Cyclic flexural fatigue resistance of NiTi Controlled Memory and Blue Technology instruments after torsional preloading

Abstract

Objective The aim of this study was to evaluate the influence of torsional preloading on the cyclic flexural fatigue resistance of thermally treated NiTi instruments.Material and Methods Ten new instruments New Hyflex CM (HF 30.06; Coltene/Whaladent Inc.), Typhoon CM (TYP 30.06; Clinician's Choice Dental Products) and Vortex Blue (VB 30.06; Dentsply Tulsa Dental) were chosen, based on geometry and specific characteristics of the manufacturing process. The new instruments of each system were tested in a bench device to determine their fatigue resistance through mean value of number of cycles to failure (Nf) (Control Group – CG). Another group of 10 new HF, TYP and VB instruments were submitted to 20 cycles of torsional straining between 0° and 180° (Experimental Group – EG) and then submitted to fatigue until rupture under the same conditions of the CG. Tested instruments were examined by scanning electron microscopy (SEM). Data were analyzed using one-way analysis of variance and post hoc Tukey’s test (α=.05).Results Higher fatigue resistance was accomplished by HF instruments, followed by VB and TYP (p<0.05). During the torsional preloading, the lowest mean torque value was observed for TYP instruments (p<0.05). The torsional preload caused a significant reduction in the Nf values (p<0.05) of about 20%, 39% and 45% for instruments HF, VB and TYP, respectively. Longitudinal cracks, generated during the torsional preloading, were present in VB files, but were not observed in the CM instruments (HF and TYP).Conclusions In conclusion, the flexural fatigue resistance of thermally treated instruments is diminished after cyclic torsional loading.

Comparison of cyclic fatigue resistance of different and novel heat-treated nickel-titanium rotary file systems at the intracanal temperature

“…HyFlex EDM files also had greater cyclic fatigue resistance values than OneCurve files (28). In one previous study, HyFlex CM was shown to have better cyclic fatigue resistance than the Typhoon CM, which also corroborates our study (6).…”

“…However, several reports have suggested that NiTi endodontic files are significantly more prone to fractures, especially during the shaping of curved or narrow root canals, which might impact the success of the root canal treatment. NiTi files are reported to be up to seven times more likely to fracture compared to stainless-steel files (5,6). Many underlying factors contribute to these fractures; however, the most common reasons for NiTi fractures are either torsional or due to cyclic fatigue.…”

“…Experiencing these frequent fractures causes major setbacks for root canal treatments in many clinics. Therefore, several experimental studies have been conducted to prevent the fracturing of NiTi rotary files and to improve their mechanical properties (6,8). These studies have included altering the operating speed, changing the file design, and attempting various metal treatments (9,10).…”

In Vitro Evaluation of a Recently Developed Rotary File: AF Rotary

“…Two were found to be most relevant factors influencing the above mentioned properties: the heat treatment and the mass of the NiTi instruments [8][9][10][11][12][13][14]. The heat treatment modifies the metallurgical properties of the instrument, and change its behavior simply changing the transition temperature of the alloy from a more austenitic to a more martensitic one [15][16][17][18][19][20]. A more martensitic file will be more resistant to flexural stress, while a more austenitic instrument will be more resistant to torsional fatigue [21][22][23][24][25][26][27].…”