## 1 Introduction  
Prevention of the apical extrusion of intracanal bacteria and debris during root canal instrumentation is among the most important objectives of the endodontic treatment of teeth with apical periodontitis [1]. Cases with apical debris extrusion are often clinically associated with pain or swelling post-endodontic instrumentation [2]. Such extrusion during root canal instrumentation (Figure 1) may also have a detrimental effect on healing after endodontic treatment [3; 4; 5]. The extrusion of debris is an involuntary result of root canal instrumentation, regardless of the types of instruments used [6]. The amount of debris extrusion may vary on the basis of kinematics, number of files used, taper,cross-section, and cutting efficacy [3; 7]. A common notion is that techniques involving a push-pull motion create a greater mass of extruded debris than do those involving a rotational action [4]. Thus, endodontic files systems that may minimize debris extrusion should be investigated. The aims of root canal instrumentation in infected oval root canals are hard to achieve, as common rotary and reciprocating files (Figure 2a) have a tendency to leave un-instrumented recesses, thus, leaving debris and unprepared root canal surfaces behind [8; 9]. This limitation led to the development of two file systems that claim to be adaptive instrumentation methods: the SAF system and, more recently, the XP-endo system (Figure 2b,c). The self-adjusting file (SAF) system (ReDent Nova, Ra'anana, Israel) was specifically designed to address the challenge of oval-shaped root canals. The SAF is a hollow,  
Figure 1: Apical extrusion of debris. During root canal treatment, debris consisting of necrotic tissue, bacteria and dentin particles is often extruded through the apical foramen and into the periapical tissues. Such debris may cause severe response (“flare up”) and may jeopardize the long-term success of the root canal treatment. (**a**) Tip of the root with an apical foramen. (**b**) Debris extruded through the apical foramen during instrumentation of the root canal. Figure 2: Adaptive nickel-titanium mechanized instruments. (**a**) Traditional rotary file, which cannot adapt itself to an oval root canal. (**b**) XP-endo shaper with a snake like shape and motion. (**c**) Self-adjusting file (SAF) which can adapt its shape to an oval root canal. nickel-titanium (NiTi) lattice that is designed specifically to be compressed and adapt itself to the walls of root canals with any cross-section, including oval canals (Figure 3). It claims to overcome most of the constraints of motorized rotary and reciprocating nickel titanium (NiTi) files when used in oval canals [5; 10; 11]. The SAF is used with a pecking motion all the way to the working length, while continuous irrigation is delivered through the hollow file. Previously, the SAF was used after establishing a minimal glide path using a #20 K-file. Recently, the manufacturers have changed the guidelines for the use of the SAF system. The updated SAF sequence switched to a motorized glide path preparation, which includes the pre-SAF OS orifice shaper, (sized #40/0.10), the pre-SAF 1 for narrow canals, (sized #15/0.02), and the pre-SAF 2 (sized #20/0.04), thus, creating a glide path. Since the SAF is not a penetrating tool, the glide path should allow passive manual insertion of the 1.5-mm SAF to the working length, prior to cleaning and shaping the root canal with this instrument [3]. The XP-endo system (FKG Dentaire, La Chaux-de-Fonds, Switzerland) consists of the XP-endo shaper (#30/0.01) and the XP-endo finisher (#25/0.00). The XP-endo shaper has a snake-like shape and is made of a MaxWire nickel-titanium alloy with a specific tip design (Figure 2b). The tip has a round non-active part, followed by six cutting edges that transit smoothly into the helical shaft. The file is affected by body temperature, which changes it into an austenitic phase to assume its functional shape.