Comparison of the superelasticity of different nickel-titanium orthodontic archwires and the loss of their properties by heat treatment

Abstract

The aim of this work is to describe and compare mechanical properties of eight widely used nickel-titanium orthodontic wires under uniform testing conditions and to determine the influence of the heat treatments on the loss of the superelasticity. Ten archwires from two batches from eight different manufacturers were evaluated. A three-point bending test was performed, in accordance with ISO 15841:2006, on 80 round nickel-titanium archwire segments of 0.016 inch. To obtain a load-deflection curve, the centre of each segment was deflected to 3.1 mm and then unloaded until force became zero. On the unloading curve, deflection at the end of the plateau and forces delivered at that point, and at 3, 2, 1 and 0.5 mm of deflection, were recorded. Plateau slopes were calculated from 3 and from 2 mm of deflection. Data obtained were statistically analysed to determine inter-brand, intra-brand and inter-batch differences (P < 0.05). The results show that at 2 mm of deflection, maximum differential force exerted among brands [Nitinol SuperElastic (1.999N)-Sentalloy M (1.001 N)] was 0.998 N (102 gf). The Nitinol SuperElastic plateau slope (0.353 N/mm) was the only one that was statistically different from 2 mm of deflection, as compared with the other brand values (0.129-0.155 N/mm). Damon Optimal Force described the gentlest slope from 3 mm of deflection (0.230 N/mm) and one of the longest plateaus. Titanol and Orthonol showed the most notable intra-brand differences, whereas inter-batch variability was significant for Nitinol (Henry Schein), Euro Ni-Ti and Orthonol. Superelasticity degree and exerted forces differed significantly among brands. Superelasticity of Nitinol SuperElastic was not observed, while Damon Optimal Force and Proclinic Ni-Ti Superelástico (G&H) showed the most superelastic curves. Intra-brand and inter-batch differences were observed in some brands. In all cases, the heat treatment at 600 °C produces precipitation in the matrix. The precipitates are rich in titanium and this fact produce changes in the chemical composition of the matrix and the loss of the superelasticity. At 400 °C these precipitates are not produced and the forces delivered by the wires are very similar with wires untreated.

Fig. 1

Loading and unloading curve of Ni–Ti archwire and all evaluated parameters of the study

Fig. 2

Box and whisker plot for all studied brands of archwires in 2 and 3 mm deformation. Test ANOVA 95 %

Fig. 3

Precipitates observed by SEM located in grain boundaries. a Ni–Ti archwire treated at 600 °C for 30 min, b Ni–Ti archwire treated at 600 °C for 1 h, c Precipitate observed by TEM. The spherical morphology can be observed. d Diffraction pattern of Ti₂Ni with zone axes

Fig. 4

Loading and unloading curve of all studied Ni–Ti archwires