Introduction

To overcome the failure of an initial root canal treatment, nonsurgical endodontic retreatment is the first clinical option [1]. The procedure should be initiated by removing the filling material in order to reach the appropriate working length [2]. The presence of intracanal filling material after retreatment may harbor bacteria and perpetuate the infection [3]; however, no technique is able to completely remove traces of gutta-percha and cement from the root canal [4,5,6], leading to the search for an ideal material and/or technique.

Reciprocating instruments have been tested for complete removal of the filling material. The Reciproc (VDW, Munich, Germany), which was designed for complete instrumentation of the root canal with a single instrument, has shown good results with respect to effectiveness in removing filling material in laboratory research [4, 7,8,9] and in clinical retreatment [10, 11]. In 2016, the same manufacturer launched the Reciproc Blue file, which has the same design as the Reciproc file but undergo a new surface heat treatment that makes the file more flexible, with a lower risk of fracture and prebending potential [12]. Both the M-Wire Reciproc and Reciproc Blue instruments were shown to be effective in removing filling materials in straight root canals with no significant differences between them [13], but their performance has not yet been tested in severely curved canals or for the removal of root canal material combined with bioceramic sealers.

It is known that the flexibility of an instrument can influence retreatment procedures, and specifically, the possibility of canal transportation [14]. Additionally, retreatment is harder to perform in the apical region and in the presence of curvatures [5, 15]. The use of bioceramic sealers may result in blockage of the apical foramen and loss of patency in some cases [16, 17]. However, some studies have indicated that these sealers are easier to remove than other epoxy-based sealers [9]. Endosequence BC Sealer (Brasseler EUA, Savannah, GA) has shown superior bond strength to dentin compared with mineral trioxide aggregate-based sealers, epoxy resin-based sealers, and dual cure resin-based sealers [18].

However, it is known that regardless of the clinical protocol used, retreatment will result in the apical extrusion of debris, a recent in vitro study showed that retreatment of root filled specimens with epoxy-based sealers resulted in apical extrusion of debris when Reciproc and ProtaperNext were used [19]. Apical extrusion has been associated with pain and/or edema due to the inflammatory response [20].

The aim of the present study was to compare the performance of the Reciproc and Reciproc Blue instruments in retreatment procedures according to microcomputed tomography (micro-CT) images and analysis of the remaining filling material, the volume of dentin removed, and the apical transportation produced after the retreatment of severely curved canals filled with AH Plus and Endosequence BC Sealer. In addition, the retreatment time and the apical extrusion of debris produced after filling removal procedures with such instruments were quantified and compared. The null hypotheses tested were that there were no differences in the performance of the Reciproc and Reciproc Blue instruments associated with the two sealers studied for all variables analyzed.

Materials and methods

This research was approved by the Institutional Research Ethics Committee (Protocol: 65455816.8.0000.5207).

Sample size calculation

The sample size of each group was calculated using the statistical software Epi Info™ (v. 6 for Windows; Centers for Disease Control and Prevention, Georgia, USA). A 5% margin of error and 80% power was established for a sample size of 15 mesial roots.

Sample selection

Three hundred and twenty first and second human mandibular molars were selected. All teeth were disinfected in a 0.1% thymol solution over 24 h and kept in purified filtered water for 30 days. These included teeth that had been extracted for unrelated reasons because they were obtained from a human tooth bank (HTB). They were examined using a stereomicroscope (LabomedLuxeo 4D, Los Angeles, CA, USA) (15× magnification) to analyze the existence of cracks. The curvature angles were chosen based on the initial radiographs using ImageJ software (Version 1.46r; National Institutes of Health, Bethesda, MD). Only teeth with a severe curvature of the mesial root (20°–40°) [21] were selected. The mean (SD) angle of curvature was 42.54° (7.0), and the mean (SD) radius of curvature was 5.32 mm (0.8) [22]. Additionally, the selected teeth were more than 17 mm in length and had fully formed apices. After a radiographic analysis, teeth with endodontic treatment, pulp calcification, and root resorption were excluded. Only teeth with two mesial canals with independent foramen were included (Vertuccitype IV) [23]. In the end, 60 teeth were selected as the sample.

