Influência do uso clínico simulado sobre a flexibilidade de diferentes sistemas endodônticos de NiTi

Abstract

Flexibility is an important characteristic of NiTi endodontic instruments. It is determined by the combination geometry and dimensions of the instruments and the metallurgical properties of the materials employed on their manufacture. The aim of this work was to assess how the simulated clinical use influences the flexibility of these instruments. The finishing instruments of the ProTaper Universal (PTU), K3 and EndoSequence (ES) systems were evaluated. Images of the longitudinal and transverse sections were used for determining diameter and cross-sectional area at 3mm from the instrument tip, position where the most important loads are concentrated during the clinical use. The structural characteristics, related to the physical properties (phase transformation temperatures) and the chemical composition of the materials employed were evaluated by conventional analytical techniques. Flexibility was evaluated in bending tests up to 45º performed according to ISO 3630-1 specification. Since these instruments are submitted to a combination of flexural and torsional fatigue during curved root canals shaping, the clinical use was simulated on fatigue bench test devices. Two types of instruments (n = 10) from each one of the systems evaluated were subjected to 20 torsion cycles from zero to 180º angular deflection each. Another similar group was tested until failure in a flexural fatigue test device to establish its fatigue life. After that, other two groups of instruments (n = 10) of each system were tested in the same device until ¼ and ¾ of their fatigue life. Then, the instruments subjected to interrupted fatigue tests had their flexibility evaluated as described before, that is, according to ISO 3630-1 specification. K3 instruments were the least flexible, that is, the ones which presented the highest bending moment (MB) values, followed by PTU and ES. A direct linear correlation was found between MB and the instruments diameter and cross-sectional area, justifying, in principle, the differences in flexibility among new files. Besides that, despite there was no chemical composition differences among the files analyzed, the transformation temperatures of K3 instruments were lower than those of the others, possibly due to differences in thermomechanical treatments. These files presented only the austenitic phase at room temperature, while in PTU and ES there was also martensite. The presence of a small amount of martensite favors the occurrence of stress induced martensitic transformation, contributing to increase the flexibility of endodontic instruments. Cyclic torsional loading caused no statistically significant differences (ANOVA, 95% confidence, p < 0.05) on the bending moment of the instruments analyzed, which kept unchanged their flexibility characteristics. After interrupted flexural fatigue experiments, statistically significant differences in the flexibility of certain instruments were observed and identified as an improvement in flexibility, which occurred for smaller as well as for higher previous number of cycles. In summary, simulated clinical use induced no detrimental effects in the flexibility of the NiTi instruments analyzed, even in situations where there was a large consumption of fatigue life.