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ObjectiveDuring the process of orthodontia, how could prevent the enamel demineral-ization that occurs adjacent to attachments is of great concern to orthodontists. Fluoride could effectively reduce caries. The methods of fluoride application in the dental clinic are: ( 1) pretreatment with fluoride before etching; (2) incorporation of fluoride in the etching solution; (3 ) application of fluoride after etching. The purpose of this study were evaluate the three methods to find the most effect method by mensurating and comparing the sheer bond strength of bracket and the consistence of Ca2+ in the acid artificial saliva and ARI.Method96 extracted human first maxillary premolars were randomly divided into 4 groups ( A, B, C, D) of 24 teeth eath group. Eath tooth was truncated at the midst of root and polished , and was enveloped the facet of root canal with wax , coated with neil varnish besided the 5 × 5mm2 area in the middle of cheek - surface. Group A was compared group,each tooch in group A was etched traditionally. Each tooch in group B was brushed with fluoride before etching. Each tooch in group C was ethched by etching solution in which fluoride was added. Each tooch in group D was brushed with fluoride after etching. All the tooth were bonded with bracket. Each one was put in a test tube that have 2ml acid artificialsaliva. All specimens were bathed with 37℃ water. After 1 day, 10 days, 20 days,30 days,every time, 6 specimens were randomly selected to determine the shear bond strength and the consistence of Ca and ARI.Result1. The shear bond strengthThe mean shear bond strength ,the first day,of group A was 14. 573 ± 1. 425MPA,of group B was 14. 192 ± 1. 047MPA,of group C was 14. 068 ±1. 368MPA,of group D was 14. 842 ± 1. 163MPA;the tenth day,of group A was 14.413 ± 1. 420MPA,of group B was 14.558 ± 1. 209MPA,of group C was 14. 197 ± 1. 283 MPA, of group D was 14. 255+1. 249 MPA; the twentieth day, of group A was 14. 138 ±1. 092MPA, of group B was 14. 330 ± 1. 558MPA, of group C was 14. 340 ± 1. 663MPA, of group D was 14. 255 ± 1. 249MPA; the thirtieth day, of group A was 13. 958 ± 1. 565MPA,of group B was 13. 755 ± 1. 294MPA,of group C was 14. 340 ± 1. 663MPA, of group D was 13. 702 ± 2. 012MPA.The difference was not significant.2. The consistence of Ca 2+ in the aid artificial salivaThe mean consistence of Ca2+ in the aid artificial saliva that in the tese tubes, the first day, of the group A was 39. 382 ±0. 783mg/L,of group B was 41. 260 ±1.01 mg/L,of group C was 39.097 ±0. 655mg/L,of group D was 39. 235 ±0.755mg/L;the tenth day, of group A was 39. 382 ±0.783mg/L,of group B was 42.690 ± 1. 413mg/L, of group C was 37. 065 ± 1. 014mg/L, of group D was 35. 860 ± 1. 082mg/L; the twentieth day, of group A was 42. 615 ± 1. 187mg/L,of group B was 42. 278 ± 1. 114mg/L,of group C was 36. 035 ±1. 004mg/L,of group D was 33. 885 ± 1. 32mg/L; the thirtieth day ,of group A was 43. 195 ±0.954mg/L,of group B was 42.690 ± 1.413mg/L,of group C was 34. 303±1.516mg/L,of group D was 32. 657 ± 1.539mg/L.The difference was significant.3. ARIIn group A, the count of ARI = 1 was 17, of ARI = 2 was 7; in group B, the
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