The aim of this study was to investigate the role of the flat-designed surface in improving the resistance to cyclic fatigue by comparing heat-treated F-One (Fanta Dental, Shanghai, China) nickel–titanium (NiTi) rotary instruments and similar prototypes, differing only by the absence of the flat side. The null hypothesis was that there were no differences between the two tested instruments in terms of cyclic fatigue lifespan. A total of 40 new NiTi instruments (20 F-One and 20 prototypes) were tested in the present study. The instruments were rotated with the same speed (500 rpm) and torque (2 N) using an endodontic motor (Elements Motor, Kerr, Orange, CA, USA) in the same stainless steel, artificial canal (90° angle of curvature and 5 mm radius). A Wilcoxon–Mann–Whitney test was performed to assess the differences in terms of time to fracture and the length of the fractured segment between the flat- and non-flat-sided instruments. Significance was set at p = 0.05. The differences in terms of time to fracture between non-flat and flat were statistically significant (p < 0.001). In addition, the differences in terms of fractured segment length were statistically significant (p = 0.034). The results of this study highlight the importance of flat-sided design in increasing the cyclic fatigue lifespan of NiTi rotary instruments.

Protaper Universal (PTU), Protaper Gold (PTG) (Maillefer, Ballaigues, CH), EdgeTaper (ET), and EdgeTaper Platinum (ETP) (Albuquerque, NM, USA) were tested for both torsional and flexural resistance. The aim of the present study was to evaluate the influence of proprietary heat treatment on the metallurgical properties of the aforementioned instruments. Four groups of 30 different instruments (size 20.07) were tested, then divided into two subgroups of 15 instruments—one for the cyclic fatigue test in a curved canal (90°—2 mm radius) at 300 rpm and 2.5 Ncm. The time to fracture (TtF) and fragment length (FL) were recorded. The other subgroup was subjected to the torsional test (300 rpm, 5.5 Ncm). The torque to fracture and TtF were recorded. All the instruments underwent a SEM analysis. The heat-treated instruments showed a significantly higher fatigue resistance than the non-heat-treated instruments (p < 0.05). No significant differences were found in the torsional resistance between the ET and PTU, and the ETP and PTG. However, when comparing all the groups, the heat-treated instruments showed less torsional resistance. The improvement from heat treatment was mainly found in the cyclic fatigue resistance.

Since there are no reviews of the literature on this theme, the aim of this narrative review is to summarize the metallurgical tests used in endodontics, pointing out their functional use and their pros and cons and giving readers a user-friendly guide to serve as an orientation aid in the plethora of metallurgical tests. With this purpose, a literature search for articles published between January 2001 and December 2021 was conducted, using the electronic database PubMed to collect all published articles regarding the metallurgical tests used in endodontics for the evaluation of NiTi rotary instruments. The search was conducted using the following keywords: “metallurgy”, “differential scanning calorimetry” (DSC), “X-ray diffraction” (XRD), “atomic force microscopy” (AFM), “energy-dispersive X-ray spectroscopy” (EDS), “focused ion beam analysis” (FIB) and “Auger electron spectroscopy” (AES) combined with the term “endodontics” or “NiTi rotary instruments”. Considering the inclusion and exclusion criteria, of the 248 articles found, only 81 were included in the narrative review. According to the results, more than 50% of the selected articles were published in one of the two most relevant journals in endodontics: International Endodontic Journal (22.2%) and Journal of Endodontics (29.6%). The most popular metallurgical test was DSC, with 43 related articles, followed by EDS (33 articles), AFM (22 articles) and XRD (21 articles). Few studies were conducted using other tests such as FIB (2 articles), micro-Raman spectroscopy (4 articles), metallographic analysis (7 articles) and Auger electron spectroscopy (2 articles).

