Effect of Bioceramic-Based Obturation Techniques on the Amount of Residual Filling Material after Retreatment: An In Vitro Study

Abstract

Objectives

The aim of this article is to quantitatively assess the amount and distribution of residual filling after retreatment of root canals obturated with a bioceramic sealer using three obturation techniques: single-cone technique (SCT), warm vertical compaction (WVC), and cold lateral compaction (CLC).

Materials and Methods

Forty-two mandibular premolars were prepared up to size F4 and randomly assigned to SCT, WVC, or CLC (n = 14/group). All canals were obturated with a bioceramic sealer. After storage for 15 days at 37 °C, retreatment was performed using the ProTaper Universal Retreatment (PTUR) without solvent. Roots were sectioned longitudinally, and residual fillings in coronal, middle, and apical thirds were quantified using stereomicroscopy and analyzed with ImageJ. Intra-examiner reliability was assessed using intraclass correlation coefficients.

Statistical Analysis

Nonparametric Kruskal–Wallis test was used to compare residual filling percentages among the three techniques, followed by pairwise comparisons with the Mann–Whitney U-test. Statistical significance was set at α = 0.05.

Results

No statistically significant differences were observed among the three obturation techniques in the percentage of remaining fillings (p = 0.0657). SCT exhibited the highest percentage of residuals (86.31 ± 14.49%), and CLC the lowest (62.66 ± 31.90%). All techniques exhibited a progressive increase in residuals toward the apical third.

Conclusion

None of the obturation techniques allowed complete removal of bioceramic-based fillings after retreatment with PTUR. Although differences were not statistically significant, CLC was associated with the lowest amount of residuals, whereas SCT exhibited the highest. The apical third remained the most challenging region to clean across all techniques.

Keywords

bioceramic sealer - cold lateral compaction - single-cone technique - warm vertical compaction - rotary retreatment - root canal retreatment

Introduction

Nonsurgical endodontic retreatment is considered the first-line conservative approach for managing failed root canal treatments. Although effective, it is often more complex and time-consuming than initial treatment.[1] The objective of retreatment is to eliminate persistent microbial contamination within the root canal system by completely removing existing root filling materials, regaining apical patency, and addressing any anatomical or iatrogenic complications that may harbor infection. A thorough removal of root filling is essential to allow irrigants and medicaments to reach the apical terminus, thereby improving the likelihood of periapical healing.[2]

The effectiveness of filling material removal during retreatment is influenced by several factors. Tooth-related variables such as canal morphology, curvature, and anatomical complexity can significantly affect the ability of instruments to contact all internal surfaces. Technique-related aspects such as file design, alloy, taper, kinematics of the instruments, and removal protocol also play a significant role.[3] [4] Additionally, the type of obturation technique and sealer material used in the initial treatment directly influences the difficulty of retreatment. Studies have shown that sealers with deep dentinal penetration or strong adhesion to canal walls pose increased difficulty during removal.[5] [6] Among the commonly employed obturation techniques are cold lateral compaction (CLC), warm vertical compaction (WVC), and the more recent single-cone technique (SCT). While WVC provides better adaptation to canal irregularities, SCT has gained popularity due to its simplicity, especially when used with premixed calcium silicate–based bioceramic sealers. However, the large volume of sealer used in SCT, combined with its high flow and chemical bonding to dentin, may lead to deeper dentinal penetration and compromise retrievability.[7]

Bioceramic-based sealers are composed primarily of calcium silicate and other bioactive ingredients known for their superior biocompatibility and ability to promote tissue healing. They promote hydroxyapatite formation and chemical bonding at the sealer–dentin interface, thereby enhancing long-term sealing ability.[8] The physicochemical interactions of calcium silicate sealers with dentin, including hydration reactions, alkalinity, and tubule penetration, have a fine particle size, resulting in deep penetration into dentinal tubules and anatomical irregularities. While these properties are advantageous for sealing, they make complete removal during retreatment significantly more difficult. As bioceramic sealers harden and integrate within dentin, they may contribute to higher levels of residual filling material after retreatment.[9]

