Apical extrusion of root canal filling material during the removal of gutta-percha and resilon

Methods Sixty extracted single-rooted teeth with single, straight canal were selected. Canals were prepared with ProTaper Universal rotary system to a size F2. Two groups (30 teeth in each) were filled with gutta-percha or resilon points, respectively. In both groups teeth were randomly divided into the three subgroups (10 teeth in each), based on the instruments used for retreatment: Hedstrom hand files and two rotary groups- ProTaper and Twisted File instruments. Apical extrusion was detected visually, using a 4-degree scoring system. Mean scores were calculated and analyzed statistically ( t -test and ANOVA). The level of significance was set at p < 0.05. Results Under tested experimental conditions, the type of canal filling material did not have significant effect on the results of apical extrusion during retreatment. Significantly more material was extruded in the resilon group when manual, Hedstrom file was used (1.80 ± 1.13) than rotary ProTaper (0.60 ± 0.70) and Twisted File (0.50 ± 0.71). rotary technique is recomended to minimize apical extrusion, especially when resilon obturation material is removed during retreatment.


INTRODUCTION
Non-surgical retreatment is often indicated as the first choice to eliminate or reduce persistent microbial infection of the root canal system. During this procedure, thorough removal of filling material is an important factor, since it enables adequate chemo-mechanical instrumentation and disinfection of the root canal system, in order to reestablish healthy periapical tissues [1]. One inherent problem related to all root canal treatment procedures is the extrusion of intracanal debris and irrigants through the apical foramen into the periapical tissue that could result in inflammation even infection, as both may be contaminated with microorganisms. This could lead to an interappointment flare-up, postoperative pain, delayed healing or even treatment failure as an undesirable occurrence, both for patient and practitioner [1,2].
It is generally accepted that none of the currently available instruments and techniques can prepare root canals or remove root-filling material without producing apical extrusion. However, selecting the appropriate retreatment technique could minimize the risk of apical extrusion, even though it may not be prevented [3][4][5][6]. During mechanical instrumentation, the number and virulence of extruded microorganisms are decisive and critical factors that determine the extent of the periradicular reaction [6].
Although this qualitative factor is not under the control of the practitioner, selecting techniques such as crowndown instrumentation to provide a gradual approach to the apical end is important. This allows the control of the amount of irritants extruded periapically [2]. One of the major tasks of dental practitioner during root canal treatment procedures is to use instruments and techniques that minimize the amount of apically extruded debris in order to avoid or minimize irritation of the periapical tissues [6,7]. The most often used hand files for retreatment are the Hedstrom files. Recently, several nickel-titanium (NiTi) rotary instruments have been specially designed to remove obturation material. The Protaper Universal Retreatment system (Dentsply, Maillefer, Ballaigues, Switzerland) contains three instruments with various tapers and tip diameters: D1 (size 30/.09 taper), D2 (size 25/.08 taper) and D3 (size 20/.07 taper). Also, a new type of instruments-Twisted File (SybronEndo, Orange, CA, USA) has become available, but it has not been specially designed for the removal of obturation materials. The Twisted File system has been developed through a specific manufacturing process [8]. These files have twisted design, not ground surface treatment, triangular cross-section, variable pitch and safe-ended tip that allow their use in retreatment cases. The manufacturer claims that Twisted Files can be used to remove obturation materials. To the author`s knowledge, no studies are present in the current literature on the apical extrusion of gutta-percha and resilon during their removal with Twisted Files and only one study evaluated the cleaning efficacy of Twisted File instruments in retreatment procedures [9].
Until now, several materials have been used to fill root canals, with gutta-percha being the most popular. However, gutta-percha has two major drawbacks: no adhesion to the canal walls and inability to strengthen the teeth [9]. Recently, a new obturation material has been developed that has some properties similar to gutta-percha. Resilon (Resilon Research LLC, Madison, CT, USA) is a thermoplastic synthetic polymer-based root filling material that bonds to dentinal walls when used in conjunction with an adhesive root canal sealer (Epiphany/Real Seal) and forms a "monoblock" within the canal [10]. The retreatment efficacy of this material has been examined, although not in such extent as gutta-percha, while the apical extrusion during resilon removal has been examined in two studies only [11,12].
The aim of this study was to compare in vitro the influence of different filling materials (gutta-percha and resilon) and different instruments (Hedstrom files, ProTaper Retreatment and Twisted Files) on the degree of apically extruded debris during retreatment.

