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[Study of mechanical properties' secular variation of thermodynamic nickel-titanium archwires].

PURPOSE
To study the changes of mechanical properties of thermodynamic nickel-titanium(NiTi) archwires under 37 degrees centigrade and in artificial saliva.


METHODS
Using a modified partial dental arch bending device, 4 types of 0.016-inch in diameter thermodynamic NiTi archwires were tested in 37degrees centigrade artificial saliva and at different moments in 8 weeks. An Instron Universal Testing Machine delivered deflections of 3mm in the lateral incisor area. The load-deflection curves were determined from the passive position to deflections of 3mm. The following variables from unloading curves were calculated, the load value at 1.5mm of unloading deflection point(UDP),stiffness between 0.5mm UDP to 2.5mm UDP after unloading. Statistically significant differences in these variables for different brands and moment were analysed using one-way analysis of variance with SPSS11.5 software package.


RESULTS
The load-deflection curves of A,B and D archwires were very similar at 0-week and 4-week, while their loading curves rised obviously at 8-week. The load-deflection curves of C archwire had significant differences at 3 moments. The load value of 1.5mm UDP of A,B and D archwires had insignificant difference at 8-week,the unloading force value of C archwire increased slightly at 8-week. There were a few fluctuations of the unloading stiffness of B and D archwires,while A and C archwires's unloading stiffness had a tendency to decrease.


CONCLUSIONS
Thermodynamic NiTi archwires always exhibited superelasticity and SME in 8 weeks. Clinically, thermodynamic NiTi archwires can still deliver forces continuously when they are loaded for 2 months even longer.