2.3.3. Bleaching protocol Two whitening sessions were carried out with intervals of one week. At the beginning of each session, dental prophylaxis was done with a brush at low speed. Stone pumice and water were used in the upper front to remove the layer from the surface of the teeth enamel so that it would not alter the effectiveness of the gels. We used a plastic lip separator, a blue gingival barrier syringe, and light-curing (Polaoffice + SDI Limited, Bayswater, Australia) to protect the soft tissue. We homogeneously applied the different gels on the vestibular surfaces of each hemiarcade. One hemiarcade was treated with 37.5% hydrogen peroxide, and the other was treated with 6% hydrogen peroxide (Polaoffice + SDI Limited, Bayswater, Australia).