Shaping ability of modern Nickel–Titanium rotary systems on the preparation of printed mandibular molars

Abstract

Introduction:

This study aimed to evaluate the shaping ability of TruNatomy (Dentsply Maillefer, Ballaigues, Switzerland), VDW.ROTATE (VDW GmbH, Munich, Germany) and ProTaper Gold (Dentsply Maillefer, Ballaigues, Switzerland) during the preparation of resin-printed mandibular molar mesial root canals.

Materials and Methods:

Thirty-three printed resin-based mandibular mesial roots with two canals were obtained from extract tooth cone-beam computed tomography (CBCT) image. The printed teeth were divided into three groups (n = 11) according to the system used for root canal preparation: TruNatomy, VDW.ROTATE, and ProTaper Gold. The specimens were scanned using CBCT imaging before and after root canal preparation. Then images were registered using a dedicated software and changes in the canal area, volume, untouched canal surface, and the maximum and minimum dentin wall wear were calculated.

Statistical Analysis Used:

Data were statistically analyzed using Shapiro–Wilk for normality, one-way ANOVA, and Tukey or Kruskal–Wallis H tests with alpha set at 5%.

Results:

No differences were observed for changes in the canal area, volume, untouched canal surface area, and minimum dentine wall wear parameters for the whole canal length (P > 0.05). The mean of untouched canal surface area for the TruNatomy, VDW.ROTATE, and ProTaper Gold was 40%, 44%, and 44%, respectively. The maximum dentine wall wear was significantly lower in the ProTaper Gold group than in the TruNatomy and VDW.ROTATE groups (P < 0.05).

Conclusions:

TruNatomy, VDW.ROTATE, and ProTaper Gold systems showed similar shaping ability in printed resin-based mandibular mesial roots without clinically significant errors. A large amount of untouched canal surface area was observed for all systems.

INTRODUCTION

One of the main goals in successful root canal shaping is to maintain the original shape of the canal configuration.[1] The newly developed Nickel–Titanium (NiTi) rotary instruments lead to minimize iatrogenic errors without changing the original root canal form.[2] However, all instrumentation techniques tend to change canal curvature and pathway. These changes may lead to ledge formation, zipping, or strip perforation. In addition, root canal walls might remain untouched after preparation procedures and if insufficiently cleaned might cause endodontic failures.[3] To overcome these limitations, companies have launched different NiTi systems with different heat-treatments and cross-section designs. Different companies have developed many NiTi systems and are claimed to cause minimal shaping errors. The TruNatomy (Dentsply Maillefer, Ballaigues, Switzerland), VDW.ROTATE (VDW GmbH, Munich, Germany) and ProTaper Gold (Dentsply Maillefer, Ballaigues, Switzerland) are different NiTi systems examples on physical and mechanical properties.

The TruNatomy (Dentsply Maillefer, Ballaigues, Switzerland) rotary system is a set of instruments made of a maximum fluted diameter of 0.8 mm NiTi and proprietary heat treatment. The TruNatomy instruments present an off-centered parallelogram cross-section design and variable taper to provide the benefits of improved performance with increased respect to the tooth anatomy during mechanical preparation.[4] VDW.ROTATE (VDW, Munich, Germany) is made of a special heat-treated “Blue-wire” NiTi alloy. According to the manufacturer, this rotary system has the double-bladed adapted S cross-section design. The design of the instruments and increased flexibility reduces canal transportation and preserve root canal anatomy.[5] ProTaper Gold (Dentsply Maillefer, Ballaigues, Switzerland) has an identical instrument design with a triangular cross-section and a variable progressive taper and was developed with special “Gold-wire” metallurgy.[6]

Extracted human teeth are undoubtedly the best substrate to reproduce clinical conditions in laboratory studies.[7] However, there are some disadvantages in using such substrate, such as ethical considerations, the possibility of cross-infection risk, and the difficulty of obtaining a good number collection to produce anatomically balanced experimental groups that effectively isolate the variable of interest.[7] With the use of cone-beam computed tomography (CBCT) data from an extracted tooth associated to three-dimensional (3D) printing technology, a physical model of a tooth with the same external and internal morphology could be obtained. Within such technology it is possible to standardize samples regarding canal anatomy and dimensions.[8] Thus, 3D-printed resin teeth can be used to evaluate the shaping ability of NiTi file systems and reduce the drawbacks that may result from the use of extracted teeth in ex vivo studies.[7]