A variety of techniques and instruments have been proposed for the effective removal of root canal fillings, including manual hand files, and engine-driven nickel-titanium (NiTi) rotary files, either continuous rotation or reciprocating systems.[10] Continuous rotation retreatment systems, such as the ProTaper Universal Retreatment (PTUR) system (Dentsply Maillefer, Ballaigues, Switzerland), have demonstrated improved efficiency compared with manual methods.[11] This rotary system facilitates softening of gutta-percha and coronal transportation of residual filling material, while the system's specialized design enhances cutting action. The PTUR sequence consists of three files with convex cross-section and variable taper: D1 (30/.09), D2 (25/.08), and D3 (20/.07), each structured to target specific portions of the canal.[12] Previous studies have reported that PTUR is effective and time-efficient for removing gutta-percha and resin-based sealers.[13] [14] Nevertheless, the literature consistently shows that no existing instrument or technique is capable of achieving complete removal of root canal fillings, particularly in anatomically complex regions.[15] Understanding the performance of NiTi systems in removing obturation materials, especially those involving bioceramic sealers, is essential for improving retreatment outcomes.[16]

Although numerous studies have investigated the retreatability of different obturation materials and sealers, only limited evidence is available comparing the residual filling material remaining after retreatment of canals obturated with bioceramic sealers using SCT, WVC, and CLC. Therefore, the present study aimed to quantitatively evaluate the amount and distribution of residual filling material after retreatment using the PTUR system in canals obturated with these three common techniques. The null hypothesis was that no significant differences would be found between obturation techniques in the quantity of residual filling material after retreatment.

Materials and Methods

Sample Size Estimation

The sample size was calculated using G*Power 3.1 software (Heinrich-Heine-Universität Düsseldorf, Germany), based on data from previous studies.[12] [17] Assuming an effect size of 0.5, a statistical power of 0.80 (1–β), and a significance level (α) of 0.05, the minimum required number of specimens was calculated to be 14 teeth per group.

Sample Selection

Forty-two extracted human mandibular premolars with single roots were collected from the Misr International University (MIU) tooth bank. All teeth were extracted for orthodontic or periodontal reasons and were selected in accordance with strict inclusion and exclusion criteria. Prior to sample collection, approval was obtained from the Research Ethics Committee of Misr International University (registration no: MIU-IRB-2122–155), and informed consent was obtained from all patients that their extracted teeth would be utilized for research purposes.

Teeth were disinfected in 2.5% sodium hypochlorite for 2 hours, then thoroughly cleaned of soft tissue remnants using an ultrasonic scaler. The canal morphology and curvature were assessed using digital radiographs taken in both buccolingual and mesiodistal views. Root curvature was calculated according to Schneider's method.[18] Teeth were examined under an operating microscope with ×20 magnification (Leica M320; Leica Microsystems, Wetzlar, Germany) to confirm anatomical suitability. The inclusion criteria were as follows: oval and straight canals with a mature apex, a single root canal, a single apical foramen, and a root curvature ≤10°. Teeth with resorption, caries, cracks, calcifications, pulp stones, or prior treatment were excluded.

Sample Preparation and Obturation

The crowns were separated using a diamond disk at 16 mm to standardize the tooth length. Apical patency was confirmed using a K-file size 10 (Mani Inc., Tochigi, Japan) till violation of the apex. Working length was established as 1 mm shorter than the length where the tip of the #10 K-file was visualized at the apical foramen. All canals were prepared under magnification by a single operator using Universal ProTaper rotary files (Dentsply Maillefer, Ballagaiues, Switzerland) up to size #F4 (40 / 06), operated by an NSK Endo Mate Endo motor (Nakanishi Inc., Tochigi, Japan) at 350 rpm. Irrigation was performed between each file with 5 mL of 2.6% NaOCl (Cerkamed, Stalowa Wola, Poland) using a 30-G side-vented needle (NOP Dental Needles, Spident, Korea), followed by 1 mL of 17% ethylene diamine tetra-acetic acid (EDTA) for 1 minute and a final rinse with saline. Apical patency was regularly checked with the #10 K-file during preparation and after final irrigation. The canals were dried with paper point size #F4, and obturated using a bioceramic sealer (Sure-Seal Root, Sure Endo, Gyeonggi-do, South Korea) with three obturation techniques: SCT, WVC, or CLC (n = 14 per group). Randomization was performed using www.randomizer.org. After obturation, access cavities were sealed with temporary filling material (Cavit G; 3M ESPE, Germany) and radiographed to confirm obturation quality; none of the samples were excluded from the study because of incomplete filling. All root canal fillings have been done by a single operator to ensure consistency. Samples were stored at 37 °C and 100% humidity for 15 days.

Retreatment Procedure

Root canal retreatment was done using PTUR files in a crown-down approach with a brushing motion at 350 rpm and 2.5 Ncm torque. The D1 file (30/.09) was used to remove coronal material, the D2 file (25/.08) for the middle third, and the D3 file (20/.07) for the apical third until full working length was reached using gentle apical pressure. Files were cleaned after each use, and canals irrigated with 2 mL of distilled water using a 30-G needle. The files were discarded after six uses. Retreatment was considered complete when the last file reached full working length and no visible filling material remained on the flutes or in the irrigation solution. All procedures were performed by a single operator, who was not blinded to the obturation technique because it was easy to differentiate the specimens after obturation.

