Gianluca Gambarini, Andrea Cicconetti, Dario Di Nardo , Gabriele Miccoli *, Alessio Zanza, Luca Testarelli and Marco Seracchiani

Abstract:

Protaper Universal (PTU), Protaper Gold (PTG) (Maillefer, Ballaigues, CH), EdgeTaper (ET), and EdgeTaper Platinum (ETP) (Albuquerque, NM, USA) were tested for both torsional and flexural resistance. The aim of the present study was to evaluate the influence of proprietary heat treatment on the metallurgical properties of the aforementioned instruments. Four groups of 30 different instruments (size 20.07) were tested, then divided into two subgroups of 15 instruments—one for the cyclic fatigue test in a curved canal (90◦—2 mm radius) at 300 rpm and 2.5 Ncm. The time to fracture (TtF) and fragment length (FL) were recorded. The other subgroup was subjected to the torsional test (300 rpm, 5.5 Ncm). The torque to fracture and TtF were recorded. All the instruments underwent a SEM analysis. The heat-treated instruments showed a significantly higher fatigue resistance than the non-heat-treated instruments (p < 0.05). No significant differences were found in the torsional resistance between the ET and PTU, and the ETP and PTG. However, when comparing all the groups, the heat-treated instruments showed less torsional resistance. The improvement from heat treatment was mainly found in the cyclic fatigue resistance.

Introduction:

The intracanal fracture of nickel–titanium (NiTi) rotary files is still one of the main concerns during root canal therapy. It has been demonstrated by many authors that rotary NiTi files essentially fail for two main reasons: flexural (cyclic) fatigue and torsional failure [1,2]. The cyclic fatigue failure of the file occurs because of the repeated compressive and tensile stresses that accumulate at the file’s point of maximum-stress, which is located at the maximum curvature point inside the root canal’s anatomy [3]. Torsional failure occurs when the tip (or some part of the file) binds (or is blocked inside) the root canal, while the motor continues to rotate until the torsional limits are overcome and a fracture occurs [4]. The mechanical resistance of NiTi rotary files has been tested in two different ways: cyclic fatigue tests and the torsional test. When investigating cyclic fatigue, instruments are usually rotated inside an artificial curved canal until a fracture occurs at the recommended rotation speed. When investigating the torsional resistance, based on the ADA testing recommendations for stainless steel manual files, the tip is usually locked at 2 mm and its rotation speed is 2 rpm. Unfortunately, there is no ADA standard that describes how to test nickel–titanium rotary files; therefore, studies and their results may differ significantly due to the different testing devices and different methodologies [5–7]. Over the past several years, improvements in design, alloys and manufacturing processes have been proposed to increase the mechanical resistance of NiTi rotary files. Several proprietary heat treatment applications have been developed. Studies have shown that thermal treatments (for files with the same geometry and design) enhance the cyclic fatigue lifespan while, at the same time, reducing the torsional resistance of a file [8–10]. Some studies have also compared heat-treated files and non-heat-treated files with different cross-sectional designs in cyclic fatigue tests, which demonstrate that heat treatment may not be the most influential parameter for flexural resistance [11]. Therefore, many other factors can influence mechanical resistance. Data have shown that different cross-sectional designs, tip and taper dimensions, pitch lengths, and operative motions could affect the resistance to both the flexural and torsional stresses [12–14]. Additional mass increases the torsional resistance and decreases the fatigue resistance [15]. However, very few studies have compared instruments of the same design (and different heat treatments) for both torsional and fatigue resistance [16,17]. Therefore, in the present study, four commercial instruments, with a similar design and tip and taper dimensions, were tested and compared, in order to assess if and how different heat treatments may affect both the torsional and flexural resistance.

Conclusions:

In conclusion, for both manufacturers, heat treatment revealed a significant impact on increasing the resistance to cyclic fatigue. In contrast, the torsional resistance was not improved by the heat treatments. Moreover, torsional failure may happen rapidly if the files become taper locked, since the recorded torque values at the point of failure were significantly lower than the operative torque values recommended by the manufacturers.

Read full article here: Influence of Different Heat Treatments on Torsional and Cyclic Fatigue Resistance of Nickel–Titanium Rotary Files: A Comparative Study