Review Of The Mechanism Of Tooth Whitening 5h3p3x

REVIEW ARTICLE

Review of the Mechanism of Tooth Whitening SO RAN KWON, DDS, MS, PHD*, PHILIP W. WERTZ, PHD†

ABSTRACT Purpose: This review integrated the current literature on diffusion of whitening agents, their interactions with stain molecules, and changes to the surface, with the aim of establishing a better understanding of the mechanism underlying tooth whitening. Materials and Methods: An electronic PubMed database search, with combinations of the following was performed: Tooth Bleaching, Tooth Bleaching Agent, Hydrogen Peroxide, Pharmacokinetics, Tooth Permeability, Oxidation-Reduction, Tooth Demineralization, and Color. Results: Tooth whitening is a dynamic process that involves diffusion of the whitening material to interact with stain molecules and also involves micromorphologic alterations on the surface and changes within the tooth that affect its optical properties. The interaction seems not to be limited to stain molecules, but rather an affinity-based interaction process that also accompanies effects on sound enamel and dentin structures. Conclusions: This review underlines that supervision by dental health professionals as recommended by the American Dental Association (ADA) Council on Scientific Affairs is critical to achieving a successful and safe whitening outcome.

CLINICAL SIGNIFICANCE The mechanism that underlies tooth whitening with the use of peroxide-based materials is a complex phenomenon encoming diffusion, interaction, and surfaces changes within the tooth. Therefore, supervision by dental health professionals as recommended by the ADA Council on Scientific Affairs is imperative to achieve a successful and safe whitening outcome. (J Esthet Restor Dent 27:240–257, 2015)

INTRODUCTION The demand for tooth whitening has been building for more than a decade as a growing number of people envision and desire a Hollywood smile. Tooth whitening now represents the most common elective dental procedure1 and has been proven safe and effective when supervised by the dentist.2 More than 1 million Americans whiten their teeth annually, driving nearly $600 million in revenues for dental offices.1 Additionally, many dentists are using tooth whitening as a tool to market additional esthetic procedures available in their clinic. The high demand is also reflected in the distribution and use of a vast variety of

whitening materials by dental professionals, as well as in over-the-counter sales of materials directly to consumers. In spite of the tremendous growth in the tooth whitening market, the basic mechanism underlying the whitening process remains unexplained. Color-producing stains within tooth structures are often organic compounds that contain conjugated double bonds. It is known from dye chemistry that decoloration can occur due to the breakup of a chromophore, and that destruction of one or more of the double bonds within the conjugated system is probably involved. Thus, the dominant theory on the

*Associate Professor, Department of Operative Dentistry, University of Iowa College of Dentistry & Dental Clinics, Iowa City, IA, USA † Professor, Oral Pathology, Radiology and Medicine, Dows Institute for Dental Research, University of Iowa College of Dentistry & Dental Clinics, Iowa City, IA, USA

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whitening mechanism is that stain molecules are oxidized into colorless compounds. The mechanism that results in changed perception of tooth color can be subdivided into three distinct phases (Figure 1): first, movement of the whitening agent into the tooth structure; second, interaction of the whitening agent with the stain molecules; and third, alteration of the tooth structure surface such that it reflects light differently. The outcome of this sequence of events would be the final color change of the tooth after whitening. Ideally, whitening procedures will optimize whitening and at the same time minimize concurrent damage to the tooth structure. Here, we integrate the current literature on the diffusion of whitening agents, their interactions with stain molecules, and changes to the tooth surface, with the aim of establishing a better understanding of the

mechanism underlying tooth whitening. It is important to point out that this review focused on the mechanism of peroxide-based whitening limiting the topic specifically to oxidative processes within the hard dental tissues and did not include whitening by means of mechanical measures nor the effect of peroxide-based materials on the soft tissue. This study was initiated with an electronic search of the PubMed database using various combinations of search selected from the Medical Subject Headings (MeSH) for PubMed. These search were: Tooth Bleaching, Tooth Bleaching Agent, Hydrogen Peroxide, Pharmacokinetics, Tooth Permeability, Oxidation-Reduction, Tooth Demineralization, and Color. This search was restricted to articles in English and published between 1968 and May 2014. Two other articles that date back to the late 1800s were found from a reference and included for this review. Additionally, textbooks related to biochemistry and tooth whitening were utilized. From a total of 2,885 titles, 379 abstracts were retrieved and considered for inclusion by two reviewers. Among the study materials represented, a total of 112 articles are covered in this review.

