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±1 °C, using a dedicated patented device, in an artificial stainless-steel canal with a 60° 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®, 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±1 °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 ≈20 mg mass subjected to 2 heating and 2 cooling thermal cycles at rates of 5 °C min-1 in flowing N2 atmosphere over a temperature range of -40 °C to +110 °C.

RESULTS: The heat-treated instruments showed a greater resistance to dynamic cyclic fatigue as compared to the non-heat-treated sample (TtF 303±18.5 s vs 220±18.4 s; p <0.05) and a higher resistance to torsional fracture, bearing a greater maximum thorque load (1.67±0.16 vs 0.82±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±25° vs 312±32°; 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-heattreated files occurred at lower temperatures (<25 °C) than those of the heat-treated files (≈50 °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-treated files, against a direct transition between austenitic and martensitic phase for non-heat-treated files.

CONCLUSIONS: According to these findings, heat treatment of the tested files provides them with microstructural properties more suited to the clinical operating conditions and improved performances in terms of torsional and flexural strength.

CLINICAL SIGNIFICANCE: Heat-treated files might be the best choice, over the traditional non-heat-treated files, when facing challenging clinical conditions, such as curved and constricted canals.

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