John A. Tesk

Failure of All-ceramic Fixed Partial Dentures in vitro and in vivo: Analysis and Modeling

Hertzian cone cracks visible at the loading site of 20 all-ceramic fixed partial dentures (FPDs), tested in vitro, led to the hypotheses that failure was due to the propagation of localized contact damage crack systems (Hertzian stress state) and that such damage was an unlikely clinical failure mode. Fractographic analysis of the 20 laboratory-failed and nine clinically-failed all-ceramic FPDs allowed for definitive testing of these hypotheses and a comparison between in vitro and in vivo failure behavior. In all cases, failure occurred in the FPD connectors (none from contact damage), with approximately 70 to 78% originating from the interface between the core and veneer ceramics. The coincidence between failure origins provides strong evidence that the in vitro test modeled aspects of structural behavior having clinical importance. The fractographic observations, coupled with the in vitro failure load data, furnished very specific boundary conditions which were applied to constrain mathematical models of FPD connector failure. Finite element analysis (FEA) of the laboratory FPDs found that maximum principal tensile stresses would occur at locations consistent with the fractographic observations only if: (1) there were appropriate elastic moduli differences between the ceramics; and (2) a small amount of abutment rotation was allowed. Weibull failure probability (Pf) calculations, incorporating FEA stress profiles, very closely replicated the laboratory failure distribution only when: (1) the veneer ceramic was much weaker than the core ceramic; and (2) the Weibull modulus of the core-veneer interface was much lower than that for the free veneer surface (i.e., the interface is of lower quality with regard to defects). This combined fractographic and mathematical analysis of FPD connectors suggests that the core-veneer interface is an important failure source and that the veneering ceramic overwhelmingly controls load-bearing capability. Observations from failed clinical restorations provided critical guidance in validating a laboratory test and focusing a mathematical failure model.

A Technique for Characterizing Casting Behavior of Dental Alloys

A technique for characterizing casting behavior of dental alloys has been developed and tested. The method employs easily reproducible specimen patterns and uses equipment and procedures generally available in dental prosthetic laboratories. A castability value is arrived at by counting complete segments of a cast alloy grid. Test results indicate that the castability evaluation procedure described can be a valuable aid in the laboratory for both selecting and using dental materials for producing cast restorations. The test method would also appear useful for the evaluation of products and techniques during research and development. An absolute minimum for the castability value determined relative to clinical and laboratory efficacy has yet to be established.

Elastic constants of two dental porcelains

The development of stress that affects the bonding in porcelain-fused-to-metal (PFM) systems can be influenced by the temperature dependence of the elastic constants of both systems. Instead of using the normal, static procedure, e.g. determining the slope of a stress-strain curve, and measuring the lateral and vertical strains, in this study the sonic resonance technique was used to determine the elastic moduli for two dental bodyporcelains. The sonic resonance technique involves the determination of both the flexural as well as the torsional resonance frequencies. From these values both Youngs,Y, and shear moduli,G, are determined. Since two elastic constants are sufficient to describe completely the elastic response of isotropic materials, it was also possible to compute, by usingY andG, the bulk modulus,B, and the Poissons ratio. Resonant frequency measurements taken at elevated temperatures resulted in correspondingly lower values for the elastic constants. Youngs and shear moduli for two dental porcelains obtained in the range from 20° C (293 K) to 500° C (773 K) are presented in this study. These data may in the future be used for refined stress calculations in PFM systems.

Transient and Residual Stress in a Porcelain-Metal Strip

Porcelain-fused-to-metal (PFM) restorations may develop cracks during processing or in-mouth service if the relative physico-mechanical properties of the porcelain and metal are highly mismatched. Precise conditions when this might occur are not known. Many processing and property variations can affect the stresses developed throughout a porcelain-metal system. To understand this, we conducted a computer simulation of stress developed in a PFM beam. The simulation considers cooling from temperatures higher than the porcelain sag-point. The following temperature-dependent factors were incorporated: the elastic modulus, shear viscosity (porcelain), and coefficients of thermal expansion. The cooling rate dependencies of the glass transition temperature, (Tg), and the temperature distribution during cooling were also included. The results suggest that transient tensile stress at the porcelain alloy interface may result in cracks in the porcelain during cooling. Occlusal forces may set up stresses to cause cracking at the surface of the porcelain if the compressive residual stress is not high enough. PFM restorations with an alloy of high thermal expansion coefficient require rapid cooling; on the contrary, PFM restorations with the alloys of lower coefficients require slow cooling. A high cooling rate can make up for thermal expansion mismatches between the alloy and the porcelain up to 2 × 10-6/°C. Finally, the results indicated that curvature was not a sensitive indication of stress for a multimaterial beam when visco-elastic relaxation and high cooling rates are involved. For the case modeled here, curvature varied inversely with a 1/2 to 1/7th power of the stress.

