| Research one: The in-vitro study on the shear bond strength of zirconia brackets Objective: To test through in-vitro experiment the shear bond strength of zirconia brackets being initialized and aged,and to compare with that of metal brackets and alumina ceramic brackets,in order to provide reference to clinical applications of zirconia brackets.Methods: 60 premolars extracted due to orthodontic treatment were collected.The 60 extracted premolars were randomly divided into three groups,Group A with 20 extracted premolars,Group B with 20 extracted premolars,and Group C with 20 extracted premolars.Each group was then further divided into 2 subgroups,which,together,were A1,A2,B1,B2,C1,and C2.Group A was bonded by alumina ceramic brackets(ORMOC,US);Group B was bonded by metal brackets(Shinye-Ortho,China);and Group C was bonded by zirconia brackets(Jingcui,China).All the three groups were processed by acid corrosion with 37% phosphoric acid for 20s;by 3M Transbond adhesive;by illumination with curing light for 10 s separately at directions of proximal gingiva,distal and middle gingiva,and occlusal gingiva,40 s of illumination in total;and were placed in 37°C artificial saliva bath for 24 h.Subgroups A1,B1,and C1 were measured the initial shear bond force(SBF)by a microforce tester.Subgroups A2,B2,and C2 were circulated 5,000 times by a thermal-cold circulator;then their aged shear bond force was measured by a microforce tester,and their shear bond strength(SBS)was calculated accordingly.The adhesive remanant indexes(ARI)were observed by stereomicroscope and recorded.Results:(1)Between Group A and Group B and Group C was there a difference of statistical significance(P<0.05)in SBS,while the difference between Group B and Group C was not statistically significant(P > 0.05).Meanwhile,statistically significant differences in SBS(P<0.05)were found in all of Groups A,B,and C.(2)Between the three groups(A,B,and C)the differences in ARI all had no statistical significance(P>0.05).Conclusions: 1.both the initial and aged shear bond strength of the zirconia brackets were found lower than that of alumina brackets and equivalent to that of metal brackets.All met clinical requirement.2.Zirconia brackets presented a higher calculate score in ARI.Their adhesive breaking more often occurred to the bracket-adhesive interface,while the enamel was at a slightly low risk of damage,which was favorable for enamel protection.Research two: The shear rebond strength and morphology of zirconia brackets processed by Er:YAG laser Objective: This study was to compare the recycling of deboned zirconia brackets via an Er:YAG laser or via the traditional chair side processing methods of flaming and sandblasting;shear bond strength and morphological changes were evaluated in recycled zirconia brackets,with aim to select the optimal solution to recycle and reuse zirconia brackets.Methods: 50 premolars extracted due to orthodontic treatment were collected and bonded with zirconia brackets.Then,these brackets were removed off by a microforce tester and recycled.The 50 zirconia brackets recycled were randomly divided into 5 groups.To treat the residual adhesive on the bottom plates,five methods--flame,sandblast method,Er: YAG laser 250 m J,Er: YAG laser 300 m J,Er: YAG laser 350 m J—were applied to the five groups,respectively.The adhesives left on the tooth surfaces were removed by low speed handpieces with dental burs and next polished by silicon ions.From each group 8 zirconia brackets were randomly selected to rebond.Of the brackets processed,their shear rebond force(SRF)was measured by a microforce tester,and their shear rebond strength(SRS)was calculated as well.For the rest 2 zirconia brackets of each group,the morphological changes were given close observation by a scanning electronic microscopy(SEM);meanwhile,the residuals on these bracket bottom plates were analyzed by a confocal Raman spectrometer(RS).Results:(1)The sandblast group showed a higher SRS than that of flame group,Er: YAG laser 250 m J group,Er: YAG laser 300 m J group,and Er: YAG laser 350 m J group,with a difference of statistical significance(P<0.05).(2)The SRS of flame group was lower than that of sandblast group,Er: YAG laser 250 m J group,Er: YAG laser 300 m J group,and Er: YAG laser 350 m J group,and the difference was of statistical significance(P<0.05).