Teeth with pulpal and periradicular involvement are commonly treated with root canal treatment. Although success rate of endodontic treatment is up to 86 to 93%, failure in endodontic treatment can be expected.1 The main causes of endodontic failure are insufficient cleaning, inadequate obturation, untreated or missed out root canals, lack of efficient hermetic sealing and survival of bacteria.2 These make nonsurgical endodontic retreatment necessary. For effective results, retreatment requires thorough debridement of former root canal filling materials including sealers.3 Debridement especially in resin based sealers that strongly adhere to the root canal dentin is more difficult.4,5 The root canal filling material in bulk can be easily removed with hand and rotary instrument leaving small amount of residue attached to the root canal dentin. Recently, Duncan and Chong have suggested the use of solvents to remove this root canal residue.5 The aim of this study was to evaluate the softening ability of two solvents Xylene & Endosolv-R on three Epoxy resin based endodontic sealers that will facilitate their effective mechanical removal.

Three epoxy resin based root canal sealers, AH 26 (Group I, Dentsply/De Trey, Konstanz, Germany), AH Plus (Group II, Dentsply/De Trey) and Adseal (Group III, Meta-Biomed, Cheongwon, Korea) were tested. Composition of each sealer is described in Table 1. A well of 6 × 1.5 mm in diameter and depth, respectively, was prepared in each of sixty Teflon disks of 12 × 2 mm in diameter and height, respectively. These sixty molds were randomly and equally divided into three groups, each containing 20 Teflon molds (n = 20). Each sealer was then mixed according to the manufacturer's instructions and placed into the well of the mold. The sealer specimens were then allowed to set for 2 weeks at 37℃ and 100% humidity. The design of the sealer specimen was such that only one surface of the specimen was exposed to solvent. Twenty set sealer specimens in each group were further randomly and equally divided into two subgroups (n = 10) based on solvents to which they were exposed to Xylene (Subgroup A, Lobachemie, Mumbai, India) and Endosolv-R (Subgroup B, Septodont, Cedex, France).

Initial Vicker's microhardness (HV) of each fully set sealer specimens were calculated using a Mitutoyo microhardness testing machine (Instrument No. 810-117E, Mitutoyo, New Delhi, India) with a Vicker's microhardness indenter. The indenter was applied at three predetermined points on the specimen surface with the load of 10 grams for 10 seconds. The indentations in the sample surface were measured under 100 times magnification with the microscope attached to the same machine. The mean of three was taken for each sample.

Each sealer specimen was then immersed in a petridish containing the corresponding solvent (i.e. Xylene and Endosolv-R) for 1 minute. Each specimen was then retrieved from the solvent and air dried. At this point of time, the microhardness of each sealer specimen was reassessed by similar procedure as mentioned above. Each sealer specimen was then again exposed to corresponding solvents for additional 1 minute. (i.e. totally 2 minutes.) and similar procedure to evaluate microhardness was followed as mentioned above. In this way ten sealer specimens from each group were tested for reduction in Vicker's microhardness (HV) after 1 and 2 minutes of exposure to solvent.

Vicker's microhardness (HV) for each sealer at initial stage, after 1 and 2 minutes exposure to solvents was summarized (Tables 2 and 3) in terms of means and standard deviations. Reduction in the microhardness of each sealer at one and two minutes was expressed as percentage with reference to the respective initial hardness. To determine the effect of solvents and sealer types on the microhardness according to time, two-way analysis of variance (ANOVA) with repeated measures was performed. The assumption of sphericity was assessed using Mauchly's test. Additionally, to determine the significance of difference in the mean hardness across groups at each time point, one-way ANOVA was used followed by Tuckey's post-hoc pairwise comparison. The analysis was performed using SPSS 16.0 (SPSS INC., Chicago, IL, USA) software.

The descriptive statistics in terms of mean and standard deviation were obtained for each sealer type according to time, for each solvent Xylene and Endosolv-R (Tables 2 and 3). It was evident that the mean Vicker's hardness reduced significantly with time for all sealer and solvent combinations. However, their extent of reduction was different. To assess the statistical relevance of change in the hardness due to sealer type and solvent exposure, two-way ANOVA with repeated measures was performed independently for two solvents.

For endodontic retreatment to be successful, it is necessary to completely remove all previous obturation material.3,6 An ideal root canal sealer should be easily removed if retreatment is necessary to allow access for antimicrobial agent and medicament to all root canal ramification.7 Shin et al. advocated the use of Gates Glidden and Profile systems for retrieval of resin based root canal sealer.8 Recently, Duncan and Chong suggested many methods for removal of root filling materials including use of the hand files, rotary files, ultrasound, heated pluggers and solvents.5 Hand and rotary instruments are commonly being used for effective removal of root canal fillings.2,3 The bulk of these filling materials can be removed within 2 to 3 minutes, but still remnants of gutta percha and sealer remains commonly attached to root canal dentin.6,9 With better sealing and bonding, resin based root canal sealers are of interest to many clinicians.4,10,13 The complete debridement of the remnants of resin based sealers that strongly attaches to the dentin is prolonged tedious task. So to remove these fillings and sealer out of the fins and aberration of root canal systems, literature has suggested 'wicking action' is necessary which can be provided by solvents.5,11,12 Therefore, it will be helpful to use solvents along with hand and rotary files to remove root canal debris.

