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Doi:10.1016/j.jdent.2006.02.002

j o u r n a l o f d e n t i s t r y x x x ( 2 0 0 6 ) x x x – x x x a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . i n t l . e l s e v i e r h e a l t h . c o m / j o u r n a l s / j d e n The bleaching of teeth: A review of the literature Unilever Oral Care, Quarry Road East, Bebington, Wirral, CH63 3JW, UK Objectives: To review current knowledge of tooth whitening with respect to external Data: The scope is the external bleaching of vital teeth and focuses on mechanisms; in vivoand in vitro measurement methods, and factors influencing the efficacy of the whiteningprocess.
Sources: ‘‘Medline’’ and ‘‘ISI Web of Science’’ databases from 1966 and 1974, respectively were searched electronically with key words tooth, teeth, colo*r, white*, bleach* and Conclusions: The importance of tooth whitening for patients and consumers has seen a dramatic increase in the number of products and procedures over recent years, with a concomitant rise in publications on this topic. Literature suggests that the mechanisms of tooth whitening by peroxide occur by the diffusion of peroxide through enamel to causeoxidation and hence lightening of coloured species, particularly within the dentinal regions.
A number of approaches are available for measuring changes in tooth colour. These includevisual measurements by trained clinicians and instrumental measurements using spectro-photometry, chromameters and digital image analysis. The key factors that affect toothwhitening efficacy by peroxide containing products are concentration and time. In general,higher concentrations are faster than lower concentrations. However, lower concentrationscan approach the efficacy of higher concentrations with extended treatment times. Alter-native bleach systems to peroxide have received only minor attention. The efficacy of lightactivated systems versus non-light activated controls in clinical studies is limited andconflicting. Other factors which can influence tooth bleaching outcome include type ofstain, initial tooth colour and subject age.
# 2006 Elsevier Ltd. All rights reserved.
Mechanism of tooth bleaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Clinical measurement of tooth whitening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
In vitro models for tooth whitening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Factors influencing tooth whitening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Type of bleach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concentration and time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
* Tel.: +44 151 641 3000; fax: +44 151 641 1806.
0300-5712/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
j o u r n a l o f d e n t i s t r y x x x ( 2 0 0 6 ) x x x – x x x Heat and light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aesthetics of the teeth is of great importance to patients, including tooth colour. For examples, in the UK it has beenreported that 28% of adults are dissatisfied with the appear- Bleaching is a decolourisation or whitening process that can ance of their and in the USA that 34% of an adult occur in solution or on a surface.The colour producing population are dissatisfied with their current tooth colour.In materials in solution or on a surface are typically organic addition, in a survey of 3215 subjects from the UK 50% compounds that possess extended conjugated chains of perceived they had some kind of tooth discolouration.
alternating single or double bonds and often include The colour of the teeth is influenced by a combination of heteroatoms, carbonyl, and phenyl rings in the conjugated their intrinsic colour and the presence of any extrinsic stains system and are often referred to as a chromophore.
that may form on the tooth surfaceIntrinsic tooth colour is Bleaching and decolourisation of the chromophore can associated with the light scattering and adsorption properties occur by destroying one or more of the double bonds in of the enamel and dentine, with the properties of dentine the conjugated chain, by cleaving the conjugated chain, or by playing a major role in determining the overall tooth oxidation of other chemical moieties in the conjugated Extrinsic stains tend to form in areas of the teeth that are less chaiHydrogen peroxide oxidises a wide variety of accessible to tooth brushing and the abrasive action of a organic and inorganic compounds. The mechanisms of toothpasteand is often promoted by smoking, dietary intake these reactions are varied and dependent on the substrate, of tannin-rich foods (e.g. red wine) and the use of certain the reaction environment, and catalysis.In general, the cationic agents such as chlorhexidine, or metal salts such as mechanism of bleaching by hydrogen peroxide is not well understood and it can form a number of different active Tooth colour can be improved by a number of methods and oxygen species depending on reaction conditions, including approaches including whitening toothpastes, professional temperature, pH, light and presence of transition metals.
