The effectiveness of passive ultrasonic irrigation and the easy-clean instrument for removing remnants of filling material.

OBJECTIVE: This study aimed to evaluate the efficacy of passive ultrasonic irrigation (PUI) and the easy clean instrument by micro-computed tomography (CT) for removing remnant filling materials during endodontic retreatment.
MATERIALS AND METHODS: Forty mesial roots of mandibular molars were divided into four groups (n = 10) according to the agitation system and sealer used: Group 1: PUI/AH Plus; Group 2: PUI/TotalFill; Group 3: Easy Clean/AH Plus; and Group 4: Easy Clean/TotalFill. The groups were compared by micro-CT analysis according to the volumes of the obturation and the remaining material before and after the agitation systems were used.
RESULTS: There was no difference between the groups for total filling volume and initial and final remaining filling volume (P > 0.05).
CONCLUSIONS: Both the PUI and easy-clean instrument are effective for removing remnants of filling material with no difference between the groups. Copyright:

INTRODUCTION

Removal of the filling material, instrumentation, disinfection, and refilling of root canals are the main objectives of a successful endodontic retreatment.[1] The removal of filling material constitutes a major risk mainly due to the anatomic complexity of the root canal system (RCS). This stage is fundamental for the cleaning and disinfection of the canals, as the components of the filling materials can favor the persistence of microorganisms, increasing the risk of irritation in the periradicular tissues.[1] The filling material can be removed by manual instruments, heated instruments, solvents, and rotary and reciprocating systems.[2] However, regardless of the technique used, complete removal of the obturator material is not achieved, particularly in the apical third of the root canals.[3]

Therefore, additional methods have been proposed for improving the removal of these filling materials.[4] Passive ultrasonic irrigation (PUI) is the activation of an irrigant using an ultrasonic insert positioned in the center of the root canal, which induces acoustic flow and/or cavitation of an irrigating solution. This procedure has been used for improving root canal cleaning; increasing disinfection; and removing the smear layer, debris, and intracanal medications, even in complex anatomical areas.[56] However, the efficacy of PUI for removing remnant filling materials during retreatment remains controversial.[789101112131415] Da Rosa et al.[8] and Barreto et al.[7] observed via micro-computed tomography (micro-CT) that PUI did not increase the removal of gutta-percha and AH Plus sealer remaining in maxillary molars and the isthmus regions of mandibular molars, respectively. Martins et al.[9] also observed by micro-CT that PUI did not increase the removal of gutta-percha and Endofill sealer in premolars. In contrast, Grischke et al.[10] and Castro et al.[11] observed via different microscopy techniques that PUI improved the removal of remnant gutta-percha and AH Plus sealer during retreatment. Moreover, some authors observed via micro-CT that PUI significantly improved the removal of remnant gutta-percha and AH Plus, AH 26, and Pulp Canal Sealer sealers in mandibular incisors and molars.[12131415]

The easy-clean agitation device (Easy Equipamentos Odontológicos, Belo Horizonte, Brazil), consisting of an acrylonitrile butadiene styrene plastic instrument, 25/0.04 in size, and an aircraft wing-shaped cross-section, was designed to be used in either reciprocating or rotatory motion. In a recent study, Rodrigues et al.[16] evaluated via scanning electron microscopy the wall cleanliness of uniradicular teeth after using the PUI and easy-clean agitation systems during endodontic retreatment of roots filled with gutta-percha and endofill sealer. They observed that the easy-clean instrument used in continuous rotation was as effective as PUI for removing the remaining filling material.

Calcium silicate-based bioceramic sealers are premixed, biocompatible, have antibacterial properties, are chemically stable, do not change volume after setting, and have good flow and penetration into dentin.[1718] Due to their chemical composition, such sealers are known as difficult-to-remove materials after stabilization, which raises concern when there is a need for retreatment.[19] Uzunoglu et al.[20] observed via stereomicroscopy that a greater amount of remnant filling material in the groups in which the iRoot SP sealer (a calcium silicate-based material) was used when compared to the resin-based AH 26 sealer.

