Factors influencing tooth whitening

Type of bleach

The majority of contemporary tooth whitening studies involve
the use of either hydrogen peroxide or carbamide peroxide.
This latter material is an adduct of urea and hydrogen

peroxide which on contact with water breaks down to urea

and hydrogen peroxide. For example, a 10% (w/w) carbamide

peroxide gel would yield a maximum of 3.6% (w/w) hydrogen

peroxide. In general, the efficacy of hydrogen peroxide

containing products are approximately the same when

compared with carbamide peroxide containing products with

equivalent or similar hydrogen peroxide content and delivered

using similar format and formulations, either tested in vitro

or in vivo. For example, Nathoo et al. demonstrated in a

clinical study that a once a day application of either a 25%

carbamide peroxide gel or a 8.7% hydrogen peroxide gel both

gave a statistically significant tooth shade lightening after 2

weeks use compared to baseline, but found no statistically

significant differences between products.

An alternative source of hydrogen peroxide is sodium

percarbonate and this has been used in a silicone polymer

containing product that is painted onto the teeth forming a

durable film for overnight bleaching procedures. The

peroxide is slowly released for up to 4 h and gave significant

tooth colour improvement after 2 weeks versus baseline.

However, the relative clinical or in vitro efficacy of sodium

percarbonate versus hydrogen peroxide tested in the same

product format and conditions has not been reported.

A tooth bleaching system based on sodium chlorite

applied to the tooth surface and activated under acidic

conditions has been described in the literature, however,

no efficacy data has been reported to date. Similarly, other

potential vital tooth bleaching systems have been outlined in

the literature with limited supporting evidence for their

efficacy. These include sodium perborate, peroxymonosulphate, peroxide plus metal catalysts and oxireductase

enzymes. The long-term acceptability and relative

efficacy of these alternative tooth bleaching systems requires  

significant further research

Concentration and time

 Two of the key factors in determining overall tooth whitening

efficacy from peroxide containing products are the concentration

of the peroxide and duration of application. For

example, Sulieman et al. compared the in vitro tooth

bleaching efficacy of gels containing 5–35% hydrogen peroxide

and found that the higher the concentration, the lower the

number of gel applications required to produce uniform

bleaching. Similar results were found by Leonard et al. who

compared the in vitro tooth bleaching efficacy of 5%, 10% and

16% carbamide peroxide gels and found the whitening was

initially faster for the 16% and 10% than the 5% concentration.

However, the efficacy of the 5% approached the higher

concentrations when the treatment time was extended. In a

clinical study using custom made bleaching trays, Kihn et al.

showed that a 15% carbamide peroxide gel gave significantly

more tooth whitening than a 10% carbamide gel after 2 weeks

use. This result was confirmed in another clinical study

reported by Matis et al. However, in this latter study, by

extending treatment time to 6 weeks, the differences in

tooth lightness were no longer of statistical significance. The

initial faster rate of bleaching for higher concentrations of

carbamide peroxide has also been observed when bleaching

tetracycline stained teeth in vivo over a 6 months period.85systems. Typically, an image of the anterior teeth is

captured under controlled lighting conditions by a digital

camera together with suitable calibration tiles or standards

and then subsequently analysed via computer software to

determine the colour of the individual teeth, often expressing

them in terms of CIE Lab values. For example, after 14 days use

of a 10% carbamide peroxide tray-based system, the mean

change from baseline in L* and b* were 2.07 and 1.67,

respectively.

Heat and light     

The rate of chemical reactions can be increased by increasing

the temperature, where a  8C rise can double the rate of

reaction. The use of high-intensity light, for raising the

temperature of the hydrogen peroxide and accelerating the

rate of chemical bleaching of teeth was reported in 1918 by

Abbot.Other approaches for heating the peroxide have

historically been described to accelerate tooth bleaching, such

as heated dental instruments.However, excessive heating

can cause irreversible damage to the dental pulp.Contemporary

approaches and literature has focussed on accelerating

peroxide bleaching with simultaneous illumination of

the anterior teeth with various sources having a range of

wavelengths and spectral power, for examples, halogen curing

lights, plasma arc lamps, lasers and light-emitting diodes.

For some light sources, significant increases in pulpal

temperatures have been measured using in vitro models

during tooth bleaching.The light source can activate

peroxide to accelerate the chemical redox reactions of the

bleaching process. In addition, it has been speculated that

the light source can energise the tooth stain to aid the overall

acceleration of the bleaching process. Some products that

are used in light activated bleaching procedures contain

ingredients that claim to aid the energy transfer from the light

to the peroxide gel and are often coloured materials, for

examples, carotene and manganese sulphate.

Case studies have demonstrated the efficacy of light

activated peroxide tooth bleaching systems. However,

the literature evidence from in vitro and clinical studies

for the actual effect of light on tooth bleaching versus a

suitable non-light control is limited and controversial. An in

vitro study using naturally coloured extracted human teeth

showed that the application of various light sources significantly

improved the whitening efficacy of some bleach

materials, but not for others. Other in vitro studies have

clearly shown significant tooth whitening benefits for peroxide

plus light versus suitable control conditions

However, these studies artificially stained the tooth specimens

with, for examples, black tea, coffee, tobacco and red

wine, i.e. ingredients commonly found to promote extrinsic

stains. These chromophores are likely to be different to that

which may be found naturally inside the tooth.

Tavares et al. conducted a tooth whitening clinical study

to compare 15% hydrogen peroxide gel illuminated with a gas

plasma light source versus 15% peroxide alone versus placebo

gel plus light, all treatments lasting 1 h. The change in Vita

shade from baseline for peroxide plus light, peroxide alone

and placebo plus light were 8.35, 5.88 and 4.93, respectively,

with peroxide plus light being significantly different to the

other two groups. In contrast, Hein et al.demonstrated no

additional effect of any of the three light sources tested over

the bleaching gel alone for three commercial products in asplit mouth clinical design. Thus, further work is clearly

required in order to unequivocally demonstrate the additional

efficacy benefit of light activated tooth whitening systems

versus their non-light activated controls.