Cleaning and shaping

Preparation, obturation, and removal filling procedures were performed by the same operator. The crowns were sectioned using a diamond disk to standardize the samples with 17 ± 0.5 mm length. After endodontic access, the glide path was performed with a K-file #15 (Dentsply Maillefer, Ballaigues, Switzerland). The working length (WL) was determined to be 1 mm below the apical foramen. A thin layer of molding material made of silicone was used to simulate the periodontal ligament in acrylic resin blocks. The root canals were prepared using the Reciproc R25 (25/.08) instrument. The file was introduced into the root canal until resistance was felt, and three forward-backward movements were performed with slight apical pressure. The instrument was removed from the canal and cleaned. At each insertion, the patency was confirmed using a K-file #10. Irrigation was performed with 2.5% sodium hypochlorite (NaOCl) after each insertion of the file. An irrigation with 2 mL of 17% EDTA was completed to remove the smear layer, and final irrigation was performed with 2 mL of 2.5% NaOCl. The sample was divided into the following four experimental groups according to the selected root sealer type and the instrument used for filling removal: EndoSequence BC Sealer/Reciproc (BCRC group), EndoSequence BC Sealer/Reciproc Blue (BCRB group), AH Plus/Reciproc (AHRC group), and AH Plus/Reciproc Blue (AHRB group). Pairs of mesiobuccal and mesiolingual canals were balanced and randomly distributed between the groups (n = 15) with the aid of digital software RANDOM.ORG, and both systems were tested on the same root.

Root canal filling procedures

The single cone technique was selected to be the root canal filling procedure in this study for being relatively easy to execute, to have less operator interference, and also to be described in comparative studies between obturation performed with epoxy resin and bioceramic sealers [24, 25]. All samples were filled using the single cone technique [26] with a Reciproc System R25 gutta-percha cone (VDW, Munich, Germany) adjusted to WL 16 mm. Two groups (n = 30) were filled with the endodontic sealer AH Plus, and the other two groups (n = 30) were filled with EndoSequence BC Sealer. The canals were dried with paper points. Patency was reconfirmed before obturation. The AH Plus sealer was manipulated according to the manufacturer’s recommendation. After cone/sealer insertion, the gutta-percha was cut at the level of the orifice and slightly condensed with a plugger. The filling procedure with EndoSequence BC Sealer was performed by positioning the syringe tip inside the root canal and injecting the sealer according to the manufacturer’s recommendation. Then, the gutta-percha was adjusted to the WL. The gutta-percha cone was cut similarly to the previous group. The teeth were radiographed buccolingually and mesiodistally to assess the quality of the filling procedure. The crowns were sealed with temporary filling material (Cavit; 3 M ESPE, St Paul, MN) and stored at 37 °C and 100% humidity for 30 days.

Micro-CT scanning

Scanning was performed individually using micro-CT (SkyScan 1172; Bruker microCT, Kontich, Belgium). Each tooth was positioned on the turntable with the roots facing up. The parameters used were as follows: 100 kV voltage and 100 μA current with 17.87 μm resolution; 360° rotation, 1-mm thick copper + aluminum filter, 0.5 rotation step, and 34-min exposure. After scanning, the images were reconstructed using the software NRecon (Bruker microCT), producing 700–800 images per tooth.

Root canal retreatment

First, the gutta-percha in the coronal third was removed (3 mm) using a number 2 Gates Glidden drill. Then, a 25/.08 instrument from each system was inserted to remove the filling material until the WL was reached, and a 40/.06 instrument was used for retreatment enlargement (n = 15). The instruments were used with the VDW Silver motor, as recommended. The files were introduced into the root canal in three forward-backward movements with slight apical pressure. After each movement, the files were removed from the root canal and cleaned. Canals were irrigated with approximately 2 mL of 2.5% NaOCl before file reinsertion using a 30-G NaviTip between each instrument change, and a total of 20 mL was delivered per canal. Then, 3 mL of 17% EDTA was used for 3 min, and a final rinse with 2 mL of bidistilled water was performed. Removal of the root filling was finished when there was no more filling material left in the file [6, 24], as confirmed by radiographic analysis. Each instrument was used in only one tooth and then discarded. The removal filling procedures were performed under an experimental model used to evaluate debris extrusion, and the time required for retreatment was recorded. The total time was considered to be the effective action time of the instruments in each root canal. After retreatment, the sample was scanned again by micro-CT using the same parameters as previously mentioned.

