Two weeks before the bleaching procedures, selected volunteers from the city of Nova Friburgo, (Rio de Janeiro, Brazil) received a dental screening and a dental prophylaxis with pumice and water in a rubber cup and signed an informed consent form.
This was a randomized, single-blind (evaluators), parallel and equivalence trial with an equal allocation rate between groups.
A total of 61 participants were examined in a dental chair to check if they met the inclusion and exclusion criteria. A total of 47 participants were included in this study. The participants were randomly divided in the 2-day and 7-day groups.
During the in-office sessions, participants had the gingival tissue isolated of the teeth using a light-cured resin dam (Top Dam, FGM, Joinville, SC, Brazil). After that, a 2% Potassium nitrate and Sodium fluoride desensitizer gel (Desensitize 2%, FGM) was applied for 15 minutes before dental bleaching. This gel was removed and the 35% hydrogen peroxide gel (Whiteness HP, FGM, Joinville, Brazil) was applied in three 15-min applications according to the manufacturer's directions in all upper and lower incisors, canines and premolars. After 2 days (2-day group) and 7 days (7-day group), this procedure was repeated using the same protocol.
Color evaluation was performed before and 30 days after the end of the bleaching treatment using a subjective (value oriented shade guide Vita Lumin, Vita Zahnfabrik, Bad Sackingen, Germany) and an objective method (Easyshade spectrophotometer, Vident, Brea, CA, USA). Color evaluation was done in a room under artificial lightning conditions without interference from outside light.
For the subjective examination, the shadeguide's 16 tabs were arranged from highest (B1) to lowest (C4) value. Changes in the scale were treated as representing a continuous and approximately linear ranking. Color was measured in the middle third of the facial surface of the right upper canine at baseline and 30 days after bleaching to calculate the variations in color towards the lighter end of the scale (Delta Shade Guide Units - DSGU).
One calibrated operator measured the colour in all participants using the Vita Easyshade spectrophotomer (Easyshade, Vident, Brea, CA, USA) before and 30 days after the bleaching therapy. The L\*, a\* and b\* parameters were recorded. L\* represents the value from 0 (black) to 100 (white) and a\* and b\* represent the shade, where a\* is the measurement along the red-green axis and b\* is the measurement along the yellow-blue axis. Color change was calculated between the two assessment periods (DE) using the formula: DE = \[(DL\*)2 + (Da\*)2 + (Db\*)2\]1/2.
The patients recorded their perception of TS during the first and second bleaching sessions using the visual analogue scale VAS. This scale employs a 10-cm horizontal line with words ''no pain'' at one end and ''worst pain'' at the opposite end. Participants recorded whether they experienced TS during the treatment up to 1 h after the bleaching, from 1 h to 24 h and from 24 h to 48 h after bleaching. In the 30-day recall, patients were also asked if they experienced pain during this 30-day period. As two bleaching sessions were performed, the VAS score obtained in both bleaching sessions was considered for statistical purposes. These values were computed for the maxillary arch.
