A Micro-Computed Tomographic Assessment of Oval-Shaped Root Canals Obturated with Bioceramic Sealer Using Different Obturation Techniques

Background/Aim: To evaluate void volumes in oval-shaped root canals obturated with three different obturation systems using microcomputed tomography (micro-CT). Material and Methods: Twenty-four distal roots of mandibular molar teeth canals were prepared and randomly divided into three groups (n = 8 in each group). The canals were obturated using single cone (SC), GuttaCore (GC), and warm vertical compaction (WVC) techniques. Each specimen was scanned using a micro-CT device. The volumes of the voids were calculated, and the data were statistically analyzed using a one-way analysis of variance and post hoc Tukey’s (P < .05). Results: The void volumes in the WVC group were significantly lower than those in the other groups (P < .05). When different root canal parts were examined, the void volumes increased significantly from the apical to the coronal level in the GC and SC groups (P < .05). In the WVC group, most voids were observed in the middle part (P < .05), whereas there was no significant difference in void volumes in the coronal and apical parts (P > .05). Conclusions: None of the systems provided void-free root filling. However, the void volume using the WVC system was significantly lower than that observed using the other systems.


Introduction
An ideal root canal filling should obturate the whole root canal space, with a homogeneous density in all three dimensions. When root canals are not homogeneously filled, voids may remain within the canals. Those voids prevent successful adaptation of filling materials to the root canal walls and create a suitable medium for bacterial leakage 1,2 . Viable bacteria within the root canal system may leak into periapical tissues and cause tissue inflammation or contribute to existing inflammation in the periapical area after root canal treatment. Previous research showed that apical periodontitis that developed after endodontic treatment was related to voids between the canal wall and canal filling 3 . Thus, a radiographic assessment of the quality of obturation after root canal treatment is important for the prognosis of endodontic treatment 1,2 .
Various canal filling methods, including single cone (SC), GuttaCore (GC), and warm vertical compaction (WVC) techniques, have been developed to improve the quality of root canal filling 4 . The single cone (SC) technique involves filling the root canal with guttapercha after root canal preparation, which is performed using nickel-titanium rotary files with a size and taper similar to those used in the last phase of preparation. The most important advantage of the SC technique is that the dentist can complete canal filling more easily and in a shorter time than is possible using other methods 5 . Major disadvantages of this technique are the relatively higher volume of sealer, possible leakage because of the sealer dissolving over time, and incomplete adaptation to the anatomic structures of root canals 5,6 . Whaledent) solution for 1 min. The irrigation protocol was completed by irrigating the root canals with 3 ml of distilled water. The root canals were dried using paper points (Brasseler USA). In all the irrigation procedures, a 31-gauge Navi Tip double side-port needle (Ultradent Products Inc., South Jordan, UT, USA) was used. The teeth were then randomly divided into three groups for root canal filling (n = 8 in each group).

Root canal filling
Group 1: SC technique Size 45.04 EndoSequence BC (Brasseler USA) master cones were tugged back. The master cones were then covered with EndoSequence (Brasseler USA) root canal sealer, and the root canals were filled using the SC cone protocol. Following the filling procedure, excessive material was removed using a heat source and vertically compacted.
Group 2: GC technique After reaching the working length, the size verifiers were tugged back. The EndoSequence (Brasseler USA) root canal sealer was applied to the canal walls using paper points that had the same taper size of the final file. GuttaCore (Dentsply Sirona, Ballaigues, Switzerland) with a 40.04 taper preset to the working length was used. The gutta-percha was softened by heating in an oven (ThermaPrep 2 Oven; Dentsply Sirona) according to the instructions of the manufacturer and placed in the canals at a length 0.5 mm shorter than the working length. The stem parts of the obturators were removed by cutting using a rounded diamond burr.

Group 3: WVC technique
Pluggers that can penetrate the canal at a length 0.5 mm shorter than the working length were used. All pluggers were suitable for the apical, middle, and coronal regions of the canal. The EndoSequence BC points and master cones (45.04) were tugged back. The apical parts of the master cones were cut by 1 mm. The EndoSequence points (45.04) were then covered with EndoSequence canal pat and pushed until they reached the working length. The root canal fillings were cut using a heated plugger (Calamus Dual 3D Obturation System; Dentsply Sirona) set to 200° C, leaving 5 mm gutta-percha at the apical. Excessive gutta-percha material was removed from the coronal. The gutta-percha material in the apical part was compacted using the plugger. The part of the Calamus Dual 3D Obturation System (Dentsply Sirona) carrying the gutta-percha was used for filling the rest of the parts of the root canal fillings, and the plugger was used to compact the gutta-percha material in an apical direction.
For all the specimens, access cavities were dressed using temporary filling material (Cavit G; 3M Espe, Seefeld, Germany). The specimens were stored at 37° C (100% humidity) for 1 week to achieve complete hardening of the canal path. The same clinician performed all the procedures ( Figure 1).
To better adapt to the irregularities of root canals, the root canal filling material (gutta-percha) is heated, and the root canal walls are covered with a thin layer of sealer 7 . Injection of thermo-plasticized gutta-percha, thermomechanical compaction of gutta-percha, and carrier-based filling methods are performed to soften the gutta-percha.
The aim of the present study was to assess void volumes between dentin wall and obturation materials applied to mandibular molar teeth with oval-shaped distal canals using different filling methods (SC, GuttaCore [GC], and warm vertical compaction [WVC]) utilizing micro-computed tomography (micro-CT). The null hypothesis was that there would be no difference between root canals filled using different filling methods in terms of void volumes between the root canal walls and dentin.

