MICRO-COMPUTED TOMOGRAPHIC EVALUATION OF REMOVAL AND EXTRUSION OF ROOT FILLING MATERIAL DURING CANAL RETREATMENT

This study aimed to assess the efficiency of manual and rotary techniques for removing root filling material in endodontic treated canals and to evaluate the extrusion of debris via apical foramen. After access preparation, thirty human lower central incisors were instrumented, filled and divided into two groups (n=15): MNmanual retreatment technique; RTrotary retreatment technique. The teeth were submitted to micro-computed tomographic scanning before and after the retreatment procedure, for quantifying the remaining filling material by relating the initial and final conditions. The analysis was conducted separately in all root thirds. The volume of material extruded through the apical foramen was also quantified. The data were statistically analyzed using t-test and two-way analysis of variance (ANOVA) at 5% of significance. No significant differences were detected between remaining filling material for the techniques evaluated (p=0.74), even considering the apical third only (p>0.75). Significant differences were found for the remaining filling material verified between the cervical third and middle/apical thirds, with lower values being observed at the cervical third for both groups (p<0.05). Apical extrusion of filling material was verified in seven specimens (2 from MN group and 5 from RT group). Both techniques showed similar performance for removing filling material from the root canal, considering the whole tooth or the root thirds independently. The apical extrusion of filling material observed was not sufficient to indicate which technique has increased tendency to induce more events during


INTRODUCTION
One of the most important points for the success of endodontic retreatment is the straight access to the root canal walls by removing root filling materials (BUENO et al., 2006).Necrotic pulp tissues or bacterial remnants may remain protected under the filling, being responsible for the maintenance of periapical infectious processes (HAMMAD, QUALTROUGH;SILIKAS, 2008).Therefore, the filling material should be carefully removed during endodontic retreatments, since pulp/dentin tissues, bacteria and material debris of filling materials can be extruded through the apical foramen, thus working as irritating agents (HUANG et al., 2007).
Despite the diversity of techniques for removing endodontic filling in canal retreatments and the methods used to evaluate their efficiency, there is still no consensus about the most efficient clinical approach for endodontic retreatments.Our hypothesis is that differences between the manual and rotatory instrumentation techniques would be detected in the percentage of remaining filling material and in the amount of extruded debris after the retreatment procedures.Thus, the aim of this study was to assess the efficiency of manual and rotary techniques for removing root filling material in endodontic treated canals and to evaluate the extrusion of debris via apical foramen using microcomputed tomography (µCT) analysis.

