Comparative Evaluation of Obturation Techniques on the Retrievability of NeoSealer Flo Bioceramic Sealer: An Ex Vivo Study

Summary

This ex vivo study investigates the impact of single cone and cold lateral compaction obturation techniques on the retrievability of NeoSealer Flo bioceramic sealer during endodontic retreatment. Results demonstrate that the single cone technique allows for more effective retrieval than cold lateral compaction.

Abstract

The successful retreatment of root canal-treated teeth depends largely on the effective removal of existing filling materials. This study aimed to evaluate the influence of two obturation techniques, single cone (SC) and cold lateral compaction (CLC), on the retrievability of NeoSealer Flo bioceramic sealer during endodontic retreatment. Twenty-four extracted single-rooted mandibular premolars with standardized root canal morphology were mechanically prepared and randomly allocated into two equal groups according to the obturation technique employed (n = 12). Following obturation with NeoSealer Flo, specimens were stored for 14 days under controlled conditions to allow complete setting of the sealer. Retreatment procedures were then performed using rotary retreatment files according to manufacturers’ recommendations. The amount of residual filling material remaining after retreatment was assessed in the coronal, middle, and apical thirds of each root using stereomicroscopic examination and quantified through ImageJ software analysis. Additionally, the total time required to complete retreatment procedures was recorded. Statistical analysis was performed with a significance level set at p < 0.05. The results demonstrated that canals obturated using the CLC technique had significantly greater residual filling material in the middle and apical thirds compared to the SC technique. No statistically significant difference was observed between groups regarding retreatment time. Within the limitations of this in vitro study, the findings suggest that NeoSealer Flo is more readily removed when used with the single cone obturation technique, highlighting the importance of obturation method selection when bioceramic sealers are employed in clinical practice.

Introduction

A successful endodontic outcome depends on achieving and maintaining an effective three-dimensional seal of the root canal system, thereby preventing bacterial recontamination and supporting long-term periapical health¹. In recent years, the advent of bioceramic sealers has shifted the obturation paradigm from one focused primarily on mechanical filling to an approach that emphasizes bioactivity and chemical interaction with dentin². Resin-free bioceramic materials such as NeoSealer Flo are formulated to release calcium hydroxide during their setting reaction, promoting an alkaline environment with antimicrobial properties and facilitating the formation of a hydroxyapatite-like interfacial layer^3,4. This bioactive bond is intended to reduce interfacial gaps, enhance sealing ability, and potentially improve periapical healing.

Despite these advantages, the same physicochemical properties that enhance the clinical performance of bioceramic sealers may compromise their retrievability during orthograde retreatment. Unlike conventional epoxy resin-based sealers, bioceramic materials exhibit increased hardness after setting and a chemical affinity for dentin, which may hinder complete removal from the canal walls. Previous studies have reported that residual bioceramic sealer is frequently observed after retreatment, even when contemporary rotary or reciprocating systems are used^5,6,7. The obturation technique employed may further influence retreatment outcomes, as sealer distribution, thickness, and penetration into dentinal tubules differ among techniques.

The single cone (SC) technique, often advocated for use with bioceramic sealers, relies on sealer flow to achieve adaptation, resulting in a relatively uniform but potentially thinner sealer layer. In contrast, cold lateral compaction (CLC) introduces additional gutta-percha mass and may generate greater sealer penetration and compaction pressure, which could enhance initial sealing but complicate subsequent removal. Alternative obturation approaches, such as warm vertical compaction or carrier-based techniques, have also been associated with increased residual material during retreatment, underscoring the clinical importance of obturation method selection when long-term retreatability is a consideration.

From a practical standpoint, retreatment protocols must balance effective material removal with procedural efficiency and preservation of dentin. Understanding how obturation techniques affect the retrievability of bioceramic sealers can therefore inform clinical decision-making, particularly in cases with a higher likelihood of retreatment, such as those involving uncertain prognosis or complex anatomy. Accordingly, the aim of this ex vivo study was to evaluate and compare the effectiveness of NeoSealer Flo removal and the associated procedural time during retreatment of canals obturated using either the single cone or cold lateral compaction technique. The findings are intended to provide protocol-oriented guidance for clinicians regarding obturation strategy selection when using bioceramic sealers in routine endodontic practice.