Influence of Tempering Method on Residual Stress in Dental Porcelain

The porcelain component of a porcelain-fused-to-metal restoration is strengthened by residual (tempering) stresses which are induced by cooling procedures followed in dental laboratories. The thermophysical properties of materials and cooling rate are the main factors which determine the residual stress. In this paper, the temperatures in the midplane of body-porcelain disks were measured from a heat-soak temperature (1000°C) to room temperature during two different cooling procedures: slow cooling in air and forced-air cooling. Experimental results approximated exponential cooling wherein the cooling rates could be represented by a linear equation of temperature. Residual stresses, as affected by the tempering method and thickness of a porcelain disk, were calculated by computer simulation for regions away from the edges. The following temperature-dependent factors were incorporated into the simulation: elastic modulus, viscosity, and coefficient of thermal expansion. The cooling rate dependencies of the glass transition temperature and the temperature distribution during cooling were also included. The cooling rates used in this simulation were derived from the tempering data. The agreement between development of transient and residual stresses—calculated by computer simulation for various cooling methods, and the tendency toward failures of porcelain disks subjected to the tempering processes-was examined. Simulated residual stresses were also in good agreement with those measured by the indentation fracture method of Marshall and Lawn (1977) and Anusavice et al. (1989).

Casting of dental alloys: mold and alloy temperature effects

Abstract A casting monitor consisting of a square polyester grid pattern was used to evaluate the casting behavior of 6 non-precious dental casting alloys. A castability value, Cv, was determined and is defined as the fraction or percentage of cast grid segments. The dependence of Cv on the alloy superheat casting temperature, Ta, and mold temperature, Tm, was evaluated. It was found that the standard deviation (SD) of Cv was dependent on Cv. For curve fitting, a transformation, Cv,t, of Cv was used such that the SD(Cv,t)≠f (Cv,t). The transformed castability value is given by C v , t = ln ⁡ 2 / 3 + C v 2 / 3 + 1 / C v It was found that a single equation was capable of representing Cv,t for all 6 dental alloys at the 95% confidence level: Cv,t=a+bTa1/2Tm2. This expression has many potential uses for alloy design, product quality assurance and dental laboratory casting control.

Internal setting expansion of a dental casting investment measured with strain gauges.

A study was undertaken to investigate the feasibility of using strain gauges to evaluate the internal setting expansion of dental casting investments. A strain gauge was invested in the center of a casting ring to determine the linear setting expansion at the site of the wax pattern. The findings suggest that strain gauges can be used to measure setting expansion internally more accurately than volumetric or external surface measurement techniques. It was also found that winged strain gauges were more stable in the investment than wingless strain gauges.

Visco-elastic deformation of dental porcelain and porcelain-metal compatibility

A computer simulation using a visco-elastic stress analysis was conducted to clarify the effect of the heating rate on deformation temperature of dental porcelain during firing. In this simulation, the following temperature-dependent factors were incorporated: elastic modulus, viscosity, and coefficient of thermal expansion. The cooling/heating rate dependencies of both the glass-transition temperature and the temperature distribution in the slab were also included. Thermal expansion curves of porcelain with an applied load at various heating rates were computed. Effects of the applied stress and the heating rate on the deformation temperature of porcelain were revealed. The results suggest that the temperature where the incompatibility stress develops in the porcelain-fused-to-metal strips during cooling can be estimated closely from the deformation point of the heating curve of the porcelain with an applied stress of about 1.2 approximately 3.1 MPa. A method for measuring temperature dependence of viscosity as represented by an Arrhenius equation is proposed.

Mesh monitor casting of NiCr alloys: Element effects

A mesh monitor has previously been used for quantitative evaluation of the casting of dental alloys. A castability value, Cv was defined. For curve-fitