Shear bond strength of metal brackets on enamel

OBJECTIVE: To compare the shear bond strength of different metal orthodontic brackets. MATERIALS AND METHODS: Five types of orthodontic metal brackets were selected (S1, Victory Series, 3M Unitek, Monrovia, Calif; S2, Mini Dyna-Lock, 3M Unitek; S3, Mini Sprint, Forestadent, Pforzheim, Germany; S4, Topic, Dentaurum, Inspringen, Germany; and S5, equilibrium 2, Dentaurum). Brackets were bonded on enamel surfaces of bovine incisors (Transbond XT, 3M Unitek) and were tested for shear bond strength with an Instron universal testing machine (Instron Corp, Canton, Mass). Data obtained in newtons and megapascals were analyzed with descriptive statistics and with analysis of variance and Tukey honestly significant difference (HSD) tests. The adhesive fracture site was classified with the adhesive remnant index (ARI). RESULTS: All the specimens tested had shear bond strength adequate to resist orthodontic forces. S5 showed significantly greater bond strength when compared with the other samples, except for S1. S1, S3, and S5 showed a significantly greater bonding force. The ARI index demonstrated a large variability. Retentive structure of S1, S3, and S5 had equal validity. The enlargement of the retentive surface enhances adhesion but affects the adaptability to surface irregularity of the enamel, increasing the risk of fracture at the interface with the bracket. CONCLUSIONS: The results of this study suggest that probably the retentive base extension can be lower than 7 mm2 proposed in previous studies as the minimal area.