TOOTH WHITENING AGENT

FIGURE 1. Illustration of the dynamics of diffusion and interaction of whitening agents and surface changes at the tooth surface. Upper inset: Diffusion of Rhodamine B at the dentino-enamel junction imaged with Zeiss 710 confocal laser scanning microscope (Carl Zeiss Microimaging GmbH, Jena, ). Middle inset: Intratubular collagen fibers imaged with Hitachi S-3400 scanning electron microscope (Hitachi High-Tech, Tokyo, Japan) and possible interaction with hydrogen peroxide molecules. Lower inset: Surface changes associated with tooth whitening material imaged with Bruker Multimode 8 atomic force microscope (Bruker Corp., Billerica, MA, USA).

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The quest for the ideal agent for whitening of discolored teeth began in the 1800s. At that time, all agents employed for tooth whitening were mixed in the dentist’s office and consisted of direct or indirect oxidizers that acted not only on the chromogen but also on the organic portion of the tooth.3 The variety of whitening agents used reflects the diverse nature of discoloration: oxalic acid was used for the removal of iron stains associated with pulp necrosis and hemorrhage; chlorine was indicated for silver and copper stains produced in the process of amalgam-based restoration;4 and cyanide of potassium could be used to remove the most resistant stains that arose from metallic restorations, although this was not recommended due to its highly poisonous nature.5 In 1884, Harlan published what is believed to be the first report of the use of hydrogen peroxide, which he called

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TABLE 1. Whitening agents used in dental applications Whitening agent

Chemical formula

Molar mass

Range of percentage

Mode of action

Free radical

Hydrogen peroxide

H2O2

34.01 g/mol

5–40

Oxidation

·OH, ·OOH−, O·−2

Carbamide peroxide

CH6N2O3

94.07 g/mol

10–35

Oxidation

·OH, ·OOH−, O·−2

Sodium perborate

NaBO3

99.82 g/mol

NA

Oxidation

·OH, ·OOH−, O·−2

Chlorine dioxide

ClO2

67.45 g/mol

0.07

Oxidation

ClO2

hydrogen dioxide.6 Although a wide variety of whitening products are currently available, in most cases, hydrogen peroxide is the active agent.7 Hydrogen peroxide may be applied directly or produced in a chemical reaction from sodium perborate or carbamide.8 Common whitening agents and their dental applications are described below and their properties are summarized in Table 1. Hydrogen peroxide (H2O2) is a colorless liquid, is slightly more viscous than water, and has a molar mass of 34.01 g/mol.9 Because of its low molecular weight, it can penetrate dentin, where it releases oxygen and thereby breaks the double bonds of the organic and inorganic compounds inside the dentinal tubules.10 In dentistry, hydrogen peroxide is used at concentrations ranging from 5% to 35%.11 It acts as a strong oxidizing agent, producing reactive oxygen molecules and hydrogen peroxide anions. Hydrogen peroxide is naturally produced, controlled, used, and destroyed during normal function of the body. Indeed, the human body can protect itself against oxidative stress by harnessing the glutathione redox cycle, catalase, ascorbate, superoxide dismutase, prostaglandin E1, glutathione peroxidase, vitamin E, and plasma peroxidase.12 Carbamide peroxide (CH6N2O3) is a white crystalline solid that releases oxygen when it comes into with water.13 The concentrations used for bleaching ranges from 10% to 35%. A 10% carbamide peroxide solution breaks down into 3.35% hydrogen peroxide and 6.65% urea.14 The urea further breaks down into ammonia and water, which may provide some beneficial side effects because it tends to increase the pH of the solution.15 Additionally, urea has proteolytic properties

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that may affect the efficiency of tooth whitening.16,17 Carbamide peroxide products usually contain either a carbopol or a glycerin base. The carbopol base slows down the release of hydrogen peroxide and is thus effective over a longer period of time.18 Sodium perborate (NaBO3) is a white, odorless, water-soluble chemical compound available as a powder.13 It has been employed as an oxidizer and whitening agent, especially in washing powders and other detergents, since 1907.19 Although it is stable when dry, in the presence of acid, warm air, or water, it breaks down to form sodium metaborate, hydrogen peroxide, and oxygen.11 Sodium perborate comes in various forms—monohydrate, trihydrate, and tetrahydrate—which differ in oxygen content and thus have different whitening efficacy.20 A mixture of sodium perborate and distilled water (2 g/1 mL) has an effect equivalent to that of 16.3% hydrogen peroxide.21 Chlorine dioxide (ClO2) is a potent and useful oxidizing agent and is commonly used in water treatment and bleaching. Nondental establishments in the United Kingdom introduced the use of chlorine dioxide, which raised concerns with respect to safety.2 Despite the safety concerns, an in vitro study showed that 0.07% chlorine dioxide effectively whitened teeth at a faster rate than 35% hydrogen peroxide.22

DIFFUSION Tooth whitening is based on the premise that hydrogen peroxide penetrates into the enamel and dentin to interact with the organic chromophores. It is well

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known that the dental hard tissues are highly permeable to fluids, and that the greatest fluid flow in the enamel and dentin is in the interprismatic spaces and dentinal tubules, respectively.23–25 Therefore, enamel and dentin are expected to act as semipermeable membranes and that they allow hydrogen peroxide to move according to Fick’s second law of diffusion, which describes that the diffusion of a molecule is proportional to the surface area, diffusion coefficient, and concentration, and that it is inversely proportional to the diffusion distance.26 Despite the fact that peroxide-based tooth whitening was introduced in the 1800s, it was in 1987 that hydrogen peroxide penetration into the pulp cavity was first detected and quantified.27 In this study, extracted teeth were exposed to 30% hydrogen peroxide, and spectrophotometric measurement of sub-microgram amounts of hydrogen peroxide was performed based on the use of leucocrystal violet and horseradish peroxidase, a well-established, accurate, selective, and sensitive analytical method.28 This in vitro model proved useful for further studies investigating various factors that might influence hydrogen peroxide penetration of the pulp cavity. These studies and the results are summarized in Table 2. Overall, hydrogen peroxide penetration was found to be enhanced by the following: higher hydrogen peroxide concentrations;27,31,34,40 prolonged application;29,31 increased temperature;27,29 the large size of the openings of dentinal tubules in young teeth;36 variations in the tooth structure due to location, acid-etching or restorations;33,37,39,44,48 and light activation.38 Penetration was also improved by specific formulations and delivery systems.30,32,35,42,46 The results of all reviewed studies are basically in accordance with Fick’s second law of diffusion. The clinical relevance of these studies is not clear because the model system did not fully replicate the dynamics of the oral cavity; in particular, the presence of the positive outward pulpal pressure associated with vital teeth was not emulated. Nonetheless, it has been cautioned to increase efficiency to the point where hydrogen peroxide penetrates the pulp cavity, due to

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potentially adverse and cytotoxic effects, especially in patients with existing hypersensitivity, gingival recession, attrition, cervical abrasion, and leaking restorations.31 Particularly noteworthy findings include the observation that the dynamics of hydrogen peroxide diffusion were constant over a prolonged period, even when the material was not replenished.43,45,49 Also, the inclusion of chemical activators resulted in significantly enhanced whitening efficacy even though it was associated with a reduction in penetration that has been attributed to exhaustion of the hydrogen peroxide molecules within the tooth structure.41 The latter observation is consistent with the finding that overall hydrogen peroxide penetration does not correlate with whitening efficacy.45 Thus, low penetration does not necessarily imply increased whitening efficacy and vice versa. The relationship between the diffusion of hydrogen peroxide into the enamel and dentin and its interaction with the dentin structure also showed that diffusion does not result in merely physical age but rather in a concentration gradient that is determined by its particular chemical affinity for each dental tissue.47 Notably, hydrogen peroxide does not interact only solely with chromophores during diffusion but also with sound tooth structure. Therefore, it seems prudent to identify optimal concentration and application times, i.e., those that minimize penetration of the pulp cavity by hydrogen peroxide without compromising whitening efficacy.

INTERACTION Traditionally, tooth whitening mechanism has been represented by the “chromophore theory,” which is based mainly on the interaction of hydrogen peroxide with organic chromophores within the tooth structure. Organic chromophores are colored molecules that consist of either conjugated pi systems, such as aromatic compounds that have electron-rich areas, or bioinorganic metallic complexes, such as chelates.50 When reactive oxygen species encounter stain molecules, they convert the chains of the latter into

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TABLE 2. Studies that quantified hydrogen peroxide penetration levels Bowles and Ugwuneri 198727 Specimen

Human maxillary anterior teeth (N = 58)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

1, 10, 30% HP for 15 minutes at 37 and 50°C

Result and comments

The amount of HP penetration was concentration and temperature dependent

Rotstein and colleagues 199129 Specimen

Human premolar teeth (N = 24)

Quantification method

Ferrous ammonium chloride and potassium thiocyanate to form ferrithiocyanate complex

Concentrations used

30% HP for 5, 20, 40, 60 minutes at 24, 37, and 47°C

Result and comments

No penetration at 5 minutes regardless of temperature. Radicular penetration was time and temperature dependent.