While several studies evaluated the shaping ability of ProTaper Gold[6,9,10,11] and TruNatomy,[12,13] to the best of the authors' knowledge, little is known regarding the shaping ability of VDW.ROTATE. The present study aimed to evaluate the shaping ability of ProTaper Gold, TruNatomy, and VDW.ROTATE during the preparation of resin-printed mandibular mesial root canals. The null hypothesis tested was that there would be no significant difference in root canal preparation among these three NiTi rotary file systems.

MATERIALS AND METHODS

Sample size calculation

A power calculation was performed using G*Power 3.1 (Heinrich Heine University, Dusseldorf, Germany) software with α = 0.05 and ß = 0.96. The calculation indicated that the sample size for each group should be a minimum of 11 roots.[14] Therefore, 11 mesial roots of mandibular molars (22 mesial canals) were included for each group.

Sample preparation

One human mandibular molar was selected after the Local Ethics Committee approval (No: 2011-KAEK-2/2020/8). After decoronation and resection of the distal root, a size #10 K-file (VDW GmbH, Munich, Germany) was inserted into the canal until the instrument's tip was just visible at the apical foramen. The root had uncalcified, type IV Vertucci canals (2 separated canals).[15] The working length (WL) was established 1 mm shorter than that point. After determination of WL, the root canals were manually prepared with hand files up to size 20,[16] rinsed with distilled water and dried with papers points. The root was scanned using a CBCT (GXDP-700, Gendex Dental Systems, Hatfield, USA) with the following parameters: 90 kVp, 13 mA, exposure time 12 s, field of view 6 × 8 cm.

The CBCT data (DICOM file) was segmented with the “Tooth Segmentation” option in RealGUIDE 5.0 software (3D iemme Software Corp, Lombardia, Italy) and turned into a 3D model. The acquired CBCT scan data were converted into STL files to use the 3D printer (Ackuretta FreeShape 120, Ackuretta Technologies, Taipei, Taiwan). 12.5% of barium sulfate powder was mixed with resin (KeyVest, Keystone Ind., Gibbstown, NJ, USA) to achieve radiopacity. After being 3D printed, the roots were cleaned with alcohol for 5 min in an ultrasonic bath (CLEANI, Ackuretta Technologies, Taipei, Taiwan) and cured for 3 min in the postcuring oven (UV CURIE; Ackuretta Technologies, Taipei, Taiwan) to react the remaining monomers. For shaping, the #15 K-file was used to control apical patency.

Root canal preparation

The roots were inserted into the CBCT by designing cylindrical bases that are precisely compatible with the produced models. Due to the cascading design, the models were not confused during CBCT [Figure 1]. The apical foramen was blocked using modeling wax. All instrumentation was performed by a single-experienced endodontist (S.F).

Figure 1.

Images of cylindrical base designed for CBCT imaging with printed roots. CBCT: Cone-beam computed tomography

The instruments were used according to the manufacturers' instructions for each system. A new instrument was used for four root canal preparations. All instruments were operated using an endomotor (VDW Gold, VDW, Munich, Germany). A total of 20 mL of distilled water was used for irrigation with 30-G IrriFlex needle (Produits Dentaires SA, Switzerland) taken up to 2 mm short of the WL between the use of each instrument in all groups. Apical patency with a size #15 K-file was also performed between the use of each instrument.

• TruNatomy Group: TruNatomy set used Orifice Modifier (20, 0.08 v) followed by the Glider (17, 0.02 v) and Prime (26, 0.04 v) instruments until the WL at 500 rpm and 1.5 N. cm torque

• VDW.ROTATE Group: Root canals were prepared with Rotate 20.05 and 25.06 files taken up to the WL. Instruments were used at 300 rpm and 2 N. cm torque

• ProTaper Gold Group: Root canals were prepared with ProTaper Gold S2 (20.04), F1 (20.07), and F2 (25.08) taken up to the WL. Instruments were used at 300 rpm and 2 N. cm torque.

