## 2 The Characteristics of Endodontic Instruments
### Limitations of Standardizing Endodontic Instrument Testing to Simulate Clinical Conditions  
Several experimental setups have been proposed for instrument testing to better replicate real-world working conditions. These endeavors aim to investigate the impact of specific conditions on the resistance of endodontic instruments while also intended to enhance the simulation of real-world working scenarios and improve the reliability of the results obtained. The cyclic fatigue test, in particular, has been the subject of various protocols and standardization efforts [11]. The implementation of a vertical handpiece movement, which aims to simulate the dynamic in-and-out motion, exemplifies the challenges associated with standardization in instrument testing. While there have been a few contradictory findings concerning the Reciproc Blue instrument [18; 38], the majority of studies indicate that the time to fracture can vary under different testing conditions, potentially resulting in an overextension of cyclic fatigue life in dynamic models [18; 39; 40; 41; 42]. However, it is important to note that, with few exceptions [38], this variable does not significantly impact the direct comparison between different instruments [18; 40; 41; 42]. Therefore,  
Figure 2: Some mechanical tests employed to evaluate the strength of instruments under different stress conditions: (**a**) cyclic fatigue, (**b**) torsion, (**c**) bending, (**d**) buckling, and (**e**) cutting ability tests. notwithstanding it has been recognized that conducting root canal instrumentation through a continuous packing motion is crucial [16], the significance of this feature becomes less pronounced when considering endodontic instruments as independent variables [18]. The impact of different artificial canal taper conditions is expected to follow a similar pattern. The use of a tapered canal in cyclic fatigue testing is motivated by its ability to better follow a specific and predefined artificial canal curvature (in terms of radius and angle of curvature). This occurs because the far apical end of the tapered artificial canal provides less compensation for the instrument's tip outside the curve once it surpasses the inner point of maximum curvature. In contrast, non-tapered canals are less effective in accommodating such deviations from the original pathway. However, unless the objective of the test is to compare specific curvatures or to analyze the differences between endodontic instruments as independent variables, this argument loses relevance. Regardless of the radius or angle of curvature, the overall differences between tested instruments are comparable [12; 43]. Therefore, the selection of a specific test setting should be guided by the research question, and both tapered and non-tapered settings are valid options depending on the study objective. The test temperature is a highly debated parameter in endodontic instrument testing. Some researchers argue that a temperature close to body temperature (\(\sim\)36 \({}^{\circ}\)C) more accurately replicates real working conditions [44; 45; 46]. However, it is important to acknowledge that the specificities of a cyclic fatigue test do not align entirely with a real working situation. Several arguments support this notion:  
1. Different outcomes can be observed when exposing the instrument to temperature changes, leading to a uniform phase transformation throughout the entire instrument. It is unreliable to assume that the short duration (usually seconds) in which the instrument is within the root canal space during shaping procedures would have an equivalent effect on its crystallographic arrangement as several minutes of continuous rotation in a laboratory-based heated bath [47];
2. The surface temperature of an instrument, right after its removal from the root canal, is lower than the body temperature, ranging from 30.8 \({}^{\circ}\)C to 32.5 \({}^{\circ}\)C [44];
3.