SUMMARY The aim of this study was to evaluate the color stability of light-cured and dual-cured resin cements after artificial accelerated aging. Ten specimens (6-mm diameter and 2-mm thickness) for each of five resin cements were prepared: GC (dual-cured cement, GCem), Vb (light-cured cement, Variolink II only the base), Vbc (dual-cured cement, Variolink II base with catalyst), VV (light-cured cement, Variolink Veneer), and FR (flowable resin composite, light cured). The samples were polished and stored in an accelerated artificial aging machine for 308 hours (160 klx), with cycles of 120 minutes under light and 60 minutes in the dark. All aging was carried out in distilled water at 37°C and light irradiation at 765 W/m2. The samples were evaluated in a spectrophotometer before and after aging, and results were calculated according to CIEDE2000. The data were statistically analyzed (one-way analysis of variance and Tukey test, 95% confidence). The results of ΔE00 were statistically significant for the type of cement (p<0.001), with differences among tested groups. Variolink II (base only and base + catalyst) and the flowable resin were the cements with the lowest color variations after the artificial accelerated aging. Considering the values ΔE00 of acceptability and perceptibility, none of the tested cements showed acceptable values.

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The aim of this study was to evaluate the failure probability of two types of abutment screws after compressive load and to analyze the stress distribution with finite element method. Sixty (60) single-tooth implant restorations were assembled on titanium implants (e-fix, A.S. Technology - Titanium Fix). The groups were divided into Conventional screw (Screw neck 1.5 ø mm) and Experimental screw (Screw neck constricted with 1.2 ø mm). Specimens were subjected to single load to failure with compressive test according ISO 14801. The fractured specimens were subjected to stereomicroscopy for measurement of remaining screws inside the implant and characterization of fracture origin. Representative specimens were analyzed by scanning electronic microscopy. For finite element method (FEM), an identical 3D model of the two in vitro test groups were used with similar conditions (30º, 100 N load). The stress in the abutment screw was analyzed by von-Mises criteria. The results of strength means were 4132.5 ± 76 MPa and 4528.2 ± 127.2 for conventional and experimental groups, respectively. During microscopy, the mean (mm) of the remaining screw piece inside the implants were 0.97 ± 0.23 and 1.32 ± 0.12 for conventional and experimental groups, respectively. In FEM, the conventional group showed stress concentered in an unfavorable region (peak of 39.23 MPa), while the experimental group showed more stress areas but less concentration than the conventional group (36.6 MPa). In using the tested experimental geometry, the abutment screw can have its strength improved, and the origin of failure can be more favorable to clinical resolution.

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In recent years, several finite element analyses of a fast running crack (of opening mode in linearly elastic isotropic bodies) have been conducted by various investigators [1][2][3][4]. These investigations have basically the same approach of modeling the crack propagation by the sequential release of nodes along the crack path. As the crack tip is modeled to move from one node to the next, the condition of the released node is smoothly relaxed from that of zero displacement to that of zero external nodal force normal to the crack plane. The negative work done by diminishing the force at the released node is assumed to represent the energy release mechanism of a running crack. These finite element models [1-4] utilized the conventional four node elements.In another investigation [5] parabolic elements have been employed where nodes were released instantly one after the other to model the running crack. This sudden release of the nodes has been attributed as a deficiency in the finite element model of crack propagation since there is no release of mechanical energy at the crack tip [i-4]. On the other hand, practical experience has in general shown that for a given number of total degrees of freedom greater accuracy is achieved by use of fewer higher order elements in place of larger number of simple element. This report thus presents the investigation of a running crack with the help of parabolic elements with an energy release mechanism.The performance of an eight node element with relaxation technique was studied by analyzing a uniformly expanding crack in a square region subjected to uniform tension, ~ acting on the edges parallel to the crack trajectory.During the initial phase of crack propagation, i.e., before a wave emitted at the start of crack propagation hits the crack after having reflected from the outer boundaries, the situation corresponds to Broberg's problem [6]. The finite element breakdown, shown in Fig. i, represents one quarter of the region due to symmetry and consists of 50 quadratic elements and 351 degrees of freedom. Very stiff massless truss elements, shown in Fig. I, were employed to constrain the vertical nodal displacement along the crack path. When the running crack tip reached a node, the corresponding truss was decoupled, and the force in the truss was relaxed linearly over time duration of crack tip translation from one node to the next with the given velocity.Plane stress was assumed. The value of Poisson's ratio was taken equal to 0.3 which gives Cp/C 2 = 1.69, where Cp is plate wave velocity and C 2 is shear wave velocity.The finite element program NONSAP [7] was employed with Newmark's time integration scheme and consistent mass formulation.Two cases of crack velocity, V = 0:159 Cp and V = 0.318 Cp, were investigated.The energy release rate for each increment of crack extension was computed from the global energy balance involving the I n t Journ o f F r a c t u r e 16 (1980) R34 strain energy, kinetic energy, and potential of remote loads computed at the end of each incremen...

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