Evaluation of irrigation activation techniques for root canal filling material removal in retreatment of 3D-printed curved canals

Abstract

Background

This study aimed to evaluate the effect of canal curvature on the removal of root canal filling material using different irrigation activation techniques in nonsurgical endodontic retreatment.

Methods

A total of 117 standardized replica maxillary incisor teeth were fabricated using a three-dimensional (3D) printer and obturated with GuttaFlow. The samples were randomly assigned to nine groups (n = 13 per group) according to canal curvature (15°, 30°, and 45°) and irrigation protocol: conventional needle irrigation (CNI), sonic activation (EDDY), or ultrasonic irrigation activation (UIA). The residual filling material in the entire canal and apical third was quantitatively assessed as absolute volume (mm³) using cone-beam computed tomography (CBCT) and 3D Slicer software, and percentage values (%) were subsequently calculated relative to the total canal volume. Data were analyzed using two-way analysis of variance (ANOVA) and Kruskal–Wallis tests (p < 0.05).

Results

Canal curvature significantly affected the volume of residual filling material (F = 16.40, p < 0.001), with greater curvature associated with greater residual volume. The mean residual volume in the apical third increased from 0.51 mm³ (7.82%) in the 15° group to 1.80 mm³ (27.60%) in the 45° group. No significant differences were observed among irrigation techniques (F = 2.44, p = 0.094).

Conclusions

Canal curvature plays a decisive role in retreatment efficacy, particularly in the apical third, whereas the contribution of irrigation activation techniques appears limited under standardized conditions. Clinicians should consider canal curvature as a primary factor influencing retreatment outcomes and adapt irrigation strategies accordingly, especially in highly curved canals.

Introduction

Endodontic failure may occur even when root canal treatments appear radiographically adequate and coronal restorations are intact, particularly if the patient reports persistent symptoms or if periapical healing is not achieved radiographically [1]. In most cases, failure is caused by persistent or secondary microbial infections resulting from inadequate cleaning, shaping, or obturation of the root canal system, making nonsurgical retreatment the preferred approach [2]. The goal of retreatment is to re-access the root canal, remove the existing filling material, and thoroughly clean and disinfect the root canal system to allow for proper healing [3]. Various tools have been employed, such as hand files, heat-assisted instruments, Gates Glidden drills, and nickel-titanium rotary files [4]. However, none of these techniques effectively removes residual filling material from severely curved or anatomically complex root canals [5]. Remaining filling materials can harbor bacteria and support biofilm formation in intricate areas of the root canal system, which may negatively affect the outcome of retreatment [6]. Irrigation activation techniques have been shown to enhance the removal of these remnants after mechanical retreatment [7, 8]. Although conventional needle irrigation (CNI) combined with chemical agents remains the most commonly used approach, it often fails to thoroughly decontaminate the root canal system. Consequently, in addition to exploring alternative irrigation solutions, various activation techniques have been introduced to enhance the chemical and mechanical effectiveness of irrigation within the canal space [9]. Among these activation techniques, ultrasonic irrigation activation (UIA) stands out for its potential to increase the effectiveness of irrigation solutions.

Ultrasonic tips enhance the effectiveness of irrigation solutions through mechanisms such as acoustic microstreaming and cavitation, thereby improving their ability to clean the root canal system [10]. During this process, a small-diameter, non-cutting metal tip is introduced into the root canal and must vibrate freely to efficiently transmit energy to the irrigation solution [9]. A UIA of 1 min per canal, with three cycles of 10–20 s each (with irrigation renewal), has been considered ideal for achieving clean canals [11]. Sonic activation (SA) facilitates the removal of debris from hard-to-reach areas within the root canal system by utilizing vibrational energy and oscillatory motion. EDDY (VDW, Munich, Germany) is a notable example of devices designed for this purpose. It has been reported to operate safely within the canal, posing minimal risk of root wall damage or instrument separation [12].

After root filling removal, various methods are used to assess the amount of remaining material. Some studies use direct visualization by chemically splitting or clearing the tooth [13, 14]. These methods give a clear view but may dislodge material or damage the sample [15]. Two-dimensional radiographs are also used, but cannot fully show the complex root anatomy [16]. Today, microcomputed tomography (micro-CT) and cone-beam computed tomography (CBCT) are preferred for their non-invasive and three-dimensional imaging [17]. CBCT is faster and more accessible. It provides better detail and helps measure the remaining material more accurately [18].

