Objective: The objective of this study was to compare the loaddeflection behavior of plastic brackets made of various materials in response to repeated torque loads with each other and with steel brackets. Material fatigue during wire's play in the bracket slot, the brackets' elasticity and the torsional forces applied were analyzed.
Material and methods: Groups of ten brackets, each made of pure polycarbonate or variously reinforced polycarbonate or polyurethane, with and without a metal slot, were artificially aged and then torqued with a testing machine five times consecutively in a torque- measuring apparatus. The control group consisted of ten steel brackets. The resulting forces were recorded with the testing machine, and the wire's deflection recorded with a digital goniometer on a PC and submitted to one-way variance analysis at p < 0.05.
Results: There were strong fluctuations among the bracket types in both the play of the wire in the bracket slot as well as the brackets' elasticity. The slot of all the polycarbonate-based brackets was bent open after a single load, except for those with a metal slot. After a single load of up to 20 degrees torque, all the brackets exhibited a significant loss of torque stability ranging between 5% for pure polyurethane and 28.5% for ceramic-reinforced polycarbonate. The loss of torque stability was roughly 17% on average. This loss did not increase significantly when additional loads were applied.
Conclusions: Each bracket material requires its own torque value to transfer identical torque values onto the tooth in clinical practice. Comparison with steel brackets revealed that only plastic brackets with a metal slot are suitable for clinical use. Adding ceramic and glass fibers to polycarbonate, or using polyurethane has no benefit in terms of torque stability. In addition, after a single application of torque, all brackets lose torque stability in response to a renewed load.