Dental News - Dentinal defects after root canal preparation

Dr Taha Özyürek

Wed. 14. February 2018

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Vertical root fractures are one of the most frequent complications seen on teeth having endodontic treatment, and generally result in the extraction of tooth (Haueisen et al. 2013). The root canal treatment procedures may cause dentinal stress and consequently dentinal cracks. The emerging dentinal fractures may transform into vertical root fractures under functional loads (Barreto et al. 2012).

The physical and mechanical properties of nickel titanium (NiTi) rotary file systems my affect the incidence of cracks on dentinal surface (Adorno et al. 2011). Moreover, the type of movement used in shaping the root canals may also influence the incidence of dentinal defects. Liu et al. (2013) have reported that a continuous rotational movement causes more dentinal defects than reciprocation movement, while Bürklein et al. (2013) have reported that reciprocation movement causes more dentinal defects.

WaveOne (WO; Dentsply Maillefer) NiTi singlefile system was recently modified to WaveOne Gold (WOG; Dentsply Maillefer). While maintaining the reciprocation movement of the file, its cross-section, dimensions and geometry were changed. The cross-section of the file was altered to a parallelogram, having two cutting edges. Moreover, the off-centre design that ProTaper Next (Dentsply Maillefer) files have is used in WOG files too. The most significant change in files is the Gold heat treatment method. Gold heat treatment is based on reversing the M-Wire technology employing the pre-production heat treatment, and by heating the file after production and then slowly cooling it. The manufacturer company claims that this new heat treatment increases the flexibility of files (WaveOne Gold Brochure).

Another NiTi rotary file system made using the Gold heat treatment procedure is the recently introduced ProTaper GOLD (PTG; Dentsply Maillefer) system. Similar to ProTaper Universal (PTU; Dentsply Maillefer) system, this model consists of three shaping (SX, S1 and S2) and five finishing (F1, F2, F3, F4 and F5) files. PTG uses the continuous rotation movement at the same torque and speed settings with PTU, but the manufacturer claims that PTG files are two times more resistant to the cyclic fatigue under favour of flexibility offered by the Gold alloy (ProTaper Gold Brochure). From the aspect of metallurgical character, PTG NiTi files have not only the 2-stage specific transformation feature but also high Af temperature similar to controlled memory (Shen et al. 2011).

Recently, the patented treatments have been involved in the innovative production of new HyFlex EDM files (HEDM; Coltène/Whaledent, Altstätten, Switzerland). The main feature of these files is that they are manufactured via an electro-discharge machining (EDM) process. The EDM is a non-contact machining procedure used in engineering for manufacturing the parts that would be difficult to machine with conventional techniques. The removal of material is performed by pulsating electric current discharges that flow between an electrode and the work piece are immersed in a dielectric medium. The electric current partially melts and evaporates small portions of the material in a well-controlled and repeatable manner. The material is therefore superficially removed, leaving an isotropic surface, characterised by regularly distributed craters (Pirani et al. 2015).

In our comprehensive literature review, no study examining the dentinal defects caused by HEDM NiTi files during root canal shaping procedure was found. For this purpose, the aim of this in vitro study was to compare the incidences of dentinal defects that HEDM, WOG and PTG NiTi files create during shaping the mesial canals of mandibular molar teeth. The null hypothesis of present study was that there would be no difference between the dentinal defect formation incidences of HEDM, WOG and PTG NiTi files.

Material and methods

Specimen selection

After obtaining the ethical committee approval, 80 mandibular molar teeth that were extracted due to periodontal reasons and had < 20° of canal curvature (Schneider 1971) and two separate mesial canals were involved in this study. The soft and hard tissues around the teeth were mechanically removed using a periodontal curette. Moreover, the distal roots of teeth were removed under water-cooling. The crowns of teeth were removed from the enamel-cement junction under water-cooling, allowing 16 mm of root length. The radiographic images of teeth were taken in mesio-distal and bucco-lingual directions. Teeth that were found to have calcification, history of previous root canal treatment, involving internal and/or external resorption, or were fractured and/or had immature roots were excluded. The selected teeth were kept in distilled water at 4 °C for the experimental procedures.

