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In Vitro Comparison of Efficacy of Neolix and ProTaper Universal Retreatment Rotary Systems in Removal of Gutta-Percha Combined with Two Different Sealers

Statement of the Problem: The complete removal of filling material is an important step to regain access to the entire canal and facilitate the disinfection of the root canal system. Rotary nickel-titanium (NiTi) instruments systems have been proposed as an effective removal technique for root canal retreatment. Purpose: The aim of this study was to evaluate the efficacy of Neolix rotary system and ProTaper Universal retreatment system in the removal of gutta-percha combined with two different sealers. Materials and Method: In vivo study, eighty extracted human permanent mandibular premolars were prepared using the ProTaper Universal rotary system to an apical size 30 (F3/0.06). The specimens were randomly divided into 4 groups (n=20) and subsequently filled with lateral condensation of gutta-percha and two sealers: AH-26 and Sure-Seal Root. The teeth were stored for 4 weeks at 37°C and 100% humidity and then retreated by one of the following rotary systems: Neolix or ProTaper Universal retreatment system. Teeth were then grooved and monitored under a stereomicroscope with 8× magnification. The images were transferred to the computer and the amount of filling material that remained on the root canal walls was scored using AutoCAD 2017 version 1.1software. Results were analyzed using one-way analysis of variance test and post hoc Tukey-HSD test (p< 0.05). Results: The mean percentage of remaining gutta-percha and sealer was significantly higher in apical third in all groups (p< 0.01). Post hoc Tukey test showed that there is significantly higher residual filling material in the group filled with gutta-percha and Sure-Seal Root sealer and retreated by Neoniti rotary system compared with other groups in both coronal and middle third of the canal. Conclusion: The Neoniti rotary system was as effective as ProTaper Universal retreatment system in retreatment of gutta-percha and AH-26 sealer but was significantly less effective in groups obturated with Sure-Seal Root sealer.


Introduction
Nonsurgical root canal retreatment is indicated when previous treatment fails [1][2] The effective removal of filling materials is essential to regain the access to the entire canal, and improve the root canal disinfection and debridement and placing a consistent, homogenous filling [3].
The most commonly used filling material is guttapercha. The use of a sealer in conjunction with gutta percha has been recommended to improve the bond of gutta percha to the dentine [20]. A wide variety of sealers has been used and new products continue to be marketed. Sure-Seal Root TM (Gyeonggi-do, South Korea) is a recently introduced premixed-injectable root canal sealing material that utilizes bioceramic technology. According to the manufacturer, this biocompatible calcium silicate sealer can chemically bond to the dentin with hydroxyapatite formation [21]. AH-26 root canal sealer (Dentsply Maillefer, Ballaigues, Switzerland) is an epoxy resin based sealer and strongly adhesive to both dentine and gutta-percha [22].  [23][24][25][26].
Recently a newly NiTi rotary system was introduced to the market. Neoniti (Neolix, Châtres-la-Forêt, France) is a rotary file system with nonhomogeneous rectangular cross section and multiple taper. It consists of one C1 and three A1 with different tip sizes (#20, #25 and #40) files [14]. The manufacturer claims that this file has special abrasive properties resulting in faster root canal preparation and higher flexibility due to heat treatment process delivered to the file [27][28].
It is manufactured using a newly developed wire cut electrical discharge machining (WEDM) process. Some studies [29][30] reported that these systems are effective in retreatment procedure.
The aim of this study was to evaluate the efficacy of ProTaper Universal retreatment system and a single file Neolix system used during the removal of root fillings comprising gutta-percha and two different sealers.

Preparation of root canals
A single operator prepared all samples. The root canals were prepared by using ProTaper Universal treatment system (Dentsply, Maillefer, Ballaigues, Switzerland).
The cervical and middle thirds of the canals were flared using the ProTaper SX and S1 rotary instruments and the canals were then finished using instruments F1, F2 and F3. An electric motor powered (Endomate DT, NSK, Japan) was used for all NiTi instruments in accordance with the manufactures' recommendations for speed and torque. Instruments were discarded after preparing of four canals. A 2.5% sodium hypochlorite (NaOCl) solution was used for irrigation during cleaning and shaping and 17% ethylenediaminetetraacetic acid (EDTA) was applied for 1 min after NaOCl to remove smear layer. Then the root canals were dried using paper points.