MATERIAL AND METHODS
Human lower central incisors with single root canal were selected (gathered after informed consent approved by the Committee for Ethics in Research of the Federal University of Uberlândia: #451/11).The teeth had no previous endodontic treatment, intraradicular pins, fractures or extensive damage in crowns.The selected teeth were cleaned of debris and stored in physiological saline solution until use (PEREIRA et al., 2012).In order to attain the calculation of volume and surface area of the root canals, as well as an overall outline of internal anatomy, teeth were pre-scanned using a µCT scanner (SkyScan 1172; Bruker-microCT, Kontich, Belgium) at a resolution of 68 µm, 90 kV and 112 µA.Transverse cross sections of the inner structure of the teeth were acquired after reconstruction procedure (NRecon v.1.6.3 software;.Minimum and maximum diameters of the root canals were measured bucco-lingually and mesio-distally (DataViewer v.1.4.4;.Teeth showing apical curvature or more than one root canal were excluded.Based on these pre-scans, 30 lower central incisors were selected. Coronal opening was performed using round diamond burs (#1016, KG-Soresen, Barueri, SP, Brazil) and tapered carbide burs with noncutting tip (Endo-Z, Dentsply-Maillefer, Ballaigues, Switzerland).The root canals were located and explored with #10 K-file (Dentsply-Maillefer) and the working length determined subtracting 1 mm from the length measured when the tip of the file was first observed emerging from the apical foramen.Teeth were instrumented with a step-down technique, using K-files (Dentsply-Maillefer) up to #30 memory file.Irrigation was performed with 1 mL of 1% sodium hypochlorite after each instrument and 2 mL of physiological saline for final rinse.Canals were dried with paper points and filled with gutta-percha and mineral trioxide aggregate-based sealer (MTA Fillapex, Ângelus, Londrina, PR, Brazil).Coronal access was provisionally restored with zinc-oxide temporary cement (Interim, Biodinâmica, Ibiporã, PR, Brazil).
Afterwards, teeth were mounted on a custom attachment base and scanned with the µCT scanner at an isotropic pixel size of 19.6 µm, 90 kV, 112 µA, resulting in the acquisition of 800-1,000 transverse cross sections per tooth.The scanning procedure was carried out by 360º rotation around the vertical axis; camera exposure time of 2,600 ms, rotation step of 0.6°, frame averaging of 2 and medium filtering of the data were applied.X-rays were filtered with aluminum (500 µm) and copper (38 µm) filters and a flat field correction was performed prior to scanning to correct for variations in the pixel sensitivity of the camera.Reconstruction of the images was done (NRecon v.1.6.3) with a beam hardening correction of 15%, smoothing of 3, no ring artifact correction, and an attenuation coefficient range of −0.002 to 0.15, providing axial cross sections of the inner structure of the specimens.The evaluation of each tooth was performed considering the total lengthening of the root canal and the initial volume of filling material was calculated using the CTVol software (v.2.2.1, Bruker-microCT).The specimens were then stored (100% humidity at 37ºC) for 3 weeks to allow the complete setting of the sealer.
Sequentially, the teeth were fixed in red wax blocks to collect filling any material that could be extruded through the apical foramen during retreatment, in order to allow its quantification.The whole set was fixed in a holding assembly to stabilize the specimen throughout the next steps.Then, teeth were randomly assigned into two experimental groups (n=15): MN-manual retreatment technique; and RT-rotary retreatment technique.For MN group, #2 and 3 Gattes-Glidden burs (Dentsply-Maillefer) were used to initially remove the filling material from the cervical and middle thirds.Then, eucalyptol-based solvent (Eucaliptol; Biodinâmica) was inserted in the filling with #15 and #20 K-files (Dentsply-Maillefer) using oscillatory movements, and the material was laterally removed with Hedströem files (Dentsply-Maillefer).The canal repreparation with K-files was performed up to #35 instrument, until obtaining 0.35 mm final diameter.For RT group, D1-D3 Nickel-Titanium (NiTi) files (ProTaper Universal Retreatment; Dentsply-Maillefer) were used to prepare the cervical, middle and apical thirds, respectively.The finishing of the canal repreparation was done with F1-F3 NiTi files (ProTaper Universal; Dentsply-Maillefer) to obtain a final diameter compatible to that produced by #30 K-file.
The removal procedure was interrupted when filling material was no longer detected in the instruments.Canal irrigation and drying was performed as described before.All endodontic procedures were conducted by one experienced operator.Then, teeth were submitted to final µCT scanning using the same parameters described previously.This analysis was conducted separately in all root thirds and the volume of filling material extruded was also quantified.The percentage of remaining filling material into the root canal was calculated by relating the initial and final volumes using the following formula: ∆% = V F *100/V I , where V I and V F are the initial and final volumes (mm 3 ) of the endodontic filling before and after the retreatment, respectively.
Normal distribution of the initial and final filling volumes was verified after square root simple transformation of the final volume data; then data of each group were compared using paired t-test.The remaining filling verified after the retreatment procedure was expressed as a percentage of the initial filling volume (∆%).Normality was verified after square root simple transformation of the data, and t-test was applied for comparisons between groups.For comparing the residual volume of filling in each root third, normal distribution of the data was detected after square root simple transformation and two-way analysis of variance (ANOVA) with repeated measures was applied.All statistic tests were performed with a significance level of 5% (SigmaPlot for windows, v.12.0,Systat Software Inc., Chicago, IL, USA).The volume of extruded material (mm3) was analyzed descriptively since statistical analysis was not possible to be applied due to limited sampling.

