Metallurgical Characterization and Three-Point Bend Testing of Rotary Nickel-Titanium Endodontic File

Superelastic rotary nickel-titanium (NiTi) endodontic files (RNEF) are used in tooth canal operations with handheld motor during endodontic treatment. However, fracture at tips occurs occasionally during operations due to low cycle fatigue.

Six different brands of 4% taper RNEF: PF (#25-25mm), GTX (#20-25mm) (Dentsply Tulsa Dental Specialties, Tulsa, OK), NRT (#25-25mm, MANI Inc., Tochigi, Japan), FKG (#25-25mm, FKG, Dentaire), K3 (#25-25mm) and TF (Twisted File #25-27mm) (Sybron Kerr, Orange, CA) were studied. Characterization included elemental compositions: with EDX, XRF and ICP-OES; crystallographic structures: by X-ray diffraction (XRD); austenitic-finish transformation temperature (A_f): by Differential Scanning Calorimetry (DSC). Three-point bending test focused on the RNEF superelasticity at various temperatures were performed. A_f of the file tips materials ranged from 17.3 to 49.5°C. Austenite b.c.c. was found the major crystal structure in low-A_f (< 26°C) RNEF (TF, K3 & PF) files. High-A_f (>26°C) RNEF (FKG, NRT & GTX) was found relatively rich in monoclinic martensite. RNEF Ni content ranged from 49.8% to 51.5%. TF, PF & FKG demonstrated relatively less permanent deformation strain after 4% flexural strain deflection at 24°C. The working temperature is concluded the most critical factor to superelastic performance. Low-A_f RNEF with predominantly austenite behaves superelastically more readily.