Deformation of reinforced polycarbonate orthodontic brackets stressed by a labiolingual moment

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Bales, Jeffrey M.
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This thesis reports the results of a study designed to determine whether reinforced polycarbonate brackets experience clinically acceptable amounts of deformation over a period of time under sustained stress. A novel in vitro apparatus was designed to eliminate some of the errors present in previous studies. The apparatus simulated labiolingual crown torque by twisting a stainless steel archwire of 0.018 x 0.025 inch dimension in an orthodontic bracket with a 0.018 x 0.025 inch bracket slot. A moment of 2500 g-mm was applied for 28 days with measurements taken at initial moment application, 1 hour, 2 hours, 24 hours and then at regular 7 day intervals to determine the amount of deformation which occurred. Forty o thodontic brackets, consisting often polycrystalline ceramic, ten stainless steel and twenty metal slot reinforced polycarbonate brackets were tested. The results of this study showed that there was a significant difference in the amount of elastic, plastic and creep deformation which occurred in the metal slot reinforced polycarbonate brackets when compared to stainless steel and ceramic brackets (p $<$ 0.05). The combination of elastic, plastic and creep deformation which occurred in the polycarbonate brackets is likely to affect the ability of the bracket to transmit the desired torque. (Abstract shortened by UMI.)