A Comparative evaluation of the shaping ability, canal straightening, and preparation time of five different NiTi rotary files in simulated canals.

AIM: The aim is to compare the shaping ability, canal straightening, and the preparation time of five different nickel-titanium rotary files in simulated J-shaped canals.
MATERIALS AND METHODS: Ninety J-shaped canals in resin blocks were filled with 2% Methylene Blue solution and pre-instrumentation images were taken using a Leica microscope at a ×10. They were prepared until size 25 taper 0.04 using (n = 18 per group): T-Flex, HyFlex CM, Vortex Blue, S5, and iRace. After instrumentation, images were captured again, and composite images were made using Adobe Photoshop imaging software. The differences in canal width and canal curvature at each respective landmark were measured and compared. The preparation time and canal abbreviations were also recorded. Statistical analyses were performed using one-way ANOVA and post hoc Tukey HSD tests. The level of statistical significance was set to P = 0.05.
RESULTS: HyFlex CM demonstrated the least difference in canal width after instrumentation, but no significant difference (P > 0.05) as compared to T-Flex and Vortex Blue. The mean canal straightening ranged between 0.91° and 7.65°. T-Flex created the least canal straightening after instrumentation which was significantly less (P < 0.05) than S5, but there was no significant difference (P > 0.05) when compared to HyFlex CM. Instrumentation with the S5 file was significantly faster (P < 0.05), whereas HyFlex CM was the slowest.
CONCLUSION: T-Flex, HyFlex CM, and Vortex Blue demonstrated better shaping ability, whilst T-Flex and HyFlex CM maintained the original canal curvatures well. S5 tended to straighten the canals and caused the greatest canal transportation, but it required the least amount of time to shape the canal. Copyright:

INTRODUCTION

Thorough cleaning and shaping, followed by three-dimensional obturation of the intricate root canal system are essential in endodontic treatment.[1] A well-prepared canal should follow the original curvature and maintain the apical constriction without causing apical widening.[2] Previous studies suggested that a greater amount of debris and bacterial contamination can be removed by enlarging the canal at the apical third region to allow the irrigating solutions to reach all surfaces.[34] Nonetheless, such a concept is still not widely accepted as clinicians are concerned about the iatrogenic damage to the apex during canal preparation. Thus, achieving a balance between adequate cleaning and shaping, and maintaining the curvature and shape of the canal is challenging.

The introduction of nickel-titanium (NiTi) rotary instruments has changed the perspective on how root canal shaping is performed.[1] The use of automated NiTi rotary instruments has made endodontic procedures less fatiguing and time-saving. Several NiTi rotary systems have been introduced into the market ever since their first appearance in the early 1990s.[5] The first NiTi rotary instrument was introduced by Dr. John McSpadden back in 1992 with an ISO-standard of 2% taper.[6] S5 rotary files, manufactured by Sendoline, Taby, Sweden, is one of the NiTi rotary systems that made of conventional NiTi alloy with S-shaped cross-section.[7] In 1999, electropolishing surface treatment on the rotary file was introduced by FKG, La Chaux-de-Fonds, Switzerland.[6] These Race systems with this electrochemical surface treatment have shown to improve fatigue resistance.[8] An improved version of the Race system, iRace, was introduced in 2011 which had the same surface treatment and design as RaCe.[9]

Later in 2007, M-wire was introduced which significantly improved the fatigue-resistant since it contained both the martensite and R phases. Subsequently, in 2010, controlled memory, CM-Wire with thermal treatment technology was introduced which yielded greater fatigue resistance.[10] HyFlex CM manufactured by Coltene, Altstätten, Switzerland, is one of the NiTi rotary systems that utilized such technology thus providing better flexibility and resistance to cyclic fatigue.[11] Although HyFlex CM files undergo plastic deformation during their manufacturing process, they tend to return to their initial condition after autoclaving.[12] In 2012, the heat-treated NiTi CM alloy file, Vortex Blue, was introduced by Dentsply Sirona, Pennsylvania, United States.[6] The file claimed to have improved flexibility, better cutting efficacy, and reduced shape memory while maintaining the curvature of the canal owing to the titanium oxide layer on its surface.[13]

T-Flex, a new heat-treated blue file system manufactured using CM-Wire was introduced by Shenzhen Perfect Medical Instruments, China, in early 2020. It was made up of high purity black NiTi alloy with low content of carbon and oxygen impurities. Furthermore, it has a rectangular-shaped cross-section, along with 4 cutting edges. Currently, the data from studies regarding the shaping ability of the mentioned file systems varies.[791415] To the best of our knowledge, literature regarding the shaping ability, degree of canal straightening, and the preparation time of this new T-Flex file system is unavailable.