Protocol

This study was conducted and reported in accordance with the PRILE guidelines (Supplementary File 1), and the overall experimental workflow is summarized in the PRILE flowchart (Supplementary File 2).

Sample selection

CAUTION: Extracted human teeth represent a potential biohazard. Handle all samples using appropriate personal protective equipment (PPE), including gloves, masks, and protective eyewear. Dispose of all biological waste in designated biohazard containers according to institutional and local regulations.

The study protocol received approval from the local institutional review board (MIU-IRB-2122). A power analysis using G*Power (v3.1.9.7) based on Baranwal et al ⁽⁸⁾ determined that a sample size of 12 per group would provide 80% power to detect significant differences based on an effect size of 0.586. Twenty-four freshly extracted, single-rooted permanent mandibular premolars (Vertucci Class I) with straight, mature roots were collected from the Oral and Maxillofacial Surgery Department at Misr International University. The teeth were extracted for orthodontic or periodontal reasons unrelated to this study. Immediately following extraction, any adherent soft tissue or calculus was removed using periodontal scalers, and the teeth were washed under running water. To ensure disinfection and maintain hydration, the samples were stored in a 0.1% thymol solution at 4 °C until use. Teeth were examined under 8X magnification; those with fractures, previous endodontic treatment, calcifications, or resorptive lesions were excluded from the study.

Sample preparation

CAUTION: Use appropriate sharps containers for the disposal of all endodontic files, needles, and diamond discs.

Crowns were removed at the cemento-enamel junction using a high-speed diamond disc under water cooling to standardize root length. Working length (WL) was established by inserting a size 10 K-file until visible at the apical foramen and subtracting 1 mm. Canals were pre-flared with HyFlex EDM Glide Path files (10/.05) at 400 × g. Instrumentation was completed using a HyFlex EDM OneFile (25/variable taper) to the full WL.

The final irrigation protocol was performed using 17% EDTA followed by 2.6% NaOCl.

NOTE: Dispose of chemical irrigant waste in appropriately labeled chemical waste receptacles in accordance with laboratory safety standards. Each solution was activated ultrasonically for 30 seconds using an ultrasonic unit equipped with a size 20, .02 taper silver-nickel activator tip. The tip was placed 2 mm short of the WL and operated at a medium power setting (approximately 45 kHz) in a passive ultrasonic irrigation (PUI) mode, ensuring the tip vibrated freely without binding against the canal walls.

Obturation protocol

Samples were randomly allocated into two groups (n = 12) using a computer-generated sequence using Microsoft Excel to ensure an unbiased distribution.

• SC Group: The canal was dried with paper points. NeoSealer Flo was injected into the coronal third of the canal using the manufacturer's intra-canal tip, and a size 25 HyFlex EDM master cone was coated with a thin layer of sealer and slowly seated to WL.
• CLC Group: A .02 taper master cone was dipped in sealer and seated to WL. A size 25 finger spreader was inserted to within 1 mm of the WL to create space laterally. Auxiliary cones (size fine-fine) were coated with sealer and compacted laterally using the same spreader until the canal was full.

Accesses were sealed with Cavit. The quality of the obturation and the absence of voids were verified radiographically using a digital intraoral sensor and an X-ray unit operating at 65 kV and 7 mA. Radiographs were taken from two orthogonal angulations (bucco-lingual and mesio-distal) to ensure a three-dimensional assessment of the filling density. The images were analyzed under 2X magnification. Samples were stored at 37 °C and 100% humidity for 14 days to ensure the sealer set completely.

Retreatment

Retreatment was initiated using ProTaper Universal retreatment files (D1, D2, and D3; Dentsply Sirona, Ballaigues, Switzerland). Following the removal of the bulk of the filling material, further enlargement was completed using ProTaper Next files X2 (size 25, .06 taper) and X3 (size 30, .07 taper) at 500 × g and 2.0 Ncm torque. Irrigation with 5.25% NaOCl was performed between each file using a 30-gauge side-vented needle.

