A Comparative Study of Cyclic Fatigue of 10 Different Types of Endodontic Instruments: an in Vitro Study.

OBJECTIVE: This study was designed to test the null hypothesis that there were no significant differences between size 25 files F360, F6 SkyTaper, Hyflex EDM, iRace, Neoniti, One Shape Protaper Next, Reciproc, Revo-S and Wave One Gold in terms of resistance to cyclic fatigue and length of broken fragments.
MATERIAL AND METHODS: 300 new size 25 files of the systems studied were selected (n=30). The instruments were mechanized with a X-Smart Plus endo motor at the speed and torque recommended by the manufacturer, holding the instruments steady with a clamping mechanism, with passive adjustment and without pressure in a stainless-steel block. The time was calculated in seconds until fracture. The number of fatigue cycles was calculated as (Resistance (s) x Speed)/60. The separated fragment lengths were measured with a digital Vernier calliper. A statistical analysis was performed with the SPSS 18 programme at a 95% confidence level, using the Levene´s Test to compare variances, the Welch's Test to compare means, and the Games-Howell´s Test to reveal differences between groups.
RESULTS: The Levene's Test showed no equal variances (P<0.05). The Welch's Test (P<0.05) showed significant differences in cyclic fatigue and separated fragment lengths. The Games-Howell test (P<0.05) exhibited significant differences in multiple comparisons, (P<0.05).
CONCLUSIONS: The systems with CM-Wire (Hyflex EDM and Neoniti) were superior in resistance to the other systems for cyclic fatigue. For separated fragment lengths, F360 (conventional NiTi) and Reciproc (M-Wire) were better significantly better in terms of resistance.

Introduction

The use of Nickel-Titanium (NiTi) instruments has increased due to the advantages they present compared to manual files (, ), which are used in canal-shaping procedures to reduce bacteria levels and facilitate irrigation and obturation for long-term success (, ). Their main advantages are elasticity and cutting efficiency (, ), and also the frequent changes of characteristics and cross-section of endodontic files have increased their resistance to cyclic fatigue (-), but their major problem is fracturing in root canals (, ).

With regard to currents systems, new manufacturing processes and alloys such as CM-Wire, M-Wire and Gold-Wire have been developed to improve conventional NiTi in recent years (). Another improvement has been a reciprocating motion, an oscillating motion that rotates the files in one direction then reverses their motion before effecting a complete rotation (). The useful life of instruments can be increased by reciprocating motion, as it presents better resistance to cyclic fatigue than continuous rotation (, ). Frequency, apical strength of instrument, pressure applied by the operator and enlargement of canal are among the clinical factors which also affect the resistance of files to cyclic fatigue ().

Science advances, and single-use instrumentation systems have been created to prevent fracture from overuse. The files are used in a single tooth, in 1 to 4 root canals (as in molars) and are safe in narrow and curved canals (). However, several studies have suggested that their traction and compression strengths are a disadvantage in the case of continuous rotation instruments ().

In relation to the assessment of the separated fragment lengths, it is preferable that the longest fragment should remain inside the tooth to make its removal easier.

Therefore, this study was designed to test the null hypothesis that there were no significant differences between size 25 files F360 (Komet Dental, Lemgo, Germany), F6 SkyTaper (Komet Dental, Lemgo, Germany), Hyflex EDM (Coltene, Altstätten, Switzerland), iRace (FKG Dentaire, La Chaux-de-Fonds, Switzerland), Neoniti (Neolix, Chatres-La-Foret, France), One Shape (Micro-Mega, Besançon, France) Protaper Next (Dentsply Maillefer, Ballaigues, Switzerland), Reciproc (VDW, Munich, Germany), Revo-S (Micro-Mega, Besançon, France) and Wave One Gold (Dentsply Maillefer, Ballaigues, Switzerland) in terms of resistance to cyclic fatigue and separated fragment lengths.

Material and methods

New size 25 files of the systems studied were selected (n=30 per system, total 300). The systems were divided into groups in alphabetical order (Table 1).

Table 1

Systems and their characteristics.