In recent years, three-dimensional (3D) printing technology has emerged as a promising tool in medicine and dentistry, gaining widespread attention for its potential to transform many aspects of clinical practice [19]. In dentistry, its applications have rapidly expanded, ranging from tissue engineering and regenerative procedures to the fabrication of anatomical models, prosthetic components, and innovative drug delivery platforms [20]. More recently, attention has shifted toward developing innovative materials compatible with 3D printing. Generally, the process involves four main steps: creating a digital model using scanning technology or CAD software, slicing the model into thin layers, printing it sequentially layer by layer, and performing post-printing refinements to complete the final structure [21, 22]. Notably, 3D printing technology in experimental research has enabled the production of standardized, reproducible samples, which are essential for minimizing variability and improving the reliability of comparative in vitro studies.

Despite extensive research on irrigation activation techniques [23, 24], the combined effect of canal curvature and irrigation activation on the removal of filling material remains unclear, due to anatomical variability and a lack of standardized models. Moreover, studies integrating controlled 3D-printed canal geometries with clinically relevant imaging modalities such as CBCT are limited. Therefore, the present study aims to address this gap by providing a standardized, reproducible model to evaluate retreatment efficacy under varying curvature conditions.

This study compares the effectiveness of three different irrigation activation techniques (CNI, EDDY, and UIA) in removing root canal filling materials during retreatment. For this purpose, replica teeth with curved root canals (15°, 30°, and 45°) will be produced using 3D printing technology. The present study will assess the effectiveness of each irrigation activation technique in removing filling material from canals with different curvatures. Unlike previous studies [23, 24], the remaining root canal filling material will be evaluated using CBCT, with quantitative data obtained for analysis, highlighting the significance of this approach in providing more accurate and reliable results.

The first null hypothesis of this study is that there is no significant difference in the removal of filling material among different irrigation activation techniques. The second null hypothesis is that there is no significant difference in the volume of residual filling material among canals with varying curvature angles.

Materials and methods

Sample size calculation

This study used G*Power 3.1.9.7 (Heinrich-Heine-Universität Düsseldorf, Germany) for statistical analysis. The sample size calculation was based on a previous study [25], using an estimated effect size of 0.401, a Type I error rate (α) of 0.05, and a statistical power of 0.8. Accordingly, 13 samples were included in each group in the present study, resulting in a total of 117 samples to ensure sufficient power and account for potential variability. This study was approved by the institutional review board/ethical committee (approval number 2025/311).

3D-printed teeth with different root canal curvatures

A single maxillary central incisor with a single root and canal was selected. The presence of any fractures, cracks, or caries in the tooth was confirmed through periapical radiography. The access cavity was prepared under water cooling using an operating microscope (Zeiss, Oberkochen, Germany) at x24 magnification. A size 10 C+ file (Dentsply, Charlotte, USA) was placed in the root canal to determine the working length, and canal instrumentation was performed 0.5 mm short of this length. Canal preparation was completed using VDW R25 and VDW R40 files (VDW, Munich, Germany), with irrigation between file changes using 3 mL of 5.25% sodium hypochlorite (NaOCl; Cerkamed, Stalowa Wola, Poland) and 3 mL of 17% ethylenediaminetetraacetic acid (EDTA; Cerkamed, Stalowa Wola, Poland). The canal was then dried with paper points (Dentsply Sirona, Switzerland). For 3D printing, the prepared tooth was scanned using micro-CT (mBT80; SCANCO Medical AG, Zurich, Switzerland) at 20 × 20 μm pixel size and 20 μm slice thickness. The images were reconstructed into a 3D model in 3D Slicer (version 4.11.0; Surgical Planning Laboratory, Harvard Medical School, Boston, MA) and exported to Blender (Blender Institute B.V., Amsterdam, The Netherlands) for adjustments. To simulate different root canal curvatures (15°, 30°, 45°), the same reference point, which corresponds to the starting point of the apical curvature, was selected for all groups (Fig. 1). The selected curvature angles represent mild, moderate, and severe canal curvatures according to Schneider’s classification. Canal curvature was determined using Schneider’s method by measuring the angle formed between the long axis of the canal and a line drawn from the point of initial curvature to the apical foramen. To ensure standardization, the curvature starting point was kept constant across all models, enabling reproducible comparisons of different curvature severities.

