Influence of intracoronal bleaching agents on the ultimate strength and ultrastructure morphology of dentine.

To evaluate the effects of intracoronal bleaching on ultimate tensile strength (UTS) of sound and etched dentine and its ultrastructure morphology. Bovine dentine specimens with (e) or without previous etching with 37% phosphoric acid for 15 s were used for the intracoronal bleaching experiments. Teeth were randomly assigned to five treatments (n = 10): (C) control--no bleaching, (SP) sodium perborate, (CP) 35% carbamide peroxide, (25% HP) 25% hydrogen peroxide and (35% HP) 35% hydrogen peroxide. Bleaching was performed four times within a 72 h interval and afterwards, dentine pulp chamber blocks were obtained. The blocks were sectioned in 0.7 mm-thick slices and these were trimmed to reduce the inner dentine to a dumbbell shape with a cross-sectional area of 0.8 mm(2). Specimens were tested with the microtensile method (0.5 mm min(-1)) and data were analysed (two-way ANOVA-Tukey test, P < 0.05). Additional teeth were prepared for transmission electron microscopy (TEM) to evaluate dentine ultramorphology. The mean values of the UTS (SD) in MPa for sound dentine were: C = 48.3(8.5)a, SP = 34.6 (8.2)b, CP = 32.9 (8.9)b, 25% HP = 28.0(4.6)b, 35% HP = 26.4(6.6)b, and pre-etched dentine: Ce = 38.9(13.8)a, SPe = 31.3 (9.3)ab, CPe = 28.4 (6.2)ab, 25% HPe = 30.0 (7.9)ab, 35% HPe = 19.9(4.6)b. Significant differences between the means are indicated by the letters. TEM observations exhibited demineralization areas for all bleaching treatments. Bleaching decreased dentine UTS after treatment. Pre-etched not-bleached dentine (Ce) presented UTS similar to pre-etched bleached dentine, except for 35% HPe. The decrease of UTS of bleached dentine could be attributed to ultrastructural alterations such as loss of inorganic components.