Sample size determination
Based on a previous study 8 , a power calculation was performed using variance statistical test (G*Power 3.1 software; Heinrich Heine University, Dusseldorf, Germany), with α = 0.05 and β = 0.95. The results indicated that the sample size should be a minimum of eight teeth in each group.

Selection and preparation of specimens
After obtaining the approval of the ethics committee (no: 2019-270), the distal roots of 24 mandibular molar teeth, which had completed their development and been extracted because of orthodontic and periodontal reasons, were included in the study. The teeth were scanned using micro-CT. Only roots of teeth with oval-shaped canals were included 2 .
The roots were examined under 12× magnification (Olympus BX43; Olympus Co., Tokyo, Japan) to detect cracks, resorption, and calcification. The specimens were then stored in 0.1% thymol solution at 5° C until used.

Root canal shaping
After preparation of the access cavities of the teeth, a #10 K-type file (VDW, Munich, Germany) was then inserted into the root canal until it was visible at the apical. The working length of each root canal was determined to be 1 mm shorter than this value. The root canals were prepared using EndoSequence BC (Brasseler USA, Savannah, GA) rotary files and an endodontic motor (VDW) in accordance with the manufacturer's instructions (torque setting: 1.8-2.3 Ncm, speed setting: 500-600 RPM) until a 45.04 apical size was achieved. After each file change, the root canals were irrigated using 3 ml of 2.5% sodium hypochlorite (CanalPro; Coltene-Whaledent, Allstetten, Switzerland) solution. For the final irrigation, all the samples were irrigated using 5 ml of 17% EDTA (CanalPro; Coltene-operation, a structuring procedure was performed using Nrecon (Bruker MicroCT software. The before and after comparison of the specimens was performed using the isolog feature of DataViewer (Bruker MicroCT) software. The analyses of the specimens were performed using CTAn (Bruker MicroCT) software, and CTVox (Bruker MicroCT) software was used for imaging. The volumes of the voids were calculated using the method reported by Iglecias et al. 9 . CTVol (Bruker MicroCT) software was used for 3D imaging of the specimens.

Statistical analysis
To verify the assumption of normality, the data were analyzed using the Shapiro-Wilk test. A one-way analysis and post hoc Tukey's tests were performed for comparisons. The statistical analyses were performed using SPSS 21.0 software (IBM-SPSS Inc., Chicago, IL, USA). The statistical significance level was set at 5%.

Results
The percentage of void volumes (mean and standard deviations) in the three groups are summarized in Table  1. When the total void volume was calculated, the void volume in the WVC group was significantly lower than that in the GG and SC groups (P < .05). The total void volume in the GC group was significantly lower than that in the SC group. However, there was no significant difference in the void volume in the apical third among the three groups (P > .05). The void volumes in the middle and coronal thirds of the WVC group were significantly lower than those in the GC and SC groups (P < .05). In the GC group, the void volume was significantly lower than that in the SC group in the middle and coronal thirds (P < .05).
The void volume in the coronal third was significantly higher than that in the apical and middle thirds in the GC and SC groups (P < .05). The highest void volume was observed in the middle third in the WVC group (P < .05). There was no statistically significant difference in the void volume in the coronal and apical parts in the WVC group (P > .05).