cleaning by scaling and polishing to remove stain and tartar, Under alkaline conditions, hydrogen peroxide bleaching internal bleaching of non-vital teeth, external bleaching of generally proceeds via the perhydroxyl anion (HO À vital teeth, microabrasion of enamel with abrasives and acid, conditions can give rise to free radical formation, for placement of crowns and veneers.The scope of the example, by homolytic cleavage of either an O–H bond or current literature review is restricted to the external bleaching the O–O bond in hydrogen peroxide to give H + OOH and of vital teeth and will focus on the following topics; 2OH (hydroxyl radical), respectively.Under photochemi- mechanisms of tooth bleaching; in vivo and in vitro evaluation cally initiated reactions using light or lasers, the formation methods, and factors influencing the efficacy of the tooth of hydroxyl radicals from hydrogen peroxide has been There are a number of methods and approaches that have The mechanism by which teeth are whitened by oxidising been described in the literature for the bleaching of vital materials such as hydrogen peroxide and carbamide peroxide teeth. For examples, methods utilising different bleach are currently not fully understood.Considering the agents, concentrations, times of application, product format, available literature, evidence points towards the initial application mode and light activationHowever, three diffusion of peroxide into and through the enamel to reach fundamental bleaching approaches exist, namely, dentist- the enamel dentine junction and dentine regions. Indeed, in supervised nightguard bleaching, in-office or power bleach- vitro experiments by a number of authors have demonstrated ing and mass market bleaching productsNightguard the penetration of low levels of peroxide into the pulp bleaching typically uses a relatively low level of whitening chambers of extracted teeth after exposure times of 15– agent applied to the teeth via a custom fabricated mouth 30 min from a range of peroxide products and guard and is worn at night for at least 2 weeks.In-office The levels of peroxide measured in these experiments is bleaching generally uses relatively high levels of whitening considerably much lower than that needed to produce pulpal agents, for example 25–35% hydrogen peroxide containing products, for shorter time periods. The whitening gel is As peroxide diffuses into the tooth, it can react with organic applied to the teeth after protection of the soft tissues and the coloured materials found within the tooth structures leading peroxide may be further activated by heat or The in- to a reduction in colour. This is particularly evident within office treatment can result in significant whitening after only dentine as demonstrated by McCaslin et who showed, one treatment visitbut may require multiple treatment using hemi-sectioned human teeth mounted on glass slides, appointments for optimum whitening.Mass market that following external bleaching with carbamide peroxide, products typically contain low levels of whitening agent colour changes occurred throughout the dentine. Indeed, the (e.g. 3–6% hydrogen peroxide) that are self-applied to the treatment of dentine specimens with 10% carbamide per- teeth via gum shields, strips or paint-on product formats and oxide, 5.3% and 6% hydrogen peroxide has been shown to give typically require twice per day application for up to 2 a significant reduction in yellowness and an increase in whiteness.In addition, Sulieman et showed using j o u r n a l o f d e n t i s t r y x x x ( 2 0 0 6 ) x x x – x x x sectioned extracted teeth stained internally with black tea Another approach for measuring tooth colour is by using chromophores that significant bleaching occurred within the non-contact camera-based digital imaging and analysis dentine, particularly on the buccal surface where a 35% Typically, an image of the anterior teeth is hydrogen peroxide gel had been applied.
captured under controlled lighting conditions by a digital For tetracycline stained teeth, the colour is derived from camera together with suitable calibration tiles or standards photo-oxidation of tetracycline molecules bound within the and then subsequently analysed via computer software to tooth structures.In some cases, it is possible to bleach these determine the colour of the individual teeth, often expressing teeth to give significant and long lasting tooth whitening.