RESULTS
The paired t-test showed significant differences between the initial and final volumes of filling material for both groups (P<0.001)(Table 1).The t-test revealed no significant differences for the percentage of remaining filling material (∆%) for the two retreatment techniques (P=0.74)(Table 1).The two-way ANOVA with repeated measures showed no significant differences between the retreatment techniques (P=0.73),but significant differences were verified on the residual volume of filling material between the cervical and middle (P=0.04), and cervical and apical (P<0.001)thirds for the MN group (Table 2).For the RT group, differences on the residual volume of filling material were checked between the cervical and apical thirds (P=0.02)(Table 2).None of the retreatment techniques tested were effective in completely removing the filling material from the canals, with remnants being located mainly in the irregularities where anatomical interferences probably affected the full-action of files (Figure 1).Extrusion of filling material was observed in seven specimens following the retreatment procedure, of which, two were from MN group and five from RT group (Figure 2).The volume and percentage of extruded material were analyzed descriptively, and an increased tendency of RT specimens to present extrusion of filling after retreatment was shown (Table 3).

DISCUSSION
The development of rotary NiTi instruments especially designed to endodontic retreatments intended to overcome the previous limitations imposed by manual instruments, aiming to remove the root filling more efficiently (ZMENER, PAMEIJER;BANEGAS, 2006;SOMMA et al., 2008).However, it has been shown that a significant volume of filling material may remain retained in the root canal walls following different retreatment approaches with manual or mechanized instruments (GIULINI, COCHETTI;PAGAVINO, 2008;HAMMAD, QUALTROUGH;SILIKAS, 2008).These findings are in accordance to the results of the present investigation, which showed no significant differences in the percentage of remaining filling material after retreatment techniques performed using manual or rotary instrumentation techniques.Extrusion of filing material through apical foramen after retreatment was detected in few specimens from the two experimental groups.Thus, the hypothesis tested in this study was rejected, since no significant differences were found between both retreatment techniques evaluated.
The success in endodontic retreatment is dependent upon the efficient removal of filling material from the root canal walls (CUNHA et al., 2007;BARLETTA et al., 2008).Anatomical complexities can challenge root canal preparation due to the limitations on the appropriate adaptation of files in the canal (PEREIRA et al., 2012).Indeed, complete removal of root fillings can also be influenced by difficulties imposed to the action of files in some irregular regions of the canal walls presenting irregularities (DUARTE et al., 2010) (Figure 1).This fact is probably responsible for the several controversies existing among different studies, since root canal anatomy can influence the amount of remaining filling material after retreatment, irrespective of the technique used (BARRIESHI-NUSAR, 2002; GIULINI, COCHETTI; PAGAVINO, 2008;HAMMAD, QUALTROUGH, SILIKAS, 2008).Despite this, some investigations also showed that both retreatment techniques evaluated in this study are incapable to completely remove filling material from root canal walls (ZMENER, PAMEIJER;BANEGAS, 2006;HAMMAD, QUALTROUGH, SILIKAS, 2008;BARLETTA et al., 2008;SOMMA et al., 2008;PIRANI et al., 2009;MOLLO et al., 2012), corroborating our findings.
No significant differences were found in the percentage of remaining filling material (∆%) after the two retreatment techniques, indicating that both approaches presented similar efficacy (Table 1).Other reports have also shown similarity between these retreatment techniques (TAKAHASHI et al., 2009;RÖDIG et al., 2012), confirming our results.On the other hand, some investigations revealed superior performance of the rotary system tested in this study for retreatment procedures as compared to manual files (GIULINI, COCHETTI; PAGAVINO, 2008;GU et al., 2008).A differential to be taken into account is that the rotary retreatment technique does not require solvent solutions, allowing a reduced working time, since files work removing filling material and existing canal interferences concomitantly.By its turn, the manual technique requires solvents and may favor rectification of root canals in the apical region during the filling material removal.