Retreatment studies use different methods to evaluate the remaining material, such as digital radiography, scanning electron microscopy, stereomicroscopy, and clinical microscopy. The main disadvantage of these methods, besides the loss of the sample, is that the analysis of the remaining filling material is qualitative, and it is not possible to evaluate the full extent of the RCS.[192021] In addition, the image evaluation is two-dimensional, which renders interpretation of the results difficult.[22] Therefore, micro-CT has been widely used, as it allows precise three-dimensional analysis of the RCS and quantitative evaluation of the canal volume, the total filling material, and the remaining filling material without destroying the sample.[223]

Based on the above, the objective of the present study was to evaluate the efficacy of the PUI and easy-clean agitation systems for removing remnant filling materials during the retreatment of mesial roots of mandibular molars previously filled with gutta-percha and either the resin-based sealer AH Plus or the bioceramic TotalFill BC sealer.

MATERIALS AND METHODS

Sample selection and preparation

This study was approved by the University Hospital Ethics and Research Committee. A sample calculation was performed using a test power of 0.8 and an alpha value fixed at 0.05, which resulted in a minimum of nine samples per group to obtain statistical significance. Forty human mandibular molars with complete root formation and a root curvature angle of up to 25°, extracted for clinical reasons, were used in this study. The teeth were stored in 0.1% thymol solution at 4°C until used. Access cavities were performed with a round diamond and Endo-Z burs (Dentsply-Maillefer, Ballaigues, Switzerland) in a water-cooled, high-speed handpiece. No file was inserted through the mesiobuccal, mesiolingual, or distal canals to prevent changes in the original anatomy of the apical region.

Micro-computed tomography scanning

Micro-CT images were acquired according to the methodology of de Almeida et al.;[24] a custom-made mold of self-polymerizing resin was created for each tooth to standardize the mounting of the specimen. The teeth were placed in a micro-CT scanner (SkyScan 1173; Bruker, Kontich, Belgium) on a custom aluminum attachment. This base ensured the standardization of the images obtained initially and after the canal obturation and retreatment, as it allowed precise re-placement of the sample inside the scanner. Scanning was performed through 360° rotation with a rotation step of 0.30 using a 1.0-mm thick aluminum filter, 70-kV energy, 114-mA current, 14.8-mm pixel size, and 21.39-mm resolution.

Chemical-mechanical instrumentation and filling

Coronal third enlargement was performed with LA Axxess stainless steel burs (SybronEndo, Glendora, CA, USA), and size 10 K-files (Dentsply-Maillefer) were used during instrumentation to establish apical patency and to determine the canal lengths. After radiographic evaluation, the working length (WL) was established 1 mm short of the radiographic apex. The teeth were instrumented with K3XF files (SybronEndo) by the crown-down technique to the WL at 350 rpm with the use of a torque control endodontic motor (VDW, Munich, Germany) using the sequence recommended by the manufacturer as follows: #25/0.08, #25/0.06, and #25/0.04. All files were used passively, and apical enlargement was performed using #25.06 and #30.04 files. The pulp chamber was filled with 5.25% sodium hypochlorite (NaOCl) throughout the instrumentation procedure. The canal was irrigated with 3 mL 5.25% NaOCl after each instrument. After instrumentation, all canals were irrigated with 3 mL 17% EDTA for 3 min (1 mL/min) and 3 mL 5.25% NaOCl and dried with FM size paper points (SybronEndo). The teeth were then randomly divided into two groups of 20 teeth each according to the sealer used: The TF group used TotalFill BC (FKG, La Chaux-de-Fonds, Switzerland), #30 or #35 gutta-percha cones, and TotalFill BC Sealer (FKG). The AHP group used size FM or M gutta-percha cones (Dentsply-Maillefer) and AH Plus sealer (Dentsply-Maillefer). In both the groups, the cone was placed in the WL. After filling, the pulp chambers of all teeth were sealed with cotton and zinc oxide-based temporary material and stored for 7 days in an oven at 37°C and 100% moisture for the total setting time of the sealers. After this period, the dental elements underwent micro-CT image acquisition again, as described earlier.

