Various factors can affect the fracture resistance of NiTi rotary instruments including alloy composition, manufacturing methods, cross-sectional geometry and flute design [26, 27]. Thermomechanical technology is commonly used for improving the microstructure and transformation behaviors of NiTi instruments in order to enhance the performance of instruments during root canal shaping including the cyclic fatigue resistance [28, 29]. In the present study, HCM and TRN instruments showed greater cyclic fatigue resistance than VB and RC instruments. HCM instruments are manufactured from CM heat-treated alloy that controls the instrument memory [4], which allows superior maintenance of the original canal curvature and enhanced the efficiency of the instrument in root canal preparation [9, 11]. HCM instruments are characterized by a triangular cross-sectional design displaying three cutting edges except for the instruments with size 20/.04 and 25/.04 taper, which have a square cross-section design with four blades and four flutes [30] as in the present study (Figure 2B).
The newly developed TRN instruments are manufactured from heat-treated NiTi alloy that supposed to enhance the flexibility and fatigue resistance of the instrument [22]. TRN instruments revealed a parallelogram cross-section design (Fig. 2A) while HCM showed a square cross-section design (Fig. 2B). Both HCM and TRN instruments revealed enhanced fatigue resistance with no significant difference between them. This finding could be attributed to the manufacturing process as HCM and TRN instruments are manufactured from heat-treated NiTi alloy. The thermomechanical treatment of endodontic instruments produced instrument with different austenite finishing temperature (Af), which affects the mechanical properties especially the fatigue resistance and bending properties [31, 32]. The heat treatment of HCM instrument is based on shifting the austenite/martensite transition temperature so that a stable martensitic microstructure is obtained at body temperature [32]. Although HCM and VB instruments are manufactured from heat-treated NiTi alloy, the HCM instrument had a higher fatigue resistance. It had been reported that VB instruments had a higher degree of austenite than HCM instruments at body temperature [33]. It could be postulated that HCM and TRN instruments might be used more safely in curved canals with double curvature due to their superior fatigue resistance.
TRN instruments revealed higher fatigue resistance than VB and RC instruments. This finding could be attributed to the special heat treatment of the alloy and the design of the instruments that enhanced fatigue resistance. Heat treatment of the alloy enhances the arrangement of the crystal structure, which might improve the flexibility and strength of the NiTi instruments [10]. In addition, heat treatments of NiTi instruments during or after the manufacturing process reducing the internal stress and surface defects due to the grinding process [10].
VB instruments showed superior fatigue resistance than RC instruments. VB instrument is manufactured from NiTi Blue alloy with a reduced shape memory characteristic which improved the fatigue resistance of the instrument [2, 7, 12]. On the other hand, the RC instrument is manufactured from conventional NiTi alloy that influenced the fatigue resistance of the instrument [2, 12]. RC instruments revealed the lowest fatigue resistance among the tested instruments.
The double curvature canals (S-shaped) created more stress on NiTi rotary instruments than in single curvature canals, and consequently, the instruments fractured due to cyclic fatigue [12, 19, 29]. The S-shaped canal is one of the most challenging conditions in clinical situations during root canal instrumentation with NiTi rotary instruments [19]. In many cases, the double curvatures are not detected in conventional radiographs; consequently, the clinician should be cautious of this probability and continue carefully during root canal instrumentation [3]. In the double curvature canal, the instruments fractured first in the apical curvature followed by the coronal curvature. This finding could be attributed to the abrupt curvature in the apical area with a 2 mm radius compared with the coronal curvature with a 5 mm radius, which is in agreement with the previous studies [3, 19, 34]. There was no significant difference in the mean length of the broken fragments of tested instruments in the same curvature. The instruments fractured at or just below the centre of curvature, which is in agreement with the previous studies [3, 12, 34].
The Weibull analysis revealed that the instruments with size 20/.04 taper revealed higher reliability than instruments with size 25/.04 taper. The instruments tested in a single curvature canal revealed higher predicted cycles for 99% survival compared with instruments tested in the double curvature canal. The probability of survival was higher for HCM and TRN instruments than VB and RC instruments. RC instruments had the lowest predicted number of cycles compared with the other groups. Weibull analysis is an appropriate method to predict the survival probability of NiTi rotary instruments [24, 35].