The effects of torque on the fracture resistance of the esthetic dental brackets

Objectives: The purpose of this study was to evaluate the fracture resistance or plastic deformation of several esthetic orthodontic brackets during orthodontic torqueing.
Methods: Ten brackets for the maxillary right central incisor (0.022×0.028 in) from seven ceramic brackets (Bright-I, Perfect Clear, WOW Advanced, Maya, C-Line, Fine, 3B) and four resin brackets (OK Real, Lucid, Shine M-old, Shine M-new) were tested. Stainless steel archwire segments of 0.019×0.025 in and 2.0 cm in length were used. Resin teeth model (two maxillary central incisor) were prepared with CAD/CAM system (ZX-5SD, Manix) using Polyglass Blank (Vericom) and embedded to PMMA plate using orthodontic resin. Two maxillary right central incisor brackets were attached on the teeth model with super glue and installed to the custom-made torque measuring system controlled by computer. With the wire torsion ranging from approximately 60^o to 180^o used in this experiment, the torque force at the point of fracture or deformation was recorded.
Results: Torque expression of approximately 3-7 Ncm was recorded at 31^o-126^o of wire rotation at the points of fracture. Repeated torque application with plastic brackets resulted in the diminished torque expression. Ceramic brackets were broken by the torsional forces from the archwire while plastic brackets were deformed.
Conclusions: Although clinically appropriate torque levels are not well established, several studies have been suggesting that 0.5-2 Ncm is the clinically relevant range. Brackets of different material properties will exhibit different responses to an applied torque. Clinicians should be aware that some esthetic bracket plastic deformation or fracture could occur with a single torque application. The observed deformations on the plastic brackets show that a general recycling of plastic brackets is not recommendable.