After the instruments had been coupled to the low-speed handpiece, the height of the support was adjusted so that the instrument could be inserted into the artificial canal, without causing stress and allowing free rotation. The device used for the tests consisted of an iron base (50×30 cm) with a fixing support for the low speed handpiece (Dabi Atlante S/A Indústrias Médico Odontológicas, Ribeirão Preto, SP, Brazil). The null hypothesis is that the heat treatment of the instruments do not improve the fatigue resistance, with no difference among the instruments. The aim of this study was to evaluate the cyclic fatigue resistance of heat-treated instruments size 20/0.06 taper from HyFlex CM (Coltene –Whaledent, Altstatten, Switzerland), Edge File (Edge Endo, EDGEFILE®, Canada), Pro Design S (Easy, Belo Horizonte, Brazil) and MTwo (VDW, Munich, Germany) systems and size 25/0.06 taper from HyFlex CM, Edge File, Pro Design Logic and MTwo. The EdgeFile instruments obtained greater resistance to cyclic fatigue than the Vortex Blue and ESX instruments at three temperatures evaluated at 3 ☌, 22 ☌, 37 ☌, and 60 ☌. The Edge File system (EdgeEndo, EDGEFILE, Albuquerque, New Mexico, USA) has instruments manufactured by a heat treatment process (FireWire NiTi), which according to the manufacturer increases the flexural strength and flexibility of NiTi instruments. Removal of the filling material after the use of alternating and rotating movements in curved canals showed that the ProDesign Logic 50/0.01 file for apical preparation reduced the amount of material remaining in the apical portion. The ProDesign Logic instruments (Easy Dental Equipment, Belo Horizonte, Brazil) follow the concept of single-file, associated with heat treatment CM, with s-shaped cross-section. This treatment modifies the phase transformation of the NiTi alloy, making it martensitic at room temperature, increasing its flexibility and flexural strength. The ProDesign instruments (Easy Dental Equipment, Belo Horizonte, Brazil) are manufactured with NiTi alloy with heat treatment CM, similar to the treatment applied to the HyFlex CM instruments. The HyFlex CM (Coltene/Whaledent AG, Altstatten, Switzerland) is a NiTi System with controlled memory instruments manufactured by a thermal process, which through a specific heating and cooling process, allows for a 300% increase in flexural strength in conventional NiTi alloys. Furthermore, studies have demonstrated the superiority of thermally treated instruments related to fatigue resistance. The thermodynamic properties of NiTi alloys involving memory shape heat treatment produced instruments significantly more resistant to fatigue and also improved flexibility. Resistance to cyclic fatigue corresponds to the number of cycles an instrument can withstands under repeated cycles of tension and compression, until fracture and the dispersion of the lifetime of NiTi instruments, and their deflecting load (DL) changes during cyclic fatigue. Advances in the manufacturing of rotary Nickel-Titanium (NiTi) instruments provide a better perspective to endodontic treatment increasing resistance to fatigue.
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