The clinical endpoint for retreatment was defined as the moment the X3 file reached the full working length and the file flutes appeared free of visible filling material remnants when inspected under 8X magnification. This criterion represents a standardized clinical "stop-point" for the procedure. However, this does not imply the total elimination of all of the filling material from the canal, as microscopic remnants often persist within dentinal irregularities and were subsequently quantified during the stereomicroscopic evaluation phase.

The procedural retreatment time was recorded in minutes and seconds using a digital stopwatch. The timing interval commenced the moment the first retreatment file (D1) touched the coronal gutta-percha and was paused during irrigation, file changes, and flute cleaning to ensure only active instrumentation time was captured. The timing ended when the final apical file (X3) reached the established working length.

Stereomicroscopic evaluation

Each root was prepared for longitudinal splitting by creating two superficial grooves (approximately 0.5 mm deep) on the buccal and lingual surfaces using a diamond disc under water cooling. The roots were then cleaved by placing the blade of a sterile chisel into the grooves and applying a light tap with a mallet. Each half was photographed at 8X magnification. Images were imported into ImageJ. To ensure standardized measurements, the software's measurement scale was first calibrated. This was done by selecting the "Straight Line" tool to draw a line across the photographed physical ruler, then opening the "Set Scale" dialog box to input the known distance (1 mm) to convert pixels into millimeters (mm²). Next, the "Polygon Selections" tool was used to manually trace the exact perimeter of the total root canal space to calculate the total canal area. Subsequently, the same tool was used to trace the perimeters of all visible islands of residual sealer and gutta-percha to calculate their total combined area. The percentage of residual filling material was then calculated by dividing the total area of the remnants by the total area of the canal space and multiplying by 100.

The remnants at the three root levels, coronal, middle, and apical were evaluated by the following scoring system according to Sherif et al.⁹. Consistent with the limitations of stereomicroscopic analysis, no attempt was made to distinguish between gutta-percha and sealer; therefore, all residual debris is referred to collectively as "residual filling material”.

Statistical analysis:

All analyses were performed using the complete raw dataset generated in this study (Supplementary File 3). Ordinal data were presented as frequency and percentage values. Numerical data are presented as mean and standard deviation values. Data were checked for normality using Shapiro-Wilk's test and were found to be not normally distributed. For inter-group comparisons between the SC and CLC groups at each root third, the Mann-Whitney U test was employed. For intra-group comparisons across the three root levels (coronal, middle, and apical), the Kruskal-Wallis test was used, followed by Dunn’s post-hoc test for pairwise comparisons where significant differences were detected. To account for multiple sectional comparisons and reduce the risk of Type I errors, a Bonferroni correction was applied, adjusting the significance level accordingly. The level of significance was set at P < 0.05. Statistical analysis was performed with R statistical analysis software version 4.3.1 for Windows (Table of Materials).

Results

Following the rotary retreatment protocol using ProTaper Universal and Next files, full working length and apical patency were successfully regained in 100% of the samples across both the SC and CLC groups. No procedural complications, such as file separation, canal transportations, or perforations, were observed during the retreatment process.

Percentage of Residual Filling Material (Derived from ImageJ Area Measurements):
Based on the quantitative area measurements performed using ImageJ software on the stereomicroscopic images, the percentage of residual filling material was calculated for each root third.

Inter-group Analysis:
At the middle and apical levels, the cold lateral compaction technique resulted in significantly higher percentages of residual filling material compared to the single cone technique (P < 0.05). However, no statistically significant difference was observed between the two techniques at the coronal level (P > 0.05).

