Mechanical Properties Of Thermodynamic Nickel-titanium Archwires In A Simulated Oral Environment In Vitro

Objective: This study was to investigate the mechanical properties of different brands thermodynamic nickel-titanium(NiTi) archwires 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 0.016 inches diameter thermodynamic NiTi archwires were tested in 25℃, 33℃, 37℃, 60℃artificial saliva and at different moments in 8 weeks. An Instron Universal Testing Machine applied deflections of 3mm 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: the load value at 1.5-mm of unloading deflection point(UDP),stiffness between 0.5-mm UDP to 2.5-mm UDP and energy loss(in percentage of total energy) after unloading.Any statistically significant differences in these variables for the different brands,temperature and moments levels were analysed using one-way analysis of variance.Results:1. The load values of 1.5-mm UDP had a significant tendency to be higher with increasing temperature for all archwires; The unloading stiffness of the same brand archwires were approximate at 25℃,33℃,37℃, while their unloading stiffness increasd obviously at 60℃; When the temperature increased, the energy loss of all archwires decreased in sequence.2. The unloading stiffness and energy loss of B and D archwires were approximate,and the unloading stiffness and energy loss of A archwire were comparable to the C archwire at the same temperature.3. The load-deflection curves of A,B and D archwires were very similar at the 0 and 4 week, while their loading curves rised obviously without nBable variation of unloading curve at the 8 week. The load-deflection curves of C archwire had significant variation at three moments. The load value of 1.5-mm UDP of A,B,D archwires had little variation in 8 weeks,The unloading force value of C archwire increased slightly at the 8 week. There are a few fluctuations of the unloading stiffness and energy loss of B and D archwires. while A and C archwires's unloading stiffness had a tendency to decrease, the energy loss increased in 8 weeks.Conclusions:1. Different brands thermodynamic NiTi archwires demonstrated different mechanical behaviours, but all of them exhibited superelasticity and practical shape memory effect (SME).2. The four thermodynamic NiTi archwires displayed temperature sensitivity at the 4 test temperatures. The orthodontist can advise the patients to intake cold/hB drink for adjusting the force of thermodynamic NiTi archwires.3. 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 two months or an even longer period .