| Objective: this in vitro study was firstly to evaluate the effect of two different bleaching regimens and two weeks post-time bonding on the shear bond strength of dentin to composite resin, and observe the dentin/composite interface of each group under scanning electron microscope. And secondly to evaluate the effect of external bleaching on the composition of dentin.Methods: thirty-two sound maxillary and mandibular premolars extracted for orthodontic reasons were randomly divided into six groups, the former five groups with six teeth each, the latter one with two teeth. Group A (control): teeth were stored in artificial saliva at 37 degree C for 3 weeks, After that the teeth were ground from superficies to expose dentin, and then mounted with the buccal surfaces upward in a plastic holder with a self-cured acrylic resin.the dentin surfaces were polished using 600# sandpaper under running water. A piece of adhesive tape with a 3.24-mm diameter hole was securely adapted to the centre of the flattened dentin surface to delimit the bonding surface. A split mould was positioned and clamped over the hole in the adhesive tape.Then the ParaCem○RUniversal DC adhesive system was applied to the delimited surface according to the manufacturer's instructions. Group B (external bleaching and bonded immediately): 11% carbamide peroxide was placed on the buccal enamel surfaces of the teeth for a period of three weeks, with a daily contact of six hours. After bleaching the prepared dentin surfaces were applied with composite resin. The preparation of dentin surfaces and application with composite resin were the same as group A. Group C (external bleaching and bonded two weeks later): the teeth were bleached as group B, after bleaching teeth were stored in artificial saliva at 37 degree C for another 2 weeks then ground to expose dentin, and applied with composite resin. The preparation of dentin surfaces and application with composite resin were the same as group A. Group D (internal bleaching and bonded immediately): the roots were removed from the crowns at 2.0mm apically to the cemento-enamel junction (CEJ). The pulp chambers were exposed from occlusal surfaces of the teeth as clinically. A 2-mm-thick Glass-ionomer cements was packed into the root canal to the CEJ. The pulp chambers were etched by a 35% phosphoric acid gel, then a paste of sodium perborate and water was inserted in the pulp chambers, the occlucal surface seal was made with Zinc oxide-eugenol. The bleaching agents were changed every 7 days for three weeks, during the bleaching and unbleaching time, the teeth were stored in artificial saliva at 37 degree C. After bleaching the dentin surfaces were applied with composite resin. The preparation of dentin surfaces and application with composite resin were the same as group A. Group E (internal bleaching and bonded two weeks later): the teeth were bleached as group D, after bleaching the teeth were stored in artificial saliva at 37 degree C for another 2 weeks, then the dentin surfaces were applied with composite resin. The preparation of dentin surfaces and application with composite resin were the same as group A. After the dentin surfaces were applied with composite resin, five specimens from each group were randomly assigned to debonding test. The force was load to failure, using a crosshead speed of 1.0 mm/min. After the shear bond strength was calculated, the data was subjected to one-way analysis of variance (ANOVA) followed by Student- Newman-Keuls (SNK-q) multiple range test for comparison of mean values. The level of significance was established as P<0.05 for all tests. Statistical analysis was processed with the SPSS 11.5 software system. The specimen remained in each group were sectioned buccal-lingually, the profile of one section selected randomly was observed by the scanning electron microscope. Group F: teeth were sectioned buccal-lingually, one section randomly selected from each tooth was used as control, stored in artificial saliva at 37 degree C for 3 weeks, The other section from the same tooth was bleached as group B. After bleaching application, a dentin slab was cut from the same region of each section, cleaned with ultrasonic, and then the profile of the dentin slab was subjected to XPS.Results:⑴Means of shear bond strength obtained from debonding test were as follow (unit: MPa): Group A 18.61± 3.56, Group B 10.74±3.03, Group C 17.21±3.15, Group D 12.10±2.64, Group E 16.78±2.84. Analysis of variance (ANOVA) results indicated that there was significant difference of shear bond strength (P<0.05); Multiple comparison (SNK-q) results indicated that significant difference of shear bond strength values existed between group A and B, group A and D, group B and C, group B and E (P<0.05); no significant difference existed between group A and C, group A and E, group B and D, group C and E (P>0.05).⑵SEM examination of dentin/composite interfaces: The number of resin tags present in the dentin/composite interface was large for group A, the length of resin tags were long and uniform; The number of resin tags present in the dentin/composite interface was scare for group B and D, the length of resin tags were shorter than group A; The number of resin tags present in the dentin/composite interface for group C and E was much more than for group B and D, but not to the point of group A.⑶The XPS analysis of the teeth in group F: Bleached dentin surface seems have a slight decrease in the concentration of oxygen, calcium, phosphorus and a slight increase in the concentration of fluorine and chlorine compared with unbleached dentin.Conclusion: 1 Internal tooth bleaching has the same effect on the shear bond strength of dentin to composite resin as external tooth bleaching has. 2 Post bonding time has different effect on the shear bond strength of dentin to composite resin. 3 The use of internal tooth bleaching technique immediately before bonding significantly reduces bond strength, so does the external bleaching technique. 4 Immersion of bleached teeth (by external or internal bleaching technique) in artificial saliva for 2 weeks before bonding results in a return to control bond strength. 5 The elemental composition of dentin seemed affected by the using of external bleaching technique. |
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