Comparative evaluation of dentinal crack formation after root canal preparation with Neoendo Flex and Mani Silk files: An in vitro study

Abstract

Context:

Neoendo Flex and Mani Silk are recently launched nickel–titanium (NiTi) rotary files. Dentinal cracks and file separation remain a critical problem in the endodontic domain.

Aim:

The aim was to evaluate and compare the incidence of dentinal crack formation after root canal preparation with Neoendo flex and Mani silk files.

Materials and Methods:

Thirty-six extracted single-rooted premolars with straight, single root canals were selected for the study. The specimens were randomly assigned to three groups, with 12 samples in each group. The root canal preparation for the three groups was done with hand K-file, Mani Silk, and Neoendo Flex files, respectively, following which the specimens were horizontally sectioned at 3, 6, and 9 mm from the apex. Sectioned samples were viewed under a stereomicroscope to determine the presence or absence of dentinal cracks. The data were analyzed using one-way ANOVA and Chi-square test.

Results:

Hand K-files and Mani Silk files produced a lesser number of cracks than Neoendo Flex files; however, the difference was not statistically significant among the three file groups at 3 mm and 6 mm (P > 0.05). A statistically significant difference between the groups was seen at 9 mm, where Mani Silk files produced fewer cracks than K files and Neoendo Flex files (P = 0.007). Overall, Neoendo Flex significantly produced more cracks than Mani Silk and hand K-files (P = 0.0029).

Conclusions:

Both hand K files and NiTi rotary instruments may cause dentinal cracks. Neoendo Flex files tend to produce more dentinal cracks than Mani silk and K-file.

INTRODUCTION

Walking from past to present, rotary nickel–titanium (NiTi) instruments have completely changed the way root canal preparations are performed. The near magical properties of the corrosion-resistant alloy for making these rotary instruments were first introduced by Walia et al. during late 1988.[1] There have been many technological advancements in rotary NiTi instruments since its introduction, which have led to faster and easier shaping of root canals.

Stainless steel instruments have been considered the gold standard for root canal preparation since time immemorial.[2] Preparation of narrow, curved canals using stainless steel files is difficult, time consuming, and it increases the operator's fatigue. The apical enlargement using stainless steel files is limited, thereby reducing the efficacy of irrigation and the quality of obturation.[3]

It is only in the past two decades that NiTi rotary instruments have gained popularity. Commercially, increasing number of proprietary systems are being introduced in the market at a rapid rate.[4] NiTi rotary instruments have the advantages of increased flexibility and shortened working time, while instrument separation and dentinal crack formation are its major disadvantages.[5]

Root canal preparation procedures can damage the root canal dentin causing dentinal cracks.[6] Dentinal cracks occur when the tensile stress of the root canal wall exceeds that of the dentin.[7,8] Dentinal microcracks under repeated occlusal forces have the potential to develop into vertical root fracture, which is one of the most frustrating complications of endodontic therapy, often leading to tooth extraction.[9]

Kim et al. established a possible relationship between the design features of NiTi rotary instruments and the incidence of the vertical root fractures. He concluded that the strain concentration and the apical stress induced during root canal preparation are influenced by the design features of the file.[10]

Mani Silk (Mani, Japan) and Neoendo Flex (Orikam Healthcare India Private Limited) are two heat-treated NiTi rotary file systems that have been introduced relatively recently. Mani Silk is a fifth-generation rotary NiTi file that has a unique cross-sectional tear drop design with an off-center mass of rotation that cuts exceptionally well and resists fracture while removing debris effectively and reduces instrument stress on the canal walls. Silk eliminates the “screwing-in” effect, which is common with many other proprietary systems.

Neoendo flex is a third-generation gold thermal-treated NiTi rotary file which has a triangular cross-sectional design with sharp cutting edges and a safety noncutting tip. It has increased flexibility to negotiate canals.

To the best of our knowledge, there is no published literature comparing the incidence of the dentinal crack formation using Mani Silk and Neoendo Flex files; hence, the objective of our study was to compare and evaluate the incidence of dentinal crack formation after root canal preparation with Neoendo Flex and Mani Silk files.

