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Year : 2018  |  Volume : 8  |  Issue : 3  |  Page : 183-188

In vitro comparative evaluation of dentinal microcracks formation during root canal preparation by different nickel-titanium file systems

1 Department of Conservative Dentistry, Endodontics & Aesthetic Dentistry, AMCMET Dental College and Hospital, Ahmedabad, Gujarat, India
2 College of Dental Sciences and Research Center, Ahmedabad, Gujarat, India

Date of Web Publication25-Jul-2018

Correspondence Address:
Dr. Nishantkumar R Surti
B/302, Vrajdeep Apartment, Near L.G.Over Bridge, Maninagar, Ahmedabad - 380 008, Gujarat
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/sej.sej_23_17

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Aim: The aim of this study was to evaluate and compare dentinal microcracks formation during root canal preparation by different commercially available nickel-titanium (NiTi) file systems.
Materials and Methods: Eighty-four single-rooted mandibular premolars were selected. All specimens were decoronated and divided into seven groups of 12 each. Teeth were mounted in the acrylic block with simulated periodontal ligaments. Twelve teeth served as a control in which no treatment was performed. Experimental groups were instrumented with Hand NiTi, ProTaper universal, ProTaper Next, Silk, WaveOne, and self-adjusting files (SAF). Roots were then sectioned 3, 6, and 9 mm from the apex, and the cut surface was observed under a stereomicroscope and checked for the presence or absence of dentinal microcracks.
Results: SAF group and Hand NiTi group showed no cracks formation along with control group. ProTaper rotary files showed more number of cracks than ProTaper Next, Silk and WaveOne used in the study. However, no statistically significant difference was found among ProTaper rotary, ProTaper Next, Silk, and WaveOne (P > 0.05).
Conclusion: All rotary files created microcracks in the root dentin at all three levels, whereas the SAF and hand files presented with satisfactory results with no dentinal microcracks.

Keywords: Dentinal microcracks, nickel-titanium files, reciprocation, self-adjusting files

How to cite this article:
Langaliya AK, Kothari AK, Surti NR, Patel AR, Doshi PR, Pandya DJ. In vitro comparative evaluation of dentinal microcracks formation during root canal preparation by different nickel-titanium file systems. Saudi Endod J 2018;8:183-8

How to cite this URL:
Langaliya AK, Kothari AK, Surti NR, Patel AR, Doshi PR, Pandya DJ. In vitro comparative evaluation of dentinal microcracks formation during root canal preparation by different nickel-titanium file systems. Saudi Endod J [serial online] 2018 [cited 2022 Oct 7];8:183-8. Available from: https://www.saudiendodj.com/text.asp?2018/8/3/183/237557

  Introduction Top

Root canal therapy involves treating necrotic and vital pulp tissues so that patients can retain their teeth in normal form and function. The goal of endodontic instrumentation is to eliminate microorganisms, debris and tissue by enlarging the canal diameter and create a canal space for medicament delivery and optimized canal geometries for adequate obturation.[1]

Use of stainless steel instrument for canal preparation has been the gold standard for many years.[2] In general, preparation of narrow curved canals by manual stainless steel files is time-consuming and difficult, with limited apical enlargement thereby reducing the efficacy of irrigation and obturation.[3]

