Review and classification of heat treatment procedures and their impact on mechanical behavior of endodontic files

Objectives: The article presents an updated literature review on NiTi endodontic instruments in order to explain and analyze the influence of heat treatment process on mechanical properties of Niti shape memory alloys. The reader should understand the clinical implications of thermal heating of endodontic intruments such as enhanced flexibility and safety. Methods: The literature concerning the influence of heat treatment on mechanical properties of NiTi alloy has been analyzed using selected criteria. A closer look at multiple patents was important to understand the effects of heating on crystalline microstructure and phase transformation properties. Results: The main advantage gained by heat treatment process is the increase of Af (Austenite finish) temperature of the alloy. If "Af" is superior to body temperature, the file will be in a mixed martensitic, R-phase and austenitic structure in intracanal temperature. Therefore, heat treated instruments showed a significant increase in flexibility and flexural fatigue resistance. Only few reports describe the mechanical behavior under torsional loads. Interestingly, modifications in crystalline structure seem to have an influence on angle of deflection and torque load, which are two determinant parameters of torsional fracture. Conclusion: An analysis of the different effects of heat treatment on Niti instruments is presented, thus enabling an easier understanding of mechanical properties of new thermomecanically treated alloys. Clinical relevance: The theme is relevant for clinical Endodontics since the variety of instruments is great and clinicians need a better understanding of the different ways of their manufacturing for a better use during endodontic treatment.