Fig. 1.

Maxillary central incisor with different root canal curvatures: (A) 15°, (B) 30°, and (C) 45°

One hundred seventeen replica teeth were fabricated using Dentoguide resin (Asiga, Sydney, Australia) with the Asiga Max UV stereolithography printer (Asiga, Sydney, Australia). The replica teeth were then cleaned to remove excess resin surrounding the tooth and placed in an ultrasonic bath with alcohol for 10 min. Afterward, the teeth were placed in a centrifuge (Allegra X-15R, Beckman Colter Life Sciences, Indianapolis, Indiana, USA) for 10 min at 3,500 rpm to remove any uncured resin from the canals. Finally, the teeth were cured for 15 min using the LC 3DPrint Box (NextDent, Soesterberg, The Netherlands). The artificial root canals had a standardized length of 16 mm, with an apical diameter corresponding to size 40 and a taper of 0.06.

Root canal obturation

All 117 replica teeth were obturated using GuttaFlow (Coltène/Whaledent, Langenau, Germany). The material was mixed according to the manufacturer’s instructions and delivered into the root canals using the manufacturer-provided syringe tip. A size-appropriate lentulo spiral was used at low speed to evenly distribute the material along the canal walls and minimize voids. The canal was filled up to the working length, and excess material was removed from the access cavity. The quality of obturation was verified radiographically, and specimens exhibiting voids or inadequate filling were excluded and replaced. After filling the root canals with Gutta-Flow, a lentulo spiral was used to remove radiographically detected air voids trapped within the root canal during the injection of the Gutta-Flow [26]. The quality of obturation was verified using standardized periapical radiographs obtained with the paralleling technique. Radiographs were acquired in a single orthogonal projection, with the X-ray beam directed perpendicular to the long axis of the tooth and the image receptor. All radiographs were evaluated for the presence of voids or incomplete filling. A total of 28 specimens were deemed inadequate and excluded. These specimens were replaced with newly prepared samples to maintain the predetermined sample size (n = 117) and ensure standardization across all groups. The cavities were closed with sterile Teflon tape and Cavit (3 M ESPE, Seefeld, Germany). These prepared samples were kept at 37 °C and 100% humidity for 1 week.

Retreatment procedures and irrigation activation methods

Following obturation, the specimens were randomly allocated into experimental groups using a computer-generated randomization method. A total of 117 samples were randomly assigned to nine groups (n = 13 per group) based on two independent variables: canal curvature (15°, 30°, and 45°) and irrigation protocol (CNI, EDDY, and UIA).

Initially, specimens were stratified by canal curvature to ensure balanced representation of curvature grades across all groups. Subsequently, within each curvature category, samples were randomly assigned to one of the three irrigation protocols using a simple randomization approach. Randomization was performed by an independent researcher not involved in the experimental procedures to minimize allocation bias.

This two-factor grouping design enabled the evaluation of both the main effects of canal curvature and irrigation technique, as well as their potential interaction, under standardized conditions.

Retreatment procedures were performed using a torque-controlled endodontic motor (Woodpecker AI motor, Guilin Woodpecker Medical Instrument Co., Guilin, China). The files were operated in Reciproc mode according to the manufacturer’s recommended settings. No solvent was used during the retreatment procedures to avoid influencing the mechanical removal efficiency of the filling material. Root canal retreatment was performed using the previously described preparation technique with VDW R25 and VDW R40 files (VDW, Munich, Germany), 1 mm short of the working length. The retreatment procedure was considered complete when no visible filling material remained on the instrument flutes and the canal walls appeared free of filling remnants under direct visual inspection. Irrigation was continued until the irrigant returned clear, with no detectable debris. These criteria were applied uniformly to all specimens to ensure procedural standardization. During retreatment, canals were irrigated between file changes with 3 mL of 5.25% NaOCl. The assigned irrigation protocols (CNI, EDDY, and UIA) were applied exclusively during the final irrigation phase, after mechanical instrumentation was completed.