Apical extrusion of debris

The experimental model used to evaluate debris extrusion was as described by Myers and Montgomery [27], with some modifications (Fig. 1). One Eppendorf tube (Eppendorf AG, Hamburg, Germany) was assigned, according to the instrument used, to prepare each mesial canal. A gingival barrier (TopDam, FGM, Brazil) was used to temporarily seal the root canal entrance that was not being instrumented. An opening was created on each Eppendorf tube cap, according to each root’s anatomical configuration, and the roots were affixed with cyanoacrylate to prevent the unintentional leakage of the irrigation solution. The Eppendorf tube was individually weighted on an analytical balance to the nearest 0.1 mg (AUW220D, Shimadzu Analytical Balance, Tokyo, Japan). Five consecutive weightings were conducted for each tube, and the highest and lowest values were discarded. The arithmetic means of the three weights obtained were regarded as the initial weight of the Eppendorf tube. The Eppendorf tube was placed in an opaque bottle to prevent the operator from being able to see the root canal during instrumentation. A 27-G needle was inserted into the Eppendorf cap to equalize the internal and external pressure. After the removal filling procedures, the tooth was removed from the Eppendorf tube, and the root was washed with 1 mL of NaOCl to collect the debris that had adhered to the outer side. All tubes were incubated at 50 °C for 15 days to allow evaporation of the remaining irrigation solution from the tubes. After the incubation period, a final weighting was performed in the same manner as the initial weighing.

Fig. 1
figure 1

A representative image of the experimental model used to evaluate apical extrusion of debris

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Micro-CT analysis

DataViewer software (SkyScan, Kontich/Belgium) was used to geometrically align the two sets of images using the three-dimensional (3D) registration function. The region of interest for each canal was 10 mm from the apex of the mesial root, and it was set by integration of the regions of interest in all cross-sections. Comparisons between the original and segmented scans were performed to ensure accuracy of the segmentation. A volumetric analysis was performed, and 3D models were constructed. The volumes of obturation material and dentin before and after the removal filling procedures were measured in cubic millimeters for the entire canal and its thirds (cervical, middle, and apical). The isthmus region was not included in the analysis. It is difficult to remove filling materials from this region using only currently available instruments.

Apical canal transportation

Apical canal transportation was measured on axial sections at 1, 3, and 5 mm from the anatomic apex. Transportation was calculated in millimeters by ImageJ software using a modification of the formula |(X1-X2)-(Y1-Y2)| [14] (Fig. 2). A result of 0 from the canal transportation formula would indicate no canal transportation. X1 is the shortest distance between the mesial edge of the root and the obturated canal, X2 is the shortest distance between the mesial edge of the root and the retreated canal, Y1 is the shortest distance between the distal edge of the root and the obturated canal, and Y2 is the shortest distance between the distal edge of the root and the retreated canal. Preoperative and postoperative measurements were compared to reveal the presence or absence of deviations in canal anatomy. An operator blinded to the study results performed these measurements twice at different times.

Fig. 2
figure 2

Cross-sections of micro-CT scan images with markings showing points of apical canal transportation measurement after obturation (a, red) and after the removal of obturation material (b, green)

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Statistical analysis

The statistical measurement data were expressed as the mean, standard deviation, and percentage. F tests (ANOVA or Kruskal-Wallis) were used to compare the variances between the groups. Student’s t test was used for comparisons between materials or instruments. In the rejection of normality cases, the Kruskal-Wallis and Mann-Whitney tests were used. The margin of error used in the statistical tests was 5.0%. In all statistical analyses, SPSS software (Version 23, IBM SPSS Inc., Chicago, IL, USA) was used.

Results

The results of the present study concerning the volume reduction of obturation material, amount of dentin removed, apical transportation, apical extrusion of debris, and retreatment time between the groups are presented in Tables 1, 2, and 3, respectively.

Table 1 Analysis of the initial root filling volume and volume reduction of filling material between the groups (n = 15)
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Table 2 Analysis of the dentin amount before and after retreatment and the percentage of dentin removal after retreatment between the groups (n = 15)
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Table 3 Analysis of apical transportation, the apical extrusion of debris, and retreatment time between groups (n = 15)
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No instrument was able to remove all obturation material from the root canal. The differences in reduction in the total volume of filling material percentage among groups were not statistically significant (p > .05). Only the apical third of the BCRC group presented significantly greater reduction than the other groups (Table 1).

