The aim of this study was to evaluate cyclic fatigue resistance of WaveOne Gold (Dentsply Maillefer), EdgeOne Fire (Edge Endo) and One RECI (MicroMega); and investigate the metallurgical characteristics and determine the phase transformation temperatures of the three tested instruments. This study was conducted on a total of 66 samples from all the targeted instruments, 20 samples for cyclic fatigue resistance tests and an additional two samples for DSC analysis for each group.
Cyclic fatigue resistance tests were performed by rotating the samples in an artificial curved canal in a stainless-steel block to simulate the mode of fracture by cyclic stresses, the time required to fracture each sample was measured and used to assess the resistance to cyclic loads. The tests were conducted in two different media temperatures, 25 °C and 37 °C to simulate the effect intra-operative temperature on cyclic fatigue resistance. Differential Scanning Calorimetry analysis (DSC) is a machine that heats a sample and detects the changes on its enthalpy along the heating process. DSC was performed in order to determine at which temperature exactly the tested NiTi sample get transformed to the austenitic phase. The value of this finding is to correlate the crystal-like phase of the instrument at certain temperature to its resistance to cyclic fatigue. The results of Cyclic Fatigue resistance at room temperature (25 °C) reported a significantly superior cyclic fatigue resistance of EdgeOne Fire samples over the other instruments, it showed higher resistance at body temperature (47 °C) as well but with a much less difference to One RECI and WaveOne Gold. WaveOne Gold happened to be the soonest to reach failure in both temperatures. DSC analysis reported that WaveOne Gold has the highest austenitic finish temperature (Af) followed by One RECI. meaning those instruments possess a stable martensitic phase at both room and body temperature, that was the reason of almost no difference in cyclic fatigue resistance between the two temperatures with WaveOne Gold and low difference with One RECI. EdgeOne Fire otherwise has lower (Af), the instrument possesses completely different microstructure upon heating to 37 °C, which explains the huge difference in resistance to cyclic fatigue between the two groups.
The conclusions of the present study were:
• EdgeOne Fire instruments are superiorly more resistant to cyclic fatigue than One RECI and WaveOne Gold.
• The phase of NiTi has a massive influence on the instrument lifespan and the safety of root canal treatments, yet it is not the only factor to be used in the judgment.
• The conventional way of investigating cyclic fatigue resistance misses an extremely critical factor which is media temperature, all intra-operative conditions need to be considered and simulated as practically applicable.