RESUMEN
PURPOSE: The aim of this study was to test the hypothesis that viscoelastic finite element analyses can reliably predict the effect of geometry on maximum tensile stresses in bilayer screening tests that are used to determine thermal compatibility. MATERIALS AND METHODS: Three-dimensional viscoelastic finite element models of a beam, cylinder, disk, sphere, central incisor crown, molar crown, and posterior three-unit fixed partial denture (FPD) were used to calculate residual stresses after simulated bench cooling. Four compatible and four incompatible systems were evaluated. RESULT: The highest residual tensile stresses for all material combinations were associated with the three-unit FPD. Residual tensile stresses ranged from 5.4 MPa in the disk for a compatible combination to 262 MPa in the three-unit FPD for an incompatible system. Residual tensile stresses in the three-unit FPD ranged from 16.8 MPa to 44.0 MPa for the compatible systems and from 175 MPa to 262 MPa for the incompatible systems. CONCLUSION: Based on finite element calculations, it is predicted that all-ceramic dental prostheses with an average thermal contraction mismatch (500 degrees C to 25 degrees C) greater than +/- 1.0 ppm/K will likely exhibit a relatively high percentage of failures in clinical use compared with systems having smaller thermal contraction mismatch between core and veneering ceramics.
Asunto(s)
Porcelana Dental , Análisis del Estrés Dental , Coronas , Pilares Dentales , Análisis del Estrés Dental/métodos , Coronas con Frente Estético , Dentadura Parcial Fija , Elasticidad , Análisis de Elementos Finitos , Calor , Ensayo de Materiales/métodos , Resistencia a la Tracción , ViscosidadRESUMEN
OBJECTIVE: To test the hypothesis that bilayer ceramic cylinders and spheres can provide valid confirmation of thermal incompatibility stresses predicted by finite element analyses. METHODS: A commercial core ceramic and an experimental core ceramic were used to fabricate open-ended cylinders and core ceramic spheres. The core cylinders and spheres were veneered with one of four commercial dental ceramics representing four thermally compatible groups and four thermally incompatible groups. Axisymmetric thermal and viscoelastic elements in the ANSYS finite element program were used to calculate temperatures and stresses for each geometry and ceramic combination. This process required a transient heat transfer analysis for each combination to determine input temperatures for the structural model. RESULTS: After fabrication, each specimen was examined visually using fiberoptic transillumination for evidence of cracking. There were 100% failures of the thermally incompatible cylinders while none of the thermally compatible combinations failed. Among the spheres, 100% of the thermally incompatible systems failed, 16% of one of the thermally compatible systems failed, and none of the remaining compatible combinations failed. The calculated stress values were in general agreement with the experimental observations, i.e., low residual stresses for the specimens that did not fail and high residual stresses for the specimens that did fail. SIGNIFICANCE: Simple screening geometries can be used to identify highly incompatible ceramic combinations, but they do not identify marginally incompatible systems.
Asunto(s)
Cerámica/química , Porcelana Dental/química , Coronas con Frente Estético , Elasticidad , Análisis de Elementos Finitos , Humanos , Compuestos de Litio/química , Modelos Químicos , Estrés Mecánico , Propiedades de Superficie , Temperatura , ViscosidadRESUMEN
In recent years metal-free ceramic systems have become increasingly popular in dental practice because of their superior aesthetics, chemical durability and biocompatibility. Recently, manufacturers have proposed new dental ceramic systems that are advertised as being suitable for posterior fixed partial dentures (FPDs). Reports indicate that some of these systems have exhibited poor clinical performance. The objective of this study was to use the viscoelastic option of the ANSYS finite element program to calculate residual stresses in an all-ceramic FPD for four ceramic-ceramic combinations. A three-dimensional finite element model of the FPD was constructed from digitized scanning data and calculations were performed for four systems: (1) IPS Empress 2, a glass-veneering material, and Empress 2 core ceramic; (2) IPS Eris a low fusing fluorapatite-containing glass-veneering ceramic, and Empress 2 core ceramic; (3) IPS Empress 2 veneer and an experimental lithium-disilicate-based core ceramic; and (4) IPS Eris and an experimental lithium-disilicate-based core ceramic. The maximum residual tensile stresses in the veneer layer for these combinations are as follows: (1) 77 MPa, (2) 108 MPa, (3) 79 MPa, and (4) 100 MPa. These stresses are relatively high compared to the flexural strengths of these materials. In all cases, the maximum residual tensile stresses in the core frameworks were well below the flexural strengths of these materials. We conclude that the high residual tensile stresses in all-ceramic FPDs with a layering ceramic may place these systems in jeopardy of failure under occlusal loading in the oral cavity.
