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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. Further, an analysis of the clinical results with over 600
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Physics of Live Processes ; Introduction to Biophysics John van Noort, Physics of Life Processes Exam 28-05-2009, 13:45-16:30h This exam consists of 6 problems (2 pages). Note your name and student ID on every sheet. Give sufficient motivation for your answers. You may use books, notes and calculators 1) A model protein Proteins are built from strings of amino acids. 20 different