# Torque Range, a New Parameter to Evaluate New and Used Instrument Safety
## 1 Introduction  
During root canal treatments (RCT) instruments undergo flexural and torsional stresses that could eventually result in the instrument separation [1; 2; 3]. According to [PERSON] et al. [4], torsional failure is the most common cause of intracanal failure. This fracture occurs when the tip or another apical part of the instrument binds into the canal while the upper part continues to rotate. More precisely, the separation occurs when torsional stress exceeds the plastic limit of the metal. To date in literature several studies investigate the parameters that could influence the torsional resistance. These factors are the cross section, the metal mass of the instrument, the heat treatments and the metallurgical properties of the nickel titanium rotary (NTR) instruments [5].  
The most common method to investigate the torsional resistance of an instrument is a machine described by International Organization for Standardization (ISO) 3630-1 [6]. This method of evaluation was created for testing stainless-steel (SS) manual instruments. The above mentioned, method has a twofold problem: the torsional load is concentrated in a specific point of the instrument, while in clinical practice it is distributed along the whole operative portion of the NTR instruments. Furthermore, thetest is performed in static condition, not representative of intracanal working of the files. For these reasons, torque generated during instrumentation should be considered. Therefore, the concept of \"operative torque\" was introduced in recently published studies [7].  
Operative torque can be defined as the quantity of torque developed during instrument progression towards the apex. It is a real time measurement of dynamic forces needed to perform the shaping of the canal. This value is influenced by time needed and technique used to reach working length. Indeed, each clinician has a different sensitivity that results in different clinical use of nickel titanium (NiTi) rotary instruments (amplitude of pecking, intensity of brushing etc.) [8]. Moreover, the morphology of the root canal system should be taken into account while considering operative torque since it is influenced by several anatomic characteristics such as canal trajectories and width, hardness of dentin, etc. [9].  
The static torsional resistance test shows values of torque that should not represent the NiTi rotary instruments clinical behavior. Despite the operative torque clearly represents the clinical behavior of the files, it depends on the anatomy complexities of the canal and the technique used by the operator. Moreover, it does not show the maximum amount of torque reachable by a specific portion of the instrument. Hence, in order to overcome the problems of these two tests, the concept of torque range has been proposed by a previously published study [10].  
This new parameter investigates the mean values of operative torque test in order to compare them to the values registered during the classic torsional resistance test. From this comparison a range can be identified, which should be used as parameter to define the instrument safety. Despite in the current literature several factors influencing the torsional resistance and the operative torque have been studied; no published articles have evaluated the influence of instrumentation on both the above-mentioned parameters.  
Therefore, aim of the present study is to compare operative torque and torque at failure of two different NiTi files using both new and used instruments. Moreover, to evaluate the torque range before and after root canal shaping.