| Numerous methods have been recommended for the improvement of the appearance of discolored teeth. Compared all the methods in the principle of conservative dentistry, vital bleaching techniques have incontrovertible superiority, because the crown isn't destroyed in the treatment. Vital bleaching techniques mainly include dentist-administered vital-bleaching technique and nightguard vital bleaching (NGVB) technique. When compared the two techniques, the latter is very simple and realizes a time-saving for the dentist, what's more, due to using less agents and not involving any etching of enamel before treatment, and is both convenient and reduces the incidence of dentine hypersensitivity. A variety of case reports and small clinical studies have shown that vital bleaching is effective. The effect of bleaching on dental enamel is not yet clear, and is still a controversial subject. In the past decade, numerous studies evaluated the effects of different bleaching agents in the same time on the enamel, but the bleaching time of different agents is different. In this study, we developed a intrinsic discoloration model to determine the bleaching time of different bleaching techniques, and took use of Scanning electron microscopic (SEM) and Atomic Absorption Spectrophotometer (AAS) to evaluate the effect of the bleaching andremineralization on the enamel surface, the content of calcium ion of the enamel and the adhesion of streptococcus mutans to enamel. Objective:To develop and evaluate a intrinsic discoloration model in vitro, investigate the influence of different bleaching techniques, different remineralizing time and enamel bonding agents on the enamel surface, the content of calcium ion of the enamel and the adhesion of streptococcus mutans to enamel by means of SEM and AAS, to explore methods of reducing the adverse effect of bleaching techniques. Methods:1. Establishment of intrinsic discoloration model in vitro:The model was developed according to the method of Sulieman, based Qimen-red tea.2. Experiment:Thirty teeth which had been soaked in tea for 6 days i.e. were randomly allocated to six groups (n=5) using a random allocation table. 37 no-stained teeth were also randomly allocated to 6 groups, 9 teeth per group in the former 5 groups and 4 teeth in the 6th group. One half of each tooth was reserved as control while the other was treated. Group A (n=14): the specimens were bleached with 15% CP; Group B(n=14): the specimens were bleached with 30% HP associated with 15% CP; Group C(n=14): the specimens were bleached with 30% HP; Group D(n=14):the specimens were bleached with 30% HP and placed in the remineralizing solutions 30 minutes per day; Group E(n=14):the specimens were bleached with 30% HP and placed in the remineralizing solutions 3 hours per day; Group F(n=9):the specimens were treated in the same way of Group D, when the bleaching was over, enamel bonding agents were applied onto the surface of the enamel.The specimens were bleached until the color of the stained teeth return to Bi.The time was recorded. In each group 2 no-stainedexperimental samples were randomly taken to bacteria experiment, and then observed under Scanning Electron Microscopic (SEM). The remnant samples were examined by means of AAS. Results:1. Results of the establishment of intrinsic discolored model in vitro:The concentration of the tea was 4g/100ml, The color of the samples stained for 1 day, 2 days, 3 days was C3 or D4, while the color of the samples stained for 4 days, 5 days, 6 days was all C4. No significant color changes occurred between stained 1 day and 6 days (P>0.05).2. Results of bleaching and remineralization treatment:1) The bleaching time of Group A was 14 days, 8 hours per day; Group B: the samples were bleached with 30% HP for 30 minutes at the first day, then were bleached with 15% CP for 6 days, 8 hours per day; Group C: the samples were bleached with 30% HP for 2 weeks, 2 times per week, 30 minutes every time. No visual differences in colour were observed between the bleached specimens and the remineralization ones.2) Results of SEM: the enamel surface of the control specimen was smooth and compact, and had small amount of streptococcus mutans adhered to. The demineralization of the specimen' enamel in Group A is the most severe and some area is loose like alveolate tissues, and a large number of bacteria existed in the "honeycomb". In Group B and C, the enamel surface existed many dimples and bacteria. In Group D there were small amount of mineral deposits in the demineralizing area, and fewer bacteria adhered when compared with the bleached. In Group E there were a large number of mineral deposits on the enamel surface. In Group F the enamel surface was glossy and had a few streptococcus mutans adhered to.3) Results of AAS: The calcium content of all the bleached enameldecreased (P<0.01). There is significant variance between Group A and Group B, C (P<0.01), while no significant change between Group B and Group C (P>0.05). The calcium content of the enamel of Group D was higher than Group C (P<0.01), and lower than the control (P<0.01). In Group E the calcium content of the enamel was higher than the control (P<0.01). Conclusions:1. The intrinsic discoloration model developed at room temperature, based on tea, is a practicable method to study the dental bleaching.2. Dentist-administered vital-bleaching technique, nightguard vital bleaching (NGVB) technique with bleaching agents of higher concentration and the associated bleaching technique all cause enamel dematerialized and the number of streptococcus mutans adhered to the enamel increased.3. Mineralizing solution obviously accelerates the remineralization of the bleached enamel, and reduces the number of streptococcus mutans adhered to the enamel.4. The dematerialized enamel surface was glossier, have better polish and fewer bacteria adhered to, after the bleached sample was treated by the mineralizing solution and enamel bonding agents.The results in vitro show that during clinic bleaching treatment with higher concentration bleaching agents, if mineralizing solution are used to rinse mouth, a promising effect will be presented, including the mineral deposition being pronounced on the surface of the bleached enamel, the gloss of the enamel being enhanced and the probability of re-discoloration, dentin hypersensitiveness and caries being decreased.
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