Introduction

An effectively sealed root canal is crucial to ensuring the long-term success of endodontic treatments, as it prevents the movement of oral fluids and periradicular exudate from the crown to the root canal system. This establishes a biologically favorable environment for adjacent tissues1,2. In addition, a three-dimensional root canal filling without any voids or gaps effectively inhibits the growth of microorganisms that have shown resistance to chemo-mechanical debridement3. Therefore, achieving a void-free root filling in the root canal system is extremely important for maintaining successful endodontic results.

Over the last decade, a remarkable evolution in the field of obturation materials has been sparked by the introduction of bioceramic (BC) sealers, especially EndoSequence BC sealer (BC, Brasseler), BioRoot™ RCS (Septodont, Saint-Maur-des-Fossés, France), and Total Fill BC sealer (TFBC; FKG Dentaire, La Chaux-des-Fonds, Switzerland). The unique properties of BC sealers make them a useful alternative to epoxy resin sealers; they are highly hydrophilic, biocompatible, and dimensionally stable, and their setting can be adjusted to the surrounding moisture in the root canal and dentinal tubules4,5.

BC sealers consist mainly of calcium silicates, calcium phosphate, zirconium oxide, and tantalum oxide. These bioactive materials interact with the surrounding tissues and encourage the formation of biological crystalline structures that resemble tooth and bone apatite materials, allowing the sealer to bond to the root dentin and form a high-quality seal6,7. Moreover, BC sealers possess antimicrobial properties due to their high pH and release of calcium ion during the setting process8. In vitro studies have shown that these BC sealers are nonstaining, have excellent physicochemical properties, and exhibit volume expansion while setting, good flowability, adequate radiopacity, and nonstaining properties9,10.

The excellent biological and physicochemical properties of BC sealers led to the traditional gutta-percha (GP)-based obturation approach being replaced with sealer-based obturation and to the single-cone (SC) concept being introduced11. This technique is based on hydraulic condensation: a pre-fitted GP cone is inserted into a canal space filled with BC sealer so that hydraulic pressure is formed, allowing three-dimensional obturation throughout the root canal system if a high-quality seal is used12. This simplified technique is efficient and cost-effective, avoids the risk of root fracture associated with lateral or vertical compaction, and offers a high-quality seal comparable to that of warm vertical compaction13. In this technique, the GP point acts as a vehicle to deliver the BC sealer while allowing a retreatment procedure if required.

Interfacial gaps and voids in filled root canal systems remain some of the most critical clinical challenges, and many confounding variables may affect the quality of obturation, such as the variability and complexity of root canal anatomy, the possibility of sealer shrinkage, and the particularity of the clinician’s experience14. Several methods have been developed for evaluating voids along the dentin–sealer interface within filled root canals. Microscopic examination of interfacial quality has been widely used, but it is a destructive technique and lacks the high resolution necessary to quantify small voids; furthermore, potential artifacts may be produced during specimen preparation15,16. Micro-computed tomography (micro-CT) is a three-dimensional imaging technique that permits high-resolution images using pixel sizes as small as 100 nm through a series of 2D planar X-ray images that can be further processed into 3D models17. This technique has been widely used to quantitatively evaluate the presence of interfacial gaps and voids in filled root canals completed using different materials and obturation techniques18,19,20.

The reviewed literature reveals a limited number of micro-CT studies that have compared void volume in root canals filled with GP and BC sealer using the single-cone technique to those filled using the continuous wave technique21,22. Therefore, this study was designed to compare void volumes in root canal fillings performed using the single-cone (SC) technique with those in fillings performed using the continuous wave condensation (CWC) technique, with BC sealer used in both cases. The null hypothesis was that there would be no difference in the volume of voids produced in root canal fillings performed using SC and CWC.