Cooper and colleagues 199230 Specimen

Human anterior teeth (N = 40)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

10% and 15% , 5% and 30% HP for 15 minutes at 37°C

Result and comments

Less penetration from sources than the same HP equivalent

Hanks and colleagues 199331 Specimen

Dentin discs (N = 6)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

3% and 10% HP, 10%, 15% for 15 minutes, 1 and 6 hours at 37°C

Result and comments

The amount of HP penetration was concentration and time dependent

Thitinanthapan and colleagues 199932 Specimen

Human premolars (N = 70)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

10% from three different formulations for 25 minutes at 37°C

Result and comments

Penetration of whitening products were different though the products were all labeled to have 10%

Benetti and colleagues 200433

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Specimen

Bovine lateral incisors (N = 60)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

10% and 35% in sound and restored teeth for 60 minutes at 37°C

Result and comments

Penetration was concentration dependent with higher HP penetration in restored teeth

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TABLE 2. Continued Gökay and colleagues 200434 Specimen

Human maxillary central incisors (N = 24)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

6.5% and 14% HP for 30 minutes at 37°C

Result and comments

Significant HP penetration with whitening strips which was concentration dependent

Gökay and colleagues 200535 Specimen

Human maxillary central incisors (N = 50)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

5.3% HP, 19% NPP, 18% , 8.7% HP for 30 minutes at 37°C

Result and comments

HP from whitening strips and paint on whiteners readily penetrates into the pulp chamber at various amounts

Camps and colleagues 200736 Specimen

Human premolars (N = 36)

Quantification method

Ferrous ammonium chloride and potassium thiocyanate to form ferrithiocyanate complex

Concentrations used

20% measured at 1, 24, 48, 120 hours at a temperature not specified in the methods

Result and comments

Maximal HP diffusion and diffusive flux through dentin was higher for young than old teeth

Camargo and colleagues 200737 Specimen

Human third molars (N = 70), bovine lateral incisors (N = 70)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

38% HP for 40 minutes at a temperature not specified in the methods

Result and comments

HP penetration was higher in human teeth for any experimental situation

Camargo and colleagues 200938 Specimen

Bovine lateral incisors (N = 48)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

35% HP with LED and Nd:YAG laser activation for 20 minutes at not-specified temperature

Result and comments

LED and Nd:YAG laser activation increased HP penetration in bovine teeth

Camps and colleagues 201039 Specimen

Human premolars (N = 46)

Quantification method

Ferrous ammonium chloride and potassium thiocyanate to form ferrithiocyanate complex

Concentrations used

35% HP measured at 24, 48, 168 hours at not-specified temperature

Result and comments

Higher HP penetration when acid-etching of dentin was performed

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TABLE 2. Continued Palo and colleagues 201040 Specimen

Bovine lateral incisors (N = 72)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

Variations of Walking Bleach technique: 35% HP, 35% , SP, SP + HP, SP + for 7 days at 37°C

Result and comments

There was direct correlation between the presence of oxidative agents and HP penetration potential

Torres and colleagues 201041 Specimen

Bovine incisors (N = 104)

Quantification method

4-aminoantipyrine and phenol with HP catalyzed with peroxidase

Concentrations used

35% HP and manganese gluconate, manganese chlorite, ferrous sulfate, ferrous chloride, mulberry root extract

Result and comments

Chemical activation using metal salts increased whitening efficacy and decreased HP penetration levels

Pignoly and colleagues 201242 Specimen

Bovine teeth (N = 30)

Quantification method

Peroxi kit (Sigma Chemical Co, St. Louis, MO, USA)

Concentrations used

30% HP pH 3.0; 35% HP pH 5.0; 38% HP pH 7.0 measured every 10 minutes for 1 hour at not-specified temperature

Result and comments

No difference in whitening efficacy among different groups and no effect of pH on HP diffusion coefficient

Kwon and colleagues 201243 Specimen

Human canines (N = 20)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

40% HP measured every 10 minutes for 1 hour at 25°C

Result and comments

Time course diffusion kinetics of HP penetration showed constant HP penetration over an hour period

Palo and colleagues 201244 Specimen

Bovine teeth (N = 50)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

Walking Bleach technique with 35% HP for 7 days at not-specified temperature

Result and comments

HP penetrated through enamel, dentin and cementum with cementum being the least permeable dental tissue

Kwon and colleagues 201345

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Specimen

Human canines (N = 80)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

38% HP for 1 hour with the use of conventional versus sealed bleaching technique (SBT) at 25°C

Result and comments

SBT that does not require replenishment of material provided same whitening efficacy with lower HP penetration

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TABLE 2. Continued Ubaldini and colleagues 201347 Specimen

Human premolars (N = 18)

Quantification method

Micro-Raman spectroscopy (MRS) and Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) to measure spectra of specimens

Concentrations used

25% HP for 1 hour at room temperature

Result and comments

MRS showed that HP crossed enamel had a marked concentration at the DEJ and accumulated in dentin FTIR-PAS showed that HP modified dentin’s organic compounds demonstrated by a decrease in amides I, II, III absorption band intensities