MATERIALS AND METHODS

Thirty-six extracted single-rooted human premolars with single, straight (<5° curvature) root canals evaluated by Schneider method were selected for the study and stored in 1% Chloramine T solution (Otto Chemie Pvt Ltd).[11] Preoperative radiographs were taken from both proximal and buccal view to confirm single and straight root canals. Teeth with root fractures, cracks, open apices, excessively curved canals, multiple roots, caries, restorations, severe anatomic variations, and calcifications were excluded from the study. The root surfaces of the selected teeth were examined under a stereomicroscope at 25x (Wuzhou New Found Instrument Co. Ltd China model: XTL 3400E) to exclude external defects and cracks. The teeth were decoronated with a diamond disc (Kerr) to a length of 16 mm from apex for standardization, and the roots of the teeth were covered with a single layer of aluminum foil (Hindalco Freshwrap). Each root was embedded into acrylic resin (DPI RR Cold Cure) kept in an acrylic tube. The root was first removed from the tube, and then the aluminum foil was removed from the root. The space created by aluminum foil was replaced by a light-body elastomeric material (3M ESPE Empress). The root was then immediately inserted back into the impression material to simulate the periodontal ligament space. Apical 3 mm of the root was not covered with foil and was immersed in water during root canal instrumentation to prevent dehydration.

Thirty-six teeth were randomly assigned to three groups of 12 teeth (n = 12) each. Working length was measured by inserting a No. 10 K-file (Mani, Japan) till it was just visible at the apical foramen and then subtracting 1 mm from the measurement. A glide path was prepared using 10/02 and 15/02 K-file (Mani, Japan). Apical preparation was completed up to size 25 of each system.

20 ml of 5% sodium hypochlorite (Paras Chemical Industries, Pune) with 27G side-vented needles (Prime Dental Products Private Limited) was used for irrigation for each specimen.

Group 1: Stainless steel hand K-file (control)

Root canals were prepared using stainless steel hand K-files (Mani, Japan). The canals were enlarged apically till 25/02, and then step back preparation was done up to 40/02.

Group 2: Neoendo Flex

Root canals were prepared using Neoendo Flex files attached to an endomotor (X-Smart, Dentsply) according to the manufacturer's instructions at speed of 350 rpm and a torque of 1.5 Ncm. Coronal flaring was done with (30/08) file. Apical shaping was done with 20/06 and 25/06 up to the full working length.

Group 3: Mani Silk

Root canals were prepared using a standard pack of Mani Silk attached to an endomotor according to manufacturer's instructions in gentle in and out motion at speed of 500 rpm and a torque of 3 Ncm. Coronal shaping was done using (25/08). Apical shaping was done using 20/06 and 25/06 up to the full working length.

Sectioning and microscopic examination

Roots were sectioned at 3, 6, and 9 mm from the apex, perpendicular to the long axis of the teeth, using a diamond disc under water cooling, and each section was stained with 2% methylene blue (Merck, India) for better visualization. Slices were viewed under a stereomicroscope of ×25 magnification. Digital images of each section were analyzed using an image analysis system (Chroma Systems Pvt. Ltd., India) attached to the stereomicroscope. Each specimen was observed by a single examiner to check the presence or absence of dentinal cracks.

To define dentinal crack formation, two distinct categories were made according to the method given by Karataş et al.[12]

• Crack – Any lines, microcracks, or fractures that were present in the root dentin

• No crack – Root dentin devoid of craze lines, microcracks on the external surface of the root, and microcracks on the internal surface of the root canal wall.

Statistical analysis

One-way ANOVA and Chi-square tests were used to analyze the data using MedCalc Statististical Software version 18.2.1 (MedCalc Software Ltd,Ostend,Belgium) with the level of significance set as P < 0.05.

RESULTS

Dentinal cracks were observed in all three groups. Figure 1 represents a bar graph with the mean number of dentinal cracks at 3, 6, and 9 mm. Group 1 (Stainless steel hand K-file) and Group 3 (Mani Silk) produced a lesser number of cracks as compared to Group 2 (Neoendo Flex). According to one-way ANOVA, there was no statistically significant difference among the three file groups at 3 mm and 6 mm (P > 0.05); however, a statistically significant difference was seen at 9 mm, at which Group 3 produced fewer cracks than the other two file systems (P = 0.007).

Figure 1.

Bar graph representing a mean number of dentinal cracks at 3, 6, and 9 mm

Figure 2 represents stereomicroscopic photographs of samples at 3, 6, and 9 mm for the different file systems used.