Materials and methods

This in vitro study was carried out following the appropriate guidelines and regulations at the College of Dentistry, King Saud University in Riyadh, Saudi Arabia. The human teeth utilized in this study were obtained from the oral surgery clinic at the College of Dentistry. These teeth had been previously extracted for orthodontic or periodontic purposes, as part of the patient’s dental treatment plan. The extraction of teeth was not conducted for the purpose of research, and there was no communication between the research team members and the patients involved. Therefore, the need for the consent form was waived, and the study protocol received approval from the Institutional Review Board, Health Sciences Colleges Research on Human Subjects sub-committee, College of Medicine at King Saud University (approval no. E-22-7112).

Sample size calculation

Based on the results of a previous study22, the sample size was determined using G*Power ver. 3.1.9.6 software (Franz Faul, Universität Kiel, Germany). A power beta of 0.90 was set. It was calculated that a minimum of 19 roots per group would be required to observe the same effect (1.08). We decided to increase the sample size to 20 per group to increase the study power (for an actual power of 0.9159).

Sample selection

A total of 40 human extracted single-rooted mandibular premolars were selected. To standardize the canal shape and curvature, preoperative digital radiographs were obtained in two projections (mesiodistal and buccolingual). The inclusion criteria specified the presence of single oval canals classified as Vertucci Type I23 and a curvature of 5 degrees or less as measured using the Schneider method24. The exclusion criteria ruled out teeth with more than one root canal, root caries, fractures, previous endodontic treatment, restorations, calcifications, internal or external resorption, or open apices.

Specimen preparation

Following the routine tooth disinfection protocol for operator safety, the teeth were thoroughly cleaned by removing the hard deposits using curettes and the soft deposits by soaking the teeth in 5.25% NaOCl for 10 min. They were then stored in a sterile normal saline solution at 37 °C until use. For standardization, the crowns were sectioned using a diamond disk (Keystone Industries, Gibbstown, NJ) at the cementoenamel junction under water cooling. The root canals were rinsed with 2.5 mL of 5.25% NaOCl. A size 10 K-file (Dentsply Maillefer, Ballaigues, Switzerland) was inserted until its tip became visible at the apical foramen, and the working length (WL) was determined as 0.5 mm shorter than this measurement according to Kuttler25. If a tooth presented with an initial size larger than an apical gauge of size 25, it was excluded. Each root was fixed in a customized plastic container using a polyvinyl siloxane impression material (Optosil base, Kulzer GmbH, Munich, Germany) to clinically simulate the alveolar position and allow the radiographic survey26.

Root canal preparation and obturation

Cleaning and shaping procedures were performed for all samples by an endodontist starting with size 2 Gates-Glidden burs (Dentsply Maillefer, Ballaigues, Switzerland) at 5000 rpm for coronal flaring followed by a K3 rotary nickel-titanium file system (SybronEndo, Orange, California) with a 0.04 taper up to size 30 and coated with RC-Prep (Dentsply Maillefer, Ballaigues, Switzerland). After shaping with each rotary file, the canal was rinsed with 1 mL 5.25% NaOCl. Finally, a copious irrigation with 5 mL 5.25% NaOCl was provided, followed by final irrigation with 5 mL 17% EDTA (SybronEndo, Orange, CA) and drying with paper points (Vericom Co., Ltd., Korea). The roots were randomly divided into two groups according to the obturation technique employed.

  • CWC group (n = 20): In this group, root canal fillings were performed using the CWC technique with a TotalFill BC GP point (FKG Dentaire, La Chaux-de-Fonds, Switzerland) and TotalFill HiFlow BC sealer (FKG Dentaire, La Chaux-de-Fonds, Switzerland). The sealer was initially introduced into the canal using the plastic syringe tip provided by the manufacturer. The GP cone was then coated with the same sealer and introduced into the canal to the determined WL. A 0.06 tip of the Alpha System (SybronEndo, Glendora, CA) was inserted into the canal to a depth of 5 mm shorter than the WL for the down-pack, followed by condensation with Dental Buchanan Hand Pluggers (SybronEndo, Glendora, CA). Backfilling was then performed using a Beta Obturation Gun (B and L Biotech, Italy) and GP bars (Meta-Biomed Co. Ltd., Cheongju-si, South Korea).

