## 4 Discussion
When the same concept was applied in the XP-endo shaper plus group, it was found that each part of the procedure contributed its share to the total extrusion of debris. The XP-endo shaper and the XP-endo finisher contributed 42% and 40% of the total amount of debris, respectively. Both the SAF and XP-endo systems could be expected to cause debris extrusion, as both the SAF and the XP-endo shaper and finisher are used with repeated packing motions once/after they have reached the working length. This approach is substantially different from rotary or reciprocating file systems, in which once the WL is reached, the procedure is done. Both systems have another common apparent problem: a potential difficulty to assure accurate working length. The SAF system is using in-and-out vibrations with an amplitude of 0.4 mm, which, when added to the packing motion applied by the operator, may compromise the accuracy of the working length. Both the XP-endo shaper, with its snake-like form, and the XP-endo finisher, with its ship-shape, may become shorter or longer when expansion and contraction occur when they are rotated, with a pecking motion, in the canal. Such inaccuracy in controlling the instruments at exactly the working length may have contributed to the finding that both systems extruded measurable amounts of debris beyond the apical foramen. What could be the difference between the two file systems studied here that might explain the difference in their extrusion of debris? One such difference could be the cross-section of the apical part of the file. The guiding non-active part of the tip of the XP-endo shaper has a round cross-section, while the tip of the SAF has a rectangular cross-section [10; 20]. The operator of the XP-endo shaper is directed by the manufacturer to perform 15 pecking motion strokes to the WL [12]. When the round guiding tip of the XP-endo shaper is moved in-and-out within the round apical part of the canal, it may act as a piston, pushing and extruding debris through the apical foramen. When the tip of the SAF is compressed into the round apical part of the canal with a 0.2-mm diameter, which was prepared during glide path preparation, it assumes a rectangular cross-section of 0.12 \(\times\) 0.16 mm (two layers of its longitudinal beams, each with a cross-section of 0.12 mm by 0.8 mm) [10; 20]. This rectangular part is changing its circular position continuously, thus maintaining the apical part of the canal in its round cross-section. When this rectangular tip of the file is moving in-and-out in the circular apical part of the canal, it does not act as a piston, as 40% of the cross-section of the canal is always free for back flow [10; 20]. This difference may partially explain the difference in extrusion of debris. Another difference between the two file systems is that the XP-endo shaper plus sequence is used with intermittent irrigation, using syringe and needle between file applications. In contrast, in the SAF system, irrigation is continuous, as the irrigant is delivered into the root canal through the hollow file throughout the operation. It could be that such continuous irrigation, which carries the debris coronally and out of the canal, better prevents the accumulation of debris, thus reducing the tendency to push such accumulated debris through the apical foramen. A limitation of the present study was that there was no element representing the resistance of the periapical tissues to extrusion of debris. [PERSON] et al. [16] used 1.5% agar gel for such representation, but in such a method, only the total amount of extruded debris can be quantified. Since the present study was aimed to analyze the contribution of each stage of instrumentation to the amount of extruded debris, such a system was not applicable. Therefore, the present results may have to be considered as comparative values between the two systems, rather than as absolute amounts of debris.