2.3. Procedure 2.3.1. Determination of study groups The study was carried out with a "split-mouth" design. Bleaching agents (hydrogen peroxide) were randomly assigned to each hemiarcade. The operators (R.H. and M.P.) were unaware of the product being used. To achieve this, automixed syringes from a Polaoffice+ whitening kit were used (SDI Limited, Bayswater, Australia). The syringes contained hydrogen peroxide in the form of thixotropic gel at a concentration determined by the manufacturer and marked properly.