Effect of autoclave sterilisation and heat activated sodium hypochlorite irrigation on the performance of nickel-titanium rotary files against cyclic fatigue

The present study aims to assess the impact of heat-activated sodium hypochlorite (NaOCl) and/or autoclave sterilisation on the cyclic fatigue resistance (CFR) of heat-treated nickel-titanium rotary files used in root canal treatment. The CFR of One Curve (OC) files was evaluated under the following conditions: as received (Group 1; control), immersion in NaOCl at 23 ± 1ºC (Group 2), immersion in NaOCl at 60 ± 1ºC (Group 3), autoclave sterilisation at 135 1ºC (Group 4), combined treatment of autoclave sterilisation and immersion in NaOCl at 23 ± 1ºC (Group 5), and combined treatment of autoclave sterilisation and immersion in NaOCl at 60 ± 1ºC (Group 6). A simulated root canal in a zirconia block was utilised to test the performance of the files. All the types of treatments resulted in significant reductions in fracture resistance of the OC files. Immersion of the files in NaOCl at 23ºC revealed the smallest reduction, while combined treatment of autoclaving and immersion in NaOCl at 60ºC caused the greatest reduction. Autoclave sterilisation or exposure of OC files to 2.5% NaOCl adversely affect the cyclic fatigue life and increasing solution temperature or combined treatment caused additionally significant reduction in CFR

Experimental evaluation of cyclic fatigue resistance of four different nickel-titanium instruments after immersion in sodium hypochlorite and/or sterilization

NiTi instruments have a high risk of separation due to torsional or flexural fatigue (cyclic fatigue). Chemomechanical preparation, cleaning procedures, chemical disinfection and sterilization cause the corrosion of endodontic instruments that may weaken the fracture resistance of the instruments. OBJECTIVE: To assess the effects of NaOCl immersion and autoclave sterilization on the cyclic fatigue resistance of ProFile, FlexMaster, Mtwo and TwistedFiles NiTi instruments (tip size 25, 0.06 taper, n=160). MATERIAL AND METHODS: The instruments (n=10 for each subgroup) were dynamically immersed in NaOCl; immersed in NaOCl and sterilized in one autoclave cycle; 5 cycles immersed in NaOCl and sterilized in autoclave and not immersed in NaOCl and not sterilized (control group). Dynamic cyclic fatigue resistance was tested. The number of cycles to failure (NCF) were statistically analyzed (P<0.05). RESULTS: Kruskall-Wallis test indicated significant differences among the tested instruments in terms of NCF (P=0.000). The mean NCF of Mtwo (556.75) was higher than that of the Twisted File, Flexmaster and ProFile, 483.1, 376.12, 365.25, respectively. NaOCl immersion and autoclave sterilization have no effect on the NCF values of the tested instruments (P>.05). CONCLUSIONS: Cyclic fatigue resistance of the tested NiTi instruments cannot be adversely affected by NaOCl immersion and autoclave sterilization. Production process (TwistedFiles) or design (Twisted Files, FlexMaster, Mtwo and ProFile) of the instruments can influence their cyclic fatigue resistance