	System	Size/Taper	Alloy	Cross-section
Group 1	F360	25/0.04	conventional NiTi	Double-S
Group 2	F6 SkyTaper	25/0.06	Conventional NiTi	Double-S
Group 3	Hyflex EDM	25/∼	CM-Wire	Variable
Group 4	iRace	25/0.04	Conventional NiTi	Triangular
Group 5	Neoniti	25/0.08	CM-Wire	non-homothetic rectangular
Group 6	One Shape	25/0.06	conventional NiTi	variable
Group 7	Protaper Next	25/0.06	M-Wire	decentered rectangular
Group 8	Reciproc	25/0.08	M-Wire	S-shaped
Group 9	Revo-S	25/0.06	conventional NiTi	assymmetrical
Group 10	Wave One Gold	25/0.07	Gold-Wire	rectangular

The instruments were rotated using X-Smart Plus endo motor (Dentsply Maillefer, Ballaigues, Switzerland) (Fig. 1) at the speed and torque recommended by manufacturers. The speed and torque of each group using only the size 25 files of endodontic systems were:

Figure 1

Hyflex EDM 25/∼ file in an artificial 60º canal.

Group 1 (F360): 300 rpm, 1.8 N·cm and continuous rotation.

Group 2 (F6 SkyTaper): 300 rpm, 2.2 N·cm and continuous rotation.

Group 3 (Hyflex EDM): 500 rpm, 2.5 N·cm and continuous rotation.

Group 4 (iRace): 600 rpm, 1.5 N·cm and continuous rotation.

Group 5 (Neoniti): 400 rpm, 1.5 N·cm and continuous rotation.

Group 6 (One Shape): 400 rpm, 4 N·cm and continuous rotation.

Group 7 (Protaper Next): 300 rpm, 2 N·cm and continuous rotation.

Group 8 (Reciproc): 300 rpm, 2 N·cm and reciprocation motion.

Group 9 (Revo-S): 350 rpm, 0.8 N·cm and continuous rotation.

Group 10 (Wave One Gold): 350 rpm, 2 N·cm and reciprocation motion.

The instruments were firmly held with clamping mechanism (Fig. 2) with passive adjustment and without pressure in a stainless-steel block containing an artificial canal with the following characteristics: 60º curvature, radius of curvature 3.5 mm, length 21 mm, width 2 mm, and depth 3 mm. The characteristics of model were similar to the block used by Gambarini et al. () and Champa et al. (). The canal was lubricated with glycerin after each file.

Figure 2

Clamping mechanism of the handpiece for the X-Smart Plus endo motor.

The time was calculated in seconds (s) until fracture. The number of cycles to fracture (NCF) was calculated by the following formula: (Resistance (s) x Speed)/60. The separated fragment lengths were measured with a digital Vernier caliper (Fig. 3).

Figure 3

Measurement of separated fragment of Wave One Gold Primary (25/0.07) file with digital Vernier caliper.

Statistical analysis was performed with the SPSS 18 programme at a 95% confidence level, using the Levene´s Test to compare variances, the Welch’s Test to compare means, and the Games-Howell´s Test to reveal differences between groups.

Results

In all of the comparisons that were made, the Levene´s Test was carried out, and no equal variances were assumed (P<0.05); therefore, it was decided to carry out the Welch´s Test (Tables 2, 3 and 4).

Table 2

Means and statistics for resistance to cyclic fatigue (s).

	F360	F6	Hyflex	iRace	Neoniti	O.Shape	P.Next	Reciproc	R-S	WOG
	152.43± 11.25	190.83± 16.61	331.07± 25.22	27.37± 2.65	414.83± 25.66	56.23± 8.39	70.43± 5.19	168.67± 15.34	33.53± 5.45	188.00± 11.39
Levene´s Test
0.000
Welch´s Test
0.000
Games-Howell´s Test
	F360	F6	Hyflex	iRace	Neoniti	O.Shape	P.Next	Reciproc	R-S	WOG
F360	-	0.009	0.000	0.000	0.000	0.000	0.000	0.765	0.000	0.001
F6	0.009	-	0.000	0.000	0.000	0.000	0.000	0.599	0.000	1.000
Hyflex	0.000	0.000	-	0.000	0.001	0.000	0.000	0.000	0.000	0.000
iRace	0.000	0.000	0.000	-	0.000	0.000	0.000	0.000	0.550	0.000
Neoniti	0.000	0.000	0.001	0.000	-	0.000	0.000	0.000	0.000	0.000
O.Shape	0.000	0.000	0.000	0.000	0.000	-	0.121	0.000	0.001	0.000
P.Next	0.000	0.000	0.000	0.000	0.000	0.121	-	0.000	0.000	0.000
Reciproc	0.765	0.599	0.000	0.000	0.000	0.000	0.000	-	0.000	0.556
R-S	0.000	0.000	0.000	0.550	0.000	0.001	0.000	0.000	-	0.000
WOG	0.001	1.000	0.000	0.000	0.000	0.000	0.000	0.556	0.000	-

Table 3

Means and statistics of number of cycles to fracture (NCF).

