# Torque Range, a New Parameter to Evaluate New and Used Instrument Safety
## 4 Discussion  
In present literature, torsional resistance is the most used test to evaluate the metallurgical features of the alloy in a static rotational resistance [14]. Nonetheless, this type of test do not take into account the multitude of stresses, since NiTi rotary instruments are used dynamically [15]. In order to evaluate the instrument during motion, the operative torque could be a more valuable test. This dynamic measurement should be determined by time needed and technique used to reach the working length [7]. To ensure a precise evaluation of the instrument metallurgical features in terms of static torsional resistance and dynamic behavior, the concept of torque range was chosen in the present article. Torque range shows the correlation between torsional stresses during instrumentation and instruments' resistance to them. It has a clinical relevance due to the fact that considers both the role of torsional static resistance and dynamic operative torque. M-Two rotary files (Sweden & Martina, Padova, Italy) are an endodontic file manufactured with traditional nickel-titanium alloy without any heat treatment. The instruments present an \"S shaped\" cross sectional design with two active cutting blades along instrument surface. These rotary instruments are thought to be used in a sequence [16; 17]. The pitch is variable and increases from the tip to the handle. In order to carry out the study the M-Two 20 tip and 0.06 taper was selected. The S-One (Fanta Dental Co., Ltd., Shanghai, China) is a new NiTi rotary recently introduced in the market. This instrument has a similar S-Shaped cross-sectional design of the M-Two. Moreover, the instrument present a new heat treatment recently produced by the manufacturer the AF-H wire [18]. This, according to manufactured internal studies, ensure a more bendable file and more flexural and torsional resistance compared to other instruments. The results describe the behavior of the two rotary instruments regarding the three parameters investigated. For new files (group A), S-One showed significant higher torsional resistance compared to M-Two. This result is in accordance with part of the current literature, indeed the role of thermal treatments on torsional resistance is not completely clarified [5; 13]. Many studies declared that the presence of heat-treated files do not significant affect or even enhance, the torsional resistance of an instruments [5; 19]. No published studies investigated the torsional resistance of an endodontic file before and after intracanal instrumentation. Therefore, it is pretty unknown the influence of instrumentation on torsional behavior. In this study, after the three single-rooted instrumentation (group B), torsional resistance of both M-Two and S-One showed a significant decrease value. This result could be explained by the fact that repetitive usage of an instrument not only modify the external surface of the instrument, but also the internal surface. Therefore, the peculiar torsional \begin{table}
\begin{tabular}{c c c c c} \hline  & **Group A** & **Group B** & \(p\) &
\begin{tabular}{c} **Amount of Reduction** \\ **in Percentage** \\ \end{tabular} \\ \hline S-One & 0.760 \(\pm\) 0.010 \({}^{\text{AB}}\) & 0.550 \(\pm\) 0.020 \({}^{\text{AD}}\) & 0.011 & 27.6\% \\ \hline M-Two & 0.645 \(\pm\) 0.010 \({}^{\text{BC}}\) & 0.425 \(\pm\) 0.010 \({}^{\text{CD}}\) & 0.001 & 34.1\% \\ \hline \(p\) & 0.009 & 0.026 & & \\ \hline \end{tabular}
\end{table} 
Table 2: Mean values for torque at Failure (TaF). \(p\) value was set at 0.05. Different capital superscript letters indicates significant difference among the groups (\(p<0.05\)). \begin{table}
\begin{tabular}{c c c} \hline  & **S-One** & **M-Two** \\ \hline Group A & 0.210 & 0.300 \\ \hline Group B & 0.08 & \(-\)0.245 \\ \hline \end{tabular}
\end{table} 
Table 3: Values of torque range. pattern of fracture starts from the center of the instrument not reaching the external surface.