# The Novel Role of Solvents in Non-Surgical Endodontic Retreatment
## 3 Solvent Specificity  
Targeting the chemistry of a resin epoxy-based sealer, MEK (also known as 2-butanone or methyl ethyl ketone) and ethyl acetate (also known as 1-acetoxyethane or acetic ester) have raised attention as being novel endodontic solvents [12]. MEK is an organic, colorless, water-soluble solvent with a sweet odor that is reminiscent of acetone, and is categorized in group D (not carcinogenic to humans) [30]. It has been especially highlighted for the dissolution of one of the most commonly used endodontic sealers: AH-Plus [12]. Based on the same principle of a targeted approach to a sealer's chemistry, 10% formic acid and 17% ethylenediaminetetraacetic acid (EDTA) have recently been suggested for hydraulic sealer dissolution in the clinical retreatment protocol [31].  
Although traditional solvents have been indistinctively used for both filling materials--gutta-percha and sealers (such as resin and zinc-eugenol-based)--there has always been a special focus on their gutta-percha dissolution profile. However, different compounds have emerged as quite specific sealers, such as EndoSolv E (Septodont) (a tetrachloroethyle lone-based compound) and EndoSolv R (Septodont) (a formamide and phenyl ethylic alcohol-based compound) for zinc oxide-eugenol-based and resin-based sealers. Recently, they have been replaced by EndoSolv (Septodont), the main constituents of which are ethyl acetate (50-100%) and pentyl acetate (2.5-10%) [32]. Even though its manufacturer claims that it can be used for different types of sealers, there are no sound reports regarding its efficacy.  
The development of new rotary retreatment files may have contributed to a lesser focus on investigating solvents for NSER. However, microtomography, scanning electronic microscopy findings, and histopathological analysis have shown that, independently of the instrumentation system or the supplementary irrigating approach, filling residues and resistant biofilms still persist in root canals or dentinal tubules after NSER conventional procedures [2; 15; 16].  
The use of traditional gutta-percha solvents is mostly isolated. However, a few studies have assessed some associations for better performance. [PERSON] et al. [33] revealed that TCE potentiated the effect of OOil and eucalypttol in different types of gutta-percha and Resilon. The association between Citrol+TCE obtained the best results on Resilon's dissolution, while OOil (citrol) alone obtained the worst; however, they were still quite milder. In the same sense, Citrol+TCE and Eucalypttol+TCE were the most successful among associated and isolated compounds against EndoREZ cones. The lack of a deeper explanation and concerns regarding their biocompatibility pointed out the need for further research. Recently, with the same methodology of weight loss percentage, [PERSON] et al. [12] presented MEK as having a higher efficacy for resin-based-sealer dissolution. The values obtained reached those of chloroform; thus, they were quite different to the traditional gutta-percha solvents studied. Additionally, the authors confirmed the efficacies of two binary mixtures with MEK as a common compound and organic/essential oil as a co-solvent: MEK/TCE and MEK/OOil. A synergistic effect explained their increased efficacy for gutta-percha and resin sealer dissolution. The mixtures' performances reached the gold standard of chloroform and, importantly, with a safer profile [13].