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Year : 2014  |  Volume : 2  |  Issue : 2  |  Page : 24-27

In vitro evaluation of the microhardness of human enamel exposed to the acid solution after bleaching

Department of Preventive and Community Dentistry, College of Dental Sciences, Davangere, Karnataka, India

Date of Web Publication21-Oct-2014

Correspondence Address:
Naveen P. G. Kumar
Department of Community Dentistry, College of Dental Sciences, Davangere - 577 004, Karnataka
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2348-1471.143324

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Introduction: Acid erosion is the superficial loss of enamel caused by chemical processes that do not involve bacteria. Intrinsic and extrinsic factors, such as the presence of acid substances in the oral cavity, may cause a pH reduction, thus potentially increasing acid erosion. Objective: Aim of this study was to evaluate the microhardness of bleached and unbleached human enamels after the immersion in a soft drink for 7 and 14 days. Materials and Methods: Thirty freshly extracted human teeth were used in the experiment. The specimens were submitted to initial microhardness measurement. The specimens of groups non-hydrogen peroxide and hydrogen peroxide, each of which was exposed to one of the respectively assigned acid solutions for 5 minutes and stored in artificial saliva for 23 hours and 55 minutes to complete a period of 24 hours. This process was repeated for 7 days in a row, where all the specimens were submitted to microhardness measurement. Results: The main effects of enamel microhardness revealed the statistical significance of the bleaching agent, as a greater microhardness mean value was observed for the unbleached condition, as compared with the bleached condition.

Keywords: Bleaching, enamel, human enamel, microhardness

How to cite this article:
Vivek H P, Prashant G M, Kumar NP, Basha S, Sushanth V H, Imranulla M. In vitro evaluation of the microhardness of human enamel exposed to the acid solution after bleaching. Dent Med Res 2014;2:24-7

How to cite this URL:
Vivek H P, Prashant G M, Kumar NP, Basha S, Sushanth V H, Imranulla M. In vitro evaluation of the microhardness of human enamel exposed to the acid solution after bleaching. Dent Med Res [serial online] 2014 [cited 2022 Dec 5];2:24-7. Available from: https://www.dmrjournal.org/text.asp?2014/2/2/24/143324

  Introduction Top

The twenty-first century has been marked by an increasing concern for oral health and esthetics, leading to a greater number of patients seeking cosmetic dental procedures, particularly dental bleaching. [1]

There are many bleaching materials available on the market, ranging from over-the-counter products to sophisticated in-office bleaching systems. However, the deleterious effects of these bleaching agents on the dental structure vary, and are related to agent composition and concentration, time of exposure, pH values and type of solutions used. [1]

The mechanisms underlying tooth bleaching have not been fully elucidated. However, it has been proposed that the strong oxidative action of the free radicals generated by hydrogen peroxide (H 2 0 2 ) acts by breaking the polypeptide chain caused by the destruction of amino acids that are part of the composition of the organic substance, and the main agents responsible for tooth bleaching may be hydroxyl radicals.

Some of these factors have been connected to dental loss, and, when this occurs, loss of structural material caused by non-carious lesions, such as attrition, abrasion and erosion, is also involved. [2]

There is no general consensus on the negative effects of bleaching agents on enamel and dentin. Most studies have reported that peroxide-containing bleaching agents have no significant deleterious effects on the enamel or dentin surface or subsurface morphology, chemistry, microhardness, or ultra structure. However, some researchers have claimed that tooth-whitening procedures irreversibly damage tooth structure and may cause changes in enamel microstructure, notably a demineralization that is similar to initial caries. [2]

Patients often need further interventions after bleaching, in order to restore esthetic defects. Acid etching of the enamel is required shortly after bleaching in numerous clinical procedures, including composite restorations, replacement of old restorations, porcelain veneers, composite veneers, and bonding of orthodontic brackets. The acid etching of bleached enamel surfaces has been reported to produce a loss of prismatic shape, giving the surface an over-etched appearance. [3]

There is no diagnostic device available to clinically detect and quantify enamel hardness in vivo. Hence, an in vitro study was carried out to check the enamel hardness after bleaching.