CNI

During preparation, the root canals were irrigated with 20 mL of 2.5% NaOCl using a 30-gauge side-vented irrigation needle (Maxi Probe; Dentsply Sirona, Ballaigues, Switzerland). The needle tip was placed 1 mm short of the working length without engaging the canal walls. During irrigation, the needle was moved in a controlled apical–coronal direction with an approximate amplitude of 2–3 mm to promote effective irrigant exchange and reduce the risk of apical extrusion. This protocol was applied consistently across all specimens. Then, the canals were irrigated with 5 mL of distilled water for 1 min using a syringe equipped with a 30-gauge side-vented irrigation needle (Maxi Probe; Dentsply Sirona, Ballaigues, Switzerland) to neutralize NaOCl, followed by a final irrigation with 5 mL of 17% EDTA.

EDDY

The EDDY group followed the same conventional irrigation protocol as described for the CNI group. In addition to this protocol, sonic activation was applied during the final irrigation phase using the EDDY system. Activation was performed for 20 s using an EDDY sonic tip (28 mm length, size 25/0.04) attached to an air scaler (TA-200; Micron, Tokyo, Japan). The tip was positioned 1 mm short of the working length and moved in a controlled apical–coronal direction with an amplitude of approximately 7–8 mm, without contacting the canal walls to allow free oscillation.

UIA

The UIA group followed the same conventional irrigation protocol as described for the CNI group, without any modification during instrumentation. Following mechanical preparation, ultrasonic activation was applied during the final irrigation phase using the UltraX device (Eighteeth, Changzhou, China) with a size 15 ultrasonic tip (0.02 taper) in high-power mode. The tip was positioned 1 mm short of the working length and activated for 20 s per cycle, with a total of three cycles. The tip was kept free from canal wall contact to ensure effective oscillation of the irrigant.

Experimental groups

A total of nine groups were formed (n = 13 for each) based on the irrigation technique applied and the degree of apical root canal curvature:

• CNI groups (15°, 30°, 45°): Conventional needle irrigation was used in root canals with 15°, 30°, and 45° apical curvature angles.

• EDDY groups(15°, 30°, 45°): Sonic activation using the EDDY system was applied in canals with the same respective curvature degrees.

• UIA groups (15°, 30°, 45°): Ultrasonic activation was employed in canals with 15°, 30°, and 45° apical curvature.

Volumetric CBCT analysis

CBCT scans of the specimens were obtained using the NewTom 5G device (Verona, Italy) at a voxel size of 75 μm and a 6 × 6 cm field of view. CBCT was selected to enable three-dimensional volumetric analysis under clinically relevant conditions. The images were exported in DICOM format and imported into the 3D Slicer software for analysis.

All measurements were performed by two independent observers. Inter-observer and intra-observer reliability were evaluated using the intraclass correlation coefficient (ICC), demonstrating excellent agreement (ICC > 0.90). A specific Hounsfield Unit (HU) threshold of 1600–3000 HU was used to identify and segment the remaining root canal filling material. This predefined range enabled standardized, reproducible segmentation across all samples. The analysis was focused on the apical third of the root canals. Structures falling outside the HU threshold range were excluded, as they were assumed to lack sufficient radiopacity to be considered filling remnants.

Statistical analysis

All statistical analyses were performed using IBM SPSS Statistics 25.0 (IBM Corp., Armonk, NY, USA). Data were analyzed separately for the entire root canal system and the apical third (3 mm) (Table 1). Normality of the total canal data was assessed using the Shapiro-Wilk test. As parametric assumptions were satisfied, a two-way ANOVA was applied to evaluate the effects of irrigation technique (CNI, EDDY, UIA) and canal curvature (15°, 30°, 45°) on residual filling material. Levene’s test assessed the homogeneity of variances, and Games-Howell post hoc tests were performed when assumptions were violated. The use of both parametric and non-parametric tests was based on the distribution characteristics of the data. While total canal volume data met the assumptions of normality and homogeneity, apical third data did not, necessitating a nonparametric analysis.

Table 1.