In the last decade, the use of nickel-titanium (NiTi) rotary instruments has grown in popularity, and there has been increasing number of proprietary systems introduced commercially.[4] Increased flexibility and shortened working time are the major advantages of NiTi files.[5],[6] By the mid-1990s, the first commercially available NiTi instruments were launched.[7] First generation NiTi files had passive cutting radial lands and fixed tapers of 4% and 6% over the length of their active blades,[8] i.e. ProFile, Quantec, and GT files. Second generation of NiTi had active cutting edges and requires fewer instruments to fully prepare a canal, i.e. Endosequence, ProTaper, and BioRace.[9] Improvements in NiTi metallurgy became the hallmark of what may be identified as the third generation of mechanical shaping files. In 2007, manufacturers began to focus on utilizing heating and cooling methods to reduce cyclic fatigue and improve safety when rotary NiTi instruments work in more curved canals,[10] i.e. Twisted File, Hyflex CM, GT, Vortex, and WaveOne. Another advancement in canal preparation procedures that utilizes reciprocation which may be repetitive up and down or back and forth motion was of the fourth generation.[9] This generation of instruments and related technology has largely fulfilled the long-hoped-for single-file technique. The self-adjusting file (SAF) produces 0.4 mm vertical amplitude stroke and vibrating movement with constant irrigation.[11] The fifth generation of files has been designed such that the center of mass and/or the center of rotation are offset,[9] Commercial examples of file brands that offer variations of this technology are Revo-S, ProTaper Next, One Shape, and Silk.

At times, during instrumentation, we inevitably end up damaging the root dentin which might become a gateway to dentinal cracks and minute intricate fractures, thereby leading to failure of treatment.[1] Root fracture might occur as result of a microcrack or craze line that propagates with repeated stress application by occlusal forces.[12] Vertical root fracture is one of the frustrating complications of root canal treatment, which often results in tooth extraction.[13]

Hence, the aim of this study was to evaluate and compare dentinal microcracks formation during root canal preparation by different commercially available NiTi file systems.

  Materials and Methods Top

Eighty-four mandibular premolars having single and straight canal extracted for periodontal and orthodontic reasons were used for this study. The collection, storage, sterilization, and handling of extracted teeth followed the Occupational Safety and Health Administration (OSHA, University of California, San Diego, CA, USA) recommendation and guidelines.[14] To ensure standardization, the teeth were decoronated under constant water cooling with a diamond disc (Addler, Golden Nimbus, Mumbai, India) at 16 mm from the apex. All roots were inspected with a dental operating microscope (Carl Zeiss, Jena, Germany) at 24x magnification to detect any preexisting craze lines or cracks. Four teeth with such findings were excluded and replaced after appropriate examination. Canals were negotiated with size #10 hand K-files (Dentsply Maillefer, Ballaigues, Switzerland) and after removal of gross pulpal tissue, working length was established by advancing file into the canal until just visible at the apical foramen and then subtracting 1 mm from it. The roots were covered with a single layer of aluminum foil and embedded in acrylic resin block, followed by removal of root from block and replacement of foil by the light body silicone-based material to simulate the periodontal ligament.

Then, the specimens were divided into seven groups each consisting of 12 specimens as follows.

Group I

Twelve specimens were left unprepared and served as controls

Group II

NiTi hand K-files were used to enlarge the root canals up to size 25K using the balanced force technique.[15] Files were inserted by a quarter-turn clockwise rotation of 90° with no apical pressure and cutting was accomplished by counter-clockwise rotation of 120° applying sufficient apical pressure. Then, working length was incrementally reduced by 1 mm beginning from #30 to #60 K-file instrument.

Group III

Following the sequence of ProTaper universal (Dentsply Maillefer, Ballaigues, Switzerland) files were used to prepare the canals as recommended by the manufacturer. The Shaping file SX was used up to two-third of working length for coronal enlargement at 300 rpm and 3 Ncm torque, then S1, S2, F1, and F2 files were used at 300 rpm and keeping torque setting at 2 Ncm till working length. Here, F2 file corresponds to apical size 25 and 8% apical taper.

Group IV

Following the sequence of ProTaper Next (Dentsply Maillefer, Ballaigues, Switzerland) files were used to prepare the canals as recommended by the manufacturer. The shaping file XA was used for coronal enlargement up to two-third of the working length, then X1 and X2 files were used at 300 rpm and 2Ncm torque till working length. Here, X2 correspond to apical size 25 and 6% apical taper.

Group V

Canals were prepared with Silk files (MANI, Tochigi, Japan) at 500 rpm and torque (3 Ncm) as recommended by manufacturer. 0.08/25 file was used for coronal enlargement and 0.06/20, 0.06/25 files up to working length.

