| Objective: This in-vitro study was designed to compare the mechanical properties of five different rectangular nickel-titanium(NiTi) archwires under different oral temperature and after different times of thermocycling which would provide reference for their clinical application.Method: Five different kinds of rectangular NiTi orthodontic archwires with the same size (A:L&H Titan, B:Ormco COPPER Ni-Ti27, C:COPPER Ni-Ti35, D:COPPER Ni-Ti40, E:Grikin ,0.016×0.022inch n=8) were selected and tested in 25℃,33℃,35.5℃,40℃artificial saliva using a modified partial dental arch bending system. To simulate leveling a lateral incisor, self-ligated brackets were bonded in such a manner as to remove the tip and angulation. An Instron Universal Testing Machine applied deflections of 3mm and then unloaded to the passive position, investigated the load-deflection characteristics of these different wires. Force- deflection diagrams were determined from the passive position to an activation of 3mm and during deactivation. The following variables were calculated: the stress value at 1.5mm of unloading deflection point(UDP), stiffness from 0.5mm UDP to 2.5 UPD, stress hysteresis and hysteresis ratio. The same tests were examed after thermocycling between 5℃and 55℃for 50times, 1500times, 3000times. Any statistically significant difference in these variables were analysed by two-way classification ANOVA.Result:1. The stress value of 1.5mm UDP and the unloading stiffness had a significant tendency to be higher with increasing temperature for all five kinds of rectangular NiTi archwires; the unloading stiffness of A archwires were approximate at 25℃, 33℃, 35.5℃while the unloading stiffness of B, C, E archwires were approximate at 33℃, 35.5℃and D archwires approximate at 33℃,35.5℃,40℃. The stress hysteresis of A, C, E archwires showed no significant difference among different temperature. The smallest stress hysteresis of B archwire were at 35.5℃and were approximate at 25℃, 33℃, 35.5℃. The smallest stress hysteresis of D archwire lied in 25℃and the biggest one lied in 33℃.When the temperature raised, the hysteresis ratio of all kinds of archwires decreased in sequence. A and D archwires possesed the most stabilized unloading stiffness and tress hysteresis.2. D archwires produced the smallest stress at 1.5mm UDP while E archwires produced the largest one, and B archwires second only to it. B and E archwires were stiffest while D archwires were the softest one. E archwires possessed the biggest stress hysteresis but D archwires had the biggest hysteresis ratio while B, E ranged the last.3. The stress value of 1.5mm UDP of all five kinds of archwires after thermocycling showed no significant difference. After thermocycling 50 times, the stiffness of A, B, C, E increased but there were no significant difference from then on. D archwires possessed the most stabilized stiffness after thermocycling. The tress hysteresis of A archwires had no significant difference after thermocycling. But the stress hysteresis of B, C, D, E archwires decreased in sequence. The hysteresis ratio of A, C, E archwires decreased after thermocycling 50 times. B archwires possessed the smallest hysteresis ratio after thermocycling 3000 times and possessed the biggest one in original.Conclusion:1. Five different kinds of rectangular NiTi archwires demonstrated different mechanical behaviour, but all of them exhibited superelasticity and practical shape memory effect (SME) except for B and E archwires.2. The five NiTi archwires displayed temperature sensitivity at different test temperature. The force generated by NiTi archwires increased as temperature rised. B archwires were most obviously influenced by temperature while D and E archwires were influenced most slightly. The hysteresis ratio of all five kinds of archwires decreased while the temperature raised.3. Because D archwires had the smallest stiffness and produced the smallest orthodontic force, it can be more suitable to be applied clinically during the early stage of the treatment to obtain the three-dimensional control of the teeth. B and E archwires were too tough to be used in the early stage of treatment of severe crowding.4. The mechanics properties of five of the NiTi archwires had changed after thermocycling but the superelatisity and SME still exhibited after thermocycling. Clinically, rectangular NiTi archwires can deliver forces continuously after used two months or even longer.5. The mechanical properties of the NiTi archwires derived from copper addition and two step heat treatment were superior to the superelastic NiTi archwires. Copper addition and two step heat treatment can be beneficial to improve the mechanical properties of NiTi archwires.
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