Intra-group Analysis:
Within the single cone group, the distribution of filling remnants varied significantly by root level (P = 0.015), with the coronal third exhibiting a significantly higher volume of material=/
than the middle and apical sections. (SupplementaryTable 1)

*Different superscript letters indicate a statistically significant difference within the same vertical column; significant (p < 0.05), ns: non-significant (p > 0.05)

Remaining filling material score (Derived from Stereomicroscopic Qualitative Assessment): In addition to the exact area percentages, the cleaved roots were evaluated using the predefined 0-3 scoring system to provide a clinical checkpoint for canal cleanliness.

Inter-group Analysis
A comparative analysis between the two obturation techniques revealed that the cold lateral compaction group exhibited significantly higher residual scores in the apical third compared to the single cone group (P = 0.007). In contrast, no statistically significant differences in scores were observed between the two techniques in the coronal or middle sections (P > 0.05).

Intra-group Analysis
When evaluating the distribution of remnants across the different root levels within each group, no statistically significant differences were detected (P > 0.05). This indicates that for both the single cone and cold lateral compaction techniques, the frequency of specific scores remained relatively consistent from the coronal to the apical aspects of the root canal. (Supplementary Table 2). These qualitative differences are visually represented in the stereomicroscopic images, which demonstrate the cleaner canal walls typical of the single cone group (Figure 2A) compared to the denser residual patches observed in the cold lateral compaction group (Figure 2B).

*; significant (p < 0.05) ns; non-significant (p > 0.05)

Retreatment time(Derived from Stopwatch Measurements):
As recorded by the digital stopwatch, measuring active instrumentation time from initial file insertion to reaching working length, the overall efficiency of both techniques was evaluated.
The time needed to reach the working length in the single cone obturation technique was (04:52±01:10), which was higher than the cold lateral compaction technique (04:22±00:43) without a statistically significant difference (p = 0.301). (Supplementary Table 3)

*; significant (p < 0.05) ns; non-significant (p > 0.05)

DATA AVAILABILITY: The raw data supporting the findings of this study are available in Supplementary File 3.

Figure 1: Stereomicroscope evaluation with magnification 8X of root halves after retreatment. (A) dividing the examined root section into coronal, middle, and apical thirds; (B) analysis using the ImageJ software. Please click here to view a larger version of this figure.

Figure 2: Representative stereomicroscopic images (8X magnification) of longitudinal root halves after retreatment. (A) Single Cone (SC) Group: The image demonstrates a significantly cleaner canal wall in the apical and middle thirds, with only minimal, isolated islands of residual filling material remaining within the dentinal irregularities. (B) Cold Lateral Compaction (CLC) Group: The image illustrates a higher accumulation of residual filling material, particularly in the apical third, appearing as larger, continuous patches of sealer and gutta-percha remnants. These visual differences highlight the superior retrievability of the SC technique compared to the CLC technique in the challenging apical portion of the canal. Please click here to view a larger version of this figure.

Supplementary Table 1: Comparisons, mean, and standard deviation values of percentage of residual filling material remnants for both obturation techniques.Please click here to download this file.

Supplementary Table 2: Comparisons, frequency, and percentage values of the remaining filling material score for different techniques.Please click here to download this file.

Supplemenatry Table 3: Intergroup comparisons, mean and standard deviation values, time of retreatment (mm:ss).Please click here to download this file.

Supplementary File 1: PRILE checklist; Completed PRILE (Preferred Reporting Items for Laboratory-based Experiments) checklist indicating where each reporting item is addressed within the manuscript.Please click here to download this file.

Supplementary File 2: PRILE flowchart; Flowchart illustrating the study design, experimental workflow, sample processing, and data analysis steps in accordance with PRILE guidelines.Please click here to download this file.

Supplementary File 3: Raw dataset generated and analyzed during the current study, including all measurements used to produce the reported results.Please click here to download this file.

Discussion

Bioceramic sealers are favored for their ability to penetrate complex canal anatomy, yet this study demonstrates that no technique ensures their total removal^10,3.

Complete removal of filling materials from the root canal system is essential to expose microbial biofilms sequestered within the dentinal tubules. This debridement allows for direct contact with irrigants and intracanal medicaments, which is critical for the eradication of persistent pathogens responsible for post-treatment periapical pathosis. Furthermore, the presence of residual filling material may compromise the interfacial adaptation and bond strength of sealers or cements used for subsequent endodontic or restorative procedures, such as post cementation^11,12.