Patri and colleagues 201348 Specimen

Human incisors (N = 60)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

10% for 1 hour at 37°C

Result and comments

HP penetration in restored teeth was higher than in sound teeth

Marson and colleagues 201449 Specimen

Bovine teeth (N = 75 specimens)

Quantification method

Leucocrystal violet and horseradish peroxidase

Concentrations used

35–38% HP for 45 minutes three times at not-specified temperature

Result and comments

Whitening gels maintain more than 86% of their initial concentration of HP after 45 minutes without replenishment

= carbamide peroxide; HP = hydrogen peroxide; LED = Light Emitting Diode; Nd:YAG = Neodymium-Doped Yttrium Aluminium Garnet; NPP = sodium percarbonate peroxide; SP = sodium perborate.

simpler structures or alter their optical properties such that the appearance of the stain is diminished.51 These reactions will also yield products that are both more polar and lower in molecular weight than the original stain molecule. Both of these properties will make the products easier to remove in an aqueous environment. Although it remains to be determined how the whitening agent interacts with the stain molecules, chemical oxidation is thought to be involved. As such, it will be important to understand the roles of pH, chemical activators, temperature, and light activation at various wavelengths.52 The rate of decomposition and the type of active oxygen formed are dependent on the temperature and concentration of the peroxide, as well as on the pH and the presence of co-catalysts and metallic reaction partners.53 In

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addition, depending on which chemical bond breaks, hydrogen peroxide can give rise to a number of reactive oxygen species. These include the hydroxyl radical, hydroperoxyl radical, hydroperoxyl radical anion, superoxide radical anion, and superoxide radical cation. In an aqueous environment, the peroxyl radical will be in equilibrium with the superoxide radical. The reactive oxygen species produced from hydrogen peroxide depend on factors such as pH and the presence of metal cations. These reactive oxygen species are generally capable of abstracting hydrogen atoms from biological molecules.12 These sorts of chemical reactions can cause damage to biological membranes, but can also cause the degradation of stain molecules. Despite the well-established chemistry of hydrogen peroxide, central issues remain to be addressed. For

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example, we remain unable to detect chromophores within the tooth structure and have yet to establish the site-specific mechanism whereby whiteners bind to them. Furthermore, it will be important to analyze and quantify breakdown products associated with tooth whitening. Studies on tooth enamel using Fourier transform infrared (FTIR) and Raman spectroscopy failed to detect chromophores or their breakdown products, and as such, the chromophore theory is not fully ed.54–56 Dental enamel is the most highly mineralized and hardest tissue of the body. It is approximately 96% mineral, 3% water, and 1% organic matter by weight. The enamel interface undergoes continuous dynamic ion exchange with the oral biofilm, with calcium phosphate apatite crystals moving in both directions to maintain a proper mineral balance. Recent evidence indicates that some organic materials in the enamel originate from exogenous sources and become part of the organic matrix.57 Mature dentin is 70% mineral, 20% organic matrix, and 10% water by weight. The mineral component of dentin consists of substituted hydroxyapatite in the form of small plates, whereas the organic matrix is 90% collagen and includes small percentages of various noncollagenous matrix proteins and lipids.58 Ideally, throughout the process of tooth penetration, the oxidizing action of hydrogen peroxide should be limited to the organic chromophores. However, review of the literature suggests that hydrogen peroxide interacts significantly with the organic as well as inorganic components of both enamel and dentin. Hydrogen peroxide penetrates the enamel mainly by entering the interprismatic spaces, which are filled with enamel proteins. Because the mineralized, inorganic phase is much more compact than the organic, penetration through the hydroxyapatite crystals and interaction with them is probably very low.57 The assumption that the organic matrix of enamel and dentin is affected by hydrogen peroxide is ed by several studies. X-ray diffraction analysis of hydroxyapatite and NMR-based measurement of

