Whitening gel consists of water and a whitening agent containing hydrogen peroxide. Water represents at least 50% of the weight of the agent. With conventional teeth whitening, the gel must be applied to the teeth, where it acts for up to one hour. The success of teeth whitening depends on the time during which the tooth surface and gel are in contact and on the degree of gel activation, which can be increased by raising the temperature. Added energy (heat) increases the amount of activated H2O2 free radicals in the gel, which are essential to the process. A higher gel temperature means greater H2O2 mobility, faster breakdown of H2O2 into OH and O ions, faster diffusion of molecules into the enamel and a faster reaction between free radicals and enamel components. The operating temperature of the gel is between 10°C and 40°C.
The use of light at certain frequencies to raise the temperature of the gel was already mentioned by Abbot in 1918. Today, laser light is used for thermal activation of the gel (laser teeth whitening). Many manufacturers of laser systems for teeth whitening claim that they have improved light absorption, reduced heating of the teeth and increased photochemical activation of the whitening agent by adding activators to the whitening gel, but this concept has not yet been proven to be completely safe.
Buchalla and Attin reported that there is still insufficient direct evidence to support photochemical teeth whitening, i.e. whitening based on the direct effect of laser light on hard dental tissues; rather, whitening is the result of photochemical activation of the whitening gel, which means that it is still an indirect effect of laser light through the conventional action of H2O2 on the composition of the enamel. They additionally warned that activation of whitening gels with laser light may have negative effects on the dental pulp due to an increase in temperature exceeding the critical 5.6 °C. The temperature rises because laser light (energy) is not completely absorbed in the thin layer of gel applied to the teeth. The unabsorbed energy is therefore released in the dental tissues themselves, which means an increase in pulp temperature and may consequently lead to inflammation and necrosis of the pulp.
For teeth whitening, methods supported by the Er:YAG laser have recently become established, because the wavelengths of the generated laser light are absorbed most strongly in water, which is the main component of whitening gels, and therefore penetrate the hard dental tissues themselves to a lesser extent. All of the laser light energy is thus directed into heating the gel and thereby increasing the efficiency of chemical reactions, while reducing the risk of thermal damage to the pulp. The laser beam is absorbed in the upper surface layer of the gel (a maximum of 10 to 50µm), while the lower inner layers of the gel are heated by conductive heat transfer from the outer heated layers. Direct heating of dental tissues should thus be reduced to a minimum. Because all of the laser beam energy is absorbed in the gel, the rise in its temperature is greater and the whitening speed is higher. The teeth whitening procedure is therefore shortened five- to sixfold compared with conventional whitening. In addition, the laser beam powers used in the aforementioned teeth whitening methods are set to a maximum of 0.5J/cm2, which is below the ablation threshold for hard dental tissues, meaning that the possibility of accidental damage is minimal. The differences between the previously used diode lasers and the Er:YAG laser described above are therefore obvious, as also demonstrated by objective studies of temperature increases in hard dental tissues during the whitening process.
The image on the left shows a thermal image of a tooth during treatment with an Er:YAG laser, while the image on the right shows treatment with a diode laser. The bright red colour of the tooth indicates a higher tooth temperature, which, in view of the above, is highly undesirable.
As with all treatments, the key to success in teeth whitening is a correct diagnosis, i.e. determining the cause of tooth discoloration. Finding a solution requires an understanding of the relationship between tooth development and the various factors that cause undesirable discoloration during this process. More than 50 different causes of discoloration are known, and they may be systemic or local in origin. Based on knowledge and experience, the dentist should recommend the most appropriate method that is most likely to achieve the desired result, ranging from teeth whitening to the placement of aesthetic veneers. The method described above is most effective for whitening discolorations caused by the following:
- intrinsic discolorations in the formative phase (brown or opaque stains due to fluorosis), tetracycline discolorations
- intrinsic discolorations in the post-formative phase (due to necrotic pulp, root or crown fillings, age-related discoloration).