V displayed the results of the measurement in the VITA system 3D-Master VITA classical shade and as VITABLOCS shade or bleach index shade system[17]. Shades of the teeth were determined in the L*a*b color space which allow images to be not affected by a visual determination, such as visual perception, office lighting or time of day[18],[19].

Total color differences or distances between two colors (ΔE) were calculated automatically by the software according to the following equation:

where ΔL*, Δa*, and Δb* are the differences in the respective values.

A one-way analysis of variance was used when comparing the tested groups, when the P value indicated any significant difference post-hoc test (Pair wise test) was used to compare quantitative data within tested groups. Comparison between the two tested subgroups at each group was performed using (t-test).

  Results Top

The total color changes ΔE was perceivable by untrained observers after first staining cycle for all tested subgroups with a statistical significant difference between ΔE of subgroups (A vs B) of the tested groups [Table 3], also a statistical significant difference was found between subgroups (A or B) of the two tested groups [Table 4] and [Table 5]. Mean shade guide unit color rebound of at home bleaching regimen and for in office bleaching regimen concerning storage in saliva for first time and/or first staining cycle was showed in [Table 6].
Table 3: Statistical comparison of the subgroups of tested groups after first staining cycle using means±SD of ΔE

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Table 4: Pairwise test for subgroup A of the tested groups using means±SD of ΔE

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Table 5: Pairwise test for subgroup B of the tested groups first time staining using means±SD of ΔE

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Table 6: Vita classical shade A1-D4 after first staining cycle

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The total color change (ΔE) were perceivable by untrained observers after second staining procedure in the tested groups, with a statistical significant difference between the subgroups (A vs B) of the two tested groups [Table 7], and a statistical significant difference was also recorded concerning subgroup A of two tested groups [Table 8], while no statistical significant difference between the two tested groups concerning subgroup B [Table 9]. Mean shade guide unit color rebound of at home bleaching regimen and for in office bleaching regimen concerning storage in saliva for second time and/or second staining cycle was showed in [Table 10].
Table 7: Statistical comparison of the subgroups (A vs B) of tested groups using means±SD of ΔE

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Table 8: Pairwise test for total color change values (ΔE) subgroups A of the tested groups

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Table 9: Pairwise test for the total color change values (ΔE) subgroups B of the tested groups

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Table 10: Vita classical shade (A1-D4) after second staining cycle

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A statistical significant difference was recorded between the total color change values after staining for first vs second time in the tested groups (1, 2, and 3), while specimen preserved in saliva nonstatistical significant deference was recorded in the tested groups [Table 11].
Table 11: Comparison between two staining cycles using mean±SD of total color change values (ΔE)

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  Discussion Top

Bovine and human dental substrates have similar morphology, physical, and chemical properties[20] and behave similarly during the bleaching process[21]. The ease of obtaining bovine incisors was also decisive in the choice because human incisors with similar anatomy would be hard to find in sufficient numbers[20].

The pulp tissues of specimens were removed using endodontic broach to avoid the ingress of the necrotic elements through the dentinal tubules which might hinder the result of the bleaching treatment[22]. The crowns of the specimens were individually embedded in chemically cured acrylic resin exposing the labial surface[13] to avoid the leakage of the bleaching gel inside the pulp chamber through root canal after root sectioning thus considering it as non-vital tooth bleaching which is not the goal of this study.

Tooth discoloration is another factor to select teeth for bleaching procedures. The teeth with extrinsic stains, caries, cracks, hypoplastic enamel, white spots, or other dental defects were excluded since these could modify the final results after the bleaching procedure. The choice of the middle third of teeth crowns as the area for the evaluation of color was determined to eliminate the translucent area, the worn incisal edge or the non-flat gingival surface[23].

Colorimeters and spectrophotometers are common instruments to measure tooth color change. Depending on the type, they can be used in an in-vitro or in-vivo environment. The present study used VITA Easy shade V, which is ideal for clinical settings because of the innovative software concept in combination with the VITA vBrain neural network, guarantees exact tooth-shade determination in accordance with the internationally established shade systems VITA classical A1–D4, VITA SYSTEM 3D-MASTER and VITABLOCS, as well as the bleached shades defined by the American Dental Association[17].

The samples were stored in artificial saliva throughout the experiment, to simulate both the remineralization of the bleached specimens[24] and the impact of saliva as being significant in the formation of tooth staining[13]. This also prevented color changes due to dehydration effects.

Bleaching studies can differ in methodology, concentration of HP, mode and duration of application; therefore, it is important to understand factors related to color change following bleaching procedures used, for this specific study. The bleaching product used for the current study was Opalescence PF (15% CP) and Opalescence Boost PF 40%. During the bleaching technique in this study the teeth were kept at room temperature (23 ± 1°C)[14]. All bleaching products were applied according to the manufacturers' instructions.