Descriptive statistics of residual Guttaflow amount (mm³) according to different irrigation techniques and canal curvature angle

Irrigation Technique	Canal Curvature Angle (°)	Mean (entire root canal)	SD (entire root canal)	Mean (Apical Third)	SD (Apical Third)	n
CNI	15	4.16	2.59	0.57	0.48	13
	30	5.02	2.73	1.35	1.13	13
	45	7.58	2.42	1.41	0.94	13
	Total (CNI)	5.59	2.91	1.11	0.94	39
EDDY	15	2.77	1.40	0.44	0.32	13
	30	5.26	2.92	0.83	0.78	13
	45	5.88	3.38	1.96	1.39	13
	Total (EDDY)	4.64	2.95	1.08	1.04	39
UIA	15	3.95	1.67	0.53	0.42	13
	30	3.15	1.50	0.97	0.75	13
	45	6.38	1.95	2.02	1.85	13
	Total (UIA)	4.49	3.29	1.17	1.13	39
Overall	15	3.63	2.03	0.51	0.40	39
	30	4.48	2.70	1.04	0.90	39
	45	6.61	2.88	1.80	1.43	39
	Overall Total	4.90	3.05	1.13	1.12	117

*CNI Conventional needle irrigation, UIA Ultrasonic irrigation activation, n Number, SD, Standard deviation

The Shapiro–Wilk test indicated non-normal distribution of the data for the apical third (3 mm). Therefore, non-parametric analyses were applied. Residual filling material was primarily quantified as absolute volume (mm³), and percentage values (%) were subsequently calculated based on the total canal volume to allow normalized comparisons between groups. Differences in residual filling material among canal curvature groups were analyzed using the Kruskal–Wallis test, followed by pairwise comparisons with the Mann–Whitney U test. The level of statistical significance was set at p < 0.05.

Results

The volume of remaining filling material was evaluated separately for all groups in the entire root canal and the apical third (Fig. 2). The distribution of the remaining filling material volume throughout the root canal system was evaluated using the Shapiro-Wilk test, which indicated that the data were normally distributed (p > 0.05). Two-way ANOVA results demonstrated a significant effect of canal curvature on residual filling material volume (F = 16.40, p < 0.001), with greater curvature angles associated with greater residual material volume. No significant differences were observed among irrigation techniques (F = 2.44, p > 0.05). The interaction between irrigation technique and canal curvature was not statistically significant (F = 1.83, p > 0.05) (Table 2).

Fig. 2.

Bar graph showing the mean values of remaining sealer amounts in the apical third and the entire root canal

Table 2.

Residual filling material in the entire canal (mm³) according to canal curvature and irrigation technique

Factor	F	df (num, den)	p-value
Irrigation Technique	2.44	2, 108	0.092
Canal Curvature Angle	16.40	2, 108	< 0.001
Interaction	1.83	4, 108	0.128

*df (num, den) Numerator and denominator degrees of freedom (model and error degrees of freedom), F test value for group differences; p-value: significance level

Since Levene’s test indicated a violation of the homogeneity of variances assumption (p < 0.05), Games-Howell post hoc tests were performed. The post hoc analysis revealed significant differences between the 15° and 45° groups and the 30° and 45° groups (p < 0.05), whereas no significant difference was found between the 15° and 30° groups (p > 0.05) (Table 3).

Table 3.

Residual Guttaflow volume (mm³) by root canal curvature angle

Canal Curvature (°) (I)	Canal Curvature (°) (J)	n	Apical Third: Mean Difference (I-J)	Apical Third: p-value**	Entire Root Canal: Mean Difference (I-J)	Entire Root Canal: p-value***
15	30	39	-15.74*	0.002	-0.85	0.241
15	45	39	-29.26*	< 0.001	-2.99*	< 0.001
30	45	39	-15.08*	0.003	-2.14*	0.002

* The mean difference is significant at the 0.05 level

** Mann–Whitney U test

*** Games-Howell Post-Hoc test

The Shapiro-Wilk test for the apical third (3 mm) showed that the data were not normally distributed (p < 0.05). Therefore, nonparametric analyses were performed. The Kruskal-Wallis test showed that canal curvature significantly affected residual root canal filling material (H(2) = 33.827, p < 0.001). Pairwise comparisons revealed that the 45° apical segments exhibited significantly greater residual filling material than both the 15° (H = 32.49, p < 0.001) and 30° (H = 8.61, p < 0.05) groups. Additionally, the difference between the 15° and 30° groups was also statistically significant (H = 9.43, p < 0.05) (Table 3).

