 133 ENDO (Lond Engl) 2018;12(2):133–138 ORIGINAL ARTICLE Kleber Borgo Kill, DDS, MSc, PhD Department of Endodontics, Faculty of Dentistry, Estácio de Sá University, Rio de Janeiro, Brazil Hélio Pereira Lopes, DDS, PhD Department of Endodontics, Faculty of Dentistry, Estácio de Sá University, Rio de Janeiro, Brazil Mônica Aparecida Schultz Neves DDS, MSc, PhD Department of Endodontics, Faculty of Dentistry, Estácio de Sá University, Rio de Janeiro, Brazil Victor Talarico Leal Vieira, DDS, MSc, PhD Department of Endodontics, Faculty of Dentistry, Grande Rio University, Duque de Caxias, Rio de Janeiro, Brazil Carlos Nelson Elias, MSc, PhD Department of Materials Science, Military Institute of Engineering, Rio de Janeiro, Rio de Janeiro, Brazil Letícia Chaves de Souza, DDS, MSc, PhD Department of Materials Science, Military Institute of Engineering, Rio de Janeiro, Rio de Janeiro, Brazil Correspondence to: Mônica Aparecida Schultz Neves, Department of Endodontics, Faculty of Dentistry, Estácio de Sá Uni- versity, Av. Alfredo Baltazar da Silveira, 580 – Recreio dos Bandeirantes – Rio de Janeiro, RJ, Brazil 22790-710. E-mail: maschultzn@gmail.com Kleber Borgo Kill, Hélio Pereira Lopes, Mônica Aparecida Schultz Neves, Victor Talarico Leal Vieira, Carlos Nelson Elias, Letícia Chaves de Souza Buckling resistance of pathfinding nickel-titanium rotary instruments in narrow root canal: an in vitro study Key words buckling resistance, endodontic instruments, endodontic treatment, glide path, narrow root canal Introduction: Exploration is an important step of root canal preparation, because it allows the clinician to negotiate the canal to its terminus and determine its complexity. Different instruments have been developed specifically for this purpose. This study compared the buckling resistance of hand stain- less steel and pathfinding nickel-titanium rotary instruments in artificial canals with different depths. Materials and methods: A total of 30 PathFile, ScoutRace and C-Pilot instruments were evalu- ated. The buckling resistance was tested using acrylic resin blocks with a flat surface or with 3 mm and 6 mm deep artificial canals. The load required to generate a lateral displacement of 1 mm was recorded. Data were statistically analysed using one-way analysis of variance (ANOVA) and Student- Newman-Keuls test for multiple comparisons at a significance level of P < 0.05. Results: On the flat surface, C-Pilot showed the greatest buckling resistance. In 3 mm deep canals, PathFile showed resistance similar to that of C-Pilot files on the flat surface. In 6 mm deep canals, both nickel-titanium instruments showed greater buckling resistance than C-Pilot on the flat surface (P < 0.001). Conclusions: The buckling resistance of nickel-titanium pathfinding instruments increases with the depth of the canal.  Introduction The endodontic glide path is the clinician’s first con- tact with the internal anatomy of the root canal, through which it is possible to verify the number, direction, and diameter of the canals. Also, during this step, the clinician evaluates the possibility of accessing the apical portion of the canal. For many years, even after the introduction of nickel-titanium (NiTi) endodontic instruments, the exploration of calcified root canals has been carried out with stainless steel (SS) hand files. Recently, rotary NiTi pathfinding instruments have been proposed for this procedure 1-4 . Endodontic instruments used for exploration of calcified curved canals must have small diameters and tapers, in addition to mechanical properties that allow them to advance safely and efficiently towards the apical portion 3,5 . Among these properties, the resistance to buckling and flexibility is highlighted 1,5 . Buckling can be defined as the elastic lateral defor- mation of an instrument resulting from application of a load in the direction of its axis. Such behaviour is different from flexibility, because the latter occurs in response to application of a load that is perpendicu- lar to the long axis of the instrument 1,5,6 . Some manufacturers have developed endodontic instruments made of SS or NiTi alloy specifically for initial exploration of calcified root canals, since the inability to reach the apical portion of the root canal may compromise treatment outcomes. Normally, SS pathfinding instruments are hand-operated,