Influence of heat treatment on fatigue resistance of two NiTi endodontic files

OBJECTIVES To evaluate the cyclic, torsional fatigue resistance and phase transformation of two heat-treated and non-heat-treated nickel-titanium reciprocating instruments. MATERIALS AND METHODS Twenty non-heat-treated (Procodile, Komet, Brasseler GmbH & Co., Lemgo, Germany) and 20 heat-treated (Procodile Q, Komet, Brasseler GmbH & Co., Lemgo, Germany) files (25 mm length, #25 apical diameter and 0.6 taper) were subjected to fatigue resistance tests. The dynamic cyclic fatigue was tested at 35 & PLUSMN;1 & DEG;C, using a dedicated patented device, in an artificial stainless-steel canal with a 60 & DEG; angle of curvature, the plate containing the artificial canal performing a controlled axial upand-down movement at 8 mm/s speed. The instruments were operated with a specific reciprocating motion (Reflex Dynamic & REG;, Komet, Breasseler GmbH & Co., Lemgo, Germany). Time to fracture (TtF) was recorded and the length of the fractured tips was measured. The torsional fatigue resistance was tested at room temperature (21 & PLUSMN;1 & DEG;C) using a custom-made device manufactured according to ISO 3630-1. The instruments were fixed 3 mm from the tip and their shafts were rotated counterclockwise at a speed of 2 rpm until fracture. The maximum torque load (Ncm) and corresponding rotation angle at fracture were recorded. All the results were statistically analysed (p <0.05). Fractographic analysis was performed using a field emission gun scanning electron microscope (FEG-SEM) to disclose the fractured surface characteristics. To assess the temperature range for phase transformations, differential scanning calorimetry (DSC) was performed on small segments of & AP;20 mg mass subjected to 2 heating and 2 cooling thermal cycles at rates of 5 & DEG;C min-1 in flowing N2 atmosphere over a temperature range of-40 & DEG;C to +110 & DEG;C. RESULTS The heat-treated instruments showed a greater resistance to dynamic cyclic fatigue as compared to the non-heat-treated sample (TtF 303 & PLUSMN;18.5 s vs 220 & PLUSMN;18.4 s; p <0.05) and a higher resistance to torsional fracture, bearing a greater maximum torque load (1.67 & PLUSMN;0.16 vs 0.82 & PLUSMN;0.07 Ncm; p <0.05). No significant differences were detected between heat-treated and non-heat-treated samples in mean angular rotation to fracture (298 & PLUSMN;25 & DEG; i,�312 & PLUSMN;32 & DEG;; p >0.05) and in the mean length of the fractured fragments (p >0.05). All instruments showed both ductile and brittle fracture patterns. According to the DSC, the direct (cooling) and reverse (heating) transformations of the non-heat-treated files occurred at lower temperatures (<25 & DEG;C) than those of the heat-treated files (& AP;50 & DEG;C), the latter thus not being austenitic at room and body temperatures at which they are operated. Also, the different transformation enthalpies suggest a multi-step transformation, likely involving R-phase formation, for heat-treat-ed files, against a direct transition between austenitic and martensi-tic phase for non-heat-treated files. CONCLUSIONS According to these findings, heat treatment of the tested files pro-vides them with microstructural properties more suited to the clin-ical operating conditions and im-proved performances in terms of torsional and flexural strength. CLINICA L SIGNIFICANCE Heat-treated files might be the best choice, over the traditional non-heat-treated files, when fac-ing challenging clinical condi-tions, such as curved and con-stricted canals.