Factors affecting teeth bleaching part 1

1. 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 h73 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.

2. 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.84 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. In this case, the most rapid whitening occurred in the first month with 20% carbamide peroxide compared to 15% and
10% carbamide peroxide. In addition, clinical studies with hydrogen peroxide strip based products have shown similar concentration and time effects for tooth whitening efficacy.