A comparative evaluation of remaining dentin thickness following biomechanical preparation of teeth using different rotary file systems: An in vitro study.

Aim: The aim of the study is to compare and evaluate the remaining dentin thickness following biomechanical preparation of teeth using different rotary file systems. Materials and Methodology: Sixty noncarious mandibular premolar teeth were collected and decoronated at the level of cementoenamel junction with a diamond disc. All specimens were randomly divided into 5 experimental groups - ProTaper Next (Dentsply Mallifer), Mtwo (VDW, Antaeus, Munich, Germany), RaCe (FKG, La Chaux-de-Fonds, Switzerland), Hyflex electro-discharge machining (EDM) (Coltene-Whaledent, Allstetten, Switzerland), NeoNiTi (Neolix, France) and 1 control group of 10 teeth each. After mounting the samples on a modeling wax sheet, preoperative cone-beam computed tomography (CBCT) scans were taken. Biomechanical preparation of canals was done following the assigned protocol of manufacturers. Postoperative CBCT scans were taken and comparison was carried out with preoperative scans. Statistical Analysis Used: Difference among the groups was analyzed by post hoc Turkey and analysis of variance tests. A P < 0.05 was considered statistically significant for all tests.
Results: A comparison of preinstrumentation and postinstrumentation CBCT images revealed ProTaper Next group to remove more dentin at 7 mm as opposed to other groups in mesiodistal direction. However, no statistical difference was evident between ProTaper Next, MTwo, Race, Hyflex EDM, NeoNiTi file systems at 3 mm, 5 mm, and 7 mm in buccolingual direction. A statistically nonsignificant difference was evident between MTwo, Race, Hyflex EDM, NeoNiTi file systems at 3 mm, 5 mm, and 7 mm in both mesiodistal and buccolingual direction.
Conclusion: Race file system performed better and removed lesser dentin in both buccolingual and mesiodistal directions. More dentin was removed at the coronal in mesiodistal direction with the use of ProTaper Next, and significant difference was seen between Protaper Next group and other groups in the study. Copyright:

INTRODUCTION

Endodontic success depends on various factors such as canal preparation, disinfection, and three-dimensional obturation. Cleaning and shaping of root canals is the most important step for endodontic success.[1] Remaining dentin thickness following instrumentation directly influences the fracture resistance of root.[2] It is during root canal instrumentation that majority of dentin removal occurs. Furthermore, researchers have found that most of the dentin removal occurs in mesial and distal direction.[3] Preservation of residual dentin thickness during instrumentation is an essential requirement for strength and longevity of an endodontically treated tooth. Researches concluded a minimum of 0.3 mm of remaining dentin should be present after root canal preparation for adequate resistance against lateral forces.[4] Conventionally, endodontic instrumentation was done by hand files. However, the initial carbon steel files were susceptible to fracture, tarnish, and corrosion, leading to the use of stainless steel files, which nevertheless had their own problems such as breakage, inflexibility, and taper. To overcome these limitations, Nickel Titanium (NiTi) instruments were introduced. However, it is indeed a big challenge for every file system to get an adequate taper of the canal without cutting excessive dentin.[5] Hence the selection of file system plays an important and a very crucial role.

Therefore, the present study was conducted to study the cutting efficiency of five different file systems in relation to the remaining dentin thickness. The file systems used in the present study were ProTaper Next, Mtwo, RaCe, Hyflex electro-discharge machining (EDM), NeoNiTi.

MATERIALS AND METHOD

A total of 60 noncarious extracted human mandibular premolar teeth were collected and sterilized in 5.25% sodium hypochlorite (NaOCl) (Septodont, India) for 30 min and stored in 0.9% normal saline (Nirlife Nirma Limited, India). Patency of the canals was confirmed with preoperative radiographs. Decoronation of teeth at the level of cemento-enamel junction (CEJ) was done with a diamond disc (Joedandy disc, Durodent dental supplies, Australia), using slow speed straight hand piece (NSK EX-6B, Nakanishin Inc., Japan) and all the specimens were randomly divided into 5 experimental and 1 control group of 10 specimens each. Glide path was established with no. 10 K file (Mani Inc, Japan) and a no. 15 K file was used to take the working length (WL) of the canal subtracting 0.5 mm from this length. Samples in each group were mounted on a modeling wax sheet with small lead paper on one end to act as indicator. Preinstrumentation cone-beam computed tomography (CBCT) scan was taken for all the samples which served as a baseline against which remaining dentin thickness following biomechanical preparation by various file systems would be calculated.

In Group I, root canals were prepared with ProTaper Next file system using a gentle in-and-out motion at 300 rpm and 2-Ncm torque with a torque-controlled endodontic motor. The first SX file was used at one half of the WL and the X1 (17/.04), X2 (25/.06) were used at full WL.