Hence, this study aims to compare the shaping ability, degree of canal straightening, and the preparation time of five different rotary file systems: T-Flex, HyFlex CM, Vortex Blue, S5, and iRace in simulated canals. The null hypothesis tested was that there is no significant difference in the shaping ability, degree of canal straightening, and preparation time between the various tested rotary NiTi systems in simulated canals.

MATERIALS AND METHODS

Sample size calculation

The sample size was calculated based on the Bivariate Normal Distribution Model and Correlation (G*Power 3.1.9 for Windows, Franz Faul, Universität Kiel, Germany.) with an effect size of 0.5, α = 0.05, β = 0.95, and H0= 0 keyed into an F-test family for the analysis. The final sample size was 90 samples.

Canal instrumentation

A total of 90 J-shaped simulated canals with 30° curvature, 0.15-mm apical diameter and 16-mm working length (WL) in clear resin blocks (Endo Training-Block, 0.02 Taper, Dentsply-Maillefer, US) were used. Each resin block was numbered accordingly and filled with 2% Methylene Blue dye solution (MKCD3437, Sigma Aldrich, USA). Five landmarks (L1 to L5) were identified on the surface of each resin block [Figure 1a].[14]

Figure 1

(a) Five landmarks, from L1 to L5, were identified on the surface of each resin block. (b) Resin block was placed on a clamp during canal instrumentation

L1: Canal orifice

L2: Midpoint between L1 and L2

L3: The starting point of the curvature

L4: Apex of the curvature

L5: Endpoint of the canal preparation.

The resin blocks were viewed under a Leica microscope (Leica Microsystem Imaging Solutions, Cambridge, UK) at a standardized fixed distance and photographed at a magnification of ×10. The pre-instrumentation digital images were saved as JPEG files. The canal patency was confirmed for each resin block using ISO standard size 15 K-file (SybronEndo, Kerr Corporation, CA, US) to the WL (16 mm). They were then randomly assigned to five different groups (n = 18) and prepared with the crown-down technique using five different NiTi rotary file systems: T-Flex (Shenzhen Perfect Medical Instruments Co. Ltd., Guangdong, China), HyFlex CM (Coltene, Altstätten, Switzerland), Vortex Blue (Dentsply Sirona, Pennsylvania, United States), S5 (Sendoline, Täby, Sweden) and iRace (FKG Dentaire, Chaux-de-Fonds, Switzerland). All specimens were placed on a clamp [Figure 1b] to stabilize the resin block during instrumentation. All specimens were prepared by a single experienced operator according to the instrumentation sequence and recommended speed and torque values as described in Table 1 to achieve the final apical preparation of size 25, taper 0.04. Copious irrigation with 5 ml of normal saline solution (Rinscap NS, Malaysia) was performed after each file usage.

Table 1

Types of rotary file used and the sequences of instrumentation

File	Sequence	Taper	Size	Length	Speed (rpm)	Torque (Ncm)
T-Flex	No. 1	0.10	20	1/3 of WL	400	2.5
	No. 2	0.04	15	Full WL
	No. 3	0.04	20	Full WL
	No. 4	0.04	25	Full WL
HyFlex CM	No. 1	0.08	25	Half of WL	400	2.5
	No. 2	0.04	20	Full WL
	No. 3	0.04	25	Full WL
Vortex Blue	No. 1	0.04	15	Full WL	500	1.0
	No. 2	0.04	20	Full WL
	No. 3	0.04	25	Full WL
S5	No. 1	0.08	30	Half of WL	300	4.0
	No. 2	0.06	30	2/3 of WL
	No. 3	0.04	30	2/3 of WL
	No. 4	0.04	25	Full WL
iRace	No. 1	0.06	15	Full WL	600	1.5
	No. 2	0.04	25	Full WL

WL: Working length

Canal preparation assessment

All specimens were repositioned in the same slot after instrumentation and photographed to obtain post-instrumentation images as described previously. The pre- and post-instrumentation images were superimposed [Figure 2] using imaging software (Adobe Photoshop; Adobe System, San Jose, CA) with 50% of transparency and a central axis line was drawn for each superimposed image. The canal widths and canal curvatures before and after instrumentation were measured at five landmarks (L1 to L5) using Leica Application Suite X (LAS X) microscope imaging software. The measurements were recorded by two blinded examiners.

