Torsional creep of polycarbonate orthodontic brackets

Objective. Polycarbonate materials are known to exhibit creep. The purpose of this study was to investigate torsional creep of polycarbonate orthodontic brackets. Methods. Four polycarbonate, one metal, and one ceramic brackets were tested. An in vitro torque apparatus was constructed which applied 2000 g-mm of torque to the bracket slot via a cantilever beam perpendicular to the bracket face. The apparatus was placed in a 37 degrees C water bath. Measurements of the angle of the cantilever beam were made at various times from 1 to 28 d. The initial angular change on loading and subsequent changes with time were compared with ANOVA and Tukey tests at p < 0.5. Results. All but one bracket showed the majority of creep by 2 d. Creep was not judged to be clinically significant for the metal, ceramic and the metal slot-reinforced brackets. The polycarbonate bracket without metal slots demonstrated significantly more creep (p < 0.05) than the polycarbonate reinforced with a metal slot, metal and ceramic brackets. Significance. Clinically significant bracket slot angle (torque) change due to creep can occur when using polycarbonate brackets without metal slot reinforcement.