In vitro Analysis of Human Enamel Microhardness as Subjected to Prolonged Use of External Bleaching Agents Análise in vitro da Microdureza do Esmalte Humano Submetido ao uso Prolongado de Agentes Clareadores Externos

The aim of this study was to evaluate the effects of external bleaching agents on the microhardness of human enamel after its prolonged use. Twenty intact human third molars were submitted to mesio-distal crosscut and embedded in polystyrene resin. The specimens were submitted to finish, and half of the enamel surface of each specimen was covered with cosmetic varnish, representing the control group (G0 – did not receive bleaching agents). The sample was divided into four groups (n=10): G1 – one bleaching session with 16% carbamide peroxide; G2 – three bleaching sessions with 16% carbamide peroxide; G3 – one bleaching session with 22% carbamide peroxide; and G4 – three bleaching sessions with 22% carbamide peroxide. Each session lasted 8 hours a day over the course of two weeks, with 45 days interval between G2 and G4 sessions. During this period, the specimens were kept in artificial saliva at 37°C. Then, the Knoop hardness test was done on the middle third of each bleached and non-bleached surface. The data were submitted to analysis of variance (two-way ANOVA), Tukey’s test (p< 0.05) and Dunnet test for comparison to G0, which showed the highest superficial hardness averages, differing statistically from the other groups (306.69 KHN). Group 4 showed the lowest average (135.37 KHN). It was concluded that bleaching reduced enamel hardness. Furthermore, increasing of carbamide peroxide (22%) associated with an increased number of sessions (3 sessions) enhanced the decrease in microhardness.