In the literature, the retreatability of endodontic sealers is typically evaluated using several standardized parameters. These include the ability to reach the apical terminus to re-establish working length and patency, the time required to complete the procedure, and the quantitative assessment of residual root filling material or debris remaining on the canal walls ¹².

There are various ways to assess the amount of remaining material inside a root canal. Micro CT is a useful tool for assessment of residual filling material, which gives high-resolution three-dimensional data for analysis of results with a non-invasive technique¹². A scanning electron microscope can be used, too, as it provides data on the direct morphology of the remaining filling material¹³. A stereomicroscope is a valuable tool for assessing the amount of remaining material inside the root canal in experimental studies. The stereomicroscope was used in this study as it provides high magnification through a variety of magnification settings and illumination, which eases visualization of the root canals and the remaining filling material¹⁴.

Critical Procedural Steps and Troubleshooting

To ensure the reproducibility and success of this protocol, several critical steps must be carefully managed. During the retreatment phase, maintaining a constant crown-down approach with copious irrigation is vital. If rotary files begin to bind or if the operator encounters high resistance, a common issue when engaging hard bioceramic masses, troubleshooting dictates immediately withdrawing the file, cleaning the flutes, and replenishing the canal with NaOCl before proceeding. Forcing the file can lead to separation or canal transportation. Furthermore, the longitudinal cleaving of the roots for stereomicroscopic evaluation is a highly technique-sensitive step. It is critical that the longitudinal grooves created by the diamond disc are exactly 0.5 mm deep and positioned perfectly opposite each other on the buccal and lingual surfaces. If the grooves are too shallow, the root may fracture asymmetrically or shatter upon tapping with the mallet, rendering the sample unusable for area analysis. If an uneven split begins to occur during troubleshooting, gently deepening the groove before reapplying mallet pressure is recommended.

As reported in the results, in all samples, apical patency and full working length were reached, and no procedural errors, such as broken files or transportations, were reported. Remnants of the filling material were detected in both groups, and achieving a canal without any remnants was not possible.

The superior retrievability of the SC technique in the apical and middle segments may be attributed to the presence of a single, solid gutta-percha mass. This core is more easily engaged and extracted by rotary instruments than multiple compacted cones. Additionally, the potential for small voids between the single cone and the sealer might facilitate mechanical removal. These results contradict those shown by Aref et al.¹⁵ who assessed the Well-Root ST bioceramic sealer and showed that the single cone was associated with more remnants of filling material. This could be the result of the different sizes and taper of retreatment files used in relation to the initial preparation size.

The finding that the SC group had more coronal remnants is logical; the single cone matches the apical preparation well but leaves a larger volumetric space in the coronal third. This gap is filled with a higher volume of bioceramic sealer, which chemically bonds to the dentin, making its removal more difficult. In the CLC group, the spreader creates a thinner, more consistent layer of sealer throughout the canal, explaining why its remnants were uniform across all levels^16,17.

Comparing the different root levels within the single cone obturation technique, the coronal root level has shown statistically significantly more filling remnants than the other two-thirds. This is quite a logical finding as using the single cone obturation technique will result in more space between the gutta-percha cone and the surrounding dentinal wall, leading to more sealer filling in this gap compared to the middle and apical thirds, where the gutta-percha cone is matching and fitting more with less space all around. The more the amount of the bioceramic sealer, the more remnants will be there as the bioceramic sealer interacts with the dentinal structure, forming the interfacial layer of hydroxyapatite-like structure, chemically bonded to the dentine substrate, making its retrieval more difficult^2,16,17. Therefore, different supplemental techniques¹² and solvents¹⁸ are advocated for the retrieval of the bioceramic sealers from the root canals. On the contrary, the gutta-percha is way easier to retrieve using different engine-driven rotary nickel-titanium files ^19,20,21,22.