  • SC group (n = 20): In the second group, root canal fillings were performed using the SC technique with a TotalFill BC GP point (FKG Dentaire, La Chaux-de-Fonds, Switzerland) and TotalFill BC sealer (FKG Dentaire, La Chaux-de-Fonds, Switzerland). The sealer was initially introduced into the canal using a plastic syringe tip provided by the manufacturer. The GP cone was then coated with the same sealer and introduced into the canal to the determined WL. The Alpha System (SybronEndo, Glendora, CA) was used to cut the cone at the root canal orifice level.

All roots were obturated by a single operator. Master cone periapical radiographs were taken for all samples. The coronal access for all roots was sealed, and the specimens were stored at 100% humidity at 37 °C for 2 weeks.

Micro-CT scanning

Each sample was individually embedded in a polypropylene tube (1.5 cm in height and 1.5 cm in diameter) containing 3M™ impression material (Minnesota). The samples were then loaded into and mounted in the specimen chamber in a position pre-established in the micro-stage. All roots were scanned using the Bruker SkyScan 1172 high-resolution micro-CT system (Bruker SkyScan, Kontich, Belgium) at 90 kV and a 102 µA anode current, 316 ms exposure time, 19.70 µm image pixel size, 0.4 rotation step for a 360° angle, frame averaging of 4 (for an improved signal-to-noise ratio), and random movement of 8 to minimize ring artifacts. A flat-field correction was performed before the scanning procedure to correct variations in camera pixel sensitivity. After the scan, a reconstruction of the projected images was performed using NRecon reconstruction software ver. 1.6.9.4 (Bruker Skyscan, Kontich, Belgium) to produce reconstructed cross-sectional images. The numerical parameters required to establish the best image results were checked and adjusted. A ring artifact reduction of 5 was applied to ensure the nonuniformity of the background image taken by the X-ray camera, 25% beam-hardening compensation to prevent the specimen from appearing artificially denser at or near its surface and less dense at its central parts, and smoothing of 2 using a Gaussian kernel. A 16-bit TIF file format was selected to save the images due to variations in specimen density.

Obturation quality evaluation

©CTAn software version 1.20.8.0 (Bruker Skyscan, Kontich, Belgium) was used for a morphometric analysis. Segmentation, or binarization, was performed for each cross-sectional image of each root third (coronal, middle, and apical). Void volumes in mm3 were then measured based on the black pixels within the canal space.

Statistical analysis

Descriptive and inferential statistics were calculated using IBM SPSS ver. 25 (IBM Corp., Armonk, NY). An independent t-test was applied to compare the presence of voids in the groups (CWC and SC), as the data were approximately normally distributed. A two-way analysis of variance (ANOVA) was conducted to test the interactions between the groups and locations (coronal, middle, and apical), followed by a one-way ANOVA (the Welch test) to compare the locations within each group. Dunnett’s T3 was performed as a multiple comparison test. The α level of significance was set at a p-value of ≤ 0.05.

Results

The data are presented as mean values and standard deviations (Table 1). Voids were detected in all filled root canals in both groups regardless of the obturation technique used (SC or CWC). The overall mean void volume in the SC group (0.253 ± 0.186) was significantly higher than that in the CWC group (0.035 ± 0.043; p = 0.000; Fig. 1).

Table 1 Mean and standard deviation (SD) of voids in each anatomical third for each group.
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Fig. 1
figure 1

Overall mean void volume (mm3) in root filling material by experimental group. CWC continuous wave condensation, SC single cone.

