Shear Bond Strength of Rebonded Self Ligating Ceramic Brackets after Different Reconditioning Procedures (A comparative an In vitro Study)
Background: The aim of this study was to evaluate the effects of different recycling methods on shear bond strength and morphological changes of deboned mechanically retentive self-ligating ceramic brackets, then compare the findings with the new brackets of the same company.
Material and methods: Forty-eight Damon® Clear™ self-ligating ceramic brackets with a mechanical retentive base were divided into two groups; the first group contained twelve new ceramic brackets (the control group), while the second group contained thirty-six new brackets which were bonded to unetched and slightly wet buccal tooth surface to allow an easy debonding of these brackets by tweezer, these debonded brackets then divided into three experimental (recycled) groups (12 per group): Recycled by sandblasting, irradiation by an Er, Cr: YAG laser and irradiation by CO2 laser. After recycling, the 36 recycled brackets plus the twelve new brackets (the control) were bonded to the forty-eight premolar teeth again following standardized bonding procedure. The teeth were stored in distilled water at 37˚C for 24 hours, then thermal cycling was performed between 5˚C and 55˚C for 500 cycles, the exposure to each bath was 30 seconds, and the transfer time between the two baths was 5-10 seconds, the shear bond strength of all specimens was determined with a universal testing machine at a crosshead speed of 1 mm/min until bond failure occurred. The adhesive remnant index (ARI) was calculated under a stereomicroscope at X10 magnification. Morphological examinations of the recycled ceramic brackets bases were conducted with scanning electron microscopy.
Result: There were highly significant differences in the mean of shear bond strength values among all groups using ANOVA F-test; furthermore, the mean shear bond strength of new brackets had the highest mean value of 22.899 Mpa, followed by Er,Cr:YSGG laser group 20.288 Mpa, then sandblasted group of 11.415 Mpa, while CO2 laser group had the lowest mean shear bond strength value of 2.994 Mpa; furthermore, the results showed significant difference in adhesive remnant index (ARI). Scanning electronic microscope photographs of ceramic brackets processed by sandblasting showed that all the adhesive was removed from the bracket's base, but destroyed the bracket's base as micro retentive structures was smoothed, in Er,Cr:YGSS group, a little adhesive remnants were observed underneath the meshwork and maintained the integrity of the micro structure of the bracket's base, while the brackets that reconditioned by CO2 laser revealed a considerable amount of residual adhesive on the overall bracket's base and in the hollows of the micro structure, while its structure was kept unchanged.
Conclusion: all reconditioning methods would result in clinically acceptable shear bond strength, except CO2 laser method. The Er,Cr:YGSS recycling method can effectively remove the adhesive from the bases of ceramic brackets without damaging them; thus, this method may be preferred over other recycling methods.