Teeth selection and preparation
Sixty extracted single-rooted teeth with single, straight canals without previous root canal treatment and with completely developed root apices were selected. To standardize specimen lengths, all teeth were shortened to 16 mm by removing the crown (with a fissure diamond bur in a high-speed handpiece under copious water cooling). After the root canal orifice was identified, canal patency was confirmed with a size 10 K-file (Senseus FlexoReamer, Dentsply, Maillefer, Ballaigues, Switzerland) until it was visible at the apical foramen. Working length was determined 1 mm short from the observed length. Primary root canal preparation was performed with a NiTi rotary system-ProTaper Universal (Dentsply, Maillefer, Switzerland). Canals were enlarged in a crown-down technique to a size 25 (F2) at working length, for all teeth. Canal irrigation was performed between each successive instrument with 2 ml of 5.25% sodium hypochlorite (NaOCl). Before obturation, a final rinse was performed with 10% citric acid for 1 minute, to remove smear layer, followed by a rinse with 10 ml of distilled water. The teeth were randomly divided into 2 groups of thirty teeth each (n = 30). After drying with paper points, all root canals were filled using cold lateral compaction technique. One group was filled with gutta-percha points (Protaper Universal F2, Dentsply, Maillefer, Switzerland) and an epoxy sealer (AHplus, Dentsply, Detrey GmbH, Germany); the other group was filled with resilon points (Resilon Research LLC, Madison, CT) and an adhesive, methacrylate sealer (RealSeal, Root Canal Sealant, SybronEndo, Kerr Corporation, USA). Additional warm vertical compaction of the obturation material was carried out with pluggers. The coronal surface of the resilon group was light cured for 40 seconds, according to the manufacturer's instruction. Total length of the root canal fillings did not exceed more than 15 mm, so the volume of filling material was approximately equal for all specimens. Obturation quality was confirmed radiographically, in buccolingual and mesiodistal directions. Access openings were sealed with a temporary filling material (Citodur, Dorident, Austria) and samples were stored at 37°C in 100% humidity for 14 days, to allow for complete setting of the sealer.

Retreatment methods
Before beginning the retreatment procedure, teeth from both groups (n = 30) were randomly divided into the three groups of ten teeth each, based on the instruments used for retreatment. Each set of instruments was used to retreat maximally 5 root canals and after that discarded. All instruments were used respecting the manufacturer's instructions. Rotary instruments were used with an endodontic electric motor (X-Smart, Dentsply, Maillefer, Ballaigues, Switzerland) in a crown-down sequence.
In the group 1, hand instrumentation was performed with Hedstrom files (Senseus Hedstroem Dentsply, Maillefer, Switzerland) from size 40-20, in a circumferential quarter-turn push-pull motion and by pushing against the root canal walls until working length was reached. Re-preparation of the canal apical part was carried out with Hedstrom files from size 20 to size 40. In group 2, ProTaper Retreatment instruments (Dentsply, Maillefer, Switzerland) were applied, using D1 file to remove filling material from the coronal portion of the root canal, whereas the material from the middle and the apical third was removed using D2 and D3 files, respectively, using a brushing action with lateral pressing movements. D3 was taken to the working length. After that, ProTaper Universal files size F3 (#30) and F4 (#40) were used, to enlarge the apical preparation. In group 3, Twisted File (TF) instruments (SybronEndo, CA, USA) were used in the following sequence: TF #25/ .08 taper instrument was applied in the coronal third and followed by #30 and #35/.06 taper instruments, until reaching the working length. Then, TF #40/.04 was used to enlarge the apical portion of the canal and again TF #25/.08 to additionally clean the canal walls.
During retreatment, the flutes of all instruments were frequently cleaned and 2 ml of 5.25% NaOCl was used after each instrument and also for final irrigation of the canal. Material removal was considered complete when the working length was reached and no more material could be seen on the last instrument and during irrigation. After re-preparation, the canals were irrigated with 10% citric acid for one minute, to remove the smear layer. The canals were finally flushed with 10 ml of distilled water. The same operator performed primary root canal preparation, obturation and retreatment and the procedure was done in the same manner for all samples.

Apically extruded debris
The amount of apically extruded material during the retreatment procedure was detected visually. A different person who was blinded to the experimental group assignment performed scoring of apically extruded debris. The following score system was used [3,11] : 0 -no extruded debris, no filling material escaping through the foramen 1 -minimal extruded debris, small amounts of filling material escaping through the foramen 2 -moderate extruded debris, greater amounts of filling material escaping through the foramen 3 -severe extruded debris, even greater amounts of filling material escaping through the foramen.

Statistical analysis
The obtained data are presented in tables and numerically processed by standard descriptive methods. Mean scores of apically extruded material were calculated. The data were analyzed statistically by t-test and one-way analysis of variance (ANOVA). Analysis was performed with SPSS (version 20) at a significance level p < 0.05.