The results demonstrated an increase in residual filling material with increasing canal curvature, particularly in the apical third. No significant differences were observed among irrigation techniques.

This study measured root volumes in the apical third and the entire root canal using 3D Slicer software. The total volume of the apical third was determined to be 6.53 mm³, and the total root canal volume was determined to be 29.84 mm³.

Residual GuttaFlow volume increased with increasing canal curvature in both the apical third and the entire root canal. In the 15° curvature group, the mean residual volume in the apical third was 0.51 mm³, corresponding to 7.82% of the total apical third volume, while the mean residual volume for the entire root canal was 3.63 mm³ (12.16%). In the 30° curvature group, the apical third exhibited a mean residual volume of 1.04 mm³ (15.94%), and the total canal volume contained 4.48 mm³ of residual material (15.01%). In the 45° curvature group, the highest values were observed, with a mean residual volume of 1.80 mm³ (27.60%) in the apical third and 6.61 mm³ (22.15%) in the entire root canal.

Discussion

The present study evaluated the effect of canal curvature and irrigation activation techniques on the removal of root canal filling material using standardized 3D-printed models. The findings demonstrated that canal curvature significantly influenced the amount of residual filling material, particularly in the apical third, whereas no significant differences were observed among irrigation activation techniques. Accordingly, the second null hypothesis was rejected, indicating that canal curvature significantly affects the effectiveness of filling material removal, whereas the first null hypothesis was accepted, as the irrigation activation method did not significantly affect the outcome.

The primary objective of nonsurgical retreatment is the effective removal of filling materials, debris, and microorganisms from the root canal system [2]. However, complete removal remains challenging, particularly in curved and anatomically complex canals, where residual material may limit irrigant penetration and compromise disinfection [27]. Therefore, effective nonsurgical retreatment should include mechanical instrumentation, adequate irrigation, and disinfection. However, particularly in curved and complex root canals, traditional shaping and irrigation procedures often leave residual material and fail to completely cleanse the root canal system [28, 29]. This study used a highly standardized experimental setup to assess the influence of canal curvature on the efficacy of various irrigation activation protocols. In this context, standardized 3D-printed replica teeth with different root canal inclinations (15°, 30°, and 45°) were used to compare the effectiveness of three irrigation techniques (CNI, EDDY, and UIA) in removing filling material.

The use of standardized 3D-printed models enabled a controlled comparison by minimizing anatomical variability, thereby improving the reliability of intergroup analyses [30].

This study has several limitations that should be considered. First, although resin-based 3D-printed models are highly standardized, they do not fully replicate the physical and chemical properties of natural dentin, which may influence the interaction between filling materials and irrigants [31]. Second, only a single obturation material (GuttaFlow) was used, limiting the generalizability of the findings to other sealers with different physicochemical properties [32]. In addition, no baseline CBCT scans were obtained after obturation, which restricts the precise quantification of the amount of material removed. Finally, the experimental design was conducted under in vitro conditions and included a limited range of canal curvatures, which may not fully reflect the complexity of clinical scenarios.

Another important methodological consideration in the present study is the choice of imaging modality. Although micro-computed tomography (micro-CT) is considered the gold standard for volumetric analysis, CBCT was preferred due to its wider availability, lower cost, and faster image acquisition. Additionally, CBCT enables clinically translatable imaging conditions while providing sufficient resolution for volumetric assessment [33]. Previous studies have demonstrated that CBCT can provide reliable quantitative measurements of residual filling materials when appropriate voxel sizes and segmentation thresholds are applied [33, 34]. Therefore, the use of CBCT in the present study represents a balance between methodological accuracy and clinical applicability.

The apical third represents a critical region in retreatment, as residual filling materials are more likely to persist in this area. Consistent with previous reports, the present study demonstrated increased residual material with increasing canal curvature, particularly in the apical region [35, 36]. In the present study, the canal curvature was standardized by identifying the onset point of curvature for each specimen. Using a uniform reference point allowed for a more consistent evaluation of residual filling material across all samples, both within the canal system and in the apical third. This method reduces the influence of natural anatomical variations and ensures that the observed differences are more directly associated with the irrigation protocol and degree of curvature. Therefore, assessing the apical third separately and the whole canal provides a more reliable and clinically relevant measure of retreatment efficiency.

