Effect of debonding force direction on orthodontic shear bond strength

Introduction: The aim of this study was to determine the influence of debonding force direction in shear bond strength testing of orthodontic brackets in vitro. Methods: One hundred fifty extracted bovine permanent mandibular incisors were randomly divided into 5 groups of 30 specimens each. Before bonding with a composite adhesive, enamel surfaces were etched with 37% phosphoric acid for 30 seconds. Teeth were bonded with mesh-based stainless steel orthodontic brackets. Shear bond strength was measured with a universal testing machine. Debonding forces were directed either parallel to the bracket base (group B), toward the enamel surface (group A: 15°), or away from the enamel surface (group C: 15°; group D: 30°; group E: 45°). Results: Shear bond strength measurements were as follows: 22.90 (SD, 1.72) MPa (group A), 17.90 (SD, 2.63) MPa (group B), 12.99 (SD, 1.71) MPa (group C), 7.93 (SD, 1.14) MPa (group D), and 6.65 (SD, 1.14) MPa (group E). Analysis of variance indicated that shear bond strengths were significantly different between the groups investigated. Mean adhesive remnant index scores ranged between 1.60 in group B and 2.13 in group D. Conclusions: Shear bond strength measurements were significantly influenced by the direction of the debonding force, indicating the need for control and standardization of this testing parameter in orthodontic shear bond strength testing.