RESULTS
The mean scores and standard deviations (SD) of apical extrusion for each group of material and for each group of tested instruments are presented in Table 1 and 2. The results indicated that in both groups of materials, all of the tested instruments caused apical debris extrusion at some degree. Comparison by t-test of the mean scores for apical extrusion during gutta-percha and resilon removal ( Table 1) did not showed statistically significant differences between the two materials (p=0.101). The highest mean score for apical extrusion (Table 2) was present in the resilon group of material during retreatment with Hedstrom files (1.80±1.13), while the samples that showed the lowest mean score were observed in the gutta-percha group of material when Twisted File instruments were used (0.11 ± 0.33). The difference between these two results was statistically significant (ANOVA, Post Hoc; p = 0.027). Analysis of the results by ANOVA in both groups of materials revealed statistically significant differences between instruments only during resilon removal (p = 0.004; Table 2). Further statistical analysis with Post Hoc tests indicated that the difference was significant between manual Hedstrom files (1.80 ± 1.13) and the two rotary instruments used, ProTaper (0.60 ± 0.70) and Twisted File (0.50 ± 0.71).

DISCUSSION
Even during primary root canal instrumentation debris such as dentin chips, necrotic pulp tissue, microorganisms and irrigants may be extruded into the periradicular tissues [13]. Successfull non-surgical retreatment depends on complete removal of pre-existing filling material from the canal, where it would be crucial to clean the apical foramen [14]. However, this could promote apical transportation and force obturation material into the periradicular tissues [15]. In addition, extrusion during retretment may be accompanied by solvents, necrotic tissue, bacteria or irrigants, which might be introduced into the apical region [11].
In the present study, apical extrusion was evaluated during removal of gutta-percha and resilon. The results showed that the type of obturation material did not have a significant impact on the mean scores of apical extrusion, although the mean score was higher during resilon removal. Other studies also evaluated the extrusion of obturation material during retreatment [7,15,16,17], but these studies observed only the removal of gutta-percha. Apical extrusion during removal of different materials (gutta-percha, resilon and resin-coated gutta-percha) was compared in one study with a visual technique and a 4-degree scoring system [11]. The authors concluded that the type of filling material did not play a statistically significant role on the amount of apically extruded material, which is consistent with the findings of the presented study. Another group of authors [12] evaluated apical extrusion during gutta-percha and resilon removal using a quantitative method, however the results regarding the difference between the two materials were also not statistically significant.  Using an instrumentation technique that minimizes apical extrusion would be advantageous. Therefore, this aspect should always be investigated for a newly developed root canal instrumentation system [13]. This study evaluated three different instruments (Hedstrom, ProTaper and Twisted File) during retreatment and their impact on apically extruded material. The present study showed that in vitro, all of the tested instruments produced apical extrusion of obturation material and these results are consistent with other apical extrusion studies [7,11,12,14,16,18]. As already mentioned, no studies are present in the current literature on the apical extrusion during retreatment with Twisted File instruments. In the present study the highest mean score of extruded material during retreatment was observed in the manual Hedstrom group and the lowest when Twisted File rotary instruments were used. The results of the present study are in agreement with previous retreatment studies that also compared hand and engine-driven instruments and their impact on apical extrusion [7,12,14,16,17]. This could be explained with rotation and a crown-down preparation technique during instrumentation, which tends to pull dentinal debris into the flutes of the file and direct it toward the coronal part of the canal [3,19]. Also, rotary movements produce a certain degree of frictional heat which might plasticize the obturation material and facilitate removal [3]. Based on the results of this study, it can be concluded that Twisted File instruments, although not primarily intended for use in retreatment, can be associated with the extrusion of smaller degree of apical debris during material removal. However, these instruments should also be tested in different conditions of experimental set up and in relation to other retreatment efficiency indicators such as canal wall cleanliness, time of retreatment and frequency of instrument fracture.
The majority of investigations used a quantitative method to determine the amount of apically transported material and debris, by collecting and measuring their amount in grams [12,13,16,20,21]. In some studies the amount of apically extruded filling material during retreatment was detected visually and evaluated with a scoring system [3,11,14], as in the present study. Criticism of this kind of evaluation methodology can be made due to the existence of a certain degree of subjectivity as well as less precision in assessing the extruded material amount. However, the reaction of periapical tissues does not depend so much on the quantity of extruded material, as much of its infectious and antigenic potential and the host defense system. It must be emphasized that the results of in vitro studies should not be directly extrapolated to clinical situations. Transported material amount can be lesser in vivo because the presence of periapical tissues may act as a natural barrier against apical extrusion [6].
Further studies on material extrusion with different engine-driven instruments that can be used in retreatment will be needed for clarifying the importance of torque and rotational speed. Also, instruments with reciprocating movements should be evaluated [14,22,23]. Moreover, apical extrusion during removal of other obturation materials, such as resilon should be examined in a greater extent.

CONCLUSION
Under the conditions of this in vitro study, all retreatment techniques produced apical extrusion of filling material. The difference between results for apical extrusion for the two materials tested (gutta-percha and resilon) was not statistically significant. However, rotary Twisted File and ProTaper instruments resulted in significantly less debris extrusion compared to hand instruments (Hedstrom files) while removing resilon. There was no significant difference among the two rotary instruments. Therefore, the use of a rotary technique can be recomended to minimize apical extrusion, especially when resilon is removed during retreatment.