Nickel-titanium (NiTi) rotary file systems used in root canal treatment rotate continuously within the canal with the aid of a motor. When working in curved root canals, with each rotation of the file, one side is subjected to tension (tensile force) at the most curved point of the root, while the other side is subjected to compressive force. This constantly repeating microstress cycle leads to the accumulation of microscopic cracks in the atomic structure of the metal; this is called “metal fatigue” (cyclic fatigue). The most dangerous aspect is that this fatigue occurs suddenly without creating any visible deformation on the file surface (such as opening of grooves). Even if the file does not get stuck in the canal, it will crack in the middle of the root with a cracking sound when it reaches its fatigue limit. To prevent this accident, dentists should meticulously record the usage time and number of files, and should prefer single-use files with controlled memory (CM-wire) in excessively curved canals.