them in terms of CIE Lab values. For example, after 14 days use The mechanism by which peroxide affects the tetracycline of a 10% carbamide peroxide tray-based system, the mean stain is considered to be by chemical degradation of the change from baseline in L* and b* were 2.07 and À1.67, unsaturated quinone type structures found in tetracycline leading to less coloured molecules.However, in contrastthere appears to be a paucity of information available in theliterature regarding the nature and chemical composition of the coloured materials naturally found within the dental hardtissues and the mechanistic effects of peroxide on these The use of in vitro models is often important for the initial structures. Thus, this is clearly an area that requires further evaluation of prototypes and the optimisation of treatment research if the chemical mechanistic aspects of tooth conditions. In addition, these models can be used to gain bleaching are to be significantly resolved.
important information on the safety of the product in terms ofits effect on the hard tissues and provide mechanisticunderstanding of the bleaching process. There have been numerous in vitro models described in the literature whichhave been used to evaluate the efficacy of tooth whitening A number of methods are available for measuring the colour of products and these are summarised in The majority of teeth and the colour changes undergone during tooth these models use whole or cut human or bovine tooth whitening procedOne of the most common methods specimens and utilises their pre-existing colour. However, is the simultaneous comparison of the tooth with a standard some in vitro models increase the levels of intrinsic tooth shade guide.This has been used in a large number of tooth colour by pre-staining with black tea or blood components. In whitening studies where longitudinal changes in tooth colour general, the changes in tooth colour are measured by have been It is a subjective method and a number of factors can influence this process. For examples,lighting conditions, experience, age, fatigue of the human eye,make-up, room decor and colour blindness.Therefore, care must be taken to standardise and control these factors.
Indeed, the tooth colour discriminatory ability of individuals can be improved with training and experienceand it isoften reported that investigators undergo a number of colour The majority of contemporary tooth whitening studies involve calibration exercises and training with shade guides when the use of either hydrogen peroxide or carbamide peroxide.
This latter material is an adduct of urea and hydrogen Colourimeters are instruments designed to measure the peroxide which on contact with water breaks down to urea colour of objects. The colour is often expressed in terms of the and hydrogen peroxide. For example, a 10% (w/w) carbamide Commission Internationale de l’E´clairage (CIE) Lab colour peroxide gel would yield a maximum of 3.6% (w/w) hydrogen space. The CIE Lab colour space represents a uniform colour peroxide. In general, the efficacy of hydrogen peroxide space, with equal distances corresponding to equal perceived containing products are approximately the same when colour differences. In this three-dimensional colour space the compared with carbamide peroxide containing products with three axes are L*, a* and b*. The L* value is a measure of the equivalent or similar hydrogen peroxide content and delivered lightness of an object and is quantified on a scale such that a using similar format and formulations, either tested in perfect black has an L* value of zero and a perfect reflecting or in vivo.For example, Nathoo et demonstrated in a diffuser an L* value of 100. The a* value is a measure of redness clinical study that a once a day application of either a 25% (positive a*) or greenness (negative a*). The b* value is a carbamide peroxide gel or a 8.7% hydrogen peroxide gel both measure of yellowness (positive b*) or blueness (negative b*).
gave a statistically significant tooth shade lightening after 2 The a* and b* co-ordinates approach zero for neutral colours weeks use compared to baseline, but found no statistically (white, greys) and increase in magnitude for more saturated or significant differences between products.
intense colours.The use of a colourimeter to measure tooth An alternative source of hydrogen peroxide is sodium colour in vivo requires the fabrication of a custom positioning percarbonate and this has been used in a silicone polymer jig to ensure reproducible intra-oral positioning of the containing product that is painted onto the teeth forming a instrument’s aperture onto the tooth surfaceThis approach durable film for overnight bleaching procedures.The has been utilised in a number of studies for measuring peroxide is slowly released for up to 4 hand gave significant longitudinal changes in tooth colour following tooth whiten- tooth colour improvement after 2 weeks versus baseline.
