## 2 The Characteristics of Endodontic Instruments
### Limitations of Standardizing Endodontic Instrument Testing to Simulate Clinical Conditions
Due to thermal conductivity, the metal alloy does not uniformly and simultaneously increase in temperature throughout its entire structure when subjected to temperature elevation. Therefore, assuming that the core temperature is equivalent to the surface temperature would be inaccurate [48];
4. Intracanal temperatures in in vivo conditions do not reach body temperature and instead range from 31 \({}^{\circ}\)C to 33.5 \({}^{\circ}\)C [49];
5. An irrigating solution at room temperature introduced into the root canal system in a clinical setting takes approximately 4 min to reach 35 \({}^{\circ}\)C, which is considered body temperature under healthy conditions. Similarly, pre-heated solutions (66 \({}^{\circ}\)C) decrease in temperature to 35.7 \({}^{\circ}\)C after the same time period [50];
6. During root canal preparation, the in-and-out motions further reduce the instrument's contact with the root canal wall, thereby minimizing its exposure to the canal temperature [51]. There is compelling evidence indicating that the testing temperature can influence the outcomes of cyclic fatigue, although the extent of this impact may vary depending on the specific instruments being tested [18; 45; 51; 52; 53]. However, determining the optimal temperature for conducting the mechanical tests remains a challenge due to the temperature fluctuations and cycles experienced by the instrument during the shaping procedures. Consequently, the recommendation to perform tests at body temperature raises significant concerns [45; 46]. Taken together, these factors emphasize that while the test temperature in cyclic fatigue testing can be a subject of discussion, it does not fully replicate the complexities of a real working environment. Therefore, careful consideration should be given to the interpretation and applicability of results obtained under specific temperature conditions. Similar to cyclic fatigue tests, adjustments have been also made to the international specification for torsion tests in an effort to better simulate real working conditions. Onceagain, studies have consistently reported comparable results across various tested models when comparing different instruments [26; 54]. But this finding is expected considering that endodontic instruments adhere to fundamental metallurgical principles. In summary, the choice of test settings depends on whether the independent variable is a specific testing condition or the instruments themselves. It is essential to recognize that all mechanical tests for instruments adhere to fundamental physical and metallurgical principles. Given the multitude of variables that can influence clinical outcomes, it becomes questionable to determine a specific laboratory setting that precisely mimics real working conditions. Instead, these tests should be viewed as laboratory experiments designed to measure specific parameters under controlled conditions. Therefore, the examples highlighted earlier, such as the handpiece movement, the taper of the artificial canal, and the use of different testing temperatures, should not be used as sole criteria for assessing the reliability of a study. Although mechanical tests have inherent limitations in simulating clinical conditions, they offer a valuable approach to compare and rank various instruments by assessing their response to specific types of stress. By recognizing these limitations and understanding their intended purpose, researchers can utilize these tests effectively in their studies.