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changes in amino acids suggested that hydrogen peroxide and hydroxyl radicals do not influence the inorganic component of dentin but do influence the organic tissue.59–61 These findings led to speculation that the whitening effect could result from modification of the polypeptide chain in the organic substance rather than from the interaction of the whitener with stain molecules. This is also in accordance with the observation of advanced oxidation processes, where hydroxyl radicals interact mainly with organic matter to produce intermediates and yield harmless species such as carbon dioxide and water.62 Other studies using atomic force microscopy (AFM) and FTIR showed that the morphological changes in dentin and enamel were due mainly to partial lysis of the tooth enamel matrix protein or the organic matrix of the dentin.47,57,63–66 Moreover, significant increases in the proteolytic activities of cathepsin B and matrix metalloproteinase after tooth whitening have been demonstrated, suggesting a dynamic change within the tooth.63 The change in the inorganic chemical composition of enamel and dentin, suggesting that hydrogen peroxide interacts with the tooth structure, has been studied extensively using ion-selective electrode probes, FT-Raman spectroscopy, and a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer, and microcomputerized tomography. Despite the fact that many studies of peroxide-based materials have shown that these agents do not influence the chemistry of enamel and dentin beyond clinical relevance or may be prevented with addition of fluoride or calcium,67–72 many others have indicated significant changes in the calcium/phosphate ratio, indicating that the inorganic components of hydroxyapatite are altered.73–80 Several studies using FTIR analysis revealed that hydrogen peroxide treatment induces loss of both carbonate and proteins from enamel and dentin,63,81 and also alterations in representative biological bands of hydroxyapatite.82,83 Furthermore, the use of microcomputerized tomography showed that 35% carbamide peroxide induced the demineralization of enamel to a depth of 250 μm, although demineralization of dentin was not observed.76,78

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Overall, these studies suggest that hydrogen peroxide has the potential to interact with all the components of dentin and enamel. The clinical significance of this interaction and how it relates to tooth whitening remains to be assessed and evaluated in future studies.

the overall change was highly influenced by that in the subsurface dentin.89,90 However, results from other studies emphasized the significance of the contribution of enamel to the overall color change and suggested that this contribution was due mainly to the decrease in translucency of the enamel and thus to masking of the color of the underlying dentin.91–93

SURFACE CHANGE AND COLOR Perception of tooth color is influenced by many factors, including lighting conditions, the object being viewed, and the viewer. Given the dynamics of these three components, accurately recording tooth shade and monitoring minute color changes is extremely difficult, especially when the complex optical properties of mineralized tissues and the combined effects of intrinsic and extrinsic pigmentation are factored in.84,85 Optical characteristics, such as gloss, opacity, and translucency, and also optical phenomena such as metamerism, opalescence and fluorescence, add to the intricacy of the color perception process.86 Most inherent tooth color is attributed to diffuse reflectance from the volume of the inner dentin through the outer translucent enamel layer. Based on color vision, the human eye can only perceive the light that is reflected. Reflection occurs at the surface and from the volume of the object that can be further divided into specular reflection and diffuse reflection, summing up into the total reflection.87 A study evaluated the tooth color and reflectance as related to light scattering and enamel hardness. It specifically compared the color of the extracted teeth before and after removal of the labial enamel, and found that tooth color is determined mainly by dentin; enamel plays only a minor role, primarily through scattering at wavelengths in the blue range.88 The fact that dentin is a predominant factor in determining tooth color is also important when considering the mechanism of tooth whitening. For example, it raises the question of whether the whitening agent interacts mainly within the dentin to change the overall tooth color. Several studies that evaluated the separate contributions of enamel and dentin to overall change in tooth color during whitening concluded that

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The change in translucency of the enamel has been attributed to micromorphological alterations of the most superficial enamel through deproteinization, demineralization, and oxidation.54,92,93 Despite the fact that surface alterations related to tooth whitening have been extensively investigated by SEM, profilometry, and AFM, with many studies indicating that whitening has no effect on surface topography,94–99 there are as many in which significant changes in surface topography have been documented.100–108 Studies that specifically related optical properties of the tooth or changes in enamel surface topography to tooth whitening and color change are summarized in Table 3. It is well known that a rough or coarse surface results in more diffuse reflection, turning the object brighter, whereas a smooth surface leads to more specular reflection, and that increased backscattering of short wavelengths that are reflected as bluish-white due to opalescent effects at small structures plays a considerable role in the light scattering of teeth.84 Although an increase in surface roughness post-whitening is not necessarily anticipated, where present, it may result in increased reflectance spectra and thus in improved digital color reading.109,113,116,117 The enhanced reflection on the surface post-whitening would, in turn, render the enamel more opaque, as can also be observed in early carious lesions.92,93,111 Within the same context, it has been suggested that demineralization during tooth whitening might contribute to the whitening effect.67,70,109,118 This possibility was ed by a report of color regression in association with increased mineral uptake after tooth whitening.114 However, a reduction in laser-induced fluorescence and luminescence during Raman spectral analyses was found to be associated with tooth whitening and has been attributed to a change in the

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TABLE 3. Studies relating optical properties or a change in surface enamel to tooth color change Kwon and colleagues 2002109 Specimen

Bovine incisors (N = 5)

Methods

Diffuse reflectance measurement with UV–vis–NIR spectrophotometer, surface topography observation with SEM

Materials used

30 % HP for 0, 1, 2 and 3 days

Result and comments

Whitened samples showed slight alterations with varying degrees of surface porosity and increased reflectance spectra compared with control

Götz and colleagues 2007110 Specimen

Human molars (N = not clear)

Methods

Color measurement with digital camera, microhardness, VP-SEM, ultrastructure assessment with CLSM, micro-Raman spectroscopy

Materials used

13% and 16% HP strips for a total exposure time of 28 hours

Result and comments

Significant whitening compared with control. No changes in VHN, VP-SEM observations similar to control. Reduction of luminescence during Raman spectral analyses of enamel and dentin in samples whitened with higher concentrations.