Removal of filling material

The filling material was removed in the same manner for both groups. Initially, a #3 Gates Glidden drill (Dentsply-Maillefer) was used in the cervical 3 mm. The nickel–titanium (NiTi) ProTaper Universal Retreatment (PTUR) (Dentsply-Maillefer) sequence was employed according to the manufacturer's instructions: D1, D2, and D3 were used in the cervical, middle, and apical regions, respectively, without solvents. The removal of the filling material was considered complete when no gutta-percha residue or sealer was observed in the instrument and the WL was reached. After this step, a NiTi ProTaper Next X4 file (#40.06) (Dentsply-Maillefer®) was used up to the WL, followed by irrigation with 5 mL 5.25% NaOCl. The teeth then underwent a new micro-CT image acquisition.

Complementary cleaning stage

After the filling material had been removed, the samples underwent a complementary cleaning step and were divided into four groups according to the sealer and cleaning procedure used: Group 1: PUI/AH Plus; Group 2: PUI/TotalFill; Group 3: Easy Clean/AH Plus; and Group 4: Easy Clean/TotalFill.

Passive ultrasonic irrigation

Each canal was irrigated with 2 mL 5.25% NaOCl, and an ultrasonic tip (Helse Dental Technology, Santa Rosa de Viterbo, Brazil) was used in the Delsonic 2000 apparatus (Deldent, Mevaseret Zion, Israel) with 40-Hz power, 2 mm short of the WL, for 20 s. This procedure was repeated two more times, with a total time of 1 min. The irrigation solution was renewed every 20 s. After this activation protocol, a final flow with 5 mL saline solution was performed without activation.[25]

Easy clean

The easy-clean instrument was coupled to a micromotor and contra-angle at low speed at approximately 20,000 mm (KaVo Kerr Group, Charlotte, NC, USA) as described by Duque et al.[26] The tip was placed 2 mm short of the WL and the same sequence of irrigation solutions and time employed for the PUI group was used. The teeth then underwent new image acquisition by micro-CT.

Evaluation of images

Only the images of the mesial roots of the mandibular molars were reconstructed by NRecon software (Bruker) and analyzed using CTan software (Bruker micro CT). The images obtained before the chemical–mechanical preparation were used to verify the equivalence of the groups' anatomy. The filling volume was obtained from the image after the obturation, the initial remaining filling material volume was obtained after the removal of the obturator material with the PTUR and ProTaper Next X4 instruments, and the final remaining obturator material volume was obtained after the complementary cleaning. The difference between the values of the filling volume and the final remaining obturator material volume was calculated and resulted in the ΔV, which represents the volume of material removed from the RCS. The volumes of filling material, the initial remaining filling material, and the final remaining filling material were also obtained for the cervical, middle, and apical thirds. The percentage of remaining filling material was calculated using the following formula: final remaining filling material volume × 100/filling volume. Based on these values, the percentage of filling material remaining in the different thirds of the root canal was calculated.

Statistical analysis

Initially, the hypothesis of similar anatomical conditions between the groups regarding the RCS volume was confirmed using Student's t-test. An initial screening for data normality was performed using the Shapiro–Wilk test and showed nonnormal distribution. The initial and final remaining filling material volumes of the groups were compared using the Kruskal–Wallis test. The Wilcoxon test was used to identify the significance of the amount of residual material removed after the use of PUI and the easy clean and to compare the volume of remaining obturator material among the root canal thirds. Finally, the percentage of the residual obturator material of the root canal walls was compared between the groups (Wilcoxon test). The level of significance was 5% (SPSS v. 25; SPSS Inc., Chicago, IL, USA).

RESULTS

There was no difference between the groups regarding the initial RCS volume, confirming the anatomical similarity of the groups (P > 0.05). Table 1 shows the total filling material volume, the initial remaining filling material, the final remaining filling material, the ΔV, and the percentage value of filling material in each group. There was no statistically significant difference between the groups for total filling material volume, initial remaining filling material volume, and final remaining filling material volume (P > 0.05). However, there was a statistically significant difference between the volume of filling material and the initial and final remaining filling material volumes in all groups (P < 0.05).