	F360	F6	Hyflex	iRace	Neoniti	O.Shape	P.Next	Reciproc	R-S	WOG
	762.17± 56.25	954.16 ±83.06	2758.88 ±210.14	273.67 ±26.54	2765.55 ±171.07	374.88 ±55.90	352.16 ±25.97	843.33 ±76.68	195.61 ±31.77	1096.66 ±66.42
Levene´s Test
0.000
Welch´s Test
0.000
Games-Howell´s Test
	F360	F6	Hyflex	iRace	Neoniti	O.Shape	P.Next	Reciproc	R-S	WOG
F360	-	0.009	0.000	0.000	0.000	0.000	0.000	0.765	0.000	0.000
F6	0.009	-	0.000	0.000	0.000	0.000	0.000	0.599	0.000	0.183
Hyflex	0.000	0.000	-	0.000	1.000	0.000	0.000	0.000	0.000	0.000
iRace	0.000	0.000	0.000	-	0.000	0.049	0.002	0.000	0.010	0.000
Neoniti	0.000	0.000	1.000	0.000	-	0.000	0.000	0.000	0.000	0.000
O.Shape	0.000	0.000	0.000	0.049	0.000	-	0.999	0.000	0.000	0.000
P.Next	0.000	0.000	0.000	0.002	0.000	0.999	-	0.000	0.000	0.000
Reciproc	0.765	0.599	0.000	0.000	0.000	0.000	0.000	-	0.000	0.000
R-S	0.000	0.000	0.000	0.010	0.000	0.000	0.000	0.000	-	0.000
WOG	0.000	0.183	0.000	0.000	0.000	0.000	0.000	0.000	0.000	-

Table 4

Means and statistics of separated fragments length (mm).

	F360	F6	Hyflex	iRace	Neoniti	O.Shape	P.Next	Reciproc	R-S	WOG
	10.48± 0.19	9.45± 0.34	9.86± 0.42	8.06± 0.07	10.27± 0.30	9.09± 0.12	9.90± 0.39	11.44± 0.21	10.05± 0.55	9.85± 0.39
Levene´s Test
0.000
Welch´s Test
0.000
Games-Howell´s Test
	F360	F6	Hyflex	iRace	Neoniti	O.Shape	P.Next	Reciproc	R-S	WOG
F360	-	0.000	0.183	0.000	0.968	0.000	0.191	0.000	0.880	0.103
F6	0.000	-	0.853	0.000	0.017	0.596	0.745	0.000	0.669	0.849
Hyflex	0.183	0.853	-	0.000	0.834	0.027	1.000	0.000	1.000	1.000
iRace	0.000	0.000	0.000	-	0.000	0.000	0.000	0.000	0.000	0.000
Neoniti	0.968	0.017	0.834	0.000	-	0.000	0.874	0.000	0.999	0.752
O.Shape	0.000	0.596	0.027	0.000	0.000	-	0.010	0.000	0.042	0.016
P.Next	0.191	0.745	1.000	0.000	0.874	0.010	-	0.000	1.000	1.000
Reciproc	0.000	0.000	0.000	0.000	0.000	0.000	0.000	-	0.001	0.000
R-S	0.880	0.669	1.000	0.000	0.999	0.042	1.000	0.001	-	1.000
WOG	0.103	0.849	1.000	0.000	0.752	0.016	1.000	0.000	1.000	-

The cyclic fatigue mean values and statistics are presented in Table 2. Statistically, in terms of resistance, Neoniti and Hyflex EDM were superior to other systems (P<0.05), but there were no significant differences between Wave One Gold vs F6 SkyTaper (P=1.000), One Shape vs Protaper Next (P=0.121), Reciproc vs F360 (P=0.765), Reciproc vs Wave One Gold (P=0.556) and Revo-S vs iRace (P=0.550).