Evaluation of the Residual Filling

Following retreatment, access cavities were sealed with cotton pellets. Roots were grooved longitudinally on the mesial and distal surfaces using a precision diamond saw (Isomet Corporation, Virginia, United States) and split. The half showing more residuals was examined under a stereomicroscope at 20× magnification (Leica Microsystems, Switzerland) and photographed. Images were processed in Photoshop (Adobe Photoshop 7.0, Adobe Systems Inc., San Jose, California, United States) for segmentation and analyzed using ImageJ (v1.53a, NIH, United States) following a previously reported method.[19] The entire canal lumen was outlined using the Freehand Selection tool and then separated from the rest of the image. The set scale function was applied to present the outcome measurements in millimeter squares, and the total canal area was measured and recorded as (A_total). Each image was converted to 8-bit grayscale to standardize contrast for analysis, and residual filling material was identified using the color threshold function, and areas corresponding to residues were isolated and measured (A_residual). Measurement was done for the whole canal and separately for each third without differentiation between residual gutta-percha and residual sealer. The percentage of residual filling material was calculated as:

To assess the distribution of residuals across different sections of the canal, the root canal was divided into three equal parts: coronal, middle, and apical thirds. Each third was individually outlined using the Rectangular Selection tool, and the corresponding regions of interest (ROIs) were saved in the ROI Manager for measurements ([Fig. 1]). All measurements were done blindly by a single operator. Intraexaminer reliability was assessed by repeating measurements on ten randomly selected specimens after 2 weeks, yielding intraclass correlation coefficients of 0.92 for area and 0.89 for percentage, indicating excellent reliability.

Fig. 1 The specimens were examined under a stereo microscope at 20× magnification and photographed. (A) The half with the most intact root canal was used. (B) The photographs were processed using photographic editing software. (C) The entire canal lumen was outlined using the freehand selection tool and then separated from the rest of the image, and the total canal area was measured, as well as the total area of each third was measured. (D) Residual filling material was identified using the color threshold function without differentiation between residual gutta-percha and residual sealer. (E) Areas corresponding to residues were isolated and measured. (F) Areas covered by residuals were divided by the total area of each third, multiplied by 100 to obtain the percentage of residual filling material for each third.

Statistical Analysis

The normality of data distribution was tested using the Shapiro–Wilk test. As the data were not normally distributed, non-parametric tests were applied. The Kruskal–Wallis test was used for comparing more than two groups, followed by pairwise comparisons using the Mann–Whitney U-test. The level of significance was set at p ≤ 0.05 for all tests. Statistical analyses were conducted using IBM SPSS version 25.0 (IBM Corp., United States).

Results

Representative stereomicroscopic images are shown in [Fig. 2]. Mean and standard deviation (± SD) values of residual filling material areas (mm²) in the three anatomical thirds are presented in [Table 1] . The Kruskal–Wallis test revealed no statistically significant differences in the percentage of remaining filling material among the three obturation groups (p = 0.0657). Comparison among canal thirds showed that the middle third displayed the highest variability, with a near-significant result (p = 0.0528).

Table 1 Mean of residual filling materials area mm², ± SD, median, interquartile range (IQR), minimum, maximum, the lower and upper bounds of the 95% confidence interval (CI) values in the three anatomical thirds

		Mean	SD	Median	IQR	Min	Max	CI—lower	CI—upper
Coronal	SCT	2.72	0.52	2.76	0.53	1.58	3.51	2.43	2.98
	WVC	2.48	0.91	2.77	0.74	0.32	4.08	2.01	2.92
	CLC	2.02	1.02	2.29	0.98	0.15	3.20	1.50	2.51
Middle	SCT	2.17	0.42	2.25	0.33	1.31	2.65	1.94	2.36
	WVC	2.28	0.63	2.34	0.73	0.62	3.18	1.96	2.58
	CLC	1.47	1.01	1.76	1.90	0.03	2.81	0.93	1.93
Apical	SCT	1.38	0.55	1.54	0.65	0.28	1.99	1.09	1.63
	WVC	1.46	0.51	1.58	0.60	0.43	2.04	1.16	1.70
	CLC	1.22	0.69	1.35	1.11	0.12	2.15	0.86	1.56

Fig. 2 Representative stereomicroscopic photograph 20× showing the remaining filling material of roots obturated with single cone technique (A), warm vertical compaction (B), and cold lateral compaction (C).

