Biological and Esthetic Effects of Tooth-bleaching According to Enamel-dentin Thickness

Objectives: The aim of this study was to evaluate the whitening effectiveness and toxicity of tooth-bleaching protocols applied to enamel/dentin discs simulating mandibular incisors (IC – 2.5mm thick) and premolars (PM – 4.0mm thick).
Methods: A 10% hydrogen peroxide (H₂O₂) gel was applied for 3x15, 1x15, or 1x5 minutes on enamel of IC or PM discs and the trans-enamel and trans-dentinal diffusion products were applied to cultured human dental pulp cells. Non-bleached samples and the 35% H₂O₂ gel - 3x15 minutes were used as negative (NC) and positive (PC) control, respectively. Cell viability (MTT Assay), oxidative stress (H₂DCFDA), odontoblastic markers expression (ALP activity and Alizarin Red), amount of H₂O₂ diffused and enamel color change (ΔE) were assessed (ANOVA Tukey/Sidak; p < 0.05).
Results: In IC-group, the percentage of cell viability for the PC; 10% 3x15; 1x15; and 1x5 protocols was 8.5%; 37.6%; 86.0%; and 107.8%, respectively. For the PM-group, the percentages were 15.3%; 71.0%; 127.2%; and 120.1%, respectively (NC = 100%). For the PC as well as 10% 3x15 protocol applied in the IC- and PM-groups, the pulp cells presented viability lower than NC (p < 0.05). In IC-group, cells exposed to 10% 3x15 and 1x15 featured lower cell viability, oxidative stress and down-regulation of odontoblastic markers than those of PM-group (p < 0.05). Concerning the other experimental protocols tested only slight cells alterations occurred, with intensity inversely proportional to the enamel/dentin thickness. These effects were also correlated with high H₂O₂ diffusion in the IC-group. In this IC-group, ΔE values similar as positive control were found for the 10% 3x15 and 1x15 protocols after 4 and 6 sessions, respectively.
Conclusions: Therefore, application of a 10% H₂O₂ bleaching gel for 15 or 45 minutes to thin dental substrate (2.5 mm thick) minimizes cell toxicity and cause adequate esthetic outcomes by increasing the number of bleaching sessions.