(3)The differences in SRS between Er: YAG laser 250 m J group,Er: YAG laser 300 m J group,and Er: YAG laser 350 m J group were found no statistical significance(P>0.05).Between the five groups there was no statistically significant difference(P>0.05)found in ARI and treatments.(4)Between the five groups the differences in ARI were all not statistically significant(P>0.05).(5)The sandblast group achieved to thoroughly remove the residual adhesives off bracket bottom plates.But for the bracket bottom plates,the method led partial retention forms to fall downward;the retention grooves to be shallower;the bracket margins to coarsen;and the pits to increase in number considerably,with a relatively uniform distribution.On the bottom plates of brackets of the combustion group were visible residual adhesives and mild damage.The Er: YAG 250 m J group showed a trace of adhesives remaining on the bracket bottom plates.The condition and shapes of these plates were kept intact and good,with no obvious damage on them.For Er: YAG 300 m J and Er: YAG 350 m J groups,both treatments were found capable of clearing the residual adhesives from the bracket bottom plates.The former treatment left the bottom plates complete and good in shape and condition,with barely visible damage.In contrast,the later treatment brought seeable damages to the plates,where part of the surface materials peeled off and fractures multiplied in the surfaces of materials.Conclusions: 1.The rebond zirconia brackets processed by 250 m J,300 m J,and 350 m J Er:YAG lasers could produce shear rebond strength equivalent to that of the first bonded of zirconia brackets.2.Minimal morphology damages were found on the shape of zirconia brackets processed by 250 m J,300 m J,and 350 m J Er:YAG lasers.3.To sum up,the rebond zirconia brackets processed by 250 m J,300 m J,and 350 m J Er:YAG lasers obtained not only a good shear rebond strength but also the minimal morphology changes.Research three: Comparison of rebond microleakage between rebond zirconia brackets processed by different methods.Objective: To compare the location and depth of rebond microleakage between rebond zirconia brackets processed by Er: YAG laser,flame,and sandblast.Methods: 4 rebond zirconia brackets for rebond tests were prepared for each group in light of the group division and methods of Study Two.All test pieces were soaked in 0.1% Rhodamine B isosulfate fluorescent staining solution for 24 hrs,then sliced and polished.Next,the test pieces were placed under a confocal laser scanning microscope(CLSM)to observe and gauge the depth(?m)of microleakage at the occlusal direction and gingiva direction of bracket-adhesive interface(B-A interface)and enamel-adhesive interface(EA interface),respectively.Results:(1)Between the B-A interface microleakage of re-bonded zirconia brackets processed by different treatments,the sandblast and flame groups present a higher microleakage than that of Er: YAG laser groups with 250 m J,300 m J,and 350 m J(P<0.05);meanwhile,the microleakage of sandblast group is higher than that of flame group(P<0.05).No statistically significant difference was found among Er: YAG laser 250 m J group,Er: YAG laser 300 m J group,and Er: YAG laser 350 m J group(P>0.05).(2)For the rebonded zirconia brackets processed by the same treatment(Er:YAG laser,sandblast,or flame),no difference of statistical significance(P>0.05)was found between occlusal direction and gingiva direction in the microleakage at the B-A interface.(3)None of the differences in microleakage at the E-A interface has statistical significance(P>0.05)between rebonded zirconia brackets processed by different treatments.(4)For the rebonded zirconia brackets processed by the same treatment(Er:YAG laser,sandblast,or flame),no difference of statistical significance(P>0.05)was found between occlusal direction and gingiva direction in the microleakage at the E-A interface.Conclusions: 1.At the bracket-adhesive interface,the re-bonded zirconia brackets processed by Er:YAG laser present a smaller microleakage than that of the re-bonded zirconia brackets processed by sandblast and flame treatments.2.At the enamel-adhesive interface,there is no statistical difference in microleakage between the re-bonded zirconia brackets processed by Er:YAG laser,sandblast,and flame methods.3.All rebonded zirconia brackets processed by different treatments show no statistically significant differences in microleakage of occlusal direction and gingiva direction,while the microleakage of gingiva direction is higher than that in occlusal direction. |
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