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Root canal treatment is a non-surgical approach used to treat two distinct endodontic disease entities: (1) vital, but irreversibly inflamed pulp or (2) the non-vital or dying, infected pulp, associated with apical periodontitis (European Society of Endodontology 2006). Non-surgical root canal therapy has become a routine procedure in modern dentistry. Recent techniques and novel materials resulted in saving of millions of teeth that would otherwise be lost (Hargreaves et al. 2011). The principal goal of endodontic treatment is to render the root canal system bacteria-free and to prevent the invasion of bacteria and their byproducts from the root canal system into the periradicular tissues (Tsesis et al. 2012). There is a several material have been used to fill the root canals: resilon, silver point and the most popular material it is the gutta-percha in conjunction with different sealers. However, gutta-percha has two major drawbacks: its poor sealing ability and its inability to further strengthen the teeth (Marfisi, K., et al.2010).Post-treatment failure is associated with the persistence of microorganisms in the root canal system due to untreated canals, inadequate filling, insufficient cleaning, iatrogenic events or recolonization of the root canal space by bacteria following coronal or apical micro-leakage (Nair et al. 1990, Siqueira 2001). Additionally, nonsurgical retreatment procedures confirm mechanical failures, previously missed canals or radicular subcrestal fractures (Ruddle CJ.2002). Retreatment is a procedure to completely remove the filling material from the root canals system, followed by cleaning, shaping and obturation of the canals (Friedman et al. 1990, Bodrumlu et al. 2008). Non-surgical root canal retreatment is often indicated as the first choice to eliminate or reduce the microbial infection (Friedman et al. 1990). Retreatment is a tedious and time-consuming process leading to many procedural errors. Selecting the case for retreatment is a meticulous process where the pros and cons of tooth prognosis have to be weighed. So the duration of time plays an important role in selecting the case ( Kasam, S. 2016).  Retreatment of a root canal system may be more challenging and take longer than the initial treatment (Duncan & Chong 2008).Different methods have been followed to gain this goal; these include H-files, rotary files, solvents (Pirani et al. 2009; Somma et al. 2008; de Oliveira et al. 2006), Gates Glidden burs ,heat ,ultrasonic instruments (Wilcox 1989) and laser (Viducic et al.2003).The removal of gutta-percha using hand files with or without solvent can be a difficult procedure and time-consuming process especially when the root filling material is well compacted (de Oliveira et al. 2006). Therefore, using of NiTi rotary instruments in endodontic  retreatment may decrease patient and operator fatigue (Tasdemir et al. 2008a).Studies have shown that none of the retreatment procedures can completely remove gutta-percha from the canal walls (Vidal et al. 2016; Khalilak et al. 2013), particularly in the apical third where microorganisms are commonly present (Preetam et al. 2016). Regardless of technique, gutta-percha is best removed from a root canal in a gradual manner to prevent inadvertent displacement of irritants periapically. Gutta-percha may be initially removed from the canal in the coronal third, then the middle third, and finally from the apical third. Single cones in larger and straighter canals can be removed with one instrument in one motion (Ruddle CJ.2002).There are studies showed that the use of NiTi files for gutta-percha removal during retreatment is faster and more efficient than Hedstrom hand files (Fariniuk et al. 2017; Khalilak et al. 2013). That may be due to the frictional heat of engine-driven file which might plasticize gutta-percha and ease its removal (Betti & Bramante, 2001). Also, it has been reported that NiTi files are more efficient in the removal of filling material at the cervical and middle one-third as well as Hedstrom hand files at the apical region (Preetam et al. 2016).Recently the ProTaper Universal retreatment files (D1, D2 and D3) are designed to remove filling materials from the root canal before the reshaping procedures. They have different tapers and diameters at the tip. The D1 recommended to remove the filling materials from the coronal third, D2 from the middle third and D3 from the apical third. Similar to the shaping and finishing instruments they have a convex cross section, the D1 has a working tip that facilitates its initial penetration into filling materials (Gu et al.2008).

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