Comparing the different root levels within the lateral compaction obturation technique, no significant difference was noted. This can be explained by the technique applied, as the spreader is used to laterally compact the gutta-percha, leading to better adaptation of the gutta-percha and a minimal amount of sealer all over the root length. This result was in contrast with Baranwal et al.⁸ who reported the apical one level to show more filling remnants compared to the coronal and middle ones. This might be attributed to the larger taper of the ProTaper Universal retreatment files used in addition to the sample teeth used in the study which carries the possibility of more anatomic variation and complexities in the apical one-third.

It is noteworthy that in the qualitative assessment of canal cleanliness, the residual material scores were confined to the lower end of the scale, specifically Score 0 and Score 1. No samples in either the SC or CLC groups exhibited Score 2 or Score 3 (more than 50% coverage). This suggests that both techniques, when followed by the specific rotary retreatment protocol used in this study, allow for a relatively high degree of material removal. However, the statistical analysis remains relevant as it identified significant differences in the distribution of these low-level remnants, particularly in the apical third, where the SC technique demonstrated superior retrieval compared to CLC.

No statistically significant difference was noted in the time needed to reach the full working length between both obturation techniques (p=0.301), similar to that of Takorava et al.²³. In contrast, Al Dahman et al.²⁴ showed a difference in the retretatment time between different obturation techniques. The contradiction might be related to the difference in the bioceramic sealer used and the obturation technique tested ²⁵.

Despite the standardized protocol, this study possesses several limitations that should be considered when interpreting the results. First, as an in vitro (ex vivo) study, the experimental conditions do not fully replicate the clinical environment. Second, the use of stereomicroscopy and longitudinal cleaving, while effective for surface area analysis, is a destructive method that allows for only a two-dimensional assessment of remnants. The lack of micro-CT means that the volume of residual material within inaccessible areas, such as isthmuses or lateral canals, could not be quantified three-dimensionally. Furthermore, the exclusion of chemical solvents and the use of straight, single-rooted teeth limit the clinical translatability of these findings to more complex scenarios, such as severely curved canals or cases requiring chemical softening of the obturation mass. Future research utilizing 3D imaging and a broader range of tooth morphologies is necessary to validate these outcomes.

In conclusion, within the limitations of this ex vivo study, the single-cone technique demonstrated a higher efficiency in material removal within the middle and apical thirds of the root canal system compared to the cold lateral compaction technique. However, both techniques were found to be comparable regarding the cleanliness of the coronal third and the total time required for retreatment.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Bioceramic Sealer	Avalon Biomed, Houston, TX, USA	NeoSealer Flo
Glide Path File	Coltene/Whaledent AG, Altstätten, Switzerland	HyFlex EDM Glide Path (10/.05)
Shaping File	Coltene/Whaledent AG, Altstätten, Switzerland	HyFlex EDM OneFile (25/0.08)
Master Cone	Coltene/Whaledent AG, Altstätten, Switzerland	HyFlex EDM Gutta-Percha Cone
Ultrasonic Unit	Guilin Woodpecker Medical Instrument Co., China	Woodpecker Ultra X
Temporary Restorative Material	3M ESPE, Seefeld, Germany	Cavit
Digital Intraoral Sensor	DEXIS, Hatfield, PA, USA	Dexis Platinum
X-ray Unit	Planmeca, Helsinki, Finland	Planmeca ProX
Retreatment Files	Dentsply Sirona, Ballaigues, Switzerland	ProTaper Universal Retreatment (D1-D3)
Retreatment Finishing Files	Dentsply Sirona, Ballaigues, Switzerland	ProTaper Next (X2, X3)
Randomization Software	Microsoft Corp., Redmond, WA, USA	Microsoft Excel
Imaging Software	DEXIS, Hatfield, PA, USA	DEXIS Software
Image Analysis Software	National Institutes of Health, Bethesda, MD, USA	ImageJ
Statistical Analysis Software	R Foundation for Statistical Computing, Vienna, Austria	R version 4.3.1
Power Analysis Software	Heinrich-Heine-Universität, Düsseldorf, Germany	G*Power (v3.1.9.7)