Nonsurgical endodontic retreatment is performed to re-access the root canal system, remove previous filling materials, reduce the residual microbial load and permit renewed chemomechanical preparation and obturation [1-4]. Residual sealer and gutta-percha may block irrigant penetration, mask infected recesses and compromise the quality of reinstrumentation. For that reason, the retreatability of modern sealers has become a clinically important issue rather than a purely laboratory concern.   Bioceramic and resin-based sealers are widely used because of their favorable sealing properties, dimensional stability and compatibility with contemporary obturation techniques [5-9]. Their behavior during retreatment may differ. Calcium silicate-based bioceramic sealers are reported to interact chemically with dentin and form interfacial mineral deposits, whereas epoxy resin-based sealers such as AH Plus penetrate dentinal tubules and may create mechanically interlocked resin tags. These differences in adhesion and penetration may influence how much material remains after retreatment.   Rotary nickel-titanium retreatment systems are routinely used because they shorten treatment time and improve filling material removal compared with manual techniques [10-16]. ProTaper Universal Retreatment, D-RaCe and R-Endo represent commonly studied continuous-rotation systems with different file designs, taper progression and cutting behavior. Comparing these systems against different sealer types remains clinically relevant because retreatment decisions are often made in cases where the original sealer is already set and difficult to identify clinically.   CBCT permits three-dimensional volumetric assessment of residual filling material and can provide a more comprehensive estimate than two-dimensional radiography [17-19]. Nevertheless, CBCT is still less precise than micro-CT for detecting very thin remnants and findings should therefore be interpreted as volumetric estimates rather than absolute measurements. Stereomicroscopic scoring remains useful as a complementary method for direct surface assessment after sectioning [20].   The present study was designed to compare PTR, D-RaCe and R-Endo for the removal of bioceramic and resin-based sealers from standardized mandibular premolars. The null hypothesis was that neither sealer type nor retreatment system would significantly influence residual filling volume, retreatment time or stereomicroscopic residual score.

Study Design and Ethics Study design and ethics. This in vitro experimental study was conducted at the Department of Endodontics, University Dental Research Center, between February and August 2024. Ethical approval for the use of extracted human teeth was obtained from the institutional review board. Teeth were collected from orthodontic or periodontal extractions according to institutional policy, de-identified before laboratory processing and stored in 0.1% thymol solution at 4 C until use [21].   Sample selection. Ninety extracted human mandibular premolars with single canals and curvature <10 degrees according to Schneiders method were included [22]. Teeth with multiple canals, resorption, fractures, calcification, prior endodontic treatment or severe curvature were excluded. Crowns were sectioned to obtain standardized root lengths of 14+1 mm. The use of mild-curvature premolars improved standardization but limits extrapolation to molars, highly curved canals and complex oval anatomy [23].   Canal Preparation and Obturation Working length was established 1 mm short of the apical foramen. All canals were prepared using ProTaper Universal to F3 (30/.09 taper). Irrigation consisted of 2 mL 2.5% sodium hypochlorite between instruments, followed by 5 mL 17% EDTA for 1 minute and a final distilled water rinse. Teeth were randomly allocated to a bioceramic sealer group (EndoSequence BC Sealer; n = 45) or a resin-based sealer group (AH Plus; n = 45). Obturation was completed using the continuous wave compaction technique with a System B heat source and matching gutta-percha cones [24-25]. Specimens were stored at 37°C and 100% humidity for 14 days to allow complete setting.   CBCT Baseline Evaluation Pre-retreatment scans were obtained using a 3D Accuitomo 170 scanner (90 kV, 5 mA, voxel size 0.125 mm, exposure 17.5 seconds). Total filling volume for each canal was measured using OnDemand3D software [26]. The CBCT operator was not involved in retreatment instrumentation and measurements were performed using the same acquisition parameters for baseline and post-retreatment scans.   Retreatment Groups and Protocol Each sealer group was randomly subdivided into PTR, DRC and REN subgroups (n = 15 each). All instruments were used in continuous rotation with a torque-controlled motor according to manufacturer recommendations at 500 rpm. New instrument sets were used for every five teeth. Chloroform (0.1 mL) was applied only when needed to soften compacted coronal gutta-percha according to the same predefined protocol for all groups. Retreatment was considered complete when the working length had been reached, no material was visible on the instrument flutes and canal walls appeared clean under 3.5x magnification [27-29]. Total retreatment time was recorded from initial instrument insertion to completion of this endpoint.   Post-Retreatment Evaluation Residual filling material was reassessed on CBCT and percentage residual volume was calculated relative to baseline volume. Roots were then split longitudinally and examined under a stereomicroscope at 20x. Residual material was scored on a 4-point ordinal scale: score 0, no visible material; score 1, <25% wall coverage; score 2, 25%-50%; score 3, >50%. Two calibrated examiners independently evaluated the specimens; inter-examiner agreement was assessed with Cohens kappa [30].   Statistical Analysis Data were analyzed using SPSS version 27.0. Shapiro-Wilk testing assessed normality. Two-way ANOVA evaluated the main effects of sealer type and retreatment system on residual percentage and Tukey post-hoc testing was used for pairwise comparisons while controlling type I error. Kruskal-Wallis testing was used for ordinal stereomicroscopic scores. Statistical significance was set at p<0.05.

