Abstract: Objective:To discuss the bond strengths of three kinds of metal bottom plate brackets commonly used in clinic and after sandblasting treatment under the condition of artificial saliva,and to evaluate their bonding properties. Methods:A total of 60 premolar teeth extracted because of orthodontic treatment were collected and divided into domestic Xinya bracket group(Xinya group),imported ultra thin MBT bracket group(MBT group),and Japan TOMY lock bracket group(TOMY group)(n=20). The teeth were randomly divided into six groups after the three types of brackets fall off. The three types of new and sandblasting brackets were respectively bonded to the teeth randomly with 10 brackets in each group.The shear strength was detected by universal electronic mechanical testing machine, and the adhesive remnant index (ARI) of enamel surface in each group was obsereved,and the morphological features of three different base brackets and three shedding of brackets conducted by sandblasting were obsereved by scanning electron microscope. Results:Under the condition of artificial saliva,for the first bonding and bonding again,the shear strengths of the new brackets in TOMY goup were higher than those in Xinya group and MBT group (P<0.05),while the shear strengths had no statistical differences between Xinya group and MBT group (P>0.05).For the brackets treated with sandblasting after shedding in three groups, the shear strength in TOMY group was greater than those in Xinya group and MBT group (P<0.05),while the shear strengths in Xinya group and MBT group had no statistical difference(P>0.05). The shear strengths of brackets that were conducted by sandblasting after falling off in TOMY group and MBT group were increased compared with the original brackets(P<0.05).There were no statistical differences in ARI between various groups(P>0.05).The scanning electron microscope results showed that three kinds of brackets were crisscroped with the mesh,and they were more dense in TOMY group. After sandblasting, the sand grains were embedded in the grid,and the sand grains were more embedded in the inverted pits in TOMY group and MBT group. After sandblasting again,the sand particle embedding scope was increased,and the grid structure of the network bottom in Xinya group was destroyed,but there were no obvious abnormities in MBT group and TOMY group. Conclusion:The bond strengths of the three types of bottom plate brackets can meet the clinical needs,and the bond strength in TOMY group is superior to those in Xinya group and MBT group. The use of sandblasting to deal with the shedding of TOMY and MBT brackets can improve the bond strengths of the brackets.

Key words: bottom plate brackets, sandblasting, bond strength, shear strength, adhesive remnant index, scanning electron microscope