Figure 2.

Stereomicroscopic photographs of samples at 3, 6, and 9 mm for the different file systems used

Chi-square test revealed a statistically significant difference between Group 2 and Group 3 (P = 0.0185) and between Group 1 and Group 2 (P = 0.0019). There was no statistically significant difference between Group 1 and Group 3 (P > 0.05). Overall, among the three file groups, a statistically significant difference was seen in the crack formation with Neoendo Flex files, which produced more number of dentinal cracks across all sections than the other file systems used (P = 0.0029). Table 1 represents the percentage of teeth with a number of dentinal cracks for hand k, Neoendo Flex, and Mani Silk files.

Table 1.

Percentage of teeth with number of dentinal cracks for Hand k, Neoendo Flex, and Mani Silk files

Number of cracks observed	Number of samples showing cracks for the different file systems used			Percentage of teeth with cracks
	Hand K file	Neoendo flex file	Mani silk file
0	12	8	16	33.3
1	17	6	12	32.4
2	5	8	2	13.9
3	1	13	6	18.5
4	1	1	0	1.9

DISCUSSION

Dentinal damage can occur during procedures such as root canal preparation, obturation, and retreatment. Biomechanical preparation is a critical step to achieve endodontic success. At times, in the zeal for ideal canal preparation, clinicians inadvertently damage the root canal dentin. The contact between the instrument and canal walls during preparation creates momentary stress concentrations in the root dentin, which may lead to dentinal defects from which vertical root fracture can originate. Instrument features such as tip design, cross-sectional geometry, taper, pitch design, and flute form may be related to dentinal crack formation.[13,14,15]

In our study, all root canal shaping files, including hand K files (control group), produced microcracks in root dentin. These findings are in accordance with Yoldas et al. and Bürklein et al., who found cracks in the root canals prepared by rotary NiTi instruments but not in the root canals instrumented with hand K file.[13,16] Unlike several studies, in our study, we found cracks in the hand K file group too; this is in accordance with findings of Kesim et al.[17] He stated that examination with any kind of magnification is not fully sufficient for detecting the preexisting craze lines or cracks that may be present on the inner surface of the root.[17] Hardness and stiffness of the stainless steel K file can also contribute to crack formation. This possibly explains the contradictory result in the control group of our study.

In the present study, cracks in the coronal region were more abundant than the cracks in the apical region for all the three groups, which is in accordance with the previous studies done by Adorno et al. and Liu et al.[18,19] Bier et al. stated that taper of files influences crack formation. The orifice opener used in our study is a larger taper (8%) instrument which causes more stress concentration in the root dentin, leading to microcracks.[6] Failure to identify preexisting internal cracks that become visible after sectioning and the sectioning method itself could explain the reason for more microcracks in the coronal region.

In this study, Neoendo flex files showed a more number of cracks than Mani silk files. Both Neoendo Flex and Mani Silk are heat-treated rotary NiTi files available in tapers of 4% and 6% with a safety noncutting tip. Neoendo Flex and Mani Silk files mainly differ in their cross-sectional geometry. Mani Silk file has a unique teardrop cross-sectional design with an off-center mass of rotation, so it establishes only one point contact with the root canal dentinal wall while it rotates in the canal during biomechanical preparation. On the other hand, Neoendo Flex has a triangular cross-sectional design, so each time the file rotates in the canal during biomechanical preparation; there is at least a three-point contact between the instrument and the root canal wall which transmits more tensile stresses to the root canal dentinal wall, leading to dentinal microcracks more than that of Mani Silk files.

This is an in vitro study, so there is a possibility of the clinical scenario and outcome to be completely different. Versiani et al. stated that, in the clinical setting, it is highly unlikely that some ordinary root canal procedures could cause microcracks in a range of 40%–80%, as reported by most of the studies.[20]

The possible limitations of our in vitro study are the sectioning method, difficulty in identifying internal preexisting cracks, and the inability to standardize the speed and torque of both the rotary files used.

CONCLUSIONS

Within the limitation of our in vitro study, it can be concluded that both NiTi files and hand K files may cause dentinal cracks on the root surface. The design features of the instrument likely play a pivotal role in crack formation and propagation. Neoendo Flex files tend to produce more number of cracks as compared to Mani Silk and hand K files.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.