Objectives of the study

To evaluate the superficial microhardness of bleached and unbleached enamels exposed to soft drink for the 7 th and 14 th days.

  Materials and methods Top

The study was conducted in the Department of Preventive and Community Dentistry, College of Dental Sciences, Davangere, Karnataka.

A total of 30 extracted premolar and molar teeth was used in the experiment. Enamel specimens were prepared in the Department. The specimens were of human, non-carious permanent molar or premolar tooth (dimension: Breadth 3 mm, width 2 mm), which were embedded in acrylic block. Enamel microhardness was analyzed at Department of Preventive and Community Dentistry, College of Dental Sciences, Davangere.

This was an in vitro double-blinded study; a hydrogen peroxide solution was used as a bleaching agent. The estimated sample size was 15 in each group and so, a total of 30 samples (enamels - 30) were taken.

A total of 30 enamel tissue specimens of Dimension: Breadth 3mm, width 2 mm, which was embedded in acrylic block. Of them, 15 enamel tissue specimens were allotted to each group [Table 1].
Table 1: Groups in the study

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Prior to the experiment, baseline measurements were performed in order to evaluate reference surfaces for calculation of enamel hardness. All 30 specimens were randomly divided into two groups of 15 each: Non-hydrogen peroxide (NHP) and hydrogen peroxide (HP). The specimens were submitted to an initial microhardness measurement. NHP samples were exposed to soft drink for 5 minutes and stored in artificial saliva for 23 hours and 55 minutes, to complete a period of 24 hours; and this procedure was repeated for 7 consecutive days, without any previous hydrogen peroxide treatment. HP samples were treated with a 35% hydrogen peroxide solution for 2 minutes and each of which was exposed to soft drink for 5 minutes and stored in artificial saliva for 23 hours and 53 minutes, to complete a period of 24 hours; and this procedure was repeated for 7 consecutive days. Then the specimens were subjected for second microhardness measurement. This 7-day regimen was repeated and third microhardness measurements were recorded at the 14 th day. The specimens were placed in artificial saliva until the time of measurement of microhardness. The hardness of specimens was determined using Vickers hardness tester (XHV-1000 Digital Micro Vickers Hardness Tester). Vickers hardness number is the applied load (kgf) divided by the surface area of the indentation (mm 2 ).


F = Load in kgf

d = Arithmetic mean of the two diagonals, d1 and d2 in mm

HV = Vickers hardness

Sensitivity = 80%

Statistical analysis

Between the group comparison at different time interval was done with un-Paired 't' test. P < 0.05 was considered as statistically significant.

  Results Top

The mean and standard deviation of the two groups at different time interval are given in [Table 1]. In HP group, at baseline, the mean was 239.2 ± 15, 7 th day mean was 202.4 ± 10 and on 14 th day the mean was 163.2 ± 10. In NHP group, at baseline, the mean was 240.2 ± 19, 7 th day mean was 234.8 ± 19 and on 14 th day, the mean was 226.5 ± 19. There was a statistical difference observed at 7 th and 14 th days (P = 0.001) [Table 2] and Graph 1 [Additional file 1]].
Table 2: Mean and standard deviation of the two groups at different time interval

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  Discussion Top

Physical contact between bleached teeth and acid substances, generally contained in foods or produced by gastroesophageal disturbances, may result in severe enamel demineralization. [4],[5] Therefore, scientific studies are needed to elucidate the relationship between dental bleaching and dental erosion. In such studies, it is important that human or bovine teeth be stored and prepared appropriately. [6]

In the present study, the effect of acid substances on the enamel surface of freshly extracted human teeth submitted to bleaching or left unbleached was investigated instead of bovine enamel, which was used in other investigation. Davidson et al. [7] found that the calcium content by weight of bovine and human tooth enamel were 37.9% and 36.8%, respectively, and the calcium distribution was more homogenous in bovine enamel compared with human enamel. Several studies were conducted to check the microhardness on both human and bovine teeth. Inconsistent outcomes were suggested regarding the use of bovine substrate as an alternative to human substrate in microhardness studies, and also to extrapolate the results for in vivo conditions. Human teeth were used in the present study.

