Physico-mechanical properties of two different heat treated Nickel-Titanium instruments: in-vitro study

Aim: This study aimed to define physico-mechanical properties of recently introduced thermally treated Edge Taper Platinum (ET Platinum) Nickel-Titanium (NiTi) instruments compared to conventional NiTi Edge Taper (ET) to disclose improvements obtained by heat-treatments. Methods: ET and ET Platinum instruments (n=30/each) were rotated until fracture in a stainless-steel canal with 90°curvature angle and 5mm curvature radius. Time to fracture (TtF) and number of cycles to fracture (NCF) were calculated and results were analyzed using Mann-Whitney test (p=0.05). Fractured instruments underwent fractographic analysis under Field-Emission-Gun Scanning Electron Microscope (FEG-SEM). Superficial features of additional new ET and ET Platinum instruments were analyzed under SEM at increasing magnification with standardized angulations and positions. After inspection, ET and ET Platinum instruments were used in extracted teeth to shape 4 straight canals each to simulate clinical conditions and re-evaluated under SEM at the same points to detect superficial wear features. ET and ET Platinum brand new instruments were examined by differential scanning calorimetry (DSC) to assess transformation temperatures. Results: Cyclic fatigue test showed a significant increase of fatigue resistance on ET Platinum compared to ET instruments (p=0.05). Fractographic analysis demonstrated a pattern of fatigue propagation on both instruments. SEM analysis on brand new instruments revealed a more regular surface of ET Platinum compared with ET and both instruments appeared with minimal alterations after the simulated clinical use. DSC analysis demonstrated higher amounts of martensite at 37°C in ET Platinum compared to ET instruments. Conclusions: ET Platinum instruments displayed an improved mechanical behavior, possibly related to modified phase transformation temperatures induced by thermal treatment of the alloy during manufacturing process. These results suggest a safe use in clinical conditions.