Figure 2

Representative images of simulated canals instrumented by using (a) T-Flex, (b). HyFlex CM, (c) Vortex Blue, (d) S5, and (e) iRace

Preparation time

The time for canal preparation, i.e., total active instrumentation was recorded to the nearest second using a digital stopwatch. The time taken for glide path creation, irrigation, cleaning, and changing instruments within the sequence was not included.

Canal aberrations

The presence of canal aberrations such as danger zone, zipping, ledges, perforation, and intracanal file separation was recorded.[14]

Statistical analysis

Statistical analyses were performed using SPSS version 24 (IBM SPSS Inc, Chicago, IL, USA) software. The Shapiro–Wilk test revealed that the data were normally distributed and analyzed using one-way ANOVA and post hoc Tukey HSD tests. The level of statistical significance was set to P = 0.05.

RESULTS

In Table 2, HyFlex CM showed the smallest difference in canal width (P < 0.05), followed by T-Flex, Vortex Blue, iRace and finally S5, but no significant difference (P > 0.05) was noted among T-Flex, HyFlex CM, and Vortex Blue. The mean canal straightening ranged between 0.91° and 7.65° of which T-Flex created the least canal straightening and S5 caused significantly (P < 0.05) greater canal straightening. shows the canal width differences at different landmarks after instrumentation. A marked difference in canal width at level L3 was found between pre-and post-instrumentation images for all files except S5 which was at level L4.

Table 2

Mean canal width differences (mm), degree of canal straightening (°) and preparation time (s) of different rotary files

Rotary file	Canal width difference (mm)					Canal straightening (º)	Preparation time (s)
	L1	L2	L3	L4	L5
T-Flex	0.353 (±0.018)a	0.331 (±0.015)b	0.354 (±0.017)c	0.274 (±0.023)d	0.159 (±0.026)e	0.91 (0.28)f	108 (±21)
HyFlex CM	0.349 (±0.016)a	0.331 (±0.015)b	0.352 (±0.016)c	0.274 (±0.015)d	0.158 (±0.016)e	0.95 (0.22)f	143 (±17)
Vortex Blue	0.353 (±0.014)a	0.333 (±0.019)b	0.377 (±0.009)c	0.285 (±0.016)d	0.161 (±0.015)e	1.31 (0.37)	117 (±14)
S5	0.426 (±0.015)	0.418 (±0.018)	0.746 (±0.022)	0.896 (±0.024)	0.523 (±0.019)	7.65 (1.01)	71 (±11)
iRace	0.389 (±0.019)	0.366 (±0.016)	0.575 (±0.025)	0.424 (±0.011)	0.178 (±0.019)	2.85 (0.54)	92 (±24)
P	0.003*					0.001*	0.001*

*Significant at 0.05. Same superscript lowercase letters within column indicate no statistical difference (P>0.05)

S5 prepared the simulated canals significantly faster (P < 0.05), whereas HyFlex CM took the longest (P < 0.05). None of the NiTi rotary files experienced any fracture during instrumentation. However, danger zones in 2 samples and ledges in 3 samples were observed in the S5 group, whereas 1 sample showed a danger zone in the iRace group. Loss of WL was noted in 3 canals prepared with S5 wherein the mean was 3 mm. Otherwise, no other aberrations were noted, and all simulated canals remained patent after instrumentation.