Table 1

Mean and standard deviation of the volume of filling material, ΔV, and percentage of remaining filling material in the groups

Group	Vo	Vri	Vrf	∆V	Rem (%)
AHP + PUI	12.91±3.41a	2.97±1.84b	2.31±1.50c	0.66±0.51	17.893
TF + PUI	14.66±2.09a	3.66±3.47b	2.16±1.83c	1.49±1.82	14.733
AHP + EC	8.07±2.40a	1.95±1.26b	1.33±0.98c	0.62±0.52	16.480
TF + EC	8.45±2.88a	2.75±1.94b	1.78±1.45c	0.96±0.56	21.065

Vo: Filling volume, Vri: Initial remaining volume, Vrf: Final remaining volume, ΔV: Difference between the filling volume values and the volume of the final remaining filling material, Rem%: Percentage of remaining filling material

Table 2 presents the initial and final remaining filling material volumes in the three thirds of the canals in the different groups. There was a significant difference between the three thirds of the canals for the volume of filling material in all groups (P < 0.05). In Groups 1 and 3, the initial remaining filling material volume in the cervical third was lower than that of the middle third (P < 0.05), with no statistical difference between the other thirds of the canal (P > 0.05). In Groups 2 and 4, there was no statistical difference between the three thirds of the canal and the volume of remaining filling material (P > 0.05). For the final remaining filling material volume, there was no statistical difference between the thirds of the canals in all groups analyzed (P > 0.05), but there was a significant reduction from the initial remaining filling material volume to the final remaining filling material volume in all canal thirds (P < 0.05).

Table 2

Mean and standard deviation of the volume of filling material in the cervical, middle, and apical thirds in the groups

Group	Vo			Vri			Vrf
	C	M	A	C	M	A	C	M	A
AHP + PUI	6.88±2.21a	4.05±1.04b	1.97±0.86c	0.43±0.41d	1.26±0.78e	1.25±0.90d,e	0.25±0.23f	0.96±0.62f	1.09±0.83f
TF + PUI	7.97±1.01a	4.73±1.35b	1.95±0.76c	0.95±0.85d	1.71±1.81d,e	0.99±1.21d,e	0.38±0.45f	1.14±0.85f	0.64±0.67f
AHP + EC	4.82±1.62a	2.46±0.70b	0.74±0.30c	0.73±0.48d	0.86±0.73e	0.35±0.18d,e	0.46±0.35f	0.59±0.56f	0.26±0.15f
TF + EC	4.81±1.81a	2.57±0.87b	0.99±0.32c	1.04±0.89d	1.08±0.99d,e	0.62±0.32d,e	0.75±0.77f	0.63±0.57f	0.40±0.25f

Equal letters indicate no statistical difference (Wilcoxon and Kruskal–Wallis tests, P<0.05). Vo: Filling volume, Vri: Initial remaining volume, Vrf: Final remaining volume, C: Cervical third, M: Medium third, A: Apical third

Figure 1 shows representative images of the reconstructions obtained by micro-CT of the initial and final remaining filling material of the four groups. We observed in the reconstructions that complete removal of the material was not achieved in any of the groups evaluated.

Figure 1

Representative micro-computed tomography reconstructions from mesial roots of mandibular molars after initial filling removal (a-d) and after complementary cleaning stage (e-h) of the four groups tested: group 1 AHP + passive ultrasonic irrigation (a and e); Group 2 TF + passive ultrasonic irrigation (b and f); Group 3 AHP + EC (c and g); Group 4 TF + EC (d and h)

DISCUSSION

The present study evaluated the effectiveness of PUI and the easy clean instrument for removing remnant filling materials from the mesial roots of mandibular molars obturated with the resin-based AH Plus and the bioceramic TotalFill BC sealers during endodontic retreatment. Both agitation systems were effective for reducing the volume of the remaining filling material, without statistical differences between the systems and sealers analyzed (P > 0.05). Complete removal of the filling material, however, was not observed in any sample.

Micro-CT was used to evaluate the volume of the remaining filling materials. This methodology has been widely used in studies that analyze the RCS three-dimensionally as it offers great advances in the reconstruction of details and maintains the original data for re-evaluation.[22324272829] Therefore, it is the ideal method for studies of anatomy, instrumentation, obturation, and retreatment.[2272829]

No solvents were used during the first step of retreatment or in the additional cleaning step with PUI and the easy-clean instrument. Although solvents can promote better dissolution of the filling material, facilitating the penetration of endodontic instruments, they can also make cleaning more difficult, as they create a layer of softened gutta-percha that adheres to the root canal walls.[30] Furthermore, it has been demonstrated that solvents reduce the bond strength of endodontic sealers, a consequence that can compromise successful retreatment.[31] Martos, et al.[32] recommend using only auxiliary chemicals and avoiding solvents when removing the obturator material. Cavenago et al.[12] used the solvent xylene and observed an improvement in the removal of filling material after PUI. In contrast, Barreto et al.[7] used orange oil, which is less irritating and more commonly used in the clinic, and did not find improvement in removing filling material after PUI.