Vieira and colleagues 2008111 Specimen

Human molar teeth (N = 14)

Methods

Color measurement with spectrophotometer

Materials used

10% for 8 hours per day for 28 days

Result and comments

For all samples, a decrease in translucency and transmittance was observed that may be due to rise in diffuse reflection on a rougher surface

er and colleagues 2008112 Specimen

Human anterior and premolar teeth (N = 68)

Methods

Color measurement with colorimeter, time-of-flight secondary ion mass spectrometry (TOF-SIMS)

Materials used

Blue covarine containing toothpaste, brushing for 1 minute

Result and comments

Blue covarine was detected on enamel surface by TOF-SIMS and resulted in a negative shift in b* and a increase in whiteness index (WIO)

Ma and colleagues 200992 Specimen

Human premolars (N = 24)

Methods

Color measurement and translucency measurement with spectrophotometer

Materials used

10% and 15% for 8 hours per day for 14 days

Result and comments

10% and 15% made color changes in E-D specimens-upon removal of dentin, significant color change was observed in enamel slabs Translucency parameters were less for bleached enamel compared with control suggesting that enamel plays a key role in color change related to whitening

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TABLE 3. Continued Markovic and colleagues 2010113 Specimen

Human teeth were grouped based on maturation stage (N = 48)

Methods

Total reflectance measurement with computer assisted spectrometer

Materials used

10% and15% for 8 hours with 14 applications, 35% HP for 24 minutes repeated three times

Result and comments

Total reflectance increased post-whitening at all enamel maturation stages, for wavelength 450 and 500 nm Whitening of enamel worked at different maturation stages even in impacted teeth

Li and colleagues 2010114 Specimen

Human incisors (N = 40)

Methods

Color measurement with spectrophotometer, mineral content measurements with μ-CT

Materials used

38% HP for 10 minutes repeated three times

Result and comments

Color regression resulted by the reversal of lightness and was correlated with the presence of remineralization process within the tooth No color regression and mineral content change was found in the anhydrous environment

Sun and colleagues 2011115 Specimen

Human premolars (N = 27)

Methods

Color measurement with spectrophotometer, ATR-FTIR spectroscopy, Raman spectroscopy, AFM, microhardness test

Materials used

Neutral 30% HP versus acidic 30% HP, immersion for 4 hours

Result and comments

No significant difference in color change between acidic and neutral 30% HP Significant decrease of carbonate : mineral ratio in acidic HP, LIF was significantly reduced in both HP groups

Pedreira de Freitas and colleagues 2010108 Specimen

Human lower incisors (N = 3)

Methods

AFM for roughness (Ra and RMS) and power spectral density (PSD)

Materials used

35% HP, four applications of 10 minutes each

Result and comments

Significant increase in PSD in the range of visible light spectrum (380–750 nm) with whitening procedure The results promote more visible light scattering which leads to an opaque surface

Eimar and colleagues 201254 Specimen

Human upper incisors and canines (N = 60)

Methods

Color measurement with spectrophotometer, elemental analysis with SEM-EDS, crystallinity index analysis with Raman spectrophotometer

Materials used

1 M NaOH, 0.5 M EDTA, 30% HP immersion in solution for 4 days

Result and comments

Deproteinization with NaOH increased lightness, demineralization with EDTA decreased lightness and oxidation with HP increased lightness

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TABLE 3. Continued Grundlingh and colleagues 2013116 Specimen

Human incisors, canines and premolars (N = 99)

Methods

Color measurement with Vitapan 3D Master Shade guide and spectrophotometer, surface roughness measurement with AFM

Materials used

Ozicure oxygen activator bleach and 35% three times according to each manufacturer’s instructions

Result and comments

Surface roughness value increased twofold for 35% . Increased light scattering improved digital color reading.