The roots of teeth were wrapped with aluminium foil and then embedded into acrylic resin (Imicryl, Konya, Turkey) (Capar et al. 2014). After the acrylic set, the teeth were taken out from the resin, and the foils were removed. To simulate the periodontal ligament, the resin blocks were filled with viscous silicon impression material (Express XT Light Body Quick; 3M ESPE, Neuss, Germany) and the specimens were then placed into the resin blocks again.

Root canal preparation

The canals of teeth were penetrated using a #10 K-file (Dentsply Maillefer) until the tip of file was seen from the apex. The working length was set to 1 mm shorter than this length. For all of the specimens, the glide path was created ensuring the apical diameter of 0.20 mm. For every specimen, 20 ml 1 % sodium hypochlorite (NaOCl) was used during the preparation. The entire procedure was executed by the same endodontist, having 5 years of experience. The teeth were randomly divided into 4 groups, 20 teeth in each. And then, the following procedures were performed.

Group 1: HyFlex EDM

Using the torque-controlled endodontic motor (X-Smart; Dentsply Maillefer), the root preparation of the specimens in this group was performed by using a HEDM 25/.~ NiTi single-file system according to the manufacturer’s instructions at 500 rpm and 2.5 Ncm torque.

Group 2: WaveOne GOLD

Using the torque-controlled endodontic motor (VDW Reciproc GOLD; VDW, Munich, Germany), the root preparation of the specimens in this group was performed by using a WOG Primary (25/.07) NiTi single- file system according to the manufacturer’s instructions in “WaveOne ALL” programme.

Group 3: ProTaper GOLD

Using the torque-controlled endodontic motor (X-Smart; Dentsply Maillefer), the root preparation of the specimens in this group was performed by using a PTG NiTi rotary file system’s S1 (18/.02), S2 (20/.04), F1 (20/.07) and F2 (25/.08) files according to the manufacturer’s instructions at 300 rpm and 3 Ncm torque.

Group 4: Negative control

No intervention was made to this group and they were assigned to the negative control group.

Assessment of dentinal defects

Under water-cooling (Isomet; Buehler Ltd, Lake Bluff, IL, USA), the roots of 80 specimens were cut perpendicular to the tooth axis at 3, 6, and 9 mm distant from the apex, and 3 slices were obtained from each specimen. Trans-illumination was applied to the slices from 1 mm distance in mesial, distal, buccal, and lingual directions using a LED (LED Light; Denshine Technology, China) device. The digital images (4 images from each slice) were taken under x25 magnification using a digital camera connected to stereomicroscope (Olympus BX43, Olympus Co, Tokyo, Japan). In order to eliminate the bias of observers, the canals on digital images were masked using a circular drawing. A total of 960 digital images—240 from each group—were examined to determine if any cracks were present. The images obtained were then randomly assigned to two experienced endodontists, who were not involved in the preparation of the specimens, in order to determine the presence or absence of dentinal defects. To define crack formation, two different categories were made (i.e. ‘no crack’ and ‘crack) to avoid the confusing description of root cracks. ‘No crack’ was defined as the root dentine without cracks or craze lines either at the internal surface of the root canal wall or the external surface of the root. ‘Crack’ was defined as all lines observed on the slice that either extended from the root canal lumen to the dentine or from the outer root surface into the dentine (Shemesh et al. 2009) (Fig. 1).

Statistical analyses

In examining the intergroup incidence of dentinal defects, a Chi-Square test was used. The level of statistical significance was set to 5 %. The statistical analyses were performed using SPSS 21 (IBM-SPSS Inc., Chicago, IL, USA) software.

Results

In the present study, 960 images taken from 240 tooth slices were examined. The distribution of dentinal defects caused by the tested NiTi file systems between apical, medial, and coronal regions is presented in Table 1. In the present study, no statistically significant difference was found among the NiTi files tested and between them and control group in terms of the total number of dentinal defects (P > 0.05).