Micro-CT assessment
The specimens were scanned using micro-CT (SkyScan 1172; Bruker-microCT, Kontich, Belgium) at 10 μm/pixel resolution and 10 kV and 100 100 μA power before and after root canal preparation. To reduce the level of noise, each image was taken three times, and the mean images were obtained. All the scanning procedures were performed at 360°, and the specimens were rotated by a 0.4° angle between each scan. Aluminum and copper filters were used to prevent beam hardening. After the scanning techniques, the SC technique resulted in significantly more voids in the coronal and middle thirds of the root canals. Castagnola et al. 23 reported that thermoplastic gutta-percha used in the GC technique better adapted to the root canal anatomy and therefore contained fewer voids on micro-CT than root canals filled using the SC system. Schafer et al. 24 used a digital stereomicroscope to assess void volumes and observed no voids at distances of 2 mm and 4 mm from the apical when using the GC technique. They also found that the GC technique resulted in fewer voids in the coronal third as compared with the void volume using the SC technique. Based on a micro-CT assessment, Çelikten et al. 25 reported that the SC technique led to more voids than root canal filling using the Thermafill technique. However, Somma et al. 14 detected no significant difference in void formation in single-root teeth after root canal filling using SC, System B, and Thermafill systems. The carrier-based filling technique is similar to the SC technique in terms of placing the gutta-percha into the canal in a single step at working length. Previous research attributed void formation using this technique to failure in placing the gutta-percha into the canal or gutta-percha escaping from the carrier and adhering to the canal walls in narrow and curved canals 14 . Zogheib et al. 26 reported that carrierbased systems provided better sealing than other systems, although they created voids. Other studies reported that a reduced amount of canal sealer was needed during root canal filling using carrier-based systems 27,28 . Another study found that the pressure applied to the apical region during filling decreased the formation of voids and improved adaptation of the filling material to the root canal 16 . The results of the present study were similar to those of some previous studies 22,23,29 , although there were methodological differences between the studies. Wu et al. 30 and Somma et al. 14 emphasized that a larger volume of canal sealer was required when using the SC technique as compared with that needed when using other filling techniques. When using the SC technique, voids form in the coronal and middle thirds when the master cone is placed into irregular-shaped canals 28 . Although the SC technique can be used for filling narrow and curved canals, it is not recommended for wide and oval-shaped canals, as it creates voids in the apical and middle thirds 5,23 .
In the present study, when void volumes using the different filling techniques were compared at different root canal levels, the void volume was highest in the middle third of roots in the WVC group. Heated guttapercha expands and then shrinks by 1-2% as it cools. During this process, voids form in the root canal walls 20 . The Calamus Dual 3D Obturation system has a hybrid structure. Previous research attributed the increased void formation in the middle third to shrinkage during the cooling process, as gutta-percha is in the beta phase in the

Discussion
The quality of obturation depends on various factors, such as the anatomical characteristics of the root canal system, type of root canal sealer, experience of the clinician, and obturation method 2 . Due to anatomical variations, some regions of the root canal cannot be accessed using endodontic devices, resulting in less than optimum root canal filling 10,11 . In the present study, the quality of SC, GC, and WVC root canal filling techniques in canals with anatomical variations (i.e., oval-shaped canals) was studied.
Various methods, such as liquid filtration, staining tests, radioisotopes, scanning electron microscope, analysis, and bacterial leakage, can be used to assess the quality of root canal filling 12,13 . However, these methods create irreversible damage to experimental models. An ideal experimental model should preserve the integrity of the specimen 14 . Thus, in accordance with previous research 2, 15,16 , in the present study, we used nondestructive and repeatable micro-CT analysis to obtain quantitative values and 3D imaging of voids left different void volumes were observed in the three groups at different root canal levels in the present study. Thus, the null hypothesis was rejected. In the present study, the void volumes in the coronal and middle thirds in the WVC group were significantly lower as compared with those in the other groups. Similar to the present study, in a micro-CT analysis study, Keleş et al. 2 reported that as compared with the lateral compaction technique, the WVC technique resulted in fewer voids in oval-shaped canals. In contrast, other studies that compared WVC and SC techniques reported no significant difference in voids left after root canal filling [17][18][19] .
In the present study, the void volume using the WVC technique was lower than that using the GC technique. Other studies that compared these two filling techniques using micro-CT reported no significant difference in the canal filling quality of two techniques 15,20 . Using confocal laser microscope analysis, which is a destructive technique, Marciano et al. 21 reported no significant difference between Thermafil (Dentsply Sirona), WVC, and SC techniques in terms of void volumes. The discordant results may be due to the type of tooth, quality of root canal preparation, type of canal sealer, and different assessment methods used. Previous research reported that existing voids might be compressed by compacting gutta-percha using pluggers while filling 22 . Compacting of warm gutta-percha in a vertical direction using the WVC technique may explain the reduced void volume in the WVC group in the present study as compared with that in the other groups.
According to the results of the present study, there was no significant difference between the filling techniques in terms of void volumes in the apical third. However, as compared with the GC and WVC apical third and in the alpha-phase in middle and coronal thirds during filling.
As is well known, voids in root canals allow residual microorganisms to reach the periapical region and induce inflammation or contribute to existing inflammation. The latter can have a negative effect on the success of endodontic treatment 3 . Further studies on the relationship between void volumes in root canals and the amount of bacterial leakage into the periapex under clinical conditions would shed light to what extent the void volume in root canals contributes to disease.

Conclusions
None of the systems provided completely homogeneous, void-free root fillings. The WVC system was associated with a lower percentage of void volume as compared with that of the other systems.