(PO80) Combined Torsional and Cyclic Fatigue Resistance in Rotary Endodontic Files

Abstract:
This study presents a novel model of instrument testing that combines torsional and cyclic fatigue stresses, aiming to evaluate the impact of torque at the tip of a rotary file on cyclic fatigue resistance. Size F2 ProTaper Ultimate (PTU) and ProTaper Gold (PTG) files from Dentsply-Sirona were rotated with either a 30- or 60-degree curvature angle and a 5mm radius at a maximum radius of D12 to simulate significant coronal curvature. The tip of the file was aligned with a torque sensor to measure the braking force applied, replicating file action within the canal. The number of cycles to failure (NCF) was recorded for tip torques of 50, 100, 150, and 200 gcm. In a 60-degree canal, a 200 gcm torque tip led to a 50% reduction in NCF in PTG and a 75% reduction in PTU. In general, PTU performed better than PTG in the test at lower tip torques. However, PTU was much more sensitive to increasing torques and failed at a lower NCF than PTG when tested at 200 gcm in both canals. File separation occurred in PTU within seconds at 250 gcm. Differential scanning calorimetry confirmed that both files were made from the same alloy, and scanning electron microscope analysis confirmed no samples fractured due to torsional failure. These findings emphasize the critical role of torque control during the instrumentation of curved canals. Sole reliance on the manufacturer's recommended torque setting may create a false sense of security, highlighting the need for precise torque management to optimize treatment outcomes.