Discussion

In this study, the dentinal defects created by HEDM, WOG and PTG NiTi file systems on mandibular molar teeth’s mesial canals were evaluated. According to the results of the present study, it was determined that all of the tested NiTi files created dentinal defects but no statistically significant difference was found when compared to thecontrol group. For this reason, the null hypothesis of the present study was accepted.

The Number and Percentage of Slices with Defects at Each Level (n = 20)
Group	3mm n(%)	6mm n(%)	9mm n(%)	Total of specimens of presenting defects n (%)
Control	4 (20 %)	8 (40 %)	6 (30 %)	11 (55 %)
Pro Taper GOLD	8 (40 %)	10 (50 %)	8 (40 %)	14 (70 %)
WaveONE GOLD	6 (30 %)	9 (45 %)	8 (40 %)	12 (60 %)
HyFlex EDM	5 (25 %)	10 (50 %)	7 (35 %)	11 (55 %)

In many in vitro studies, the dentinal defects created by the NiTi file systems during root canal preparation were investigated using single- and straightrooted teeth (Karata et al. 2015b, Kfir et al. 2017). However, the increasing root canal curvature would increase the stress on the files, which were used in preparation, and consequently on the dentine. An increase in the stress on dentine would cause increasing irregularities (transportation, straightening, etc.) within the canal and lead to thinner dentinal structure in certain regions. Thinner dentine would weaken the root structure and prepare the ground for vertical root fracture formation (Kim et al. 2013). In previous studies, it has been reported that the highest level of stress occurred on the curved root canals during the root canal preparation by NiTi rotary files (Kim et al. 2013, Medha et al. 2014). For this reason, the mesial canals of mandibular molar teeth were used in present study.

NiTi file manufacturers generally recommend using the files on single. Based on four-canal maxillary first molar tooth in present study, the files were discarded after use in four canals (two specimens) in order to prevent the deformation from influencing the results (Hin et al. 2013).

It has been reported that use of larger files in shaping the root canals increase the incidence of dentinal defect formation (Capar et al. 2015). For this reason, in the present study, the apical diameter of files was determined to be 0.25 mm, and no larger file was used. Moreover, in order to protect the dentine microstructure, 1 % NaOCl was used as an irrigation solution. Thus, it was ensured that most of the dentinal defects to be related with the mechanic preparation.

It has been reported that the forces applied while extracting the teeth, and the stress during storing the teeth and obtaining the slices might cause dentinal defects (De-Deus et al. 2014). This may explain the formation of dentinal defects in the negative control group, where no intervention was made. Studies using conventional methods of sectioning have failed in determining these defects in negative control groups (Capar et al. 2014, Karata et al. 2015b, Li et al. 2015). When illumination was applied on the obtained dentine slices, the light moved along the dentine, but stopped at the point of any crack on dentine and thus the presence of crack and/or fracture could be determined (American Association of Endodontists, 2008). In Coelho and colleagues’ study (2016a, 2016b), dentinal defects could be determined in many specimens in negative control groups by employing light-emitting diode (LED) trans-illumination. Moreover, Arslan et al. (2014) have also used methylene blue in order to determine dentinal defects, and they reported non-significant differences between the experimental group and negative control group in terms of dentinal defects.

