The rate of chemical reactions can be increased by increasing the temperature, where a 10 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 a split 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.
Other factors
The type of intrinsic stain and the initial tooth colour can play a significant part in the ultimate outcome of tooth bleaching. Mild to moderate tetracycline staining tends to respond to extended bleaching regimes of 2–6 months. However, It is documented that severe tetracycline staining is more difficult to bleach with the darker the teeth at baseline, the longer it can take to lighten them. In addition, it is reported that when the tetracycline discolouration is located in the neck of the tooth, the prognosis for bleaching is the poorest; when it is dark gray or blue, the prognosis also is poor. For non-tetracycline stained teeth, a meta analysis of placebo controlled, patient applied tooth whitening clinical studies using 10% carbamide peroxide found that 93% of people who used the peroxide product and 20% who used the placebo exhibited a change of two shade guide units.
In addition, 20% of subjects who used the peroxide product achieved a mean change of five shade guide units. Ishikawa-Nagai et al. evaluated the tooth colour change of 80 subjects after using 10% carbamide peroxide in a gum shield over 14 days and found a strong correlation between total colour change and b* values, demonstrating that bleaching works efficiently for teeth with a yellow hue. Further, an analysis of the clinical results with over 600 subjects undergoing tooth bleaching, indicate that the yellower the teeth at baseline, the greater the magnitude of the whitening response. This analysis demonstrated a significant relationship between subject age and the magnitude of whitening response, with younger subjects experiencing greater tooth whitening. Further, there was a relationship between subject age and the initial colour and the magnitude of whitening response.
Older subjects with less yellow initial tooth colour exhibited the smallest mean colour change post bleaching, whereas younger subjects with more yellow initial tooth colour exhibited the greatest mean colour change post bleaching. In addition, neither gender nor coffee/tea consumption had any significant affect on the tooth whitening response. The presence on the tooth surface of pellicle and plaque has the theoretical potential to reduce the activity of peroxide by acting as a substrate for peroxide bleaching and/or degrading peroxide. Wattanapayungkul et al. has shown that the rate of peroxide degradation did not increase with the presence of pellicle on tooth surfaces in vivo over 1 h indicating that pellicle does not have a significant effect on the stability of peroxide. In addition, a clinical study by Gerlach et al.comparing the effect of immediate prebrushing with a toothpaste versus no prebrushing prior to tooth bleaching with 6.5% hydrogen peroxide over a 14-day period, suggested that toothbrushing immediately before bleaching has only a modest positive impact on overall efficacy. Thus, the modifying role of pellicle on peroxide delivery and whitening efficacyappears to be overall small.
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