## 4 Discussion  
In the present study, SRCTs with two reciprocating instruments, RB #25 and RB #40, were performed on root canals previously filled with two different techniques (SC and GF). The principal aim of the study was to evaluate the effectiveness of the reciprocating NiTi instrument in removing root-filling materials and their remnants from root canal systems characterized by an oval-shaped configuration. The persistence of remnants inside the root canal was detected by two radiographic methods commonly used in clinical activities and by one in vitro system to obtain more information on the ultra-morphology and composition of the remnants after the SRCTs. The presence of remnants and their positions in the root canal has been proposed [21; 22; 23] to be an index of the effectiveness of the SRCTs. Phosphor plate X-rays were used to measure the area occupied by the remnants. In addition, the volume of the remnants was measured using CBCT. In this way the volume and the position of the remnants were detected in a 3D configuration. Therefore, the X-rays offered a 2D evaluation of the presence/absence of remnants, but no information on the 3D positions in the canal. Thus, information on the area and volume should be combined and taken in account by clinicians during SRCTs. In this study, a greater amount of remnants was observed on the middle third of the canals. Both the X-rays and CBCT confirmed the presence of dense remnants solidly packed into the middle thirds. The CBCT evaluation demonstrated that the presence of the remnants is predominantly localized into the lingual and buccal sides of the root canals. Previous studies have demonstrated the presence of a large amount of remnants corresponding to the roots apical third [24; 25], suggesting that the conventional manual or rotary systems used to remove filling materials may induce the formation of an apical critical area [25]. In a similar way, remnants may still also be detectable using a self-adjusting file (SAF) and a hollow, compressible NiTi instrument, through which a continuous flow of an irrigant can be provided [26]. These data suggest some further considerations. The conicity, the specific design of the RB, and the reciprocating movement may have contributed to the removal of remnants from the apical third, which resulted cleaner and free from remnants compared to the middle third. Thus, it is probable that RBs may be valid instruments in removing apical debris. RB #25 and RB #40 instruments have similar sizes and taper in the middle third because of their regressive variable taper. So, despite its greater apical diameter, the RB #40 is not able to remove more remnants from the middle thirds. Obviously, the use of Gates Glidden burs in the first 4 mm of the root canals--the coronal thirds--was helpful in removing all the remnants and keep these parts of the root canals extremely clean and free from any deposits or residues. Despite a previous study reported a higher removal effect of the gutta-percha using solvents, such as orange oil, chloroform, and eucalypt [27], no solvents have been used in the present study. The use of solvents may induce a greater spreading of softened gutta-percha in the buccal-vestibular portions of the oval root canal. Moreover, the new improvements in the mechanical properties of NiTi have led clinicians to use gutta-percha solvents less [28]. In this study, we included the use of ESEM connected with EDX. ESEM is a new method using a microscopic device able to inspect humid dentine surfaces and to reduce artefacts from the dehydration of the sample. ESEM-EDX allows the microscopic evaluation of the inner surfaces of root canals to detect the presence, morphology, and atomic composition of remnants and dentin surfaces, in accordance with previous studies [10; 29; 30]. Our ESEM observations demonstrated that the mesial and distal surfaces of the root canal had a uniform dentin morphology free from remnants, but with a thin smear layer and with visible dentinal tubules.