[Evaluation of load-deflection rate changes of nickel-titanium wire after single clinical application in three different crowdings]

Desired mechanical properties of nickel-titanium wires have resulted in some orthodontists to recycle these wires and apply them repeatedly. The aim of this study was to evaluate load-deflection changes of nickel-titanium wires after single clinical application in three different types of crowding. 80 nickel-titanium wires [Global trade mark] with 0.014 inch diameter were randomly divided into four groups as: control, mild, moderate and severe crowding. Wires were used in patients for a period of 6 +/- 1 weeks Then, load-deflection experiment, using Zwick machine was done. Data were analyzed by Hottelling, Fisher's exact and Duncan tests. Hottelling analysis showed a significant difference between the groups. Fisher's exact test showed that there was a significant difference between forces at 0.5, 1.0, 1.5, 2, 2.5 mm deflection loading and unloading but there was no significant difference in 3 mm loading deflection. This study showed that although single application of nickel-titanium wire [at least for the wire used in this study] for one orthodontic visit causes significant decrease in force levels in different types of crowding, residual force after single clinical application is still enough for repeated clinical application