DISCUSSION

Curvature deviation and canal transportation are considered undesirable iatrogenic accidents that can occur during canal preparation.[16] Poorly centered instruments will result in unequal dentine removal and increase the risk of canal transportation.[9] Furthermore, irregular canal preparations will promote the growth of bacteria, favor infected debris accumulation and prevent adequate obturation of the root canal system.[917] For better comparison and standardization, size 25 with 0.04 taper was selected to be the final apical preparation as larger apical preparation with greater taper would result in greater canal straightening due to reduced file flexibility and possibly weaken the root dentine.[18]

The results of the present study suggested that HyFlex CM can create minimal canal transportation which is corroborated by previous studies.[1517] HyFlex CM instrument was reported to have the ability to prepare curved canals without causing any significant shaping error or instrument fracture during instrumentation.[1517] This can be attributed to the high flexibility of HyFlex CM as it is manufactured by a unique thermal pre-treatment technology that controls the file's memory.[1116] This also reduces the magnitude of the restoring force by changing their spiral shape during canal preparation, thereby allowing the CM wire to follow the canal curvature and preventing unnecessary iatrogenic errors.[91119] However, the result of the present study is contrary to a previous study done by Saber et al.,[9] which found that iRace had better shaping efficacy with minimal transportation as compared to HyFlex CM though no significant difference was noted.

On the other hand, T-Flex was found to exhibit comparable shaping ability with HyFlex CM probably due to similar file properties with respect to controlled-memory technology. However, a literature search could not identify any other study which compared the shaping ability of T-Flex. A greater amount of resin was removed at L1 using T-Flex as compared to HyFlex CM and this could be explained by the larger taper of the T-Flex coronal flare file. Furthermore, the results of the present study showed that Vortex Blue had slightly inferior but comparable shaping ability with T-Flex and HyFlex CM, in line with a previous study done by Bansal et al.[20] This can be explained by their similar manufacturing process. Vortex Blue is subject to proprietary postsmachining heat treatment and coated with titanium oxide layers on its surface that give the instrument its flexibility, excellent fatigue resistance, and low Vickers surface hardness.[61321] S5 revealed the poorest shaping ability and created more canal transportation, but the result obtained is not consistent with other file systems in previous similar studies.[722]

The alloy microstructure of the NiTi rotary files is one of the aspects that need to be considered when performing such a study.[521] The three main phases of alloy microstructure in the NiTi system are austenite, martensite, and R-phase. Unlike S5 and iRace systems, the microstructures of T-Flex, HyFlex CM, and Vortex Blue mostly consist of martensite, thus they exhibit less spring-back effect and prevent the straightening of the canal curvature.[621] This explains the reason T-Flex, HyFlex CM, and Vortex Blue performed better resulting in lesser canal straightening with minimal canal transportation. Other factors that can affect the shaping ability of NiTi rotary files include the cross-sectional geometry and file taper.

T-Flex was found to create the least degree of canal straightening probably due to its small rectangular-shaped cross-sectional area which makes it more flexible and thus, reducing the tendency to straighten curved simulated canals. S5 has an S shaped cross-sectional design with two cutting edges and a previous study found it resulted in a more aggressive cutting action while enlarging canals.[23] S-shaped cross-section was also found to increase the “screw-in” tendency and caused iatrogenic errors such as the loss of apical stop and extremely large canal transportation which could be the reason 3 canals instrumented using S5 experienced a loss in WL.[2123]

In the present study, the preparation time included only active instrumentation since it is difficult to standardize the time taken for irrigation, cleaning, and changing instruments. S5 was significantly faster in shaping the canal, which could probably be due to the S-shaped cross-section and the austenitic phase of the S5 file which allowed it to have increased cutting efficacy.[2123] In general, all NiTi rotary file systems used in the present study were able to prepare the canals in <3 min which is considered relatively fast. Therefore, the time difference between each file system may be of subordinate importance from a clinical point of view. Ledge and danger zone were mostly found in simulated canals instrumented with S5. Although S5 and iRace consist of conventional NiTi alloys, only one danger zone was noted in a canal instrumented by iRace which could be due to the increased fatigue resistance of iRace after surface electropolishing.[24]

CONCLUSION

Within the parameters of this study, T-Flex, HyFlex CM and Vortex Blue files demonstrated better shaping ability, while T-Flex and HyFlex CM maintained the original canal curvatures well. Although S5 required the least amount of time to shape the canal, it showed a marked tendency to straighten the canal and caused a greater degree of canal transportation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.