Concerning color rebound in the present study the total color change after first staining cycle ΔE were perceivable by untrained observers for both bleaching techniques as color rebound was from A1 to C4 concerning at home bleaching regimen and from A1 to B4, A4 or C4 concerning in office bleaching regimen [Table 6]. The effect of staining produced by beverages is usually believed as explanation for the darkening that follows bleaching over time[23].

However, one should consider that this staining is usually extrinsic and although it may affect the overall perception of whiter teeth, this can be easily removed by professional cleaning, but this did not perform in the current study to avoid the effect of brushing on bleached enamel. Some studies proved that even though bleaching did not alter the surface roughness, it increased surface wear when combined with brushing[25]. Color rebound can also result from reversal of oxidative reactions so that the shorter and lighter molecules produced by the bleaching therapy return to their original configuration and yellower color[26].

This assumption might be valuable to explain that in the present study the color change showed a statistical significant relapse after both bleaching techniques after the 24 months simulating period when bleaching was performed for the first time. In the present study the color rebound after first staining cycle revealed a statistical significant difference [Table 5] between two tested groups, where at home bleached specimens showed darker color compared to in office bleached specimens.

These findings could be explained by the readily decomposition of HP when it encounters substances with which it can react resulting in the release of free radicals. If the bleaching procedure is continued beyond the saturation point, in the course of decomposition, HP also interacts with the organic components of the tooth, such as proteins and lipids, resulting in their removal[27], this makes the surface rough. HP also dissolves the inorganic components of enamel by penetrating the intra- and inter-prismatic regions. An increased exposure to HP leads to an increase in its penetration into the enamel. This leads to a deepening of the grooves in an already roughened enamel surface[28]. As the exposure time is much higher in at home bleaching than in office, more deleterious effect happens to enamel so it become more vulnerable to staining.

Regarding efficacy, there is a positive correlation between the rebounding of mineral density of the tooth and the degree of lightening. Using the at-home bleaching technique, the teeth received a continual application of HP during which the demineralization and remineralization processes interact[29]. While in office bleaching, the color regression is primarily a result of the reversal of whitening which is due to just the remineralization process[30]. Furthermore, in office bleaching can have a dehydration effect on bleached teeth, which interferes with the evaluation of the color differences. In the present study, in order to decrease the consequences of whitening such as dehydration of teeth, the color evaluation of bleached teeth was done 2 h after the completion of the bleaching procedure rather than immediately thereafter. Even though, this time is insufficient for complete rehydration of the bleached teeth there is a limitation on delaying the color evaluation any longer because the regression of whitening might occur and interfere with the true results in terms of the degree of whitening and color regression. This procedure also has already been described by Li et al.[29].

In the present study when staining cycle was performed for the second time after second time bleaching, there was a significant relapse after at home and in office bleaching techniques after 24-month simulating period as color rebound of at home bleaching regimen was from A3 or A3.5 to C4 and for in office bleaching regimen from A3, B3 or A3.5 to C4 [Table 10].

There was no significance difference between the bleaching techniques in color rebound after bleaching for a second time [Table 9]. This mean that despite the bleaching technique performed rebound take place 2 years after bleaching retreatment. Rebound similarity between the two regimens in spite of different peroxide concentration (15% CP and 40% HP) and exposure time (56 h for at home bleaching and 80 h for in office bleaching) indicated that such negative effects may not be related to the peroxide concentration and exposure time but related to repetition of the bleaching procedure.

In the present study comparing first vs second staining cycles revealed that, second staining cycle showed a significant difference compared to first staining cycle in tested groups [Table 11]. This might be explained by that, staining susceptibility of teeth after second bleaching increased regardless of the applied bleaching regimen indicating that enamel roughness increased due to bleaching of the specimen for second time, where the bleaching agents may alter the surface roughness of the enamel even under using of artificial saliva as a preserving solution and using bleaching gel at a neutral pH level.

Some studies shown that using bleaching gels with concentrations 10 and 16% CP for long durations resulted in decreasing enamel microhardness, decrease in calcium and phosphate content and increase in surface roughness[31],[32]. In addition, it was found that bleaching increased enamel demineralization[33]. Also, some studied showed that higher bleaching gel concentrations lead to more deleterious effects on enamel as more softening, demineralization and increased surface roughness[32],[34]. These studies explain the rebound occurred after first bleaching procedure and the rebound that increased after second bleaching procedure.

  Conclusion Top

Both regimens showed a significant rebound after 2 years simulating period after first treatment, but in office bleaching was at lower rate compared to at home bleaching rebound.

Both bleaching regimens showed a significant rebound after 2 years simulating period after second treatment with same staining susceptibility of both bleaching techniques.

Staining susceptibility of both bleaching regimens increased after performing bleaching regimens for second time.

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Conflicts of interest

There are no conflicts of interest.

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  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9], [Table 10], [Table 11]


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