Three epoxy resin based sealers, AH plus, AH 26, and Adseal were used because they are mechanically harder and more difficult to remove than zinc oxide euginol based ones.5,10,13 Lee et al. have mentioned that resin based sealers attach more strongly to both dentine and gutta percha as compared to zinc oxide euginol and calcium hydroxides based ones.10 Mamootil et al. have stated that resin-based sealers have deeper and more consistent penetration into dentinal tubules than other sealers both in vitro and in vivo.15 Further microleakage has been found to be least in the case of resin based sealers.16 Cho et al. have also mentioned in their study that the bond strength of final restoration was the least affected in case of resin based sealers than the zinc oxide euginol root canal sealer.17 Kim et al. have stated that resin based sealers (AH 26, EZ fill and AD Seal) are more biocompatible and have advantage in terms of radiopacity.18

In paint industries, solvents are often used to soften resin coating materials on paints to allow their easy removal.19 Thus solvents used in removal of paints can be considered in root canal retreatment for removal of strongly adhering resin based sealers from root canal walls.19 Chloroform and Xylene as a solvent for root canal sealer have been studied, but U.S. Food and Drug Administration has banned chloroform due to its potential for carcinogenicity and cytotoxicity.5,19-21 Use of D-Limonene (Refined Orange Oil) in endodontics is becoming popular due to its confirmed biocompatibility, safety, and noncarcinogenic property, but Martos et al. and Mushtaq et al. have mentioned that the performance of orange oil as a solvent was inferior to xylene and chloroform.22-24 Because of concerns about Chloroform, clinicians and researchers have developed a renewed interest in finding an alternative solvent. Xylene is chlorinated hydrocarbon commonly considered as gutta percha solvent.25 It may also soften or dissolve the sealers and could potentially facilitate their mechanical removal.19 Use of Endosolv-R for removal of resin based sealer has suggested by Cohen, Duncan and Chong.5,11 It contains 66.5 grams of formamide and 33.5 grams of phenyl ethylic alcohol.26

In this study, only one surface of sealer was exposed to solvent to simulate the root canal conditions. Softening was defined as reduction in hardness that resulted from exposure to solvent.19 After 1 minute, Xylene was most effective against AH Plus (86.1%) followed by Adseal (65.1%) sealer and least effective against AH 26 (8.5%). After 1 minute, Endosolv-R was most effective against Adseal (69.2%) followed by AH 26 (62.5%) and AH plus (61.4%) (p < 0.001). After 2 minutes, Xylene was most effective against AH Plus (89.1%) followed by Adseal (75.1%) sealer but least effective against AH 26 (10.6%). These results are in partial agreement with the study of Kfir et al.19 After 2 minutes, Endosolv-R was most effective against AH plus (94.4%) followed by Adseal (91.9%) and AH 26 (87.3%) (p < 0.001). In other words, after 2 minutes of exposure Endosolv-R has been found to be a more effective short term softner for all the three sealers and its effect was more pronounced in case of AH plus.

Setting of epoxy resin sealers involves polymerization and cross linking of their monomers, resulting in 3D lattice.19 This set polymer is unaffected by saline or water. Hydrophobic organic solvents such as Xylene and Endosolv-R may have the ability to penetrate this 3D lattice resulting in swelling of the lattice and reduction in strength and hardness.19 Thus softening occurs that facilitates their removal by scrubbing effect provided by files.19 Ramzi et al. have stated that Endosolv-R combined with rotary files has most effectively removed filling materials from the root canals, especially in the apical third.6 Vranas et al. have reported that Endosolv-R has significant softening effect on resorcinol-formalin pastes after 2 minutes.4,25 Gambrel et al. concluded in their probe penetration study that softening effect of Endosolv-R after 20 minutes was superior to other tested solvents.4,26 Shokubinejad et al. mentioned that Endosolv-R does not affect the bond strength of newer obturation materials with root canal dentin whereas Laxmi Narayan et al. showed that Xylene causes significant reduction in enamel and dentin microhardness and thus may reduce the bond strength of newer endodontic sealers.27,28

Also, by Occupational Safety and Health Administration (OSHA) guidelines, Xylene causes irritation of eyes and mucous membranes, gastrointestinal distress and toxic hepatitis when ingested, chemical pneumonitis, hemorrhages in air spaces when inhaled, cytotoxic reaction when extruded periapically.5,20,29 However Chutich et al. have suggested that the amount of Xylene periapically extruded was too small to cause toxicity.30 Even less information regarding biocompatibility of Endosolv-R is available and it has been suggested to have fetotoxic properties.31 However, results mentioned in this study may vary in in vivo conditions based on setting characteristics of sealer in root canal system and availability of solvents to the sealer in curved & ramified canals.

Within the limitations of this study, it can be concluded that after 2 minutes, Endosolv-R was a significantly more effective short term softener than Xylene for all the tested sealers, and thus Endosolv-R can be viewed as a better substitute of Chloroform for softening and removing of epoxy resin based sealers.