## 4 Discussion  
The effectiveness of reciprocating motion instruments for the removal of filling material during root canal non-surgical retreatment procedures is currently a topic of debate in the literature. [PERSON] et al. demonstrated that the reciprocating instruments they analyzed are more effective in removing filling materials compared to instruments with continuous rotation [58]. In contrast, other authors concluded that there is no statistically significant difference in the ability to remove canal filling material between reciprocating files and continuous kinematics [59; 60]. Some authors have compared the speed of removing endodontic filling materials using reciprocating files or files with continuous rotation. [PERSON] et al. found that the continuous rotation instruments they used are significantly faster than the reciprocating system [61]. However, [PERSON] et al. concluded that there are no significant differences in the time required for the removal of endodontic filling materials between the two types of movements [55]. There is no strong evidence in the literature regarding the use of continuously rotating or reciprocating endodontic instruments to reduce the extrusion of apical debris during endodontic therapy. Recent systematic reviews have examined this issue [40; 62]. [PERSON] concluded that the type of motion does not influence the quantity of extruded debris [40]. [PERSON], while concluding that reciprocating instruments tended to extrude more dentin debris than continuous rotating instruments, noted that many of the studies included in his review showed conflicting results [62]. This review aims to assess whether there is a correlation between continuous or reciprocating rotation of the endodontic instrument and the extrusion of apical debris in cases of non-surgical endodontic retreatment. During the phases of canal shaping, dentin, pulp, bacteria, and infected tissues are removed from the root canals. In cases of retreatment, the clinician must not only remove dentin, pulp, and infected tissues but also the main substrate must be removed, which is gutta-percha [19]. This characteristic could potentially lead to a difference in the extrusion of apical debris in cases of endodontic retreatment compared to normal endodontic therapy. This difference is the reason why the authors decided to write this systematic review of the literature: the systematic reviews of the literature currently published have evaluated apical extrusion during the endodontic therapy and not during the non-surgical endodontic retreatment, in which changes the substrate to be removed from the endodontic system [40; 62]. The authors of the research included in this review do not agree on the results obtained [43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56]. Most of them conclude that there are no significant differences in the amount of apical debris extruded using instruments with reciprocating or continuous rotation motion [43; 44; 45; 46; 47; 48; 49; 50; 51; 53; 56]. However, [PERSON] concludes that the analyzed reciprocating instruments extrude a greater amount of debris than instruments with continuous rotation [52]. On the other hand, other authors conclude that reciprocating motion instruments result in less extrusion [47; 54; 55; 57]. There is, therefore, no strong scientific evidence regarding the choice of one of these movements based on the amount of extruded apical debris. The differences in the results obtained from the studies included in this systematic review may be attributed not only to the type of instrument motion used but also to other factors of the endodontic instruments used, such as:  
* The different taper [63; 64]. * Cross-section, cutting angle, groove depth, radial surfaces, cutting direction [63; 64; 65]. * Cutting efficiency [63]. * Thermomechanical treatment [66; 67]. * Torque, speed, and taper [68]. * The number of instruments involved in the system. * The influence of canal curvature [69; 70]. * The influence of working length [71; 72]. * Influence of apical diameter [73; 74; 75].