A comparison of cyclic fatigue between used and new Mtwo Ni-Ti rotary instruments

Aim To evaluate the cyclic fatigue of Mtwo Ni-Ti rotary instruments after controlled clinical use in molar teeth. Methodology Twenty Mtwo instruments of each size were selected and divided into two groups: group A consisted of 10 new instruments (control group); group B consisted of 10 used instruments. Each instrument in group B was used to clean and shape 10 root canals of molar teeth in patients. Cyclic fatigue testing of instruments was performed in tapered artificial canals with a 5-mm radius of curvature and a 60 degrees angle of curvature. In all 140, instruments were rotated until fracture and the number of cycles to failure was recorded. Data were analysed using one-way ANOVA, Tukey's HSD test and independent sample t-test to determine any statistical difference; the significance was determined at the 95% confidence level. Results A reduction of cycles to failure between new (group A) and used (group B) instruments was apparent. A statistically significant difference (P < 0.05) was noted between instruments of groups A and B in all sizes with the exception of size 40, 0.04 taper. Conclusions Clinical use significantly reduced cyclic fatigue resistance of Mtwo rotary instruments when compared with an unused control group. However, all the instruments had minimal instrument fatigue when discarded after controlled clinical use.