| Objective :To investigate the effect of etch with 35% phosphoric acid on enamel cracks.Method : The sound first maxillary premolars extracted for orthodontic reasons were collected and enamel cracks were made by application of force with dental forceps. Then 63 premolars which present enamel cracks parallel to the long axis and extend to the center of buccal surface were selected as the specimen . 3 teeth would not undergo etched as control. Other 60 were randomly divided into 3 groups of 20 , the buccal surface of teeth in each group was etched with 35%phosphoric acid gel for 15,30,or 60 seconds respectively . After etched the teeth were cut horizontally from the midpoint of clinical crown with the long axis vertically . Then cross section of teeth was examined under the scanning electron microscope (SEM), and the depth of changes was measured and analyzed .Result :The enamel of teeth which have not undergo etch show an even appearance and the edge of enamel crack was intact . For all three test groups , the enamel dissolved along prism , but the density of dissolve of each group was different . 15 second group appears disconnected dissolution , 30 and 60 second group appear connected and uniformed dissolution . At enamel crack area , enamel dissolved along crack .Within 60 second , the depth of etch increased with the etch time extend . the depth of etch at enamel crack area are significantly deeper than at the normal area , the depths of etch listed in descending order is : 60 second etch at enamel crack area (53.4±5.3 μm) , 30 second etch at enamel crack area (27.8±3.4μm) , 60 second etch at normal enamel area (24.4 ±2.2μm) , 15 second etch at enamel crack area (19.7±3.6μm), 3 0 second etch at normal enamel area (15.2± 2.1μm), 15 second etch at normal enamel area (5.1 ± 1.5μm).Conclusion : It is indispensable to examine tooth surface before orthodontic bonding , and for the teeth which present enamel crack the etching duration should be controlled exactly.
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