Load-deflection characteristics of superelastic and thermal nickel-titanium wires

Abstract

The aim of this study was to investigate the mechanical properties of superelastic and thermal nickel-titanium (NiTi) archwires for correct selection of orthodontic wires. Seven different NiTi wires of two different sizes (0.014 and 0.016 inches), commonly used during the alignment phase, were tested. A three-point bending test was carried out to evaluate the load-deflection characteristics. The archwires were subjected to bending at a constant temperature of 37°C and deflections of 2 and 4 mm. Analysis of variance showed that thermal NiTi wires exerted significantly lower working forces than superelastic wires of the same size in all experimental tests (P < 0.05). Wire size had a significant effect on the forces produced: with an increase in archwire dimension, the released strength increased for both thermal and superelastic wires. Superelastic wires showed, at a deflection of 2 mm, narrow and steep hysteresis curves in comparison with the corresponding thermal wires, which presented a wide interval between loading and unloading forces. During unloading at 4 mm of deflection, all wires showed curves with a wider plateau when compared with 2 mm deflection. Such a difference for the superelastic wires was caused by the martensite stress induced at higher deformation levels. A comprehensive understanding of mechanical characteristics of orthodontic wires is essential and selection should be undertaken in accordance with the behaviour of the different wires. It is also necessary to take into account the biomechanics used. In low-friction mechanics, thermal NiTi wires are to be preferred to superelastic wires, during the alignment phase due to their lower working forces. In conventional straightwire mechanics, a low force archwire would be unable to overcome the resistance to sliding.

Mechanical Performance of Nickel-titanium Archwires

“…Despite there is not a clear statement of the optimal force delivered to ensure an efficient orthodontic movement 13 , the interval of different lots described in the present work (104.8-161.1g = 102.7-157.8cN) is a little higher but still in accordance with literature results described above (60-239g = 58.8-234.2cN). The difference should be credited to divergences in the deflection maximum value, crosshead speed and span, which did not observe the recommendation of ISO 15841 or due to differences in the material behavior provided by different manufacturers and commercial available lots.…”

“…Herein the force delivery during unloading, comparing the force considered suitable for clinical purposes described in orthodontic journals, Gatto et al 13 showed values between 60-100g when comparing different manufacturers, Bartzela et al 14 presented an interval between 139-239g and Lombardo et al 15 disclosed values between 78.4-109.6g [13][14][15] . All the authors tested different manufacturers of superelastic orthodontic .014'archwires, with exception of Bartzela et al who tested a .016'round wire.…”

“…Different wire deflections were reported in literature up to 5 mm; some authors have used deflections between 2 mm and 4 mm and it was stated that although these values were greater than the 5% deflection limit for the span length used, they reproduce the clinical use of the material and are regularly used in the oral environment [3][4][5][6][7][8][9][10]13,14,17,18 . Mallory et al 19 reported that during 5-mm deactivation of all heat activated wires, there was a sharp decline in force production.…”

“…Assuming a limit of 100gf, GAC HA lot H240 (81.46gf), 3M SE lot AL5SY (98.43gf) and Morelli HA lot 1462504 (99.23gf) had forces below this baseline. Several authors have proposed the use of the three-point bending test to evaluate the mechanical properties of orthodontic archwires; however, a wide range of variation upon materials and methods is observed 13,17,18,30 . The results reported by Gatto et al 13 and Oltjen et al 31 include a lower range of forces during unloading, 40-50gf for GAC and 3M groups 13 and between 40-60gf 13,31 .…”

Avaliação da força gerada pela deflexão gradual de fios ortodônticos em braquetes convencionais metálicos, estéticos e autoligáveis

“…Portanto, é de maior interesse clínico conhecer as forças liberadas pelo teste de descarga do que as forças liberadas pelo teste de carga (CHEN;ZHI;ARVYSTAS, 1992;FISCHER-BRANDIES et al, 2003;MURAVIEV;OSPANOVA;SHLYAKHOVA, 2001;WATERS, 1992b;YONEYAMA et al, 1992), uma vez que a descarga representa as forças que promovem a movimentação dentária durante o tratamento ortodôntico (ANDREASEN; HEILMAN;KRELL, 1985;BARTZELA;WICHELHAUS, 2007;WARAM, 2007;BURSTONE;QIN;MORTON, 1985;TAVERNIER;JORDAN, 2005;GATTO et al, 2013;KAPILA et al, 1991;LIAW et al, 2007;LOMBARDO et al, 2012;MURAVIEV;OSPANOVA;SHLYAKHOVA, 2001;NAKANO et al, 1999;SEGNER;IBE, 1995;VAN AKEN et al, 2008). Adicionalmente, a norma ISO 15.841 também indica a avaliação dos resultados em descarga.…”