The aim of this project is to clinically evaluate patient satisfaction, whitening efficacy, tooth sensitivity, and gingival irritation during at-home whitening with 10% carbamide peroxide using an Essix-type retainer or a Vivera retainer.

Forty patients (n=40) will be randomized as to which side will receive the type of bleaching tray, essix-type retairner or Vivera retainer. The at-home bleaching will be performed with 10% carbamide peroxide (Opalescence PF 10%) for 2 hours The treatment lasted 4 weeks, although as each retainer was worn every other day, the actual application time was 2 weeks. Patient´s satisfaction will be evaluated at baseline, after one, two weeks, and 4 weeks with an apropriate questionnaire using a visual analog scale (EVA 0-10). The color will be assessed at baseline, after one, two and four weeks using subjective color guides (VITA Classical and VITA Bleachguide 3D-MASTER) and a objective digital spectrophotometer (VITA Easyshade). The intensity and absolute risk of tooth sensitivity and gingival irritation will also be determined with a visual analog scale (EVA 0-10). Color change between groups will be compared by paired Student´s t-test. The absolute risk of tooth sensitivity and gingival irritation for both groups will be compared by McNemar test. Also, odds ratios, confidence intervals, and Spearman correlation will be determined. The intensity of tooth sensitivity, gingival irritation and patient´s satisfaction will be compared by paired Student´s t-test. The level of statistical significance accepted will be 5% for all analyses.

The inclusion criteria will be the following:

• Patients over 18 years of age.
• Patients with vital teeth without restorations, single crowns, implants and/or bridges at the level of the anterior teeth.
• Absence of cavities in the teeth to be whitened.
• Patients with satisfactory oral hygiene, and periodontally healthy, who are not undergoing periodontal maintenance and who do not use chlorhexidine rinses. Patients with recessions without sensitivity are included.
• Absence of sensitivity, measured based on stimulation with the air from the syringe.
• Patients who are at least in A2-A3 color (Vita Classic Guide)

The exclusion criteria will be the following:

• Patients who have previously undergone whitening (less than 5 years).
• Patients undergoing orthodontic treatment.
• Patients with white spot or developmental alterations (dentinogenesis imperfecta, amelogenesis imperfecta) or with stains (tetracyclines or fluorosis).
• Patients with a history of trauma to anterior teeth.
• Patients who require internal whitening.
• Smokers.
• Pregnant or lactating women.
• Allergy to any component of whitening.
• Alterations of the oral mucosa (desquamative gingivitis, OLP, leukoplakias, etc.).
• Medical conditions considered by the researchers that may compromise the study or the individual safety of the patient.
• Patients with poor oral hygiene.
• Patients with previous hypersensitivity.

It will be a single-blind study in which only the evaluator will not know the designation of the groups because he has not participated in the study's randomization and implementation process. Due to the tray test and demonstration of the bleaching procedure, the operator and the participant cannot be blinded.