summerize: Abstract
Objectives

To evaluate the effects of experimental protocols on bleaching effectiveness and hydrogen peroxide (HP) diffusion through enamel and dentine.
Methods

Enamel/dentine discs were subjected to six bleaching sessions, consisting of 1 or 3 applications of 17.5% or 35%-HP gel for 5/15 min, or 37% carbamide peroxide (CP) gel for 10/20 min. Discs undergoing the regular protocol (35%-HP; 3 × 15 min) constituted the positive control group. Colour change (ΔE) was assessed (CIE L*a*b* system) after each session. HP diffusion was quantified (sessions 1, 3, and 6) in enamel/dentine discs adapted to artificial pulp chambers. Data were analysed by Pillai's Trace and Bonferroni test, or by one-way ANOVA and SNK/Tamhane's test (α = 5%).
Results

All tooth-bleaching protocols significantly increased the ΔE values. A reduction in HP diffusion and no significant difference in ΔE compared with the positive control were observed for the following bleaching protocols: 17.5%-HP 3 × 15 min, at the 4th session; and 35%-HP 1 × 15 and 3 × 5 min, at the 5th session. HP diffusion in the 37%-CP 3 × 20 min bleaching protocol was statistically similar to that in the positive control. The other experimental bleaching protocols significantly decreased HP diffusion through enamel/dentine discs, but the ΔE values were statistically lower than those observed in the positive control, in all sessions.
Conclusion

Shortening the contact time of a 35%-HP gel or reducing its concentration produces gradual tooth colour change and reduced HP diffusion through enamel and dentine.
Clinical significance

A reduction in HP concentration, from 35% to 17.5%, in a bleaching gel or shortening its application time on enamel provides a significant tooth-bleaching improvement associated with decreased HP diffusion across hard dental tissues. Therefore, these protocols may be an interesting alternative to be tested in the clinical situation.
Introduction

An at-home tooth-bleaching procedure with 10% carbamide peroxide (CP) gel has been considered the safest method for bleaching teeth with minimal adverse effects.1, 2, 3, 4, 5 However, since this tooth-bleaching modality is patient-applied, there is a risk of gel application over exposed dentine in patients with gingival recession and abfraction/abrasion lesions. Also, the inadequate use of the tray may result in gel overflow, with extended soft-tissue exposure and likely material ingestion.3, 6 Therefore, it seems evident that this kind of aesthetic therapy should be performed entirely under professional supervision.

For the in-office tooth-bleaching technique, in which the procedure is performed by clinicians, bleaching gels with high concentrations (35–38%) of hydrogen peroxide (HP) are applied in a 30- to 45-min chair-side session.1 This therapy allows for a clinically perceptible dental colour improvement even after one clinical appointment.6, 7, 8, 9, 10 However, tooth sensitivity is a side-effect commonly reported in the literature when this technique is applied to vital teeth.7, 8, 9, 10, 11 The low molecular mass of HP and its sub-products favours its diffusion through mineralized dental tissues to reach the pulp chamber.12, 13, 14 The contact of pulp cells with these reactive oxygen species (ROS) results in oxidative stress generation, due, at least in part, to an imbalance between the amount of ROS and endogenous/exogenous antioxidants.10, 11 It has been shown that HP and its sub-products can reduce cell viability, as well as cause cell membrane damage and proteolytic enzyme activation, extracellular-matrix degradation, tissue inflammatory reaction, and even partial pulp necrosis.11, 15, 16, 17, 18, 19, 20, 21, 22

Previous studies demonstrated that the higher the concentration and the length of time of the bleaching agent application on enamel, the higher the HP penetration of the pulp chamber, and the more intense are the adverse effects to pulp cells.15 Thus, changes in in-office tooth-bleaching techniques may provide an interesting alternative for maintaining bleaching effectiveness, while preventing or at least minimizing the negative effects of this kind of aesthetic therapy on pulp tissue. Therefore, the present study aimed to assess and correlate the bleaching effectiveness of different experimental in-office tooth-bleaching protocols and their capacity for HP diffusion through enamel and dentine.
Section snippets
Sample preparation

Enamel/dentine discs (5.6 mm diameter, 3.5 mm thick) were obtained from the buccal surfaces of sound bovine central incisors (24–30-month-old bullocks).9 An intrinsic stain model was carried out as described by Sulieman et al.23 The dentine surfaces of experimental and control discs were etched with 37% phosphoric acid (3M ESPE, St. Paul, MN, USA) for 60 s. The acidic product was then rinsed with distilled water spray. The discs remained in contact with a standardized solution of black tea (Leão
Bleaching effectiveness

The graphic representation of Δa, Δb, ΔL, and ΔE according to the different tooth-bleaching sessions is shown in Fig. 1. When each experimental group was analysed during the sessions, a significant reduction in Δa values was observed for G2 (sessions 3–6), G4 (session 6), G3 (session 6), and G6 (sessions 5–6) (p < 0.05). No significant difference was observed in Δb through the sessions for all experimental groups (p > 0.05). All groups presented a significant increase in mean ΔL values, which
Discussion

Currently, post-operative sensitivity claimed by patients subjected to tooth-bleaching therapies is believed to be a consequence, at least in part, of HP diffusion through enamel and dentine into the pulp chamber. This phenomenon results in pulp inflammation, with release of inflammatory mediators and pulp sensory nerve stimuli.16, 18 In some cases, the sensitivity is so intense that treatment is discontinued.7, 11 Therefore, a reduction in enamel/dentine-HP diffusion and, consequently, its
Conclusions

Based on the data obtained in the present study, it can be concluded that the application of 35%-HP gel to enamel for only 15 min or 17.5%-HP gel for 45 min (3× 15 min) produces gradual tooth colour change and increases its lightness. Both protocols, which presented bleaching performance similar to that of the traditional in-office protocol (35%-HP gel – 3× 15 min), significantly reduced HP diffusion through enamel and dentine. The 37%-CP gel presented the worst tooth-bleaching performance,