No significant difference in dentin removal was observed between groups (p > .05) (Table 2).

Groups obturated with EndoSequence BC Sealer took longer to retreat (AHRC < AHRB < BCRC < BCRB). However, only the AHRC group retreated faster than the other groups (p > .05) (Table 3).

Apical transportation and apical extrusion of debris occurred in all groups, but there were no significant differences among groups (p > .05) (Table 3).

Discussion

The null hypothesis was partially accepted since there were no differences (p > .05) in the percentage of obturation material reduction, apical transportation, volume of dentin removed, or apical extrusion of debris in the situations tested. Only the time required for retreatment significantly differed.

Reciprocating movement is implemented to increase the centralization of the preparation and to reduce the risk of deformation of the canals [28]. These points are all related to relief of the torsional and flexural tensions that occur during reciprocating motion [28]. Instruments with this movement have exhibited the ability to maintain the original axis of curved canals during preparation [29]. The Reciproc and Reciproc Blue systems have already been evaluated and demonstrated positive results in retreatment procedures [6, 8, 9, 13, 30]; however, to date, no research has evaluated their effectiveness in the retreatment of severely curved root canals, nor in removing bioceramic sealers.

The Reciproc instrument is made from a nickel-titanium alloy with an M-Wire (Memory Wire, Dentsply Tulsa Dental Specialties, Tulsa, OK) thermal treatment that provides the instruments with a greater resistance to cyclic fatigue [31] and torsion [32]. The Reciproc Blue instrument is made from a nickel-titanium alloy with a Blue-Wire surface heat treatment. This type of alloy shows better overall performance than M-Wire, and it demonstrates better flexibility, fatigue resistance, and microhardness reduction while maintaining similar surface characteristics [12]. It is advantageous that instruments have both flexibility and strength for the removal of obturator material in curved canals.

The roots selected in the present study had two separate and distinct canals from the pulp to the apex classified as Vertucci type IV [23]. Mesiobuccal canals of the mandibular molars tend to have more pronounced curvature than mesiolingual canals [33], and therefore, they were equally distributed between the groups to minimize the number of variables. In addition, the initial obturation volumes were similar between the groups in the middle and apical thirds (p > 0.05).

Each canal of the mesial root was retreated with a different type of instrument, the Reciproc or the Reciproc Blue, thus allowing a balanced distribution of the sample. The analysis of the reduction in filling material and apical transportation each canal was considered extremely reliable, as the analysis was performed using micro-CT. Thus, each canal was evaluated individually in two steps without the samples being destroyed and allowing intergroup and intragroup comparisons.

To ensure standardization in the experiment, the initial diameter of the last tip used in both instruments was .40, and both maintained the same taper. Therefore, the differences in the flexibility and microhardness of the instrument only influenced the percentage of filling material reduction in the apical third, regardless of the sealer used. Within this context, the Reciproc and Reciproc Blue instruments had similar cutting power in the cervical and middle thirds.

Although neither the Reciproc nor Reciproc Blue system was originally developed for retreatment purposes, the manufacturer has indeed recommended these instruments for such cases. Several studies have evaluated the efficacy of the Reciproc in obturation removal. The results have been promising due to its good performance [6, 34], and even better performance has been reported compared with rotary instruments developed for retreatment [7, 11].

In the present study, apical transportation was evaluated at 1, 3, and 5 mm from the apical foramen. The null hypothesis was accepted for this parameter because there was no significant difference between the groups (P > 0.05). Both the Reciproc and Reciproc Blue instruments remained centralized in the apical third of the root canal and preserved the original anatomy. It can be concluded that both the Reciproc and Reciproc Blue systems had similar performance in the retreatment of severely curved root canals (Fig. 3).

The highest apical transportation occurred at 1 mm from the apex in this study and in other studies [6, 35]. Although there was no significant difference, the groups filled with BC Sealer showed a tendency to transport less in this region. However, there is a greater tendency for filling materials to be removed in the cervical, middle, and apical thirds [6, 8], which corroborates the results of the present study.