However, the relative clinical or in vitro efficacy of sodium j o u r n a l o f d e n t i s t r y x x x ( 2 0 0 6 ) x x x – x x x Table 1 – Summary of in vitro models used for evaluation of tooth bleaching materials CP – carbamide peroxide, HP – hydrogen peroxide.
percarbonate versus hydrogen peroxide tested in the same most rapid whitening occurred in the first month with 20% product format and conditions has not been reported.
carbamide peroxide compared to 15% and 10% carbamide A tooth bleaching system based on sodium chlorite applied peroxide. In addition, clinical studies with hydrogen peroxide to the tooth surface and activated under acidic conditions has strip based products have shown similar concentration and been described in the literature,however, no efficacy data has been reported to date. Similarly, other potential vital toothbleaching systems have been outlined in the literature with limited supporting evidence for their efficacy. These includesodium peroxymonosulphate,peroxide plus The rate of chemical reactions can be increased by increasing metal catalystsand oxireductase enzymes.The long- the temperature, where a 10 8C rise can double the rate of term acceptability and relative efficacy of these alternative reaction.The use of high-intensity light, for raising the tooth bleaching systems requires significant further research.
temperature of the hydrogen peroxide and accelerating therate of chemical bleaching of teeth was reported in 1918 by Other approaches for heating the peroxide havehistorically been described to accelerate tooth bleaching, such Two of the key factors in determining overall tooth whitening as heated dental instruments.However, excessive heating efficacy from peroxide containing products are the concen- can cause irreversible damage to the dental Con- tration of the peroxide and duration of application. For temporary approaches and literature has focussed on accel- example, Sulieman et compared the in vitro tooth erating peroxide bleaching with simultaneous illumination of bleaching efficacy of gels containing 5–35% hydrogen peroxide the anterior teeth with various sources having a range of and found that the higher the concentration, the lower the wavelengths and spectral power, for examples, halogen curing number of gel applications required to produce uniform lights, plasma arc lamps, lasers and light-emitting diodes.
bleaching. Similar results were found by Leonard et al.who For some light sources, significant increases in pulpal compared the in vitro tooth bleaching efficacy of 5%, 10% and temperatures have been measured using in vitro models 16% carbamide peroxide gels and found the whitening was during tooth bleaching.The light source can activate initially faster for the 16% and 10% than the 5% concentration.
peroxide to accelerate the chemical redox reactions of the However, the efficacy of the 5% approached the higher bleaching In addition, it has been speculated that concentrations when the treatment time was extended. In a the light source can energise the tooth stain to aid the overall clinical study using custom made bleaching trays, Kihn et al.
acceleration of the bleaching Some products that showed that a 15% carbamide peroxide gel gave significantly are used in light activated bleaching procedures contain more tooth whitening than a 10% carbamide gel after 2 weeks ingredients that claim to aid the energy transfer from the light use. This result was confirmed in another clinical study to the peroxide gel and are often coloured materials, for reported by Matis et al.However, in this latter study, by extending treatment time to 6 weeks, the differences in tooth Case studies have demonstrated the efficacy of light lightness were no longer of statistical significance. The initial activated peroxide tooth bleaching systems.How- faster rate of bleaching for higher concentrations of carbamide ever, the literature evidence from in vitro and clinical studies peroxide has also been observed when bleaching tetracycline for the actual effect of light on tooth bleaching versus a stained teeth in vivo over a 6 months period.In this case, the suitable non-light control is limited and controversial. An in j o u r n a l o f d e n t i s t r y x x x ( 2 0 0 6 ) x x x – x x x vitro study using naturally coloured extracted human teeth yellow initial tooth colour exhibited the smallest mean colour showed that the application of various light sources signifi- change post bleaching, whereas younger subjects with more cantly improved the whitening efficacy of some bleach yellow initial tooth colour exhibited the greatest mean colour materials, but not for others.Other in vitro studies have change post bleaching.In addition, neither gender nor clearly shown significant tooth whitening benefits for per- coffee/tea consumption had any significant affect on the tooth oxide plus light versus suitable control However, these studies artificially stained the tooth speci- The presence on the tooth surface of pellicle and plaque mens with, for examples, black tea, coffee, tobacco and red has the theoretical potential to reduce the activity of peroxide wine, i.e. ingredients commonly found to promote extrinsic by acting as a substrate for peroxide bleaching and/or stains. These chromophores are likely to be different to that degrading peroxide. Wattanapayungkul et al.has shown which may be found naturally inside the tooth.
