Torsional creep of polycarbonate orthodontic brackets

Abstract

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.