The percentage of residual material (8–15%) in the present study was higher than that reported in the study by Rödig et al. [36], who detected a 3.7–11.3% residual volume in curved canals after retreatment with Hedstrom instruments and two NiTi retreatment systems, ProTaper Universal Retreatment, and D-RaCe instruments. In another study by Rödig et al. [4], a 5.1–8.3% residual volume among Hedstrom, ProTaper Universal Retreatment, and Reciproc was reported, and these results were more similar to the study by reference [6] (13–15%) after canal retreatment with Reciproc and ProTaper NEXT instruments. Moreover, in a study by reference [24], the residual volume of filling material was 6% after retreatment with Waveone Gold and Reciproc instruments. Reciprocating movement has been reported to be significantly more effective than rotatory instruments in clearing straight canals [7] and similarly effective in clearing curved canals [6, 24] (Fig. 4).

Fig. 3
figure 3

Cross-sections of micro-CT scan images. a Sample images after obturation with EndoSequence BC Sealer and AH Plus at 5 mm, 3 mm, and 1 mm distances from the apical foramen. b Sample images after the removal of obturation material with RB and RC files at the respective cross-sections (RB: Reciproc Blue; RC: Reciproc)

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Fig. 4
figure 4

Tridimensional reconstructions of representative samples. a Obturation material after root canal filling procedures. b Remaining obturation material after retreatment with RB and RC files (RB: Reciproc Blue; RC: Reciproc)

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In the current study, apical enlargement by two sizes beyond the initial preparation was performed to reduce the amount of residual filling material [4], and to create a better space to conduct effective irrigation. Therefore, the use of two instruments with different tapers may have contributed to more touches on canal walls and a higher percentage of obturation material removal.

A recent study [13] evaluated the effectiveness of Reciproc in straight root canals and showed that the percentage of remaining material was lower (approximately 7%) than in the present study, which was performed in severely curved canals.

Several variables may have contributed to the discrepancy between the remaining volumes in previously published studies, although no system has been able to completely remove the material from a curved root canal. One factor to consider regarding the variation in the results of these material reduction percentages is the different obturation techniques used in previous studies. Nevares et al. [6] used a thermoplastic technique, while [4, 36] used a lateral condensation technique. Fruchi et al. [24] filled the canals using a single-cone technique, as in the present study; however, they used solvent to facilitate the removal of gutta-percha. In the present study, solvent was not used, to avoid a chemical plastification of gutta-percha and the adhesion of a thin layer of this material to the canal walls [37]. The soft gutta-percha could be pushed into areas of irregularities, making the cleaning process difficult. To obtain better disinfection of the root canals, the volume of dentin removed is an important consideration. Thus, in the present study, this factor did not present a difference among any groups.

The time required for the retreatment procedure was similar between the BCRB, BCRC, and AHRB groups. The mean time required to remove the filling material was longest in the BCRB group, followed by the BCRC, AHRB, and AHRC groups. The mean differed from 2.4 to 1.29 min between groups. The longer times required in the BCRB and BCRC groups can be attributed to the additional time and effort required to regain patency in several samples filled with EndoSequence BC sealer. Regarding the time for retreatment, the results of the present study were the same as those reported in studies by Zuolo et al. and Oltra et al. [16, 17].

Additionally, calcium-silicate sealers had better retreatment capacity results than AH Plus. Less remaining sealer [9] was observed, which is consistent with the results of the present research.

The results of the present study are also consistent with the results of a study by Suk et al. [38], which showed no difference in retreatment between EndoSequence BC Sealer and AH Plus using the rotatory technique. Several previous studies have also shown limitations in retreatment with both rotary and reciprocating instruments in root canals obturated with calcium silicate material compared with resin-based and zinc oxide materials [39, 40].

Kim et al. [40] evaluated the retreatment capacity of EndoSequence BC and AH Plus sealers using rotary scanning electron microscopy. They concluded that both materials showed retreatment characteristics at a similar penetration in the dentinal tubules. Ersev et al. [41] also stated that there was no difference in the capacity of retreatment between AH Plus and EndoSequence BC, as evaluated by radiography. The root filling removal of the two sealers has only been studied by a few authors, namely, Oltra et al. [17], Suk et al. [38], Hess et al. [39], Kim et al. [40], Ersev et al. [41], Agrafioti et al. [42], and Chybowski et al. [43], but there is no consensus among studies. On the other hand, AH Plus is an epoxy-based endodontic sealer that is already well established and has been used extensively in research studies. A greater amount of remaining material in the canal after retreatment with AH Plus followed by EndoSequence BC sealer was observed in both [42] and the present study.