Asunto(s)
Cerámica/química , Dentadura Parcial Fija , Análisis del Estrés Dental , Análisis de Elementos Finitos , Imagenología Tridimensional , Modelos Teóricos , Resistencia a la TracciónRESUMEN
OBJECTIVE: To test the hypothesis that shear stress relaxation functions of dental ceramics can be determined from creep functions measured in a beam-bending viscometer. METHODS: Stress relaxation behavior was determined from creep data for the following materials: (1) a veneering ceramic-IPS Empress2 body ceramic (E2V); (2) an experimental veneering ceramic (EXV); (3) a low expansion body porcelain-Vita VMK 68 feldspathic body porcelain (VB); (4) a high expansion body porcelain-Will Ceram feldspathic body porcelain (WCB); (5) a medium expansion opaque porcelain-Vita feldspathic opaque porcelain (VO); and (6) a high expansion opaque porcelain-Will Ceram feldspathic opaque porcelain (WCO). Laplace transform techniques were used to relate shear stress relaxation functions to creep functions for an eight-parameter, discrete viscoelastic model. Nonlinear regression analysis was performed to fit a four-term exponential relaxation function for each material at each temperature. The relaxation functions were utilized in the ANSYS finite element program to simulate creep behavior in three-point bending for each material at each temperature. RESULTS: Shear stress relaxation times at 575 degrees C ranged from 0.03 s for EXV to 195 s for WCO. SIGNIFICANCE: Knowledge of the shear relaxation functions for dental ceramics at high temperatures is required input for the viscoelastic element in the ANSYS finite element program, which can used to determine transient and residual stresses in dental prostheses during fabrication.
Asunto(s)
Cerámica/química , Porcelana Dental/química , Silicatos de Aluminio/química , Coronas con Frente Estético , Elasticidad , Análisis de Elementos Finitos , Calor , Humanos , Compuestos de Litio/química , Ensayo de Materiales , Modelos Químicos , Dinámicas no Lineales , Compuestos de Potasio/química , Análisis de Regresión , Estrés Mecánico , ViscosidadRESUMEN
OBJECTIVE: To characterize the high temperature viscoelastic properties of several dental ceramics by the determination of creep functions based on mid-span deflections measured in a beam-bending viscometer (BBV). METHODS: Six groups of beam specimens (58 x 5.5 x 2.5 mm) were made from the following materials: (1) IPS Empress2 body--a glass veneer ceramic (E2V); (2) an experimental glass veneer (EXV); (3) Vita VMK 68 feldspathic body porcelain--a low-expansion body porcelain (VB); (4) Will-Ceram feldspathic body porcelain--a high-expansion body porcelain (WCB); (5) Vita feldspathic opaque porcelain--a medium-expansion opaque porcelain (VO); and (6) Will-Ceram feldspathic opaque porcelain--a high-expansion opaque porcelain (WCO). Midpoint deflections for each specimen were measured in a BBV under isothermal conditions at furnace temperatures ranging from 450 to 675 degrees C. Non-linear regression and linear regression analyses were used to determine creep functions and shear viscosities, respectively, for each material at each temperature. RESULTS: The shear viscosities of each group of dental ceramics exhibited bilinear Arrhenius behavior with the slope ratios (x) ranging from 0.19 for WCB to 0.71 for EXV. At the higher temperature ranges, activation energies ranged from 363 kJ/mol for VO to 386 kJ/mol for E2V. SIGNIFICANCE: The viscoelastic properties of dental ceramics at high temperatures are important factors in understanding how residual stresses develop in all-ceramic and metal-ceramic dental restorations.
Asunto(s)
Porcelana Dental , Análisis del Estrés Dental , Elasticidad , Calor , Análisis de Regresión , Reología , ViscosidadRESUMEN
OBJECTIVE: To test the hypothesis that the Weibull moduli of single- and multilayer ceramics are controlled primarily by the structural reliability of the core ceramic.Methods. Seven groups of 20 bar specimens (25 x 4 x 1.2 mm) were made from the following materials: (1) IPS Empress--a hot-pressed (HP) leucite-based core ceramic; (2) IPS Empress2--a HP lithia-based core ceramic; (3 and 7) Evision--a HP lithia-based core ceramic (ES); (4) IPS Empress2 body--a glass veneer; (5) ES (1.1 mm thick) plus a glaze layer (0.1 mm); and (6) ES (0.8 mm thick) plus veneer (0.3 mm) and glaze (0.1 mm). Each specimen was subjected to four-point flexure loading at a cross-head speed of 0.5 mm/min while immersed in distilled water at 37 degrees C, except for Group 7 that was tested in a dry environment. Failure loads were recorded and the fracture surfaces were examined using SEM. ANOVA and Duncan's multiple range test were used for statistical analysis. RESULTS: No significant differences were found between the mean flexural strength values of Groups 2, 3, 5, and 6 or between Groups 1 and 4 (p>0.05). However, significant differences were found for dry (Group 7) and wet (Groups 1-6) conditions. Glazing had no significant effect on the flexural strength or Weibull modulus. The strength and Weibull modulus of the ES ceramic were similar to those of Groups 5 and 6. SIGNIFICANCE: The structural reliability of veneered core ceramic is controlled primarily by that of the core ceramic.