Phase transformation and metallurgical changes in reciprocating endodontic nickel–titanium files: A systematic review

Introduction: Reciprocating nickel–titanium (NiTi) instruments demonstrate improved cyclic fatigue resistance compared with continuous rotation, largely due to thermomechanical processing that modifies phase transformation behavior. This systematic review synthesized metallurgical evidence on reciprocating NiTi instruments and examined the relationship between phase constitution and mechanical performance under clinically relevant temperature conditions. Materials and Methods: Six electronic databases and gray literature were searched. Laboratory studies evaluating phase transformation and metallurgical characteristics of reciprocating NiTi instruments using differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, or transmission electron microscopy were included. Considerable heterogeneity was observed in instrument systems, temperature protocols, and reported outcomes. Therefore, meta-analysis was not performed. Findings were synthesized narratively. Results: Fourteen studies met the inclusion criteria. Heat-treated instruments exhibited distinct transformation temperature profiles, with several systems retaining martensitic and/or R-phase fractions at intracanal temperature (34°C). Instruments with Af temperature near or above intracanal temperature were generally more flexible and demonstrated greater cyclic fatigue resistance. In contrast, instruments predominantly austenitic at this temperature showed increased stiffness and shorter fatigue life. Mechanical behavior reflected interactions among phase constitution, instrument geometry, and surface characteristics, including manufacturing defects that may act as crack initiation sites. Conclusions: Phase constitution at intracanal temperature is a major determinant of reciprocating NiTi instrument performance. Heat-treated systems that retain martensitic and/or R-phase fractions near clinical temperature generally demonstrate greater flexibility and improved cyclic fatigue resistance. However, performance is also strongly influenced by instrument design and surface integrity.