Hess et al. [39] evaluated the removal capacity of BC sealer and AH Plus related to patency and loss of WL using scanning electron microscopy. Oltra et al. [17] reported that BC sealer was impenetrable by small hand instruments when sealer was present in the apical foramen; we obtained similar results in this study. Oltra et al. [17] also reported that WL and patency were restored to 100% of AH Plus samples but could not be restored to all teeth sealed with BC sealer. However, in the present study, 100% of the samples had their WL and patency restored, consistent with the report by De-Deus et al. [13]. These results are different from those found by Hess et al. [39], in which patency was recovered in only 80% of samples. In addition, establishing patency and WL in cases of retreatment has been shown to significantly improve periapical cure rates [44].

These results clearly indicate that calcium silicate-based sealers are harder and more resilient to common retreatment techniques but are not impenetrable [42], corroborating the results of the present research. Calcium silicate-based sealers increased retreatment time by approximately two-fold the retreatment time of BC sealer [42], and according to present research, it may increase the time up to approximately 1 min.

Successful removal of a bioceramic sealer requires patience and additional time and effort compared with conventional sealers. The time commitment necessary for retreatment can sometimes compromise proper root canal system cleaning and modeling and affect the success of nonsurgical endodontic therapy [42]. The present study is aligned with a previous study that stated that silicate-based sealers can be removed from canals in the same manner as AH Plus [41]. It was also confirmed that AH Plus is a common sealer that is difficult to completely remove [40].

The amount of extruded debris can be influenced by the design and kinematics of endodontic instruments [45] and by cross-cutting and cutting efficiency [46]. However, these variables did not exist in the present study. Thus, the cutting efficiency of these instruments involves an interrelationship of several factors, such as design, cross-section, debris removal capacity, angles, metallurgical properties, and surface treatment [47]. Previous research on the Reciproc has shown that this instrument has excellent cutting power [48]. The highest cross-cutting efficiency of the Reciproc is due to its “S” cross-section with two cutting edges [49], which is retained in the Reciproc Blue instrument [12].

Regarding the extrusion of debris in this study, the results showed that the quantification was similar between groups, regardless of the endodontic sealer and files used for retreatment. It is known that Reciproc [6, 30, 50,51,52,53,54] and Reciproc Blue [55], when used in retreatment, promote the apical extrusion of debris, but there are no comparative data between the systems or even data associated with different bioceramic endodontic sealers in severely curved canals.

The morphology and kinematics of the instruments analyzed in this study were the same, and therefore, the groups were compared only in relation to the root canal retreatment procedures. All of them demonstrated similar apical extrusion of debris, and no significant differences were found among the studied groups. Thus, the manufacturing process, which is the only difference between these instruments, did not interfere in the apical extrusion of debris.

Reproducing clinical conditions in apical extrusion studies in vitro is difficult, which is an important point to consider. Clinically, periapical tissues may act as a physical barrier to the apical extrusion of debris [56]. However, in the present study, all efforts were made to standardize the methodology employed. The apparatus for collecting debris has been slightly modified to make it simpler, more practical and accessible from what was previously proposed by Myers and Montgomery [27]. In the present study, the Eppendorf tube was placed in an opaque vial, limiting the operator’s sight during the retreatment procedure. This method eliminated the possibility of contamination throughout the study procedure, because there was no direct contact between the operator’s fingers and the device.

In previous studies, the creation of a periapical barrier was attempted using agarose gel [57] and a floral sponge [58]. However, the gel density did not simulate the same conditions as periapical or periradicular lesion tissues [57], and the sponge absorbed extruded irrigation and debris solutions, making volumetric quantification difficult [46]. No method has yet been able to reproduce an optimal simulation of periapical tissues.

Conclusions

It can be concluded that the Reciproc and Reciproc Blue systems obtained similar efficacy in severely curved canal filling material removal procedures. Although no system completely removes the filling material, both can be indicated for root canal retreatment. There were no significant differences between groups in relation to the amount of material remaining, dentin removal, apical transportation, or debris extrusion. However, canals filled with EndoSequence BC Sealer may require more time during retreatment. As none of the systems could completely remove the obturation material, additional techniques should be used to improve root canal cleansing during retreatment procedures.