Group VI

Reciprocating WaveOne (Dentsply Maillefer, Ballaigues, Switzerland) size 0.08/25 file was used in a reciprocating, slow in and out pecking motion with a 6:1 contra-angle handpiece powered by a torque-limited electric motor (WaveOne™ motor, Dentsply Maillefer, Ballaigues, Switzerland) at 350 rpm as recommended by manufacturer.

Group VII

Canals were prepared with SAFs (1.5 mm) (SAF, Re-Dent Nova, Ra'anana, Israel) using a special RDT handpiece-head at 5000 rpm and with constant irrigation at 4 ml/min from VATEA irrigating pump according to the manufacturer's instructions. First pre-SAF (0.04/20) used then SAF was used in a sequence.

All canals were prepared by the same operator, who had more than 6 years' experience in root canal therapy. In all the groups, canals were irrigated with freshly prepared 2.5% sodium hypochlorite between each instrument during the procedure. All roots were kept moist throughout the procedures by frequent immersion in purified distilled water.

Sectioning and microscopic observations

All roots were cut horizontally at three levels (3, 6, and 9 mm) from the apex with diamond disc under constant water cooling. Sections were then viewed under stereomicroscope (Olympus, Tokyo, Japan) at 20X magnification. The appearances of cracks were registered by the digital pictures. The root cracks were divided into two categories [Table 1].[16],[17]
Table 1: Categories used to evaluate the crack type

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Statistical analysis

The results were expressed as the number and percentage of cracks in each group. The data was analyzed with a Chi-square test.

  Results Top

The no preparation, hand NiTi and SAF presented no cracks [Figure 1]a and [Figure 1]b at all three levels. Cracks [Figure 1]c and [Figure 1]d were found in roots instrumented with ProTaper, ProTaper Next, Silk, and WaveOne [Table 2]. Among the specimens depicting cracks, ProTaper files showed the highest number of cracks at all three levels and WaveOne showed least number of cracks at all three levels [Figure 2]. Among the cracked specimens, the results representing the number of cracks in roots instrumented with ProTaper, ProTaper Next, Silk, and WaveOne were statistically insignificant (P = 0.99).
Figure 1: (a and b) Sectioned specimens without microcracks. (c and d) Sectioned specimens showing microcracks

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Table 2: Quantity of microcracks for each group at each root level

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Figure 2: The number of microcracks in each group

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  Discussion Top

Biomechanical preparation is one of the principal steps in achieving endodontic success. Inadvertent instrumentation during canal preparation and retreatment cases may end up with the complications such as perforations, canal transportation, ledge, zip formation, and separation of instruments.[12],[18],[19]

During preparation, the contact between the instrument and canal walls creates momentary stress concentrations in dentin which may lead to dentinal defects wherein vertical root fracture can initiate. Microcracks formation may be related to instrument features such as tip design, cross-sectional geometry, taper, pitch design, and flute form.[12]

In this study, all teeth were inspected for pre-existing cracks or fracture. However, ruling out cracks before the start of the experiment is difficult because some of these cracks could be internal and may not be visible externally. The control group, however, showed no cracks, signifying that the microcracks seen were as a result of the preparation procedures with NiTi rotary files.

In the current study, the control and hand NiTi file groups showed no microcracks formations. The findings were similar to Bier et al.[20] findings. Hand instrumentation did not cause damage to the root dentin due to its less aggressive movements in the canal compared with engine operated files.[21]

In this study, SAF showed no cracks along with the control and hand file groups. This result is in accordance with the results of Yoldas et al.[12] SAF does not have blades, instead, removes dentin using a grinding motion similar to the use of sandpaper. SAF easily compresses into the canal and then attempts to regain its original dimensions; thus, apply a constant delicate pressure on the canal walls, which allows for uniform removal of dentin along the whole perimeter of the root canal cross-section.[22] It might be the reason why SAF did not create any cracks in experimental samples.