All specimens completed baseline and post-retreatment assessment. Baseline filling volumes did not differ significantly among groups (p = 0.624), indicating comparable starting conditions before retreatment. Two-way ANOVA showed significant main effects for sealer type (p<0.001) and retreatment system (p = 0.002), whereas the sealer-system interaction was not significant (p = 0.318), suggesting that the relative ranking of the tested systems remained similar across both sealer types (Table 1).   Table 1: Residual Filling Material Percentage by Sealer Type and Retreatment System Retreatment system Bioceramic sealer (%) Resin-based sealer (%) p-value ProTaper Universal Retreatment 8.42+2.18 14.28+3.46 <0.001 D-RaCe 11.36+3.24 18.52+4.12 <0.001 R-Endo 12.84+2.96 19.74+3.88 <0.001 Between-system p-value 0.003 0.004 -   PTR showed the lowest residual percentage for both sealers. In every retreatment system, resin-based sealer left more residual material than bioceramic sealer. None of the groups achieved complete removal. These findings were corroborated by the stereomicroscopic scores, which also favored PTR and bioceramic sealer removal (Table 2).   Table 2: Mean Retreatment Time (Seconds) by Sealer Type and Retreatment System Retreatment system Bioceramic sealer Resin-based sealer p-value ProTaper Universal Retreatment 186.4+28.6 248.2+36.4 <0.001 D-RaCe 224.8+32.4 298.6+42.8 <0.001 R-Endo 242.6+38.2 324.4+48.6 <0.001 Overall mean 217.9+38.4 290.4+52.6 <0.001   Retreatment time was consistently shorter in the bioceramic groups and shortest with PTR. The longer time required for resin-based sealer removal is clinically relevant because it may reflect stronger dentin interaction and may increase operator fatigue and instrument stress during retreatment (Table 3).   Table 3: Distribution of Stereomicroscopic Scores by Group Group Score 0 Score 1 Score 2 Score 3 Median BC-PTR 4 (26.7) 8 (53.3) 3 (20.0) 0 (0.0) 1 BC-DRC 2 (13.3) 7 (46.7) 5 (33.3) 1 (6.7) 1 BC-REN 1 (6.7) 6 (40.0) 6 (40.0) 2 (13.3) 2 RB-PTR 1 (6.7) 5 (33.3) 7 (46.7) 2 (13.3) 2 RB-DRC 0 (0.0) 4 (26.7) 7 (46.7) 4 (26.7) 2 RB-REN 0 (0.0) 3 (20.0) 6 (40.0) 6 (40.0) 2.5 Inter-examiner agreement was excellent (Cohens kappa = 0.86). Regional distribution analysis showed that the apical third contained the largest share of the remaining filling material in all groups, accounting on average for 52.4% of the total residual volume, followed by the middle and coronal thirds.

This study showed that complete sealer removal was not achieved by any of the tested retreatment systems. That finding is consistent with the broader retreatment literature and highlights the inherent difficulty of completely debriding prepared canals, particularly when modern sealers are used and canal walls include inaccessible recesses and apical irregularities [31].   Bioceramic sealer was easier to remove than the resin-based sealer in the present design. The resin-based sealer likely resisted retreatment because of deeper dentinal tubule penetration and stronger micromechanical interlocking. By contrast, although bioceramic materials are often described as bonding chemically to dentin, the present results indicate that under the present preparation, obturation and aging conditions, they left less residual material and required less retreatment time than AH Plus [32-33].   PTR produced lower residual percentages and shorter retreatment times than D-RaCe and R-Endo. The likely explanation is instrument design, including active cutting geometry and progressive taper sequence, which may improve coronal debulking and apical penetration of the retreatment files. Because the sealer-system interaction was not significant, the superiority of PTR appeared consistent across both sealer categories rather than being limited to only one obturation condition.   The apical third remained the most difficult region to clean. This is clinically important because persistent sealer remnants in the apical segment may shield bacteria and limit irrigant penetration during retreatment. The finding also reflects the restricted contact of rotary files with canal walls in the apical third, particularly in canals that are not perfectly round. Even in standardized premolars, anatomical variation and instrument-centered preparation may still leave untouched surfaces [23,31].   Chloroform was used only when necessary and this should be considered when interpreting both removal percentage and retreatment time. Although solvent use facilitates gutta-percha softening, it may also alter the consistency of filling remnants and influence how effectively the files engage the material. For that reason, future studies should standardize solvent application more strictly or compare solvent-assisted and solvent-free retreatment protocols directly.   This study has several limitations. It was performed in vitro using mildly curved mandibular premolars, so the findings cannot be extrapolated directly to molars, highly curved canals or more complex anatomy. CBCT volumetric analysis is practical and three-dimensional but it remains less precise than micro-CT for very small remnants. Instrument deformation or separation was not evaluated in this protocol. Accordingly, the present findings should be interpreted as controlled laboratory data rather than direct predictors of clinical retreatment success.   Clinically, the findings suggest that sealer composition should be considered when planning retreatment. Rotary retreatment files alone are unlikely to achieve complete material removal, especially apically. Additional steps such as ultrasonic activation, finishing instruments or enhanced irrigation may be required, particularly when resin-based sealers are being retreated.

Within the limitations of this in vitro study, none of the evaluated retreatment systems completely removed filling material from the root canals.   ProTaper Universal Retreatment showed the best overall performance and bioceramic sealer was easier to remove than resin-based sealer under the present conditions.   Residual material persisted mainly in the apical third, indicating that supplementary cleaning strategies may be necessary during retreatment, especially when resin-based sealers are involved.   Ethical Approval Approval for the use of extracted human teeth was obtained from the institutional review board.