Within the SCT group, the mean residual material values were relatively uniform across all thirds, and the Mann–Whitney U-test confirmed no significant intragroup differences (p > 0.05). The WVC group exhibited consistent results, particularly in the apical third, but again without statistically significant differences among thirds. The CLC group showed an increase in residual material from the coronal to the apical third (coronal < middle < apical), as illustrated in [Figs. 3] and [4]; however, these differences were not statistically significant (p > 0.05).

Fig. 3 Heatmap displays quantitative measurements of residual filling materials in millimeter squares for three obturation methods (single cone, warm vertical compaction, and lateral compaction) across three root canal thirds (coronal, middle, and apical).

Fig. 4 Box plot comparison of residual filling material area (mm²) across root canal regions and obturation techniques. The plot displays the distribution of residual filling material measurements in coronal, middle, and apical thirds of root canals for the three different obturation groups: Group A (single-cone technique), Group B (warm vertical compaction), and Group C (lateral compaction).

The overall percentage of root canal area covered by residual filling material ranged from 62.66 to 86.31% across all specimens. The SCT group exhibited the highest mean percentage of remaining material in the full root (86.31 ± 14%), while the CLC group demonstrated the lowest (62.66 ± 32%). Despite these numerical differences, there were no statistically significant differences in the mean residual percentages between the three groups. In all groups, a progressive increase in residual filling material from the coronal to the apical third was observed ([Fig. 5], [Table 2]), although this trend was not statistically significant.

Table 2 Mean ± SD and intergroup comparison of residual filling materials percentage for the three thirds employing the tested obturation techniques

	Coronal%	Middle%	Apical%	p-Value	Total remnant
SCT	84.73 ± 18.35	87.32 ± 16.72	90.19 ± 7.76	0.7651	86.31 ± 14.49
WVC	75.41 ± 31.51	81.72 ± 22.94	86.66 ± 13.65	0.9408	77.22 ± 22.61
CLC	61.21 ± 32.96	75.15 ± 22.76	82.33 ± 16.47	0.3567	62.66 ± 31.9
p-Value	0.196	0.4083	0.6055		0.0657

Fig. 5 Box plot comparison of residual filling material percentage in Group A (single cone), Group B (warm vertical compaction), and Group C (lateral compaction). For each group, the percentage of area fraction of the residual filling material is shown for the coronal, middle, and apical thirds of the root canal.

Discussion

The complete removal of root canal filling material during retreatment remains a significant challenge in endodontic treatment. Several factors influence the effectiveness of filling material removal, including root canal anatomy, the obturation technique, and the physicochemical properties of the sealer used.[20] The present study aimed to evaluate the amount of residual filling material after the retreatment of root canals filled with a bioceramic sealer using three different obturation techniques. While no statistically significant differences were found among the groups, the results suggest a trend indicating that the obturation technique may influence the ease and effectiveness of filling material removal; thus, the null hypothesis cannot be rejected. The lack of statistically significant differences among groups agrees with previous reports that no current system or technique ensures the complete removal of filling materials from the canal walls.[6] [21] Residual obturation material may remain adhered to canal irregularities, isthmuses, or dentinal tubules, even after the use of advanced rotary systems. In the present study, residual filling percentages ranged from 62.66 to 86.31%, consistent with findings by Tomar et al[3] and Rödig et al,[10] who reported incomplete removal of gutta-percha and sealers, regardless of the technique used.

The higher mean percentage of residual material observed in the SCT group may be attributed to the greater sealer-to-gutta-percha ratio characteristic of this obturation method. Bioceramic sealers used in the single-cone approach penetrate deeply into dentinal tubules due to their fine particle size, hydrophilicity, and chemical bonding capacity with calcium phosphate precipitation at the sealer–dentin interface.[22] [23] These properties, while beneficial for sealing and biocompatibility, hinder retrievability during retreatment. Muedra et al[24] demonstrated that premixed calcium silicate–based sealers can undergo mineral infiltration into dentin and form hydroxyapatite crystals, reinforcing the interface and resisting mechanical removal. The present findings align with those of Kim et al,[25] who also reported that calcium silicate sealers and resin-based sealers have similar efficacy in dentin penetration and retreatment efficacy. Also consistent with Uzunoglu et al,[26] who assessed three different endodontic sealers and found that none of the tested sealers were completely removed. These results also came in accordance with a previous study, which demonstrated that the highest percentage of remaining filling was found in SCT with bioceramic sealer, while the lowest percentage was found in CLC with resin sealer.[27] This is attributed in part to the bioactivity of the calcium silicate sealers and the formation of strong chemical bonding with root dentin.[23] Also, the finding is in accordance with a previous study comparing WVC to CLC using micro CT.[28]

In contrast, the CLC group showed the lowest percentage of residual filling material. This may be explained by the reduced sealer volume and mechanical spreading of gutta-percha in CLC, which limits the depth of sealer penetration and facilitates dislodgment during retreatment. Athkuri et al[28] similarly found that the fillings placed using CLC were easier to remove than those placed using the thermoplasticized technique, particularly when bioceramic sealers were employed. In all groups, residual filling material increased progressively from the coronal to apical third, a pattern consistent with earlier studies.[28] [29] [30] This finding can be attributed to the washing-out effect of irrigation, the reduced canal diameter, limited irrigant flow, and increased anatomical complexity in the apical third.[25] Moreover, the smaller size and taper of the D3 ProTaper Universal Retreatment file (20/.07) may limit cutting engagement in the apical region compared with the larger final preparation size (F4, 40/.06).[31]

Although rotary systems such as PTUR are widely regarded as effective for retreatment, the present study confirmed that significant residue remained, particularly in the apical third. In previous studies, the use of engine-driven instruments, whether in rotating or reciprocating motions, results in high cleaning ability.[2] [32] The impact of motion kinematics, file design, and the number of files used for root canal filling removal on the remaining fillings has been studied previously.[33] [34] The absence of radial land and negative cutting angle in PTUR produces both cutting and softening action on the gutta-percha, while the triangular cross-section of the files reduces the contact area with dentin walls and leads to a low risk of fracture and less debris extrusion.[35] In the present study, the PTUR system demonstrated effective but incomplete filling removal, consistent with prior research, which showed that no technique or instrument can completely remove root canal-filling material during retreatment.[36] [37] [38] This could be attributed to the fact that rotary files, which have a helical design and a spiral blade, prepare the root canal into a round cross-section when rotating. This process often leaves untouched areas on the buccal and lingual sides of the canal, resulting in incomplete removal of the root canal filling material.[39] Importantly, this study intentionally excluded solvent use to avoid the formation of a thin layer that is hard to remove and to facilitate the evaluation of the residual filling material after filling removal.[40]

The findings of this study should be interpreted in light of certain limitations. The relatively small sample size and the absence of simulated apical resistance may limit the generalizability of the results. Under clinical conditions, periapical tissues act as a natural barrier that can reduce both apical debris extrusion and influence the amount of residual material retained within the canal system. Therefore, the current in vitro findings may not fully replicate in vivo outcomes.[7] Additionally, the operator performing the retreatment procedures was not blinded to the obturation technique, which may have introduced a potential source of performance bias. Although standardized protocols and a single experienced operator were employed to minimize variability, blinding could further strengthen the reliability of future investigations. The use of two-dimensional stereomicroscopic and digital image analysis, while reproducible, provides only surface measurements and may underestimate the true volume of residual material compared with three-dimensional micro-computed tomography (micro-CT) analysis. Furthermore, clinical factors such as curved canal anatomy, variations in dentin hardness, and the use of solvents were not simulated in the present study, all of which may influence the retrievability of bioceramic sealers under clinical conditions.

Future research incorporating micro-CT or confocal microscopy is recommended to provide more detailed volumetric and spatial characterization of residual filling materials, particularly in teeth obturated with bioceramic-based sealers. Studies with larger and more diverse sample sizes are also warranted to validate these findings and to explore the chemical composition of remnants after retreatment. Despite these limitations, the present results highlight the clinical importance of obturation technique selection. When using bioceramic sealers, clinicians should consider that single-cone fillings may pose greater challenges during retreatment and may benefit from adjunctive strategies such as ultrasonic activation, reciprocating files, or solvent-assisted methods to enhance cleaning efficiency and retreatment outcomes.

Conclusion

Within these experimental conditions, all obturation techniques used in this study, in conjunction with a bioceramic sealer, resulted in substantial residual filling material following retreatment with the ProTaper Universal Retreatment system. Although no statistically significant differences were found between the groups, the SCT technique demonstrated the highest percentage of residual material, while CLC showed the lowest. A trend of increasing residuals from coronal to apical thirds was observed in all groups, with the apical third remaining the most difficult area to clean. These findings suggest that the choice of obturation technique may influence the retrievability of bioceramic-based materials and should be considered during case planning.

Conflict of Interest

None declared.

Acknowledgments

The authors thank Ajman University for supporting the publication of this article.

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Publication History

Article published online:
04 February 2026