In Group II, Mtwo file system was used with a torque controlled endodontic motor used at 280 rpm. The following files were used: 10/.04 with 1.2 torque, 15/.05 with 1.4 torque, 20/.06 with 2 torque, 25/.06 with 2.2 torque.

In Group III, Xtreme RaCe file system was used with only one rotary file #25/0.06 with a torque-controlled endodontic motor. RaCe file system was used at 300–600 rpm in crown down operative sequence using a short and delicate pressure with push-pull movement.

In Group IV, Hyflex EDM file system was used at 500 rpm and torque of 2.4Ncm with a torque-controlled endodontic motor. The following files were used: 25/.12, 10/.05, 25/~ files.

In Group V, NeoNiTi file system was used at 300–500 rpm and torque of 1.4Ncm with a torque-controlled endodontic motor. Single file, A1 #25/0.06 was used.

In all the groups, Glyde (Dentsply Maillefer, Switzerland) was used with each file and irrigation was performed with 3% Sodium hypochlorite between every file.

For the control group, the teeth were left unprepared.

After biomechanical preparation, postinstrumentation CBCT scan was obtained and compared with preinstrumentation CBCT scan at three different levels – 3 mm, 5 mm, 7 mm from the apex and evaluated for remaining dentin thickness following root canal preparation.

Statistical analysis

Descriptive and comparative statistics were performed. Difference among the groups was analyzed by post hoc Turkey and analysis of variance tests. A P < 0.05 was considered statistically significant for all tests. Variables were expressed as means ± standard deviation.

RESULTS

This study evaluated the working efficiency of 5 file systems: Protaper Next, MTwo, Race, Hyflex EDM, and NeoNiTi.

The mean values of removed dentin at each level in mesiodistal and buccolingual directions are shown in Tables 1, 2 and Figures 1, 2, respectively.

Table 1

Comparison of mean removed dentin in mesiodistal dimension in six groups

Mean	3 mm	5 mm	7 mm
Group 1 ProTaper next	0.16	0.135	0.38
Group 2 MTwo	0.165	0.195	0.21
Group 3 race	0.155	0.145	0.145
Group 4 Hyflex EDM	0.2	0.19	0.22
Group 5 NeoNiTi	0.195	0.21	0.19
Group 6 control	0	0	0

EDM: Electro-discharge machining

Table 2

Comparison of mean removed dentin in buccolingual dimension in six groups

Mean	3 mm	5 mm	7 mm
Group 1 ProTaper next	0.145	0.1	0.39
Group 2 MTwo	0.13	0.12	0.265
Group 3 race	0.135	0.075	0.055
Group 4 Hyflex EDM	0.185	0.11	0.16
Group 5 NeoNiTi	0.14	0.08	0.058
Group 6 control	0	0	0

EDM: Electro-discharge machining

Figure 1

(a) Preoperative cone-beam computed tomography at 3 mm, (b) Preoperative cone-beam computed tomography at 5 mm, (c) Preoperative cone-beam computed tomography at 7 mm, (d) Postoperative cone-beam computed tomography at 3mm, (e) Postoperative cone-beam computed tomography at 5 mm, (f) Postoperative cone-beam computed tomography at 7 mm

Figure 2

(a) Graph comparing mean removed dentin in mesiodistal dimension in six groups, (b) Graph comparing mean removed dentin in buccolingual dimension in six groups, (c) Diagrammatic representation of preinstrumentation measurement in mesiodistal and buccolingual direction and diagrammatic representation of postinstrumentation measurement in mesiodistal and buccolingual direction

The Protaper Next file system (Group 1) showed the significantly higher mean value of removed dentin at 7 mm and least at 5 mm while Race file system (Group 3) showed least mean value of removed dentin at 3 mm and 7 mm in mesiodistal direction. On the other hand, MTwo, Hyflex EDM, and NeoNiTi file systems had no significant difference among them in mesiodistal direction.

MTwo file system (Group 2) showed the significantly least mean value of removed dentin at 3 mm and Race file system (Group 3) showed the least mean value of removed dentin at 5 mm and 7 mm in buccolingual direction while there was no significant difference seen among other file systems: ProTaper Next, Hyflex EDM, and NeoNiTi.