Hydrogen peroxide is one of the most important bleaching products available today, and is a substance capable of altering the enamel surface. Its action is generally a result of its oxidizing properties, which removes the organic material without dissolving the enamel matrix, eventually producing clearer compounds. However, long exposure could result in dissolution of the enamel matrix.

Other studies have reported high solubility and decreased resistance of enamel, dentin and cementum, when exposed to bleaching agents. Demineralization, protein matrix destruction and deposition of small amorphous mineral precipitates have been reported to affect prism periphery. [8],[9]

The results of this study show that the action of the bleaching agent was significant. The mean microhardness value for the unbleached teeth was greater than that observed for the bleached teeth on 7 th and 14 th days [Table 1]. The microhardness values of the unbleached and bleached specimens followed a decreasing trend throughout the immersion periods of 7 th and 14 th days.

The present study and other reports found in the literature suggest that human enamel could be affected by chemical substances, depending on substance composition and acidity level, as well as on the time of exposure to these substances. Thus, deleterious effects on dental enamel can be observed when it comes into frequent contact with acidic drinks, particularly when associated to a low saliva flow rate and poor oral hygiene habits.

Further investigations are required to better elucidate the acid demineralization process. We suggest that such studies include the variables of consumption frequency, swallowing speed, consumption temperature, period of exposure to gingival fluid and remineralization ability.

  Conclusions Top

The time period of contact between acid solutions and teeth directly affected the structural demineralization of enamel: The longer the exposure time, the greater the surface demineralization.

The enamel microhardness values for unbleached teeth were higher than those of bleached teeth.

  References Top

1.Zanet CG, Fava M, Alves LA. In vitro evaluation of the microhardness of bovine enamel exposed to acid solutions after bleaching. Braz Oral Res 2011;25:562-7.  Back to cited text no. 1
2.Torres-Rodríguez C, González-López S, Bolaños-Carmona V, Sánchez-Sánchez P, Rodríguez-Navarro A, Attinf T. Demineralization effects of phosphoric acid on surface and subsurface bovine enamel bleached with in-office hydrogen peroxide. J Adhes Dent 2011;13:315-21.  Back to cited text no. 2
3.Rajesh AG, Ranganath LM, Prem Kumar KS, Sunil Rao B. Surface morphological changes in human enamel following bleaching: An in vitro scanning electron microscopic study. J Contemp Dent Pract 2012;13:405-15.  Back to cited text no. 3
4.4 Nucci C, Marchionni S, Piana G, Mazzoni A, Prati C. Morphological evaluation of enamel surface after application of two 'home' whitening products. Oral Health Prev Dent 2004;2:221-9.  Back to cited text no. 4
5.Attin T, Betke H, Schippan F, Wiegand A. Potential of fluoridated carbamide peroxide gels to support post-bleaching enamel re-hardening. J Dent 2007;35:755-9.  Back to cited text no. 5
6.Serra MC, Messias DC, Turssi CP. Control of erosive tooth wear: Possibilities and rationale. Braz Oral Res 2009;23:49-55.  Back to cited text no. 6
7.Davidson CL, Boom G, Arends J. Calcium distribution in human and bovine surface enamel. Caries Res 1973;7:349-59.  Back to cited text no. 7
8.Bartlett DW, Evans DF, Smith BG. The relationship between gastro-oesophageal reflux disease and dental erosion. J Oral Rehabil 1996;23:289-97.  Back to cited text no. 8
9.Bitter NC. A scanning electron microscope study of the long-term effect of bleaching agents on the enamel surface in vivo. Gen Dent 1998;46:84-8.  Back to cited text no. 9


  [Table 1], [Table 2]


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