This study explores how heat treatment less significantly influences (increasing or decreasing) torsional resistance when compared to the high increase in flexibility and fatigue resistance reported in many published articles. Moreover, torsional fracture occurs extremely rapidly when an instrument’s tip becomes blocked.
Introduction
The main mechanisms of nickel-titanium (NiTi) endodontic instrument fracture have been revealed as two modes of failure, one being torsional failure and the other cyclic fatigue. The former contributes to a significant proportion of failures.1 Cyclic fatigue fracture is caused by repetitive compressive and tensile stresses on the outermost fibres of a file 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 instruments has been assessed extensively.2 In contrast, there is less information available on torsional fracture resistance tests.3 The main method of testing for static rotational fracture is the comparison of the torsional resistance of the instruments as described by ISO 3630-1. According to this specification, the 3 mm file tip must be fixed 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 endodonticmotor: torque settings can be selected to prevent excessive torsional load on the instruments. It has been shown that the correct preset torque value for each instrument is very difficult to determine.4 If it is too high, safety becomes dependent on the clinician’s skill in avoiding over-engagement and/or blockage of the file. If it is too low, the rotary instrument will be loaded by repeated locking and release through use of the torque-controlled motor or auto-reverse function. However, in narrow canals, where instruments are subject to higher torsional stresses than in wider canals, the chance of experiencing these repetitive torsional loads is increased.
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 that is considered safe (i.e. capable of preventing shear fracture of the instrument) is not completely accurate. Therefore, NiTi rotary instruments should ideally exhibit a good resistance to torsion in all cases and in curved canals should also be flexible and resistant to cyclic fatigue.
Many factors can affect resistance to torsion, including design, dimensions, manufacturing process and motion.5 In the present study, two NiTi rotary instruments, similar in dimensions and design, were tested to compare torque at failure. The null hypothesis was that any differences found would be related to different manufacturing processes.
Methodology
The following instruments from two different systems were tested and compared: ProTaper Next (Dentsply Maillefer) and EdgeFile X7 (EdgeEndo). For each brand, ten 17/0.04 instruments were subjected to a repetitive torsional test. The test was performed using a torque-controlled endodontic motor (MASTERsurg, KaVo). The motor allowed for precise recording of torque values during the instruments’ use. The accuracy and reliability of the device had been validated in a previous study.6 To perform the test, the apical 3 mm of each file was firmly secured, embedded in a resin block produced with a mixed autopolymerising resin (DuraLay, Reliance Dental Manufacturing). Each file was then rotated clockwise at a speed of 300 rpm until fracture occurred. The torque limit was set at 5.5 N cm, to ensure recording measurements ranging from 0.1 to 5.5 N cm. The torque values at failure 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 significance level of α = 5%.
Results
Table 1 shows the results from the present study. The ProTaper Next files demonstrated no significantly different levels of resistance in terms of maximum torque at failure compared with the EdgeFile X7 instruments (p < 0.05). Similarly, no statistically significant differences were found between the two instruments in terms of time to failure (p < 0.05).
| Table 1: torque at failure (N/cm) | Maximum torque (SD ) | Time to failure ( seconds) |
| Edge ENDO X7 | 0, 57 (± 0,1) | 0,42 (± 3,5 ) |
| Protaper Next | 0,51 (± 0,1) | 0,39 (± 2,9) |
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 instruments that rotate at speeds much higher than 2 rpm or for the specific motors with torque control and auto-reverse mode.7 Therefore, in the present study, torsional resistance was assessed by using a different speed: the clinical speed (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’s website, EdgeFile X7 files exhibit a higher flexibility and a greater resistance to cyclic fatigue than competitors’ instruments do. In stainless-steel instruments, flexibility and torsional resistance are usually inversely proportional, which 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.8 In the present case, mass and dimensions were very similar, and torsional resistance was similar, showing no statistically significant difference between the two instruments. The null hypothesis was therefore rejected.
Hence, the present study showed that heat treatment does not significantly influence torsional resistance in contrast to the high increase in flexibility and fatigue resistance derived from heat treatment as reported in many published articles.9, 10
References:
- Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel-titanium files after clinical use. J Endod. 2000 Mar;26:161–5.
- Plotino G, Grande NM, Cordaro M, Testarelli L, Gambarini G. A review of cyclic fatigue testing of nickel-titanium rotary instruments. J Endod. 2009 Nov;35(11):1469–76.
- Pedullà E, Grande NM, Plotino G, Gambarini G, Rapisarda E. Influence of continuous or reciprocating motion on cyclic fatigue resistance of 4 different nickel-titanium rotary instruments. J Endod. 2013 Feb;39(2):258–61.
- Gambarini G. Advantages and disadvantages of new torque-controlled endodontic motors and low-torque NiTi rotary instrumentation. Aust Endod J. 2001 Dec;27(3):99–104.
- Xu X, Eng M, Zheng Y, Eng D. Comparative study of torsional and bending properties for six models of nickel-titanium root canal instruments with different cross-sections. J Endod. 2006 Apr;32(4):372–5.
- Gambarini G, Seracchiani M, Piasecki L, Valenti Obino F, Galli M, Di Nardo D, Testarelli L. Measurement of torque generated during intracanal instrumentation in vivo. Int Endod J. 2019 May;52(5):737–45.
- Pedullà E, Lo Savio F, La Rosa GRM, Miccoli G, Bruno E, Rapisarda S, Chang SW,Rapisarda E, La Rosa G, Gambarini G, Testarelli L. Cyclic fatigue resistance,torsional resistance, and metallurgical characteristics of M3 Rotary and M3 ProGold NiTi files. Restor Dent Endod. 2018 Apr 23;43(2)
- Gambarini G, Testarelli L, Galli M, Tucci E, De Luca M. The effect of a new finishing process on the torsional resistance of twisted nickel-titanium rotary instruments. Minerva Stomatol. 2010 Jul-Aug;59(7-8):401-6.
- Braga LC, Faria Silva AC, Buono VT, de Azevedo Bahia MG. Impact of heat treatments on the fatigue resistance of different rotary nickel-titanium instruments. J Endod. 2014;40(9):1494–7
- Plotino G, Testarelli L, Al-Sudani D, Pongione G, Grande NM, Gambarini G. Fatigue resistance of rotary instruments manufactured using different nickel-titanium alloys: a comparative study. 2014;102(1):31–5