Torque Stability of Plastic Brackets Following Multiple Loading and Artificial Material Aging – an In-Vitro Comparison

Objective:The objective of this study was to compare the loaddeflectionbehavior of plastic brackets made of various materials inresponse to repeated torque loads with each other and with steelbrackets. Material fatigue during wire’s play in the bracket slot, thebrackets’ elasticity and the torsional forces applied were analyzed.Material and Methods:Groups of ten brackets, each made of purepolycarbonate or variously reinforced polycarbonate or polyurethane,with and without a metal slot, were artificially aged and thentorqued with a testing machine five times consecutively in a torque-measuring apparatus. The control group consisted of ten steelbrackets. The resulting forces were recorded with the testing machine,and the wire’s deflection recorded with a digital goniometeron a PC and submitted to one-way variance analysis at p < 0.05.Results:There were strong fluctuations among the bracket types inboth the play of the wire in the bracket slot as well as the brackets’elasticity. The slot of all the polycarbonate-based brackets wasbent open after a single load, except for those with a metal slot.After a single load of up to 20° torque, all the brackets exhibiteda 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 loadswere applied.Conclusions:Each bracket material requires its own torque valueto transfer identical torque values onto the tooth in clinicalpractice. Comparison with steel brackets revealed that only plasticbrackets with a metal slot are suitable for clinical use. Addingceramic and glass fibers to polycarbonate, or using polyurethanehas no benefit in terms of torque stability. In addition, after asingle application of torque, all brackets lose torque stability inresponse to a renewed load.