Structural characterisation and mechanical FE analysis of conventional and M-Wire Ni-Ti alloys used in endodontic rotary instruments.

Diogo Montalvão,Francisco Manuel Braz Fernandes,Francisca Sena Alçada,Sancho De Vilaverde-Correia

doi:10.1155/2014/976459

Abstract

The purpose of this study is to understand how the M-Wire alloy conditions the mechanical flexibility of endodontic rotary files at body temperature.Two different rotary instruments, a Profile GT 20/.06 and a Profile GT Series X 20/.06, were selected due to their geometrical similarity and their different constituent alloy. GT series X files are made from M-Wire, a Ni-Ti alloy allegedly having higher flexibility at body temperature. Both files were analysed by X-Ray Diffraction and Differential Scanning Calorimetry to investigate phase transformations and the effects of working temperature on these different alloys. Mechanical behaviour was assessed by means of static bending and torsional Finite Element simulations, taking into account the nonlinear superelastic behaviour of Ni-Ti materials. It was found that GT files present austenitic phase at body temperature, whereas GT series X present R-phase at temperatures under 40°C with a potential for larger flexibility. For the same load conditions, simulations showed that the slight geometrical differences between the two files do not introduce great disagreement in the instruments' mechanical response. It was confirmed that M-Wire increases the instrument's flexibility, mainly due to the presence of R-phase at body temperature.

Highlights

Stainless steel instruments are rigid and unsuited for large apical enlargement in thin curved canals [1]
Two as-received Ni-Ti rotary files with similar cross-sections—but different material properties—were selected for this study (Figure 1): Profile GT 20/.06 (GT) is made from conventional Ni-Ti alloy and GT series X 20/.06 (GTX) is made from M-Wire
A more detailed analysis of these results shows that: (i) the austenite B2(110) peak’s intensity decreases from 40∘C downwards with a corresponding slight increase of the R-phase peaks’ intensity for the GT Series X 20/.06 (GTX) file while for the GT file this is more noticeable only below 20∘C; (ii) R-phase peaks’ intensity increases significantly during cooling, until −40∘C is reached, in both files; (iii) martensite peaks’ intensity increases more significantly for temperatures below −40∘C, for both files; (iv) Ni4Ti3 precipitates are present in both files

Summary

Introduction

Stainless steel instruments are rigid and unsuited for large apical enlargement in thin curved canals [1]. Some studies claim that there is evidence [5] that Ni-Ti instruments yield better clinical prognosis in endodontics when compared to their stainless steel counterparts considered alone. Some published studies show that Ni-Ti instruments fracture at a frequency ten times greater than their stainless steel counterparts [8, 9]. Ni-Ti fragments are up to seven times less likely to be removed from inside root canals, even by experienced endodontists [10]. This explains the current interest in the subject by several researchers [11,12,13,14,15,16,17]

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