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.
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