| Objective: The esthetics of the natural dentition can be improved by bleaching and this process can be applied to intrinsically and extrinsically stain teeth. The use of peroxide-based tooth-whitening materials has increased substantially in the past few years, despite many unanswered questions about their use. Thus, little is known of their biological and physical effects, particularly their effects on dental restorative materials and on the shear bond strength of orthodontic adhesives to human tooth enamel. Because some adults who are interested in orthodontic treatment might have also had their teeth bleached or might be interested in bleaching, it seems important to determine whether bleaching would significantly influence the bonding strength of orthodontic bracket adhesives to the enamel surface. The purpose of our study was to 1 observe the influence of 30% hydrogen peroxide bleaching technique on the enamel surface morphology, 2 determine the effect of 30% hydrogen peroxide bleaching on the shear bond strength of metallic orthodontic brackets and 3 evaluate the adhesive remnant index after debonding.Method: Under 10×magnification, 48 noncarious premolars fleshly extracted for orthodontic reasons were used in this study. Teeth with hypoplastic areas, crack, tetracycline or fluorotic of the enamel structure were excluded. The criteria for tooth selection dictated no pretreatment with a chemical agent such as alcohol, formalin, or hydrogen peroxide, or any other form of bleaching. The teeth were stored in artificial saliva at 37℃after all stains and debris were removed using a dentifrice and rubber cup rotating in a slow speed handpiece. The liquid was changed weekly to avoid bacterial growth.The 48 premolars were randomly divided into 4 groups of 12 teeth. Group A that was not subjected to any pretreatment and evaluated as a control group, was prepared for bonding brackets directly. The teeth in the other groups were immersed in 30% hydrogen peroxide solutions for 30 min every day and then stored in artificial saliva. This procedure was repeated over 7 days. Following bleaching, Group B was prepared for bonding brackets immediately; Group C was stored in artificial saliva for 7 days; Group D was stored in artificial saliva for 28 days.Paired teeth were selected random from Group A, B and C. Premolars were transversely sectioned with diamond discs in a buccal-lingual direction and removed the roots. The enamel buccal surface was remained 1/3 in the mesio-distal and cervico-occlusal directions, and then made into the sample. Samples were washed with water spray, dehydrated with 70% alcohol, and dried with air. After samples evaporated and coated with 15um of gold in a Polaron EIKO.IB-3 scanning electron microscope coating unit, the specimens' configuration were examined microscopically and photographed with a Hitachi S-3500N scanning electron microscope.According to the recommendations provided by the manufacturers of enamel adhesion system, the buccal surface of the tooth was etched with 37% phosphoric acid and applied the liquid primer. Following that the pastes were mixed symmetrically, edgewise brackets were bonded to location with enamel adhesive in 10 teeth from each group respectively and press lightly, excess composite removed with a probe. Five minutes curing later, the specimens were kept in artificial saliva at 37℃ for 24h to prepare the shear bond strength test. The operator himself completed all procedures.After this process, the root of each tooth was mounted vertically in a cylinder gypsum pedestal so that the crown was exposed. The embedded specimen was secured in a jig attached to the base plate of a universal testing machine. A steel knife-edge was placed parallel to the specimen surface so that the shear force applied the load directly to the bracket soleplate. A crosshead speed of 0.5 mm/min was used, and the maximum load necessary to debond the bracket was recorded. The force required to remove the brackets was measured, and the shear bond strength w... |
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