μ-CT = microcomputerized tomography; AFM = atomic force microscopy; ATR-FTIR = attenuated total reflection–Fourier transform infrared spectroscopy; CLSM = confocal laser scanning microscopy; = carbamide peroxide; EDS = energy dispersive spectroscopy; HP = hydrogen peroxide; LIF = laser-induced fluorescence intensity; RMS = Root Mean Square; SEM = scanning electron microscopy; VHN = Vickers hardness number; VP = variable pressure.

organic matrix within the enamel rather than with surface mineral loss and changes in roughness.54,110,115,119 Efforts have also been made to assess tooth shade as it relates to the chemical composition and crystallography of the enamel. Based on a study using crystallography, tooth hue was associated with the size of hydroxylapatite (HA) crystals in the enamel, whereas chroma was associated with the carbonization of these HA crystals, and lightness was associated with both the size and the carbonization of the HA crystals. This partly explained the reduction of lightness observed with aging, as crystal size increases with aging.120 Other innovative approaches that have been applied toward understanding the optical phenomenon introduced chemicals such as covarine into whitening agents, with the purpose of depositing a material on the tooth surface that alters the optical properties of the tooth. The results indicated that blue covarine produced a greater negative shift in chroma in the yellow blue axis and also increased the measured whiteness index.112 All of the studies evaluating changes in the optical properties of teeth suggest that the assumption that the “chromophore effect” is dominant in tooth whitening cannot be sustained. It seems necessary to modify this theory to reflect the complexity of interactions and optical changes associated with tooth whitening.

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DISCUSSION AND CONCLUSIONS With the high demand for instant whitening results from patients in a highly appearance-driven society, tooth whitening products continue to become ever more prolific and diverse. To keep pace with this trend, and to provide a solid foundation for the development of innovative, new whitening technologies, scientists have put great effort into elucidating the mechanism underlying tooth whitening. The mechanism that underlies tooth whitening, however, has proven to be a complex phenomenon, and no model fully integrating the details of the mechanism that leads to the final outcome of color change toward increased lightness and reduced chroma has been developed. Therefore, it is imperative to develop a stain retention model that takes into the amount and type of chromophore within a given tooth. Most studies to date have been concerned mainly with the evaluation of tooth discoloration based on visual or instrumental color measurements without trying to directly assess the density of chromophore molecules trapped within the dentition. However, it is precisely the amount of stain intercollated within the dental hard tissues that needs to be correlated with spectroscopic methods in order to allow for assessment of the true character of staining and possible changes associated with tooth whitening. With the development of a proper stain retention model, several significant outcomes become possible. For instance, if the stain has an ultraviolet absorption spectrum, that spectrum can

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be used to assess the amount of stain still intercollated in the tooth structure after the completion of tooth whitening. This model could also be used to assess the amount of stain or breakdown products present in both enamel and dentin, which will lead to a better understanding of the nature of perceived color, i.e., the stain could be present primarily in the dentin but perceived through the enamel even though the enamel itself may have very little stain present. Furthermore, surface changes and concomitant changes in optical properties could be additionally determined through this model. A more distant but still reachable outcome could be the development of innovative whitening alternatives. For instance, if the chromophore never had an opportunity to bond with the organic or inorganic content of the dental hard tissues by means of an anti-staining treatment that would act like dental Teflon, a dentist could prescribe a rinse once or twice a week that could potentially prevent tooth staining in the first place. This review on the integration of three aspects of tooth whitening mechanism covers diffusion, interaction, and surface changes related to the optical properties of the tooth. Although a vast body of literature indicates that surface changes and interactions with minerals in sound tooth structures are associated with tooth whitening, other studies suggest that they are not. Based on our review, it is evident that tooth whitening with peroxide-based materials is a dynamic process initiated by the movement of the whitening material into the tooth structure to interact with stain molecules and also involves micromorphologic alterations on the surface and changes within the tooth that affect its optical properties. The interaction seems not to be limited to the organic stain molecules, but rather an affinity-based interaction process that also accompanies effects on sound enamel and dentin structures. The clinical relevance and significance of these minute alterations remain to be investigated in further detail. The conclusions of this review underline that careful monitoring and supervision by dental health professionals as recommended by the ADA Council on Scientific Affairs is critical to achieving a successful and safe tooth whitening outcome.2

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DISCLOSURE AND ACKNOWLEDGEMENT The authors declare no potential conflicts of interest with respect to the authorship and/or publication of this article. The authors express their sincere gratitude to University of Iowa College of Dentistry librarian Christine White for her assistance in the database search. Special thanks also to Patricia Conrad at Technology & Media Services for her help with the illustration.

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Reprint requests: So Ran Kwon, DDS, MS, PhD, Department of Operative Dentistry, University of Iowa College of Dentistry & Dental Clinics, 801 Newton Road #45, S235 DSB, Iowa City, IA 52242-1001, USA; Tel.: 319-335-8871; Fax: 319-335-7267; email: [email protected]

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