The shear bond strength of orthodontic bracket base shapes to human enamel, an in vitro study

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Esmail, Ziana
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Introduction: There is a growing demand for esthetic appliances during orthodontic treatment, with a specific interest in WildSmiles shaped brackets amongst children and adolescents. Bond failure of brackets over the course of orthodontic treatment can result in increased treatment time, increased overhead costs, and feelings of frustration for the patient and practitioner. Objective: The purpose of this study was to the evaluate the effect of orthodontic bracket base shape on shear bond strength (SBS) to human enamel, in vitro, at two time points, as well as the nature of debonding fractures using the Adhesive Remnant Index (ARI). Methods: Orthodontic brackets with different shaped bases (flower, heart, rectangular) were bonded to 120 extracted human third molars. SBS was measured using a Servohydraulic Test System at 24 hours and 2 months after initial bonding. ARI scores were evaluated under 10x magnification to assess the amount of resin left on the tooth. Results: The control bracket with a rectangular base shape had the highest mean SBS (26.78 ± 8.19 MPa), and significantly differed from flower (17.19 ± 4.45 MPa) and heart (18.86 ± 3.50 MPa) base shapes (p < 0.001). The mean SBS of flower and heart shaped brackets did not differ considerably from each other (p > 0.05). The mean SBS between debond times at 24 hours (21.50 ± 7.46 MPa) and 2 months (20.38 ± 6.69 MPa) were not statistically significant (p > 0.05). Analysis of ARI scores showed a significant difference between Flower-24 hours vs Heart- 2 months (p=0.039), Flower-24 hours vs Heart- 24 hours (p=0.004), and Control-2 months vs Heart-24 hours (p=0.015). The heart shaped bracket had a greater proportion of adhesive bond failures at the enamel-resin interface. Conclusion: Bracket base shape has an effect on SBS, with the rectangular base shape having a higher mean SBS compared to flower and heart base shapes. All bracket shapes had bond strengths above the clinically acceptable range of 6-8 MPa, and thus may provide adequate SBS in a clinical situation. Variations in ARI scores occurred based on bracket shape and were of a mixed adhesive-cohesive nature.
Orthodontic bracket base shapes, Shear bond strength, Orthodontic bonding, WildSmiles