DISCUSSION

Success of endodontic therapy is based upon triad of thorough root canal debridement, effective disinfection, and three-dimensional obturation of root canal space. Biomechanical preparation of root canal system determines the success of all successive steps. The entire root canal space should have adequate taper to allow effective irrigation and to facilitate obturation but on the other hand, over-enlargement unnecessarily weakens the root structure.[6] Hence, selection of an appropriate instrument for instrumentation is crucial for the outcome of root canal treatment. A study done by Moore, Walter and Parashos 2009 proved that hand instrumentation may not be conservative of apical root dentin when paralleled to rotary NiTi instruments and NiTi instruments prepared the canal to larger apical sizes with minimal iatrogenic errors.[7] Remaining dentin thickness following instrumentation can be the most important iatrogenic factor which is related to the fracture resistance of the tooth.[3] A study done by Raiden et al. 2001 revealed that radiographs often show greater thickness of dentin than what is actually present. In the present study, CBCT was used as it permits observations of the root canal space in three – dimensions (transverse, axial, and tangent) and also allows preinstrumentation and postinstrumentation measurements of the amount of dentin removed following instrumentation.[8] Three areas of root canal system were evaluated: 3 mm, 5 mm, and 7 mm which represent apical, middle, and coronal thirds of root canal, wherein there is a high vulnerability of iatrogenic mishaps.[9]

In the present study, freshly extracted human single-rooted mandibular premolars were chosen as they were readily available. All the teeth were decoronated at CEJ to eliminate any variables in access preparation and facilitate instrumentation. Preinstrumentation CBCT was taken for all the samples. All specimens were prepared up to the same apical diameter but file systems have different designs, file numbers, speed, torque, and metallurgy. Throughout preparation, root canal irrigation was done using 3% NaOCl. It is the simplest solely available root canal irrigant with organic tissue dissolving properties. Smear layer was removed with 17% Ethylenediamine tetra acetic acid (EDTA). Most of the lower premolars have oval-shaped canals having narrow width in mesiodistal direction than buccolingual direction and rotary file systems are round in cross-section. Therefore, most of the dentin is removed in mesial and distal direction.[10]

In the present study, Race file system reported lesser removal of dentin as compared to other groups this may be due to the design of file system. It has a cross-sectional design of triangular shape with two alternating cutting edges and no radial lands which eliminates threading into the canal wall and also ensures efficient evacuation of debris. Instrument surface of RaCe file system has also been modified by electropolishing.

The results show that Protaper Next file system removed more dentin at 7 mm, showed lower mean value of removed dentin and least cutting at the middle level, i.e., 5 mm from apex in mesiodistal direction. The most probable reason for more dentin removal at coronal level could be attributed to the fact that Protaper Next has less taper in the apical than in the coronal level.[11] The reason for superior performance at middle level may be due to the design of file system. ProTaper Next is a novel system designed with the new M-Wire alloy which enhances its flexibility and cyclic fatigue resistance over conventional NiTi.[12]

Venino et al. 2016 revealed that Hyflex EDM performed better in the apical and coronal thirds whereas Protaper Next achieved better results in middle third. The most probable reason for the better performance of Hyflex EDM is that HyFlex EDM rotary instruments are manufactured via EDM process, have controlled memory with increased flexibility and cyclic fatigue resistance.[13]

In the present study, MTwo file system showed higher mean value of removed dentin at middle level in buccolingual direction. The most probable reason for this is that the Mtwo instruments have S-shaped cross-sectional design with two cutting edges and a positive rake angle which enhances the engagement of file edges to canal walls.[14]

NeoNiTi has nonhomothetic rectangular cross section, controlled memory and is manufactured by EDM process same as HyFlex EDM instruments. This process allows sharp cutting edges, produces rough surface, built-in abrasive properties which result in faster preparation of root canal system, progressive flexibility and variable changing profiles. This design could explain that NeoNiTi System showed higher mean value of removed dentin at 5 mm (middle) in mesiodistal direction as compared to other file systems.

Da Silva Limoeiro et al. 2016, Venino et al. 2016, and Rubio et al. 2017 have compared the remaining dentin thickness following instrumentation, however, no study has been attempted to compare remaining dentin thickness in two dimensions – mesiodistal and buccolingual direction.[13141516] Hence, the present study focused on measuring the dentin removed in both mesiodistal and buccolingual direction.

The results of this in vitro study need to be validated by similar in vivo studies with larger sample size. The design features and metallurgy of NiTi endodontic instruments need a continuous improvement to decrease the dentin removed during instrumentation and reduce the number of endodontic failures. Keeping all the limitations of the present study in mind, it was revealed that the design features of some of the rotary files have a negative impact on the remaining dentin thickness but the benefits of these rotary instruments outweigh these detrimental effects.

CONCLUSION

The present study was conducted to compare the remaining dentin thickness following biomechanical preparation of teeth using different rotary file systems. Within the limitations of the study, it was found that different file systems showed variable amount of dentin removal at coronal, middle, and apical level. Race file system showed least mean value of removed dentin as compared to other file systems at apical and coronal level in mesiodistal direction. ProTaper Next file system showed least mean value of removed dentin as compared to other file systems at middle in mesiodistal direction. All file systems used till date have shown different cutting efficiency at three different levels, i.e., at 3 mm, 5 mm, and 7 mm. However, Race file system has shown overall better performance as compared to other file systems.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.