Arias et al. (2014) reported in their study that masking is important for eliminating observer bias because of observing which of the specimens had been shaped or not. For this reason, the canal-masking method was used in the present study in order to eliminate any bias. The movement kinematic of NiTi files may affect the amount of dentinal defects during canal root shaping by files. Under favour of the reciprocation movement in clockwise and counterclockwise directions, the file is protected from being stuck within the canal while shaping the root canals (Yared 2008). Some of the studies have reported WaveOne files to cause less dentinal defects than ProTaper Universal files (Kansal et al. 2014, Li et al. 2015), while some other studies have reported that reciprocation systems create more dentinal defects (Bürklein et al. 2013). Besides that, in some studies, no statistically significant difference between the reciprocation systems and rotary systems has been reported (Arias et al. 2014, Karata et al. 2015a, Coelho et al. 2016b). Li et al. (2015) have examined the dentinal defect formation incidences of ProTaper Universal, ProTaper Next and WaveOne files in curved root canals of molar teeth. The researchers have reported that ProTaper Next file system created less dentinal defects than other files. El Nasr and El Kader (2014) have reported ProTaper Universal F2 files operating based on the same movement kinematic with WaveOne system to cause less dentinal defects. Similar to the results of other studies (Capar et al. 2014, Li et al. 2015), the researchers attributed these results to the heat treatment, to which WaveOne files are exposed. Karata et al. (2015a) have examined the dentinal cracks created by ProTaper Universal, ProFile Vortex (Dentsply Maillefer), ProTaper GOLD, Reciproc (VDW, Munich, Germany), and F360 (Komet Brasseler, Lemgo, Germany) files systems in mandibular incisor teeth, and reported that there was no statistically significant difference between ProTaper Universal, ProFile Vortex, ProTaper GOLD and Reciproc groups in terms of dentinal defect formation. It is believed that the reason for the difference in the present study originates from the differences in methodologies used. Similar to the present study, Coelho et al. (2016b) have used LED in investigating the dentinal defects created by ProFile (Dentsply Maillefer), TRUShape (Dentsply Maillefer) and WaveOne GOLD systems on mandibular molar teeth’s mesial canals, and reported statistically non-significant differences between the negative control group and experimental groups in terms of dentinal defects.

Capar et al. (2014) have examined the dentinal defects created by HyFlex CM, ProTaper Universal and ProTaper Next NiTi file systems on mandibular premolar teeth during preparation procedure. The researchers have reported ProTaper Next and HyFlex CM files to cause less dentinal defects than the ProTaper Universal files. Ashraf et al. (2016) have examined the dentinal defects created by ProTaper Universal, ProTaper Next and HyFlex CM NiTi file systems in mandibular premolar teeth by using sectioning method. Researchers have reported that HyFlex CM files caused less cracks than ProTaper Universal and ProTaper Next files did. In our literature review, it was determined that HEDM files’ dentinal defect incidence had not been studied before. For this reason, it is not possible to directly compare the results of the present study to those of others. In a finite elements analysis, it has been shown that increasing the taper of files also increased the stress on root canals during shaping procedure (Kim et al. 2010). Bier et al. (2009) reported that the taper of files might influence dentinal defects on roots during the shaping procedure. Yolda et al. (2012) have alleged that the tip design, cross-section, constant or variable taper, and groove and pitch structure of NiTi files might be related with the formation of dentinal defects. However, it is not exactly known how the taper of files affected the results of present study, because the taper of files used were not same and the taper of HEDM file was not known. The similar results obtained are thought to originate from the fact that the files were made of alloys having no shape memory (Gold and CM).

Versluis et al. (2006) have reported that the level of stress on coronal and medial third during root canal shaping was three times more than that on the apical third. Despite that, Kim et al. (2010) have reported the stress on the apical third during shaping of the curved root canals to be more than that on the middle and coronal third. According to the results of the present study, there was statistically non-significant differences between the dentinal defects created by NiTi file systems on the apical, middle, and coronal third, and this finding is believed to originate from the fact that the files were made of alloys having no shape memory (Gold and CM).

Even though it was important to simulate the clinic conditions in a laboratory environment in the present study, especially in the studies on examining the mechanical properties of teeth, many external factors such as storing the teeth after extraction and until the sectioning procedure affected the results of study (Bürklein et al. 2013). For this reason, as stated by Coelho et al. (2016b) in their study, the use of teeth extracted using periodontal reasons, which require very low level of force during extraction, and the careful storage of these teeth until the sectioning procedures would allow for more successful outcomes. Another limitation of present study is the difficulty of standardisation of apical pressure applied by the operator during root canal shaping procedure and that this may influence the results.

Conclusion

Within the limitations of the present study, no statistically significant difference was found among the HEDM, WOG, PTG and the control group in terms of the total number of dentinal defects.

Editorial note: A complete list of references are available from the publisher. This article was published in roots - international magazine of endodontology No. 04/2017.

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