that the rate of peroxide degradation did not increase with the Tavares et conducted a tooth whitening clinical study presence of pellicle on tooth surfaces in vivo over 1 h to compare 15% hydrogen peroxide gel illuminated with a gas indicating that pellicle does not have a significant effect on plasma light source versus 15% peroxide alone versus placebo the stability of peroxide. In addition, a clinical study by Gerlach gel plus light, all treatments lasting 1 h. The change in Vita et al.,comparing the effect of immediate prebrushing with a shade from baseline for peroxide plus light, peroxide alone toothpaste versus no prebrushing prior to tooth bleaching and placebo plus light were 8.35, 5.88 and 4.93, respectively, with 6.5% hydrogen peroxide over a 14-day period, suggested with peroxide plus light being significantly different to the that toothbrushing immediately before bleaching has only a other two groups. In contrast, Hein et al.demonstrated no modest positive impact on overall efficacy. Thus, the modify- additional effect of any of the three light sources tested over ing role of pellicle on peroxide delivery and whitening efficacy the bleaching gel alone for three commercial products in a split mouth clinical design. Thus, further work is clearlyrequired in order to unequivocally demonstrate the additionalefficacy benefit of light activated tooth whitening systems versus their non-light activated controls.
The importance of tooth whitening for patients and con- sumers has seen a dramatic rise in the number of toothwhitening products and procedures. Concomitantly, there has The type of intrinsic stain and the initial tooth colour can play been a rapid increase of published in vivo and in vitro tooth a significant part in the ultimate outcome of tooth bleaching.
whitening studies. Indeed, it is clearly evident that there is an Mild to moderate tetracycline staining tends to respond to extensive literature describing their efficacy and safety.
extended bleaching regimes of 2–6 However, It However, some of this literature is conflicting, and these is documented that severe tetracycline staining is more topics warrant further careful evaluation as they were outside difficult to bleachwith the darker the teeth at baseline, the scope of the current review. A number of approaches to the longer it can take to lighten In addition, it is measuring tooth colour changes following tooth whitening reported that when the tetracycline discolouration is located exist, each with their own advantages and disadvantages, and in the neck of the tooth, the prognosis for bleaching is the this topic is likely to be an area commanding further research poorest; when it is dark gray or blue, the prognosis also is in the future. With the continued interest in tooth whitening amongst basic and clinical researchers, the further mechan- For non-tetracycline stained teeth, a meta analysis of istic understanding and optimisation of the factors controlling placebo controlled, patient applied tooth whitening clinical the tooth whitening process will continue to expand. This will studies using 10% carbamide peroxide found that 93% of give further improvements to the tooth whitening products people who used the peroxide product and 20% who used the and procedures, and give significant benefits to the field of placebo exhibited a change of two shade guide units. In aesthetic dentistry. This will ultimately lead to the enhance- addition, 20% of subjects who used the peroxide product ment of patient compliance and satisfaction with the achieved a mean change of five shade guide units.
Ishikawa-Nagai et al.evaluated the tooth colour changeof 80 subjects after using 10% carbamide peroxide in a gumshield over 14 days and found a strong correlation between total colour change and b* values, demonstrating thatbleaching works efficiently for teeth with a yellow hue.
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