The NCF were described in Table 3. Neoniti and Hyflex EDM proved to be statistically superior to other systems (P<0.000). However, there were no significant differences between One Shape vs Protaper Next (P=0.999), Wave One Gold vs F6 SkyTaper (P=0.183), Neoniti vs Hyflex EDM (P=1.000), Reciproc vs F360 (P=0.765) and Reciproc vs F6 SkyTaper (P=0.599).

The separated fragment length mean values and statistics are presented in Table 4. The highest values were obtained by F360 and Reciproc. F360 was significantly superior (P=0.000) vs F6 SkyTaper, One Shape and iRace. Reciproc was significantly superior to all the other systems (p≤0.001).

Discussion

In the present study, files with continuous and reciprocating motion and with different alloys, cross-sections, tapers, speeds and torques were studied. The results showed that the systems with CM-Wire and Gold-Wire alloys, reciprocating motion and conventional NiTi instruments with an S cross-section offered better resistance to cyclic fatigue.

A complete bio-mechanical preparation of root canals is an essential factor for endodontic success. Shaping and cleaning of the canal are performed during this phase, and they present great difficulty in curved canals (-). Goldberg et al. () reported that apical enlargement may produce defects such as apical transportation or zipping, with a risk of endodontic treatment failure.

The cross-section design, the chemical composition of the alloy and the thermo-mechanical process used during the manufacture of the alloy all influence cyclic fatigue (-). In the present study, F6 SkyTaper, with a S-shaped cross-section, obtained a higher NCF than other conventional NiTi systems such as iRace, One Shape and Revo-S, indicating that an S-shaped cross-section offered more flexibility and resistance. On the other hand, CM-Wire alloy (Neoniti and Hyflex EDM) obtained the best results compared to the other systems studied, showing that CM-Wire with memory control is more resistant than the other alloys.

The cyclic fatigue findings of this study (Tables 2 and 3) showed that Group 4 (iRace, 27.37±2.65s, 273.67±26.54 NCF) and Group 9 (Revo-S, 33.53±5.45s, 195.61±31.77 NCF) obtained worse results than the other groups. In contrast, Group 3 (Hyflex EDM, 331.07±25.22s, 2758.88±210.14 NCF) and Group 5 (Neoniti, 414.83±25.66s, 2765.55±171.07 NCF) achieved significantly better results than the other systems. It may also be observed that the S-shaped cross-sectional systems (F360, F6 SkyTaper and Reciproc) obtained better results than the conventional NiTi and M-Wire systems (iRace, Protaper Next, One Shape and Revo-S). Reciprocating motion (Reciproc and Wave One Gold) was found to improve the cyclic fatigue results compared to almost all the continuous motion systems studied (F360, iRace, Protaper Next, One Shape and Revo-S). Regarding separated fragment lengths, F360 (10.48±0.19mm) and Reciproc (11.44±0.21mm) obtained the highest values, while iRace (8.06±0.07mm) and One Shape (9.09±0.12mm) presented the lowest fragments length.

Aminsobhani et al. () compared the cyclic fatigue and separated fragment lengths of size 25 Neoniti, Race, Mtwo, TF and Protaper Next files with continuous rotation in 3 simulated canals in a stainless-steel block. In all 3 canals, the Neoniti system obtained better results than other systems: between 400 and 1600 NCF. Statistically, Neoniti was superior to other systems, concluding that CM-Wire (Neoniti) is better than conventional NiTi, R-Phase and M-Wire. For separated fragment lengths, the Race system obtained the lowest mean and TF the highest in group 1. In group 2, Neoniti had the lowest average whereas Race had the highest; and in group 3, Neoniti obtained the lowest mean and TF the highest. In the present study, unlike Aminsobhani et al., Neoniti obtained better fatigue results with continuous rotation also, but both studies agree that Neoniti (CM-Wire) was more resistant than Race with conventional NiTi. As regards separated fragment lengths, the present study is congruent with Aminsobhani et al.’s findings for Group 1, where Race obtained a lower average than Neoniti, unlike in Groups 2 and 3, probably because of the characteristics of each artificial canal in this case.

