A comparative study of cyclic fatigue of 6 endodontic systems. An in vitro study

Background Mechanical preparation and the formation of space for adequate obturation are included in root canal shaping, but the complex root canal anatomy may be affect it. Manufacturers have created different alloys like M-Wire, Blue-Wire, Gold-Wire or R-phase. Objective: This investigation was performed to verify the null hypothesis that there were not significant differences between size 25 instruments ESP Files Thermoflex, Protaper Ultimate, Protaper Next, Blueshaper, One Curve and 2Shape about cyclic fatigue and length of broken fragments. Material and Methods 180 new size 25 files of the systems investigated were selected (n=30). Files were used with Endo Mate DT endo motor with speed and torque recommended by manufacturers, holding the instruments with clamping mechanism, with passive adjustment, glycerine and without pressure in a stainless-steel block. The time was calculated in seconds until fracture. Number of fatigue cycles was determined as (Resistance (s) x Speed)/60. Separated fragment lengths were calculated with digital Vernier caliper. Statistical analysis was carried out with the SPSS 18 programme at a 95% confidence level, using Levene´s Test to compare variances, Welch’s Test to compare means, and Games-Howell´s Test to reveal differences between groups. Results Levene’s Test determined no equal variances (P<0.05). Welch’s Test and ANOVA (P<0.05) showed significant differences in cyclic fatigue and separated fragment lengths. Games-Howell’s and Bonferroni´s Test established significant differences in multiple comparisons (P<0.05). Conclusions ESP Files Thermoflex was superior in cyclic fatigue. About separated fragment lengths, ESP Files Thermoflex, Protaper Ultimate and Blueshaper obtained longer lengths. Key words:Cyclic fatigue, continuous movement, separated fragment lengths, rotary systems.


Introduction
Mechanical preparation and the formation of space for adequate obturation are included in root canal shaping, but the complex root canal anatomy may be affect it. For this reason, different instruments and alloys were designed (1). The fracture of instruments remains a pro-blem in root canal treatment, despite improved file performance (2). Thanks to the physical transformation of martensitic and austenitic phases, the instruments have great flexibility, but bending fatigue or torsional failure lead to fracture (3). Manufacturers have created different alloys like M-Wi-re, Blue-Wire, Gold-Wire or R-phase. M-Wire increases resistance to fracture of conventional NiTi with heat treatment, but files have elastic memory (4). However, instruments with Gold-Wire have control memory and more resistance and flexibility than M-Wire files (5,6). The Blue-Wire of new Blueshaper files, was designed with special thermal process and stable martensitic phase, increasing the flexibility and resistance to fracture by cyclic fatigue, and avoiding elastic memory (7)(8)(9). About R-phase Thermoflex Control with heat treatment of new ESP Files system, provides to instruments with memory control and more flexibility, avoiding traction to recuperate the anatomy inside the root canal. Apart from alloy, there are multiple factors that affect the resistance to fracture of the instruments, such as the number of uses, the apical pressure of the operator, the design or the type of movement (10,11). Regarding the length of separated fragments, it is desirable that the longest fragment remain inside the root canal to perform its removal with extractor system. Therefore, this investigation was performed to verify the null hypothesis that there were not significant differences between size 25 instruments ESP Files Thermoflex (Jata Endo, Madrid, Spain), Protaper Ultimate (Dentsply Sirona, Ballaigues, Switzerland), Protaper Next (Dentsply Sirona, Ballaigues, Switzerland), Blueshaper (Zar-c4Endo, Gijón, Spain), One Curve (Micro-Mega, Besançon, France) and 2Shape (Micro-Mega, Besançon, France) about cyclic fatigue and length of broken fragments.

Material and Methods
Similar methods to those mentioned in another scientific paper, which published in 2019 by the same authors, were used (12). 180 new size 25 instruments of systems investigated were selected (n=30). In alphabetical order, systems were divided into groups (Table 1). Files were used with X-Smart Plus endo motor (Dentsply Sirona, Ballaigues, Switzerland) ( Fig. 1   The instruments were firmly held with clamping mechanism ( Fig. 2) without pressure and with passive adjustment in stainless-steel block with artificial canal with following characteristics: radius of curvature 3.5 mm,

Results
No equal variances were assumed (P<0.05) with Leve-ne´s Test, so Welch´s Test was used to contrast means consequently (Tables 2-4).
The cyclic fatigue mean values and statistics were described in Table 2. About statistics, ESP Files Thermoflex was superior to the other systems (P<0.05), but there were no significant differences between Protaper Ultimate vs Blueshaper (P=0.943) and 2Shape vs Blueshaper (P=0.270).
The NCF were presented in Table 3. ESP Files Thermoflex was superior statistically (P<0.000) except in comparison with One Curve (P<0.506). Nevertheless, there were not significant differences between Blues-

Discussion
In the present research, files with continuous movement but with different cross-section, taper, alloy, torque and speed were investigated. Results showed that ESP Files with R-Phase Thermoflex obtained better resistance to cyclic fatigue.
In the 1980s, Walia et al. introduced NiTi instruments in endodontics, and the manufacturing process has been developing with improvements to avoid failures like broken files and to have better mechanical properties of alloy. Actually, endodontic instruments can be classified in files with higher austenite phase (NiTi, R-Phase or M-Wire), or with superior martensite phase (CM-Wire, Blue-Wire or Gold-Wire). The first files are indicated to shape slightly curved or straight canals, and the instruments with higher martensite phase are suitable to treat severe or double curvatures of root canals, due to the greater flexibility and control memory (15)(16)(17). According to Goldberg  One Curve 0,000 0,000 -0,000 0,000 0,000 P. Next 0,000 0,000 0,000 -0,000 0,000 P. Ultimate 1,000 1,000 0,000 0,000 -0,000 2Shape 0,000 0,000 0,000 0,000 0,000 -  (31), in this investigation were showed similar results of One Curve. Regarding to Uygun et al. (32), One Curve was significant better than system with Gold-Wire in this study also, but One Curve (5.55±0.06mm) did not get the shortest value in separated fragment length (Protaper Next, 4.57±0.05mm).