Evaluation of in vitro the shear bond strength of brackets with different base designs

The purpose of this study is to compare the shear bond strengths of three orginal and recycled brackets with different base designs (Sprint with integral base, Gemini with single mesh base, Discovery with laser structured base) and their bond failure sites after shearing tests. Upper and lower 120 human premolar teeth extracted for orthodontic purposes were evaluated in this study. For the three different bracket types, six groups each consisting 20 teeth were formed by distinguishing the upper and lower premolar. Teeth were kept in 0,1% thymol solution and teeth were held in distilled water, one week prior to bonding. Transbond XT was used as bonding agent and curing time was set as 20 seconds with light cure for brackets. After the bonding procedure, samples were kept in distilled water at 37 °C for 24 hours. Shear bond strength (SBS) was tested using a universal testing machine at a crosshead speed of 1 mm/minute in the occlusogingival direction. Residual adhesive amounts on the tooth surface after the shearing tests were scored by the ARI system. Debonded brackets were sandblasted with 90 micron aluminum oxide particules until all the composit resin was removed from bracket base in order to reuse. Adhesive remnants were cleaned of using tungsten carbide burs, from debonded teeth enamel surfaces . Adhesive removal was considered complete when all visible residues were removed. Then, the sandblasted brackets were rebonded onto the same tooth, shear bond strengths and bond failure sites were evaluated again. For evaluation of the data, the descriptive statistical methods were used, Kruskal Wallis and Tamhane Post Hoc analyzes were used to compare mean shear bond strenghs of all groups. The first and second shear bond strength values of all study groups were compared with Mann-Whitney U and t-test in independent groups. ARI scores were compared with Chi-Square test. For all bracket groups, mean values of bond strength were found higher than the minimum suggested orthodontic bond strength. The comparison of original bracket groups revealed higher bond strength for Discovery upper brackets and lower bond strength for Gemini lower brackets. The comparison of the original and rebonded shear bond strengths showed that Discovery brackets had significantly lower mean shear bond strengths than they were originally bonded. However, shear bond strengths of the other bracket groups were not significantly effected with sandblasting. In conclusion, it will be beneficial if different bracket base designs evaluate in clinical conditions. Besides further investigations are needed to determine the effects of sandblasting on these base structures. Standardization in bonding procedures should be developed in order to compare researches.