The present study’s findings are consistent with those of Radeva et al. [35], who compared the effectiveness of different irrigation techniques for calcium hydroxide removal and reported that canal curvature did not significantly affect efficacy. However, Swimberghe et al. [37] reported that ultrasonic activation was more effective in removing filling material in canals with 60° curvature. One likely explanation for this discrepancy is the difference in the range of curvatures examined: their study included curvatures up to 60°, whereas ours was limited to 45°. Greater curvatures may reduce the penetration and activation efficiency of irrigation solutions, thereby increasing the relative effectiveness of ultrasonic activation. Furthermore, differences in measurement methods may also contribute to inconsistent results; Swimberghe et al. [37] quantified biofilm removal as a continuous percentage variable, while our study evaluated the absolute volumes of residual filling material in the apical third and the entire canal. These methodological distinctions and limitations underscore the importance of interpreting the findings within the specific experimental framework of this study. Therefore, the lack of a significant difference between irrigation techniques in the present study supports the first null hypothesis, indicating that the irrigation activation method does not significantly affect filling material removal under the conditions tested. This outcome can be attributed to both the limited range of curvature examined and methodological differences.

Increased canal curvature has been associated with greater residual filling material, likely due to reduced instrument maneuverability and limited irrigant penetration [38]. This outcome has been attributed to limited instrument maneuverability and insufficient irrigation flow in curved anatomies. Likewise, Radeva et al. [35] evaluated moderately curved canals (20°–30°) using micro-CT and reported that none of the tested irrigation activation methods, including sonic and ultrasonic techniques, achieved complete calcium hydroxide removal. The authors further observed that specimens with greater curvature exhibited significantly higher residual medicament volumes, underscoring the decisive role of canal anatomy in retreatment efficacy. Taken together, these findings indicate that increasing curvature compromises the removal of both intracanal medicaments and filling materials, which is consistent with the results of the present study.

The same file system was used across all samples in this study to eliminate the effect of different file designs on the results. Indeed, in highly curved canals, the flexibility limits of the files are challenged, reducing mechanical efficiency and resulting in greater residual filling material in the apical third [39]. Moreover, increased curvature causes files to contact the canal walls more extensively, leading to transportation and shaping irregularities, further hindering adequate cleaning of the apical region [40]. Consistent with these findings, the present study observed a significant increase in the volume of residual filling material in the apical third with increasing canal curvature. Consequently, the second null hypothesis is rejected, indicating that canal curvature significantly affects the effectiveness of filling material removal. The present findings suggest that anatomical complexity, particularly canal curvature, may play a more dominant role than irrigation activation techniques in determining retreatment efficacy. This highlights a critical clinical consideration: even advanced irrigation activation systems may be less effective in highly curved canals due to limited irrigant penetration and reduced mechanical action.

Therefore, future studies should focus on developing combined mechanical and chemical strategies tailored to complex canal anatomy.

Conclusions

Within the limitations of this study, canal curvature was identified as a significant factor influencing the amount of residual filling material, particularly in the apical third. In contrast, irrigation activation techniques did not demonstrate a significant effect under the tested conditions. These findings highlight the importance of considering canal curvature in nonsurgical retreatment planning and suggest that anatomical complexity may outweigh the contribution of irrigation activation strategies. Further research is warranted to develop more effective approaches for improving cleaning efficiency in curved canals.

Authors’ contributions

İ.EA. and S.D. gathered the data and analyzed it. S.D. and M.Ç. administered the project. B.Y. and M.Ç. made contributions in methodology. S.D., B.Y., and M.Ç. wrote the main manuscript text and prepared figures. All the authors reviewed and edited the manuscript.

Funding

No funding was obtained for this study.

Data availability

The data of this study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

Ethical approval was obtained from the Erciyes University Clinical Research Ethics Committee (Approval number: 2025/311). This study did not involve human participants or human tissue samples. All procedures were performed on simulated teeth (resin-based 3D-printed models) in accordance with relevant laboratory safety and institutional guidelines. Therefore, informed consent was not required.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.