# The Novel Role of Solvents in Non-Surgical Endodontic Retreatment
## 1 Introduction  
Non-surgical endodontic retreatment (NSER) is a conservative option for managing persistent apical periodontitis (AP) associated with root-filled teeth, or where a new disease has emerged after root canal filling. Its main objective is to reduce the interradicular bacterial load to levels that are compatible with periapical healing, relying on maximum filling removal, repreparation through the most complete and canal-centered shaping techniques, and disinfection protocols [1]. However, the current therapy still focuses on the main root canal.  
Reducing old filling remnants is crucial, as they may harbor intraradicular biofilms, the main cause of post-treatment AP [2]. The relative difficulty of NSER is related to variables such as the design of the retreatment/instrumentation systems, the age and type of the root canal filling, and previous preparation errors, besides the complex root canal anatomy [3]. After regaining access to the apical foramen, chemo-mechanical preparation (repreparation) aims to further remove filling residues and disrupt persisting adhered biofilms. Current retreatment techniques include rotary files, ultrasonic instruments, heat, laser, hand files, and solvent solutions [3]. Although their combination is generally required, removing the bulk of the obturations has greatly improved with the development of nickel-titanium (NiTi) rotary systems. This improvement has led to clinicians rarely using solvents.  
Two main strategies have been proposed to optimize disinfection before the new filling: (i) a further apical enlargement [4], with the risk of weakening the root structure; or(ii) using adjunctive procedures, such as sonic/ultrasonic processes or recently developed finishing instruments, to activate the standard sodium hypochlorite (NaOCI) irrigating solution [5; 6]. Laser and photoactivated therapies have also been mentioned, despite their inherent high costs [7; 8]. The subsequent sealing and the remaining cervical and radicular dentin structure have also been considered to be factors of favorable outcomes [9; 10]. However, the current state-of-art, which involves combining adequate mechanical shaping and activated antimicrobial irrigating solutions, namely NaOCI, is still not able to provide a predictable outcome for NSER. Furthermore, there is no evidence for an improvement in the periapical status of populations that are concomitant with the extensive advance in endodontic knowledge and research [10].  
Following the evolution path of NiTi instruments, new engine-driven NiTi instruments, for purposes other than shaping, such as glide path preparation, retreatment, or irrigation enhancement, have emerged. The ProTaper retreatment (Dentsply Maillefer), the self-adjusting file, or the XP-endo Finisher (FKG Dentaire) are some elucidative examples [11]. On the other hand, recent investigations using proposals that are safer and as effective as chloroform, such as methyl ethyl ketone (MEK), ethyl acetate, and novel solvent mixtures (MEK/tetrachloroethylene (TCE) and MEK/orange oil (OOil)), have highlighted an additional role for endodontic solvents. There is no intention of promoting the use of solvents per se, but essentially the purpose is to uncover different paths for optimizing disinfection in retreatment procedures without neglecting all of the available options. Apart from filling dissolution, its antibofilm efficacy, enhanced by agitation, opens new perspectives in the current retreatment disinfecting protocol [12; 13; 14]. Built on the literature search and the previous work of the team, one of the major goals was to identify and summarize new solvent proposals concerning their specificity, their moment of use, their enhancement through agitation and biocompatibility, their effects on dentin structure, and their antimicrobial/antibiofilm activity in NSER.