Kaval et al. () investigated the cyclic fatigue of F6 SkyTaper, One Shape, K3XF and TRUShape 3D (Dentsply Tulsa Dental Specialties, Tulsa, USA) files using model with similar characteristics of the block of this study; they observed significant differences between all groups, with F6 SkyTaper obtaining the highest resistance (959±92 NCF). Similar to the findings in the present study, Kaval et al. considered that a double-S cross-section could improve the resistance to cyclic fatigue. Furthermore, F6 SkyTaper was significantly superior to One Shape, as found by Kaval et al.

Pedullà et al. () compared size 25 Hyflex EDM, Reciproc and Wave One files in an artificial canal with 60º curvature with a 3 mm radius. Hyflex EDM gave better results than Reciproc and Wave One, but no significant differences were observed between the latter two. The authors concluded that Hyflex EDM was more resistant than other systems, determining that CM-Wire was more resistant than M-Wire. In relation to movement, the reciprocating motion did not affect the results. Like Pedullà et al., the Hyflex EDM results were statistically superior to Reciproc in a canal with the same curvature in the present study. Unlike Pedullà et al., in this investigation the reciprocating motion improved the results of other files except in instruments with CM-Wire.

Ersoy et al. () studied F360, TF, FlexMaster and Race in a stainless-steel block with an artificial canal measuring 1.5 mm in diameter with a 60º curvature. They observed that F360 was significantly more resistant than the other systems; TF was significantly more resistant than FlexMaster and Race, and there were no significant differences between FlexMaster and Race. They concluded that F360 was the best system, showing that the S-shaped cross-section in systems with continuous rotation improved the cyclic fatigue resistance between systems with shape memory; furthermore, TF was shown to be superior to FlexMaster and Race. The results of the present study, which also compared the files in a 60º canal, are similar to the results obtained by Ersoy et al. in that F360 was significantly better than Race.

Topçuoğlu et al. () examined size 25 Wave One Gold, Reciproc and Wave One files in an artificial double S canal, 1.4 mm in diameter and 18 mm in length. The results showed that Wave One Gold obtained the best results in apical and coronal curvature. In the statistical analysis, Wave One Gold was significantly better than Reciproc and Wave One in both curvatures, whereas Reciproc was superior to Wave One in the apical curve; however, there were no significant differences between the latter two systems in the coronal curvature. In conclusion, the authors found that Wave One Gold offered the best resistance in an artificial double S canal. In the present study, Wave One Gold was superior to Reciproc, with similar results to those of Topçuoğlu et al, but there were significant differences in number of cycles to fracture, probably due to the speed and alloy of Wave One Gold.

A study by Keskin et al. () compared the resistance to cyclic fatigue and the separated fragments length of size 25 Reciproc Blue, Reciproc and Wave One Gold files at 60° curvature with different radius of curvature. The authors determined that Reciproc Blue obtained the highest significant resistance to cyclic fatigue, and Wave One Gold was significantly better than Reciproc as found by Topçuoğlu et al, confirming that Gold-Wire improved the characteristics of M-Wire. Regarding the length of the separated fragments, they observed no significant differences (P>0.05). In contrast to Keskin et al., the fracture times in the present study were similar for Wave One Gold and Reciproc except in NCF, and significant differences in separated fragment lengths were observed.

Gündogar et al. () examined size #25 One Shape, Hyflex EDM, Wave One Gold and Reciproc Blue files at 60º curvature and a 5mm curvature radius. Hyflex EDM obtained significantly better resistance and One Shape was significantly the worst system; they found that the new alloys were better than the conventional NiTi. As regards separated fragment lengths, they found no significant differences. They concluded that Hyflex EDM showed the highest resistance to cyclic fatigue fracture. The results of the study by Gündogar et al., are similar to those obtained in the present study: the Hyflex EDM was better than Wave One Gold and One Shape, determining that CM-Wire alloy with memory control improved the results of Gold-Wire and conventional NiTi. However, significant differences were observed in separated fragment lengths.

Conclusions

The systems with CM-Wire (Hyflex EDM and Neoniti) were superior to the other systems for cyclic fatigue. For separated fragment lengths, F360 (conventional NiTi) and Reciproc (M-Wire), the lengths were longer.