The microcracks formation with ProTaper files might be attributed to stiff nature of these files,[23] that generate comparatively higher tensile and compressive stresses in dentin.[21] The SX is a very large and aggressive instrument. This probably accounts for the presence of a lot of cracks in the coronal portion of the canal.

This could be attributed to the high number of rotations associated with more number of files, progressively increasing taper, smaller pitch, and cross section design all of which removes relatively more dentin compared to other systems.[24] On the other hand, microcracks formation with silk files may be attributed to few of its design characteristics. They are selective heat treatment, teardrop shape, and off-center cross-sectional design.[25] Although, both Silk and Protaper Next have off-centered cross-section, such difference may be due to their metallurgy. The different design features of Protaper Next include offset cross-sectional design and M-wire technology.

According to the findings of this study, WaveOne file created lesser cracks as compared to multiple rotary files system. Reciprocation is based on the fundamental of balanced force technique by Roane et al.[26] which probably minimizes torsional and flexural stresses. Moreover, WaveOne instrument is manufactured with M-wire, which is a more flexible variant of the NiTi alloy. Kansal et al.[27] conducted a similar study and found that when ProTaper F2 and WaveOne files were used in reciprocating motion, it induced lesser dentinal damage as compared to ProTaper F2 file used in rotary motion. The study results are contradictory to results of Bürklein et al.[28] that reciprocating single file created more defects than Protaper and Mtwo files used in full sequence. This may be due to the fact that Bürklein et al.[28] did the apical enlargement directly with Reciproc R40 files (40/0.08) and did not use methods to simulate periodontal ligament which could have attributed to more defects. Furthermore, the instruments used in the present study as master apical files were of smaller sizes as compared to Bürklein et al.[28] to standardize the final canal preparations with different systems as close as possible.

In the present study, although cracks were observed in all groups, cracks in the coronal region were more than cracks in the apical region which is in accordance with the previous studies done by Adorno et al., and Liu et al., respectively.[29],[30]

The standardization of speed and torque settings for different file systems could be a limitation of the present study. Furthermore, it was difficult to standardize the downward force used during each instrumentation. Also, teeth with straight root canals without anatomical complexities were mounted on resin blocks, which might not always reproduce a true clinical presentation. A drawback of using resin blocks is heat generation and softening of the resin material [31]

In this study, as in the previous studies, teeth were sectioned at different levels, observing for microcracks with a stereomicroscope [27],[32] which has a significant disadvantage related to the deleterious effect of sectioning procedure.[33] However, in this study, this might not have been the situation as we had no microcracks defects in our control group. Another nondestructive system such as micro-computed tomography (micro-CT) imaging was proposed to investigate the same by rotary instrumentation.[33],[34] However, it was reported that use of such X-ray and computed tomographic imaging procedures produced variable heating. It is imperative to think that an increased temperature from the use of high-resolution micro-CT scans can induce dehydration of the samples leading to augmentation of the cracks affecting the outcome of the research.[35] However, further studies using other methods such as optical coherence tomography or infrared thermography should help in minimizing the apparent disadvantages of the mentioned techniques to achieve predictable results. Versiani et al. stated it is rather unlikely that, in the clinical setting, some ordinary canal procedure could cause microcracks in a range of 40%–80%, as reported by most of the studies.[36] It seems important to critically appraise the different methods used in microcracks detection studies not to overstress the correlation between the results obtained in this type of study and the clinical reality.[28]

  Conclusion Top

Within the limitations of this study, we can conclude that different NiTi instruments tend to produce varying degrees of dentinal damage during root canal preparation. Various factors cause dentinal cracks, but the flexibility of file due to heat treatment, kinematics of the file and the basic architecture of the file are the most significant ones. Hand instrumentation and SAF file represented satisfactory results with no microcracks defects.


We are grateful to Dr. Sujal Parkar (Asst. Professor, Public Health Dentistry, Siddhpur Dental College, India) for helping us with the statistical analysis and Dr. Mayur Dudhat (Surat, India) for helping us with microscopic analysis.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2]

  [Table 1], [Table 2]


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