Heat treatment-irrigation interaction governs the fatigue life of NiTi endodontic files: Insights from acoustic emission and photoelastic stress mapping.

This study investigates the combined effects of 96% ethanol irrigation and C-Wire heat treatment on the fatigue durability of NiTi endodontic files. One Curve® instruments were subjected to cyclic root canal instrumentation in PMMA blocks until fracture, with real-time acoustic emission monitoring and photoelastic visualization of isochromatic fringe patterns. Heat treatment (HT) increased the number of blocks until failure by a factor of 1.6 to 2.1 depending on irrigation status, while irrigation increased it by a factor of 2.4 to 3.3 depending on heat treatment status, consistent with a significant statistical interaction between the two factors, most robustly evidenced by the acoustic damage index (AEDI, p < 0.001). Their combined application extended the average instrument lifespan fivefold (9.2 ± 1.5 NBF vs. 1.8 ± 0.8, p < 0.001). Furthermore, photoelastic analysis revealed the residual stress field induced after instrument passage. Scanning electron microscopy fractography showed predominantly ductile, helical fracture patterns in heat-treated files, whereas untreated instruments exhibited brittle, transverse fractures. The observed interaction between heat treatment and ethanol irrigation underscores the potential of lubricating irrigants to enhance the fatigue resistance of NiTi files, while also emphasizing the value of multi-signature approach for characterizing their damage mechanisms.