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In the CWC group, although the percentage of voids inside the filling material decreased from the coronal to the apical third, the percentage between the root canal thirds showed no statistically significant difference in any comparisons (p > 0.05). In the SC group, the percentage of voids was significantly higher (p = 0.000) in the coronal third (0. 197 ± 0.159%) than in the middle third (0.048% ± 0.063%) and apical third (0.008% ± 0.015%) (Table 1, Fig. 2). When the percentage of voids within each anatomical third was compared by group, the percentage of coronal voids was found to be significantly higher (p = 0.000) in the SC group than in the CWC group. Similarly, the percentage of voids in the middle third in the SC group was significantly higher (p = 0.017) than that in the CWC group. The percentage of voids in the apical third was the lowest among the thirds, and there was no statistically significant difference between the groups (p = 0.981) in this value, at 0.008 ± 0.020% in the CWC group and 0.008 ± 0.015% in the SC group (Fig. 2).

Fig. 2
figure 2

Mean void volume in filling material in each anatomical root third (coronal, middle, and apical) by group (CWC and SC). CWC continuous wave condensation, SC single cone.

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Cross-sectional and three-dimensional micro-CT images revealed that the apical thirds of the roots were well-sealed, with few voids, and there was no significant difference between the groups. However, in the middle and coronal thirds, the SC group had significantly more voids (Figs. 3, 4, 5 and 6; Table 2).

Fig. 3
figure 3

Micro-CT images of the three different root thirds (apical, middle, and coronal) of samples from the CWC group (top) and the SC group (bottom). Each sample in this figure demonstrated the lowest volume of voids within its respective group. However, more voids can be seen in the SC group (0.01358 mm3) than in the CWC group (0.00119 mm3). CWC continuous wave condensation, SC single cone.

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Fig. 4
figure 4

Colored 3D images of a sample from the CWC group (top) and another sample from the SC group (bottom). The orange areas represent the filling material, and the green areas represent voids. Each sample in this figure demonstrated the fewest voids within its respective group. However, voids can be seen more in the SC group sample (0.01358 mm3) compared to the CWC group (0.00119 mm3). CWC continuous wave condensation, SC single-cone.

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Fig. 5
figure 5

Micro-CT images of three different root thirds (apical, middle, and coronal) of samples from the CWC group (top) and the SC group (bottom). Each sample in this figure demonstrated the highest volume of voids within its respective group. However, more voids can be seen in the SC group (0.3401 mm3) than in the CWC group (0.09621 mm3). CWC continuous wave condensation, SC single cone.

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Fig. 6
figure 6

Colored 3D images of a sample from the CWC group (top) and another sample from the SC group (bottom). The orange areas represent the filling material, and the green areas represent voids. Each sample in this figure demonstrated the lowest volume of voids within its respective group. However, more voids can be seen in the SC group (0.3401 mm3) than in the CWC group (0.09621 mm3). CWC continuous wave condensation, SC single cone.

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Table 2 Multiple comparison test between different anatomical thirds for each group.
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Discussion

High-quality, void-free, three-dimensional obturation of the root canal system is crucial in maintaining the long-term success of endodontic treatment3. This micro-CT study evaluated the quality of obturations accomplished with the SC technique in comparison to those achieved with the CWC technique, with BC sealer used in both groups. A literature search revealed that few micro-CT studies have evaluated void volume in root canals filled with GP and BC sealer using the SC technique in comparison with the CWC technique21,22. However, these studies used 3D-printed teeth. The current study used extracted human mandibular premolars with single oval-shaped canals, which are known to be difficult to adequately shape and obturate27,28,29,30. The wide buccolingual dimension of the canal and the presence of unprepared areas could explain the presence of voids within the root canal filling in both groups in this study, regardless of the obturation technique used.

The results of the present study showed a higher void volume in the SC group than in the CWC group in the coronal and middle thirds. This is consistent with previously documented studies that suggested the superiority of the CWC technique in filling anatomically complex root canals, mainly in the middle and coronal thirds31,32,33. This could be explained by the use of thermoplastic GP during backfilling with the CWC technique, as the GP flows when heated and fills the root canal system34. In 2021, Zhang et al. evaluated the obturation quality of root canals filled with GP and iRoot SP sealer using SC and CWC techniques. They assessed void volumes in the isthmus area in artificial teeth, finding that the CWC technique was superior to the SC technique21.

