Investigation of Static versus Dynamic Cyclic Fatigue Resistance in NiTi Endodontic Instruments with Different Alloy Treatments at Body Temperature.

Introduction: This study evaluated the fatigue resistance of 2 nickel-titanium engine-driven file systems with identical geometries and different heat treatments tested under static and dynamic conditions in simulated root canals.

Methods: Cyclic fatigue tests were conducted using ProTaper Universal (PTU) and ProTaper Gold (PTG) instruments with a curvature of 35° and a radius of 6 mm in both static and dynamic modes at body temperature using a customized cyclic fatigue testing device. The number of cycles to fracture was recorded. A scanning electron microscope was used to examine the fracture surfaces of fractured fragments.

Results: The cyclic fatigue resistances of PTU and PTG varied depending on the testing methods (static or dynamic). Statistically significant differences were observed between the 2 file systems, with PTG demonstrating superior fatigue resistance in both testing modes. Notably, dynamic testing resulted in a higher number of cycles to fracture for both PTU and PTG compared to static testing. However, the relative improvement in the cyclic fatigue resistance of PTG over PTU remained consistent under both conditions. The scanning electron microscope analysis of the cross-sectional and longitudinal aspects of the fractured specimens revealed microcrack initiation regions, overloaded quick fracture areas, and microcracks. PTG demonstrated a lower concentration of microcracks compared to PTU.

Conclusions: This study confirmed that heat treatment significantly enhances the cyclic fatigue resistance of NiTi instruments and static mode reduces the fatigue resistance of the file. Heat-treated files showed better property due to their martensitic phase, which improves flexibility and reduces crack initiation.