The easy-clean instrument was used in continuous rotation, as it has been recently demonstrated that it is more efficient for cleaning the isthmus area and the canal walls when used in continuous rotation rather than in reciprocating motion.[26] The authors observed that the instrument removed more debris when compared to other agitation systems. This result can be explained by the difference in the rotation speed that produced turbulence of the irrigating solution, favoring the removal of isthmus debris. In addition, agitation occurs throughout the instrument, promoting similar cleaning along the entire canal.[3] In their study, Rodrigues et al.[16] also used the easy-clean instrument in continuous rotation for removing remnant filling materials during endodontic retreatment and also demonstrated its effectiveness for this purpose.

Here, in Groups 1 and 3 where the AH Plus sealer was used, the amount of initial remaining filling material in the cervical third was lower than that in the middle third (P < 0.05), with no statistically significant difference between the other canal thirds (P > 0.05). This result can be explained by the possible presence of the isthmus in the middle third region and by the fact that the cervical third is more accessible to the instruments, facilitating cleaning in this area.

There was a statistically significant difference between the initial and final remaining filling material volumes in all groups (P < 0.05). These results are in agreement with Grischke et al.[10] and Castro et al.,[11] who observed via different microscopy techniques that PUI improved the removal of remnant gutta-percha and AH Plus sealer during retreatment. In addition, our results are in accordance with that of Cavenago et al.,[12] Bernardes et al.,[13] Silveira et al.,[14] and Kaloustian et al.,[15] who observed via micro-CT that PUI significantly improved the removal of gutta-percha and AH Plus, AH 26, and Pulp Canal sealers remaining in incisors and mandibular molars. However, our results differ from those of Da Rosa et al.[8] and Barreto et al.,[7] who observed by micro-CT that PUI did not increase the removal of the remaining gutta-percha and AH Plus sealer. This discrepancy can be explained by the different methodologies used by these authors. Da Rosa et al.[8] used maxillary molars with curvature, while Barreto et al.[7] evaluated the isthmus of mandibular molars, an area of great anatomical complexity, making it difficult to remove obturator materials. Our results also differ from that of Martins et al.,[9] who observed using micro-CT that PUI did not increase the removal of filling material during endodontic retreatment. The authors used mandibular premolars filled with Endofill sealer that is different methodological conditions from those used in the present study. According to the authors, the teeth used present less anatomical complexity when compared to the elements used in other studies, which favors contact of the instrument with the canal walls and consequently improves cleaning.

Our results show that the easy-clean instrument demonstrated significant removal of remaining obturator materials during endodontic retreatment in all thirds of the canal, with no difference when compared to PUI. These results corroborate with that of Rodrigues et al.,[16] who observed via scanning electron microscopy that the easy-clean instrument used in continuous rotation was as effective as PUI for removing remnant filling material during endodontic retreatment of maxillary lateral incisors filled with Endofill sealer.

Complete removal of filling material was not observed in any sample. This is in agreement with previous studies that have also demonstrated that no additional cleaning technique during endodontic retreatment was able to completely remove the material from the root canals.[78910111213141516] The percentage of filling material remaining after the use of PUI and easy-clean agitation systems was 14%–21%, and there was no statistically significant difference between the groups. Although no system completely removed the remaining obturator material, the filling material was significantly reduced when compared to the initial retreatment step, demonstrating the importance of performing an additional cleaning step during retreatment of the RCS. In addition, our results demonstrate that the easy-clean instrument is effective for final cleaning during retreatment; thus, is a more practical and accessible alternative to PUI, as it is used in the contra-angle and does not require inserts or an ultrasound device.

CONCLUSIONS

Based on our results, we conclude that the PUI and easy-clean agitation systems are effective for removing remaining filling materials from the mesial roots of mandibular molars filled with gutta-percha and AH Plus and TotalFill sealers, without a statistically significant difference. However, neither agitation system was able to completely remove residual material from the RCS.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.