When analyzing the efficacy of the retreatment techniques in the root thirds independently, no significant differences were detected between both approaches tested (IMURA et al., 2000;PIRANI et al., 2009;ERSEV et al., 2012).However, significant differences on the residual volume of filling material were verified between the cervical and the middle/apical thirds and between the cervical and the apical thirds for the MN and RT groups, respectively (Table 2).The easier access and the increased diameter of the root canal in the cervical region may explain the better results found in this region.Some studies have shown increased removal of root filling material in the apical and middle thirds when using rotary instruments (SAE-LIM et al., 2000;SAAD, AL-HADLAQ;AL-KATHEERI, 2007), while others, verified more effectiveness for manual files in cleaning canal walls in the apical third (ZMENER, PAMEIJER;BANEGAS, 2006;SOMMA et al., 2008).The complete removal of filling material from apical thirds is clinically relevant as endodontic treatment failures are usually related to bacterial remnants at the apical portion of root canals (CROZETA et al., 2016).The presence of remaining filling material in this region must be avoided when performing retreatments since the success of the endodontic intervention will depend on appropriate cleaning of this area will be impossible (SILVA et al., 2015).
Several studies on extrusion of root filling used methods in which the extruded material was freely released, without any barrier to block debris (IMURA et al., 2000;HUANG et al., 2007;SOMMA et al., 2008;MOLLO et al., 2012).This fact does not correspond to the clinical reality, because periodontal structures prevent larger amounts of debris to reach the apical region (SELTZER et al., 1968).Thus, red wax blocks were used to simulate this situation and collect the extruded filling material (Figure 2).Besides, most studies that evaluated filling material during retreatment procedures only registered whether the event occurred, not quantifying the volume of debris (SOMMA et al., 2008;MOLLO et al., 2012).Our concern was not only to detect material extrusion, but also to quantify its volume.Despite statistical analysis was not possible due to reduced sampling, an increased tendency of RT specimens to present extrusion of filling after retreatment was verified (Table 3).
In overall view, none of the retreatment techniques evaluated in this study was capable to completely removed the endodontic filling material from the root canals, once the action of files was probably affected by the irregularities present in the canal walls.However, both retreatment techniques proved their efficacy since the mean percentage of remaining filling material was lower than 5% for both groups.Regarding the extrusion of filling material, none of the techniques provided enough specimens for carrying out statistical analysis and this probably occurred due to the barrier used to collect debris.According to the methodology used and on the basis of the results of the present study, the choice of a manual or rotary technique for performing endodontic retreatments should consider aspects such as cost-benefit and technical ability in view of the similar performance presented by both approaches.

Figure 1 .
Figure 1.Micro-tomographic assessment of the specimens showing the initial and final filling material after each retreatment approach: MN-manual retreatment technique; RT-rotary retreatment technique.

Figure 2 .
Figure 2. Evaluation of the filling material extruded through the apical foramen: A-tooth fixed in red wax block to collect filling material; B and C-bucco-lingual and mesio-distal micro-tomographic images of the specimen showing remaining filling material in the middle third and extrusion of material through the apical foramen.

Table 1 .
Mean initial and final volume of filling material (mm 3 ) and percentage of remaining filling material *Different capital letters indicate significant differences within groups (columns).Different small letters indicate significant differences between groups (grows).Different symbols indicate significant differences between the ∆% (p < 0.05).

Table 2 .
Mean residual volume of filling material (mm 3 ) for each root third according to the retreatment *Different capital letters indicate significant differences between groups.Different small letters indicate significant differences within MN root thirds.Different symbols indicate significant differences within RT root thirds (p < 0.05).