In contrast, the findings of other investigations contradict the results of the present study. Soma et al. found no significant difference in void distribution after root canal obturation when they compared the SC and CWC techniques based on micro-CT analysis35. Kim et al. reported that the CWC group had significantly higher void volumes than the SC group in the coronal third. The authors explained that voids in the coronal third in the CWC group might have emerged in the transition site at the cut-off level when the injected GP heated up and could not be properly condensed22. This discrepancy in the results of previous studies could be due to differences in the types of sealers used, types of teeth used, sample size, insertion technique, and level of clinical experience of the operator.

In the current investigation, the two obturation techniques (SC and CWC) showed comparable results in apical void volumes, which could be attributed to the fact that the techniques applied the same method of adjusting the matched-cone size and taper to the apical preparation. The significant increase in void volume in the SC obturation technique from the apical third to the coronal third could be attributed to the affinity of the matched cone size for the apical preparation size, and the tendency of the canal to have a circular shape in the apical third36.

All root canal filling materials and techniques are associated with gaps and voids18,37,38,39. Yet the introduction of BC sealers produced a paradigm shift in the obturation process by enabling the SC approach. Penha da Silva et al. reported that the quality of obturation achieved with the SC technique and BC sealer was comparable to that achieved with the cold lateral technique38. However, some studies have questioned the quality of obturations performed using the SC technique, even when BC sealers were used. It was reported that root canal fillings with BC sealer had the same void volumes as any other type of sealer when used with the SC technique37. Furthermore, when used in the SC method, BC sealer had higher void volumes compared to the cold lateral approach and Thermafill18.

BC sealers are characterized by excellent flowability, good dimensional stability, and strong sealing ability10,40,41. However, conventional BC sealers will not tolerate the high temperature present during the down-pack stage of the CWC process42. Therefore, in the present study, the TotalFill HiFlow BC sealer (FKG Dentai0re, La Chaux-des-Fonds, Switzerland) was used with the CWC obturation technique, as recommended by the manufacturer, and the conventional TotalFill BC sealer was used with the SC obturation technique. The Hiflow BC sealer is more expensive than the conventional BC sealer; its only advantage is its heat tolerance, which was not necessary for the SC obturation technique. The current study aims to assist clinicians in selecting the most effective obturation technique for use with BC sealers while minimizing associated costs.

These two sealers have similar chemical compositions, with similar percentages of carbon, oxygen, and silicon; however, the amounts of calcium and zirconium vary43. Previous studies have shown that under high temperatures, HiFlow sealers have lower viscosity, better flowability, and shorter setting times than conventional BC sealers; the manufacturer recommends that they be used with warm condensation techniques and temperatures up to 200 °C44. On the other hand, the conventional TotalFill BC sealer, under heat, had an extended setting time, increased film thickness, decreased flow, increased shrinkage, and surface deformation42. On the contrary, another study showed that, when heated, both sealers’ chemistry was modified. However, both recovered when cooled. Therefore, they recommended using TotalFill BC sealer with the warm vertical compaction technique because its efficacy resembled that of HiFlow at a lower cost45. Micro-CT was used in this study due to its high accuracy, reproducibility, and noninvasiveness46. It provides detailed 3D images and evaluations of the quality of root canal obturations, including the presence of gaps47.

This in vitro study has some limitations. First, the obturated root canals had minimum curvature. Therefore, the results cannot be generalized to all root canals and morphologies. Second, decoronation of the roots was performed with all samples for standardization purposes, which does not reflect the real clinical scenario. Finally, different BC sealer formulas were used for each group based on the manufacturer’s recommendation. The improved flowability of the HiFlow sealer with the CWC obturation technique at high temperatures44 could be attributed to the superior performance of the CWC approach compared to the SC obturation technique in reducing void volumes. Therefore, for future studies, it is recommended to use the same BC sealer formula with different obturation techniques.

Conclusion

The obturation quality achieved with the CWC technique and TotalFill Hiflow BC sealer was superior to that of the SC technique with conventional BC sealer in oval-shaped premolars, particularly in the middle and coronal thirds.