[Mechanical properties of thermodynamic nickel-titanium archwires in a simulated oral environment in vitro].

OBJECTIVE: This study was to investigate the mechanical properties of thermodynamic nickel-titanium (NiTi) archwires of different brands under controlled temperature and in artificial saliva to provide some reference for its clinical application. METHODS: Using a modified partial dental arch bending system, four types of thermodynamic NiTi archwires were tested under controlled temperature (25, 33, 37, 60 degrees C) and in artificial saliva. An Instron Universal Testing Machine was used to apply 3 mm deflections of archwires in the lateral incisor area. The load-deflection curves were determined from the passive position to deflections of 3 mm. The following variables from unloading curves were calculated: Stiffness between 2.5 mm UDP (unloading deflection point) to 0.5 mm UDP after unloading. Any statistically significant differences in these variables for the different brands, temperature and moments levels were analyzed using one-way analysis of variance. RESULTS: Unloading values of four types thermodynamic nickel-titanium archwires increased with controlled temperature rising. The unloading stiffness of the same brand archwires were approximate at 25, 33, 37 degrees C, while their unloading stiffness increased obviously at 60 degrees C. CONCLUSION: Different brands thermodynamic NiTi archwires demonstrated different mechanical behaviours, but all of them exhibited super elasticity and practical shape memory effect. The four thermodynamic NiTi archwires displayed temperature sensitivity at the four test temperatures.