Effects Of Tooth Bleaching And Desensitization On Shear Bond Strength Of Orthodontic Brackets

Objective: To evaluate the effects of beyond cold light whitening and desensitization on shear bond strength of orthodontic metal brackets.Materials and Methods: Ninty eight extracted human premolar teeth were randomly divided into seven groups of fourteen each.The first group of teeth had no treatment before bonding serving as control.The second group of teeth was stored in distilled water after teeth bleaching and was bonded orthodontic brackets after twenty four hours.Desensitizer alone was used in the third group,then all teeth were stored in distilled water twenty four hours before bonding. The fourth group of teeth was bleached the same as the second group and then the desensitizer was applied the same as the third group.After that all teeth were stored in distilled water twenty four hours before bonding.The fifth group of teeth was treated the same as the fourth group,the only difference is that the orthodontic brackets were bonded one week after the treatment.The sixth group of teeth had the same treatment as the fourth group and was bonded orthodontic brackets after two weeks.The seventh group of teeth with the same treatment was bonded orthodontic brackets after three weeks.All the samples were stored in artificial saliva at tempreture of 37℃ after the treatment and were bonded orthodontic brackets with light cure composite resin manufactured by 3M company and cured with halogen light.The shear bond strength of all samples was measured with a universal testing machine(INSTRON 5848) at a crosshead speed of 1 mm/min and converted into megapascals(Mpa).To evaluate the amount of resin left on the enamel surfaces and the brackets base surfaces after debonding,the adhesive remnant index(ARI) scores were used. All teeth surfaces both after treatments and after 37% phosphoric acid corrosion were observed by laser scanning microscope and then images magnification of 2000 times were analyzed. Statistical analyses were processed with the SPSS 19.0 software system. The shear bond strength values wre analyzed with ANOVA and pairwise comparison were made by the Tukey’s test.The ARI scores were analyzed using a chi-square test.The level of significance was established as p<0.05 for all statistical tests.Result: The brackets bonded twenty four hours after bleaching revealed significantly lower(p<0.05) shear bond strength(7.5 ± 1.77 Mpa) than that of the control group(mean:12.24±3.71 Mpa). The shear bond strength(mean:11.68±3.49 Mpa) of group which were bonded orthodontic brackets twenty four hours after desensitization slightly lower compared with the control group,but no statistically significant difference(p>0.05). Comparing with the control group,the shear bond strength(8.29±2.01 Mpa) of the group bonding brackets twenty four hours after bleaching and desensitization was significantly lower(p<0.05) and that(12.95±3.35 Mpa) of the group bonding brackets one week after bleaching and desensitization was no significant difference(p>0.05).The shear bond strength(11.24±2.92 Mpa) of the group bonding brackets two weeks after bleaching and desensitization and the shear bond strength(11.27±2.73 Mpa) of the group bonding brackets three weeks after bleaching and desensitization gradually restored to that of the control group,the two groups were not statistically different(p>0.05) comparing with the control group.The adhesive remnant index(ARI) scores revealed no significant difference(p>0.05) between the seven groups.There was a greater frequency of ARI scores of 3、4 and 5 in all groups.Conclusions:1.The shear bond strength of the brackets which were bonded twenty four hours after bleaching and bleaching joint desensitization significantly reduce,but the values could meet the needs of clinical orthodontic. So we could bond the brackets twenty four hours after treatments; 2.Nano-biomaterials desensitizing agents used don’t reduce the shear bond strength of the brackets, but also can repair the damage to the teeth of bleaching; 3.The impacts of bleaching and desensitization gradully reduce and return to normal levels after one week.