F 40 new NiTi instruments (20 S-One 25.06 taper and 20 AF Blue S4 25.06 taper) were tested in the present study. Both instruments were rotated at 300 rpm and with a torque setting of 2 Ncm) using an endodontic motor in the same artificial canal (90° angle of curvature and 3 mm radius). A Student's t-test was performed to determinate the differences in terms of time to fracture (TtF) and the length of the fractured portion between the two different instruments. Significance was set at P < 0.05. Results: The differences in terms of TtF for cyclic fatigue resistance between the two files were statistically significant (P < 0.05). In terms of fractured length, not statistically significant differences were found (P > 0.05). Scanning electron microscopy images showed patterns of cyclic fatigue striations. Conclusion: The results of this study confirm and enhance the role of cross-section in increasing the cyclic fatigue resistance of NiTi rotary instruments....

The aim of this study was to assess the role of the crystallographic phase of Nickel-titanium (NiTi) rotary instruments in determining their torsional resistance during different bending conditions, such as different degrees and angles of curvature. 200 F-One 20.04 instruments (Fanta Dental, Shanghai, China) were used, 100 austenitic instruments and 100 martensitic instruments. Each group was divided in 5 subgroups according to the different bending conditions (straight canal, 90° or 60° of curvature degrees and 3 mm or 5 mm of radius of curvature). The static torsional test was performed by using a device composed of an electric motor capable of recording torque values (N·cm); a vice used to secure the instruments at 3 mm from the tip; and artificial canals, which allow instruments to remain flexed during test. Each instrument was rotated at 500 rpm with a torque limit set to 5.5 Ncm until its fracture. Torque at Fracture (TtF) was registered. A scanning electron microscopy (SEM) observation was conducted. The collected data confirm that an increase in the angle of curvature and a decrease in the radius of curvature of the artificial canals lead to an increase of TtF values with a statistically significant difference (p < 0.05), both in the austenitic and martensitic groups. Regarding the comparison between austenitic and martensitic groups in the same bending condition, a statistically significant difference was found only when the torsional test was performed in the canals with the degrees of curvature of 90° and the radius of curvature of 3 mm and 5 mm, with the austenitic instruments showing a higher TtF than the martensitic ones. In conclusion, it can be stated that the crystallographic phase influences the maximum torque at fracture when the instruments are subjected to severe bending and that the radius of curvature significantly influences their torsional resistance.

Previously published studies have investigated the influence of instrument access on cyclic fatigue resistance. However, no studies have evaluated the relationship between angulated access and torsional resistance. The aim of this study was to investigate the influence of the angle of access on the torsional resistance of endodontic instruments. One hundred and eighty instruments were selected: 90 F-One Blue 25/04 and 90 HeroShaper 25/04 instruments. Three subgroups (n = 30) for each instrument type (A and B) were established according to the angle of insertion of the instruments inside the artificial canal (0°, 10° and 20°). The tests were performed using a custom-made device consisting of the following: a motor that can record torque values of 0.1 s; interchangeable stainless-steel canals with different curvature (0°, 10° and 20°) that allow the instrument’s angulated insertion and keep it flexed during testing procedures; and a vise used to secure the instrument at 3 mm from the tip. Torque limit was set to 5.5 Ncm, and each instrument was rotated at 500 rpm until fracture occurred. Torque to fracture (TtF) was registered by the endodontic motor, and the fragment length (FL) was measured with a digital caliper. Fractographic analysis was performed using a scanning electron microscopy (SEM) evaluation to confirm the cause of failure. TtF values and fragment length (FL) values were statistically analyzed using one-way analysis of variance (ANOVA) test and the Bonferroni correction for multiple comparisons across the groups with significance set to a 95% confidence level. Regarding the F-One Blue instruments, the results showed a higher TtF for group A3 (20°) than for group A1 (0°) and group A2 (10°), with a statistically significant difference between group A3 and the other two groups (p < 0.05), whereas no statistically significant difference was found between group A1 and group A2 (p > 0.05). Regarding the HeroShaper instrument, the results showed the highest TtF for group B3, with a statistically significant difference between the three subgroups B1, B2 and B3 (p < 0.05). The results showed that the torsional resistance increases as the angle of instrument access increases with a varying intensity, according to the crystallographic phase of the instrument selected.