A critical analysis of research methods and experimental models to study the physical properties of NiTi instruments and their fracture characteristics

Abstract

The aim of this review is to provide a critical overview of the physical properties (surface hardness, cutting efficiency, bending properties, flexibility and cyclic fatigue resistance) of NiTi instruments. Frequently used experimental models regarding these aspects will be presented and discussed with regard to their strengths and weaknesses. For all these aspects, a plethora of experimental models have been described. Based on a critical appraisal and especially taking the appropriate translation of experimental findings to clinical endodontics into account, suggestions for future research based on clearly defined and valid experimental methodologies will be provided. Up to now, very few attempts have been made to assess which particular physical properties of NiTi instruments exert an impact on the clinical outcome of root canal treatment. Departure from merely focusing on physical properties and fracture characteristics towards more biological aspects in terms of treatment outcome is essential.

Effect of Rotational Speed on the Resistance of NiTi Alloy Endodontic Rotary Files to Cyclic Fatigue—An In Vitro Study

“…Furthermore, they did not carry out any additional measurement methods [ 23 ]. Additionally, some reviews have been conducted with the aim of analyzing the mechanical and metallurgical behavior of endodontic instruments under different testing conditions and methodologies [ 24 , 25 , 26 ].…”

Nickel–Titanium Rotary Instruments: Mechanical and Metallurgical Characteristics

“…Since the 1990s, the world of endodontics has made considerable progress due to the great number of studies in recent years, leading to a deeper understanding of the mechanical and metallurgical behaviour of NiTi instruments [ 1 , 2 ]. The improvements in endodontic instruments are a result of a fundamental focus on two aspects, that aim to reduce their intracanal fracture as much as possible: the instruments’ design and geometry and the alloy with its corresponding heat treatments [ 3 , 4 ]. In fact, the most remarkable limitation of NiTi instrumentation is the possibility of instrument failure during shaping procedures.…”

“…It is not uncommon to have instruments with a high cyclic fatigue and torsional resistance, requiring a high apical pressure to reach a working length. This could be directly related to the intrinsic properties of the alloy, such as the hardness of the surface or the crystallographic phase, and to the geometry and design of the instruments [ 4 , 15 ].…”

“…As a consequence of the different mechanical behaviours of NiTi alloys depending on temperature, it is fundamental to perform in vitro static and dynamic tests at simulated body temperature in order to obtain clinically significant results. Tests that are conducted above body temperature (35.1 °C ± 1 °C) should be regarded as ineffectual, and thus should not be considered as representative of the effectiveness of NiTi alloys [ 4 , 17 ].…”

Future Trends in Endodontics: How Could Materials Increase the Long-Term Outcome of Root Canal Therapies?

“…Recently, the knowledge of the factors influencing the mechanical resistance of endodontic instruments has been expanded, but the dynamic interaction between flexural and torsional stresses remains unclear. In fact, it has demonstrated that the cyclic fatigue resistance is determined by the mass and the crystallographic phase of the instrument [ 14 ]. On the contrary, the torsional resistance is not determined by the mass in terms of absolute value, but instead by its distribution in relation to the centre of rotation [ 15 ].…”