Filling of root canals
All canals were filled with lateral condensation technique up to the orifice level. Then the samples randomly were divided in two groups based on the sealers used and in the next part each group was divided into two subgroup based on the retreatment techniques. In groups 2 and 4, ProTaper Universal system (Dentsply, Maillefer, Ballaigues, Switzerland) was used and root canal preparation was done using crowndown technique according to the manufacturer's instructions by using the electrical VDW motor. The D1, D2, and D3 files were used sequentially in a pecking motion [29] toward the apex until they reached the working length. Each NiTi instrument was discarded after being used four times.
During filling removal, irrigation was done using a total of 25 mL 2.5% NaOCl solution per tooth followed by irrigation of 5 mL 17% EDTA for 3 min to remove the smear layer. Then final irrigation with 5 mL 2.5% NaOCl for each specimen was performed.

Statistical Analysis
The One-way analysis of variance test and post hoc Tukey-HSD test were used to identify differences between the experimental groups at the apical, middle, and coronal levels (p< 0.05).

Results
The mean percentage value of remaining filling material in coronal, middle and apical thirds and total canal area following each technique and sealer are given in Table 1. With regard the comparison of canal thirds, the mean percentage of remaining filling material was significantly higher in apical third in all groups (p< 0.05).
When comparing the sealers, the only significant difference was detected between AH26/Neolix group and SureSeal Root /Neolix group in both middle (p< 0.01) and coronal thirds (p< 0.01) and total canal area of the canals (p< 0.01). In comprising the retreatment technique, Tukey test showed significant differences between Sure-Seal Root/ProTaper and Sure-Seal Root/ Neolix groups in coronal (p< 0.01) and middle (p< 0.01) thirds and total canal area (p< 0.01).

Discussion
A major concern for all clinicians is complete removal of preexisting filling material from root canals to achieve successful nonsurgical root canal retreatment [32]. This procedure is essential to allow further cleaning, shaping, and removing the infected residual necrotic tissue and microbes in the root canal system [32][33]. While different techniques were used to achieve this purpose [4,7,9,34], the present study investigated two NiTi rotary file system to retreat obturated root canals with different sealers.
Various methodologies have been used to access the remaining filling material in the canals such as radiography [19], splitting the roots longitudinally [35], and computed tomography. In this study, the teeth were grooved longitudinally and split. Then directly visualized under a stereomicroscope and photographs were taken as proposed in other studies [36][37]. The examination under scanning electron microscopy enhanced the inspection of filling material residues and it has been shown to be more effective compared with radiographic techniques [38].
This technique is considered a simple and efficient method for inspection of the root canal walls but it has an objection, which is displacement of the filling material that might occur during splitting and affect the accuracy of scoring [39]. In the current study, root canal walls were evaluated in three sections of coronal, middle and apical. The results of this study revealed that independent of the type and technique of retreatment the highest amount of filling material was remained in apical portion. This result was in consistent with results of some other studies [19,37,40]. This has been attributed to the increased anatomical variation in apical portion of the canal and the less accessibility to clean this section [19].
In this study, only significant difference was observed between the sealer and filling material residues in the groups retreated by Neolix. Two types of sealer were used in this study: a bioceramic sealer (Sure-Seal Root) and a resin based sealer (AH-26). The results of the present study showed that regardless of the retreat- Table 1: Mean ± Standard Deviation (SD) percentage value of remaining filling material for the experimental groups the coronal, middle and apical third and total canal area.  which is related to the ease of removal [42]. In a study conducted by Nagas et al. [43], it was shown that bioceramic sealer has higher dentin bond strength than resin-based sealer. According to the manufacturer, Sure-Seal Root is a bioceramic sealer and has a good dentinal bond strength, which explains the difficulty of removal from dentinal walls during retreatment procedure.
In the present study, two rotary files were evaluated in retreatment procedure: ProTaper Universal retreatment system and Neolix file. The results showed that considering the amount of obturation residues, there is no significant difference among the debridement ability of these retreatment files. Khoshbin et al. [44] in an in vitro study reported that no significant difference was observed between the debridement ability of Neolix and ProTaper systems in both 0-5 and 5-