## 1 Introduction  
A successful endodontic procedure requires a proper shaping and irrigation. The linear movement and rotation of mechanical instruments in the root system produce the smear layer, a crystalline structure with 1-2 u thickness, whose components (pulp residues, dentinal debris, bacteria, and their products [1]) can be found on the canal walls, root canal branches, and pressed into the dentinal tubules [2, 3]. Moreover, during a canal retreatment, calcium hydroxide is used as intra-canal medicaments, in order to achieve the decrease of the bacterial amount. However, the remnants of this medicament, which have to be completely removed before permanent root canal filling, could obstacle the penetration of sealers and disinfectant, compromising the successful result of the endodontic therapy [4]. In addition,the existence of different canal systems and anatomical variation (e.g., c-shaped canals, oval-shaped canals) complicates the treatment procedure. If residual intra-radicular infection occurs, a retreatment is required in order to eliminate the filling material and to reduce the presence of microorganisms. However, these goals are not always simple to achieve and may require adjunctive steps [5; 6]. The only way to eliminate all of this type of debris is irrigation, a procedure that guarantees a positive impact on those areas of the canal system that cannot be reached by mechanical instruments [7]. One of the most common and profitable irrigation protocols involves the use of sodium hypochlorite (NaOCl), which solves necrotic tissues and reduces the bacterial load [8]. To fulfil its function, the irrigation procedure should provide the dispersion of the solution also within the inaccessible areas of the prepared canal walls, and, for this reason, several manual and mechanical irrigant agitation procedures have been proposed. In fact, a recent study by [PERSON] et al. [9] demonstrated that irrigant activation improves tissue dissolution, overcoming the limitation of the conventional needle irrigation procedure (apical positive pressure), which is unable to clean the most apical portion of the root canal system. Examples of irrigant activation procedures are represented by the passive ultrasonic irrigation, the Self-Adjusting File system, or by the TRUShape 3D Conforming File. The passive ultrasonic irrigation technique activates the irrigant through ultrasonically oscillating small files or smooth noncutting wires, respecting the canal preparation [10]. The Self-Adjusting File system involves the use of hollow and thin cylindrical nickel-titanium instruments, which adapt to the cross-section of the root canal and operate with a constant flow of irrigant that is continuously replaced during the procedure [11]. Finally, the recent TRUShape 3D Conforming File is an S-shaped curve and blue color instrument, which preserves more tooth structure than ordinary NiTi instruments [12]. Recently a new mechanical cleaning system has been introduced: XP-Endo(r) Finisher (FKG Dentaire. La-Chaux-de-Fonds, Switzerland), that, with its revolutionary design, may enhance the ability to clean root canals with particular anatomical characteristics, reaching areas previously impossible to treat, but always preserving dentine. XP-Endo(r) Finisher is available in two sizes, ISO 025 and ISO30 (with 0% taper), and it can be used for retreatment cases (Finisher R, Reinforced) and for initial treatment (Finisher, a smaller version). It is made of nickel-titanium (NiTi) MaxWire alloy and, thanks to its small core size and its zero taper, it guarantees flexibility and excellent resistance to cyclic fatigue. Exploiting the shape-memory principles of NiTi alloy, this instrument is able to pass from the martensite-phase (straight shape, room temperature) to the austenite-phase (spoon-like shape) when exposed to body temperature, adapting to the specific morphology of the root canal.