Three-dimensional modeling and evaluation

After root canal preparation, the root canals were dried with absorbent paper points and the teeth were submitted to a new CBCT scanning. Then, DICOM images were rendered 3D in RealGuide software again. [Figure 2] shows the real mandibular molar mesial root and its 3D-printed replica with CBCT images. Finally, the 3D rendered models were overlaid with the STL file of the unprepared model in Rapidform software (INUS Technology, Inc., Korea). After superpose, only the 3D version of the canals was obtained, and the differences between root canals before and after preparation were evaluated [Figure 3]. The changing canal area (Δ canal area), changing canal volume (Δ canal volume), untouched canal surface area, and maximum–minimum dentine wall wear parameters were calculated with Rapidform software by an experienced operator [Figure 4]. According to the analysis, the distances between corresponding areas of the before and after preparation models were computed and complemented by visualization of the 3D color-coded maps in which the blue to red fields indicated that the after preparation of the model was wider than the prepreparation model and blue-to-red fields indicated that maximum–minimum dentine wall wear. The tolerance of range was set at ± 0.50 mm. In addition, the deviation analysis was carried out, and the percentages (%) of untouched canal surface area within the tolerance range were calculated. Volumetric assessment of root canals was also calculated for each tooth investigated. These values represented the degree of matching between the pairs of before and after preparation models.

Figure 2.

(a) Image of the chosen mandibular mesial root, (b) image of the 3D-printed root, CBCT images of (c and d) respectively. CBCT: Cone-beam computed tomography, 3D: Three-dimensional

Figure 3.

Root canal preparation analysis in the superposition of the pre- and post-preparation of the root canals. The colored map shows the dentine wall wear (minimum blue, maximum red)

Figure 4.

Software superpose images of root canals before and after preparation; VDW.ROTATE, ProTaper Gold, TruNatomy

Statistical analysis

The normality of the data was tested with Shapiro–Wilk. Mean and standard deviations were calculated for each group. The Δ canal area, Δ canal volume and untouched canal surface area were compared using one-way ANOVA. Homogeneity of variances was examined by Levene's Test. Maximum and minimum dentine wall wear were compared using Kruskal–Wallis test. The level of significance was set at P < 0.05.

RESULTS

Table 1 shows the mean and standard deviations regarding the parameters in each group. Considering the initial Δ canal area, Δ canal volume and minimum dentine wall wear after each preparation protocol, no statistically significant difference was observed in all groups (P > 0.05). For the percentage of the untouched canal surface area was observed no statistically significant difference (P > 0.05) with the TruNatomy (40% ± 3%) followed by the VDW.ROTATE (44% ± 7%) and the ProTaper Gold (44% ± 6%). Moreover, maximum dentine wall wear was significantly lower in the ProTaper Gold group than in the TruNatomy and VDW.ROTATE groups (P < 0.05).

Table 1.

Mean and standard deviations of canal area, canal volume, maximum dentine wall wear, and untouched canal surface; and median, minimum and maximum values of minimum dentine wall wear in the different experimental groups

Parameters	ProTaper Gold	TruNatomy	VDW.ROTATE
Canal area (mm2)
Initial	51.52	51.52	51.52
After instrumentation	105.37±10.85	115.8±9.85	112.13±9.33
Δ∞	53.85±10.85	64.28±9.85	60.61±9.33
Canal volume (mm3)
Initial	13.63	13.63	13.63
After instrumentation	32.36±5.94	33.96±2.84	32.19±3.55
Δ#	18.73±5.94	20.33±2.84	18.56±3.55
Dentin wall wear (mm)
Minimum*	0.0004 (0–0.001)	0.0003 (0–0.0006)	0.0003 (0–0.002)
Maximum*	0.258 (0.254–0.266)a	0.265 (0.260–0.274)b	0.265 (0.245–0.273)b
Untouched canal surface (%)∞	44±6	40±3	44±7

∞One-way ANOVA, ^#ANOVA (Welch), *Kruskal–Wallis H-test, ΔThe changing amount of parameter, Different superscript letters in a row indicate statistical significance (P<0.05)

DISCUSSION

In this study, TruNatomy, VDW.ROTATE, and ProTaper Gold were chosen for evaluating the effects on root canal geometry using 3D-printed resin teeth CBCT images. Despite dissimilarities in these systems, preparation revealed no differences in the Δ canal area, Δ canal volume, and minimum dentine wall wear, and untouched canal surface area. However, maximum dentine wall wear was significantly lower in the ProTaper Gold group. Therefore, the null hypothesis was rejected. The results with the VDW.ROTATE system cannot be compared with others because, currently, similar studies are not available.

In experimental groups, apical preparation sizes were #25 (VDW.ROTATE, ProTaper Gold) and #26 (TruNatomy). In a previous study, the #25 apical size effect of reducing bacteria was not significantly different from the other groups with greater apical sizes.[17] In addition, as the apical size increases, the flexibility of the file decreases, and the risk of shaping errors increases.[18] Furthermore, the apical taper of VDW.ROTATE, ProTaper Gold, and TruNatomy were 0.06, 0.08, and 0.04, respectively. No significant differences in all parameters except maximum dentine wall wear were observed between groups despite these differences (P > 0.05). These file systems, made of heat-treated NiTi alloy, may explain these results. Special heat-treated NiTi alloys are mostly available in the martensitic phase and increase instrument flexibility.[19] Due to special heat-treated alloy, TruNatomy had a similar shaping ability, with regressive tapers and a slim structure, to remove dentin with VDW.ROTATE and ProTaper Gold.

The NiTi instruments with different metallurgical properties and geometric designs compared in this study have left a relatively high mean percentage of untouched canal surface area. Untouched areas are crucial for post-treatment apical periodontitis because of harbor residual bacterial biofilms. Therefore, chemomechanical preparation acts mechanically and chemically on bacterial communities colonizing the root canal.[20,21] The mean range of the untouched canal surface areas of ProTaper Gold was similar compared with the previous micro-CT study.[11] However, the untouched canal areas of TruNatomy were lower than the previous report,[12] probably because of the different methodology of studies. No difference was observed among the experimental groups in this study; we think it is possible because of the off-centered cross-sectional design of the TruNatomy, the double-bladed adapted S cross-section design of VDW.ROTATE, and triangular cross-section of ProTaper Gold. All systems have different cross-section designs, but none of them could touch the total surface of the canal walls. However, the cross-sectional design of the TruNatomy and VDW.ROTATE may affect maximum dentine wall wear. In addition, the off-centered and double-bladed adapted S cross-section designs give more space for dentine removal than the triangular cross-section design of ProTaper Gold. Although the assessment of centering ability and transportation was not on the focus of the current study, no gross error was verified during the sample's evaluation. This leads us to believe that none of the tested systems was able to abruptly change the trajectory of the root canals. Even so, future studies should be performed evaluating such parameters for the tested systems.

In the present study, lower molars were due to the anatomy of the mesial root canals and the concave and convex irregularities in the canal surface.[22] To provide standardization, a resin model was obtained from a CBCT scan image obtained from natural human teeth. Thus, the length of the root canal, its apical diameter, and thickness could be standardized. Furthermore, the standardization of canal morphology in each sample increases validity and eliminates potential biases that could confuse between-group results. Reymus et al.[8] reported that the significant benefits of 3D-printed teeth are good standardization of tooth and canal shape and dimensions before preparation, especially those with complex and rare anatomy way.[7]

Since the resolution of the resin layers used in the 3D printer is between 16 and 32 μm, the initial diameter of the root should not be less than ISO 15 size.[23] Therefore, in the present study root canals were prepared with a size 20 K-file before the CBCT scan. There are discussions in the literature about the difference in radiopacity and hardness between resin and human dentine.[23,24] However, Reymus et al. reported that none commercially available ones could mimic human dentine in hardness and radiopacity.

CONCLUSIONS

Under the conditions of this study, there were no significant differences between TruNatomy, VDW.ROTATE, and ProTaper Gold in all the parameters except maximum dentine wall wear. The maximum dentine wall wear was significantly lower in the ProTaper Gold group. TruNatomy touched the highest percentage of root canal surface, but it is not a statistically significant difference. All the files used were able to clean and shape 3D-printed resin mandibular mesial roots.

Financial support and sponsorship

This study was supported by Afyonkarahisar Health Sciences University Scientific Research Projects Coordination Unit under grant number 20.GENEL.023.

Conflicts of interest

There are no conflicts of interest.