Papers by Author: Vicente Tadeu Lopes Buono

Abstract: This study evaluated the influence of cyclic straining in torsion on fatigue resistance and torsional properties of ProTaper (S1, S2, F1, F2, F3) NiTi instruments. The mechanical behavior was evaluated by means of torsion and bending tests, performed according to ISO 3630-1 specification, while the fatigue resistance was evaluated using a bench test device employing an artificial canal with 5mm curvature radius and angle of curvature of 45º. The dimensional aspects, diameter (D3) and cross-sectional area (A3) at 3mm from the tip of the instruments were determined by means of image analysis of optical and scanning electron microscopy (SEM). The surface characteristics of instruments before and after they were submitted to cyclic straining in torsion and the morphological aspects of the fractured surfaces were analyzed by SEM. The maximum torque and the bending moment at 45º were higher for instruments with larger D3 and A3. The F1 and F2 instruments followed the Coffin-Manson’s relation, i.e., their number of cycles to failure (NCF) varied inversely with the strain amplitude. With the exception of S1 and S2 instruments, there was a tendency of decreasing the maximum torque after the instruments have been submitted to cyclic straining in torsion. With the exception of S1 instruments, there was a tendency of decreasing the NCF after torsional cycling. The analyses by SEM showed the presence of longitudinal cracks on the instruments previously cycled in torsion. The morphological aspects of the fractured surfaces of instruments tested in torsion or flexural fatigue did not changed by the cyclic straining in torsion. These changes in mechanical properties with torsional cycling can be related to the generation of imperfections such as longitudinal cracks on the instruments.

Abstract: Superelasticity is closely related to shape memory effect. It refers to the property presented by some materials submitted to large strains (usually up to about 8%) to restore their original shape immediately after unloading without the need of heating. This phenomenon results directly from a diffusionless transformation of the material from an austenitic to a martensitic phase (martensitic transformation). The recovering mechanism is the reverse transformation, from martensite to austenite. This paper compares fatigue live curves obtained in bending-rotation fatigue tests carried out on wires of NiTi alloys with three different microstructures, stable austenite, unstable austenite (superelastic), and stable martensite. These curves are also compared to data from the literature. The tests were strain controlled and the wires were submitted to strain amplitudes from 0.6% to 12.0%. To minimize changes in material properties, the wire temperature was monitored using a thermocouple and controlled by its rotation speed. For strain amplitudes up to 4%, the εa-Nf curve for superelastic wires was consistent with those reported in the literature, closely approaching the curve of the stable austenite wire. For higher strain amplitudes, fatigue life of superelastic wires increased with strain until it approached the fatigue life curve of stable martensitic wire. This unusual behavior results in a “Z-shaped” curve for high strain values. It is possibly linked to the changes in microstructure and fatigue properties that occur when the superelastic material is deformed.

Abstract: The aim of this study was to characterize metallurgical properties and the dimensions of three types of rotary NiTi endodontic instruments and to establish a correlation between these characteristics and the flexibility of the instruments. Their chemical composition and constitution were characterized by energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD). Transformation temperatures were assessed by means of differential scanning calorimetry (DSC). Computer software was used to analyse images of the longitudinal and transverse sections for determining instrument diameter and cross-sectional area at 3mm from the tip. The flexibility of the instruments was evaluated in bending tests performed according to the ISO 3630-1 specification, in which the instruments are clamped at 3mm from their tip and bent by 45° along their longitudinal axis. The values of bending moment at 45° were correlated with instrument diameter and cross-sectional area at 3mm from the tip. The results of EDX, XRD and DSC showed that physical and chemical properties of the materials differed slightly among the files analyzed. A direct relationship was found between bending moment and the geometric characteristics of the instruments. Resistance to bending of NiTi root canal instruments depended on their geometrical shapes and metallurgical properties, but the cross-sectional configuration can be seen as an important parameter affecting this property.