## 4 Discussion  
This systematic review had the objective of assessing the root canal cleaning effectiveness of the XP-Endo finished instrument, both during initial endodontic treatment and retreatment, compared to the efficacy of other irrigant activation procedures. Studies included in this review tested the cleaning efficacy of different protocols using teeth with different anatomical morphologies: Straight or oval-shaped root canals, c-shaped canals, single-rooted teeth, or mandibular molars/premolars. The collected data suggested that CNI is the less effective irrigation technique, since it is not able to adapt to the particular anatomical characteristics of the root canal system and it does not allow to clean the root canal apical third [12, 16, 19, 22, 23, 24, 29]. Given its characteristics, the XP-Endo Finisher instrument surely provides a wider contact with canal walls, increasing cleaning efficacy, also in anatomically complex root canal systems, but it is still unclear whether it is able to guarantee greater debris removal than passive ultrasonic irrigation technique. PUI produces acoustic microwaves, which enhance the reaction of debris dissolution, generating hydrodynamic shear stress [19]. However, the ultrasonic tip could bind to the canal walls of curved canals, interfering with the acoustic streaming [20]. The laser activation procedure has been proven to be more effective in apical third than the XPF, EA, and CB methods [4, 25, 26, 29], since it generates a shock wave effect, allowing a deeper penetration of irrigation solutions into dentimal tubules [26]. The SAF technique showed no significant differences from the XPF and PUI techniques in the study by [PERSON] et al. [24], but it has been demonstrated to be less effective in the study by [PERSON] et al. [10]. Our review highlights that further investigations are needed in order to establish which final irrigation activation procedure could reach the maximum root filling material reduction during endodontic treatment. Studies included in this paper also demonstrated that none of the above-mentioned methods ensure the total removal of root canal debris. Different types of root canals and different concentrations of irrigation solutions could be limitations of our study, leading to inhomogeneous results. Sodium hypochlorite (NaOCl) was used at 2, 2.5, 4, or 5.25% concentrations and ethylenediaminetetraacetic acid (EDTAA) was used at 17% only. Moreover, root canal preparation of teeth samples in the included articles was performed by using different instruments, a condition that does not guarantee homogeneity. Three studies used the BT-Race rotary system [15, 17, 29]. Reciproc rotary files were used by [PERSON] et al. [5, 20], [PERSON] et al. [4, 25], and  
Figure 2: Distribution of scores for removing root canal debris in a sample of 222 teeth. Scoring system by [PERSON] et al. and [PERSON] et al. \(\rightarrow\) Score 0: Groove was empty; Score 1: Present in less than half of the grooves; Score 2: Present in more than half of the grooves; Score 3, The groove was completely covered [27, 28]. by [PERSON] et al. [23]. [PERSON] et al. [18], [PERSON] et al. [19], and [PERSON] et al. [12] prepared the root canal system with the ProTaper Universal rotary system. Vortex Blue files, XP-Endo Shaper, the WaveOne Small File, and EndoSequence Endodontic File System were used only in one study, respectively [10; 16; 23; 26]. [PERSON] et al. [23] tested the efficacy of the XP-Endo Finisher instrument compared with the passive ultrasonic irrigation technique and conventional needle irrigation. In all the three groups, significantly different hard-tissue debris reduction values were obtained depending on the type of the root canal preparation procedure. In the PUI group, the AHTD reduction values were 77.3 after the XP-Endo Shaper instrumentation and 64.2% after Reciproc rotary file preparation. In the XPF group, the values were 63.1% and 68.4%, respectively.