N L Y A L S O N F E S SI O O R T A L P N E D PUBLISHED IN DUBAI www.dental-tribune.me July-August 2020 | No. 4, Vol. 10 Torsional resistance of two nickel-titanium rotary instruments: A comparative study By Prof. Gianluca Gambarini, Italy Introduction The main mechanisms of nickel- titanium (NiTi) endodontic instru- ment fracture have been revealed to be two modes of failure, one being torsional failure and the other cyclic fatigue. The former contributes to a signifi cant pro- portion of failures.1 Cyclic fatigue fracture is caused by repetitive compressive and tensile stresses on the outermost fi bres of a fi le rotating in a curved root canal, and torsional failure occurs when the tip of the instrument binds to the canal wall, even in a straight root canal.1 Cyclic fatigue resistance of NiTi in- struments has been2 assessed ex- tensively. In contrast, there is less information available on torsional fracture resistance tests. The main method of testing for static rota- tional fracture is the comparison of the torsional resistance of the instru- ments as described by ISO 3630-1. According to this specifi cation, the last 3 mm of the fi le tip must be fi xed with brass and a rotational speed of 2 rpm applied to create a continuous torsional load until fracture occurs.3 Torsional load can be limited during intra-canal rotary instrumentation by the torque-controlled endodon- tic motor: torque settings can be se- lected to prevent excessive torsional load on the instruments. It has been shown that the correct preset torque value for each instrument is very dif- fi cult to determine4. If too high (the same happens when the clinician ap- plies maximum torque), safety be- comes dependent on the clinician’s skill in avoiding over-engagement and/or blockage of the fi le. If too low, the rotary instrument will be loaded by repeated locking and release through use of the torque-controlled motor or auto-reverse function. How- ever, in narrow canals, where instruments are subject to higher torsional stresses than in wider ca- nals, the chance of experiencing these repetitive torsional loads is in- creased. To this point, torque value at failure according to the ISO test has not been commonly used to determine torque settings in torque-controlled motors. In most cases, values are higher than torque at failure. As a consequence, the concept that the use of a preset torque value is consid- ered safe (i.e. capable of preventing shear fracture of the instrument) is not completely accurate. Therefore, NiTi rotary instruments should ide- ally exhibit good resistance to tor- sion in all cases and in curved canals should also be fl exible and resistant to cyclic fatigue. File system Maximum torque at failure in Ncm (mean ± SD) Time to failure in seconds (mean ± SD) EdgeFile X7 0.57 (± 0.10) ProTaper Next 0.51 (± 0.10) Table 1: Results Many factors can affect resistance to torsion, including design, dimen- sions, manufacturing process and motion5. In the present study, two NiTi rotary instruments, similar in dimension and design, were tested to compare torque at failure. The null hypothesis was that differences related to the different manufactur- ing processes would be found. Methodology Instruments from the following two different systems were tested and compared: ProTaper Next (Dentsply Maillefer) and EdgeFile X7 (EdgeEndo). For each system, ten 17/.04 instruments were subjected to a repetitive torsional test. The test was performed using a torque- controlled endodontic motor (MAS- TERsurg, KaVo). The motor allowed precise recording of torque values during the instruments’ use. The ac- curacy and reliability of the device had been validated in a previous study. To perform the test, the apical 3mm of each fi le was fi rmly secured, embedded in a resin block produced with a mixed auto- polymerising res- in (DuraLay, Reliance Dental Manu- facturing). Each fi le was then rotated clockwise at a speed of 300 rpm until fracture occurred. The torque limit was set at 5.5 Ncm, to ensure record- ing measurements rang- ing from 0.1 0.42 (± 3.50) 0.39 (± 2.90) to 5.5 Ncm. The torque values at fail- ure were recorded by the integrated software of the motor and analysed using spreadsheet software. The data was analysed using one-way analysis of variance and a Tukey test with a signifi cance level of a = 5%. Results Table 1 shows the results from the present study. The ProTaper Next fi les demonstrated no signifi cantly different resistance in terms of max- imum torque at failure compared with the EdgeFile X7 fi les (p < 0.05). Similarly, no statistically signifi cant differences were found between the two instruments in terms of time to failure (p < 0.05). Discussion The ISO torsional resistance static test was developed more than 50 years ago to test manual stainless- steel instruments and is probably not ideal for testing rotary instru- ments that rotate at speeds much higher than 2 rpm or for the spe- cifi c motors with torque control and auto- reverse mode.5 Therefore, in the present study, torsional resist- ance was assessed by using a differ- ent speed: the clinical one (300 rpm). The tested instruments were similar in dimension and design, but had been produced through different manufacturing processes (alloys and heat treatments). According to the manufacturer, EdgeFile X7 fi les ex- hibit a higher fl exibility and a greater resistance to cyclic fatigue than com- petitors’ instruments do. In stainless- steel instruments, fl exibility and torsional resistance are usually in- versely proportional. This is mainly due to the mass and/or dimensions of the instruments. The greater the mass, the more rigid and resistant to static torsion the instrument is.7,8 In the present study, mass and dimen- sions were very similar and torsional resistance too was similar, showing no statistically signifi cant difference between the two instruments. The null hypothesis was therefore re- jected. Hence, the present study showed that heat treatment does not sig- nifi cantly infl uence torsional resist- ance, in contrast to the high increase in fl exibility and fatigue resistance derived from heat treatment as re- ported in many published articles. 9,10 Editorial note: A list of references is available from the publisher. About the author Prof. Gianluca Gambarini is head of endodontics and restorative dentistry at the Sapienza University of Rome in Italy and director of the den- tal school’s master of endodontics pro- gramme. He maintains a private practice limited to endodontics in Rome, where his focus is on endodontic materials and clinical endodontics. As an international lecturer and re- searcher, Gambarini has held more than 500 presentations at the world’s most renowned international congresses and universities. He has also received several awards and led research projects funded by national and international grants. In addition to that, Gambarini is an active consultant in the development of new technologies, surgical procedures and materials for root canal therapy. Further- more, he holds patents concerning en- dodontic technologies he has developed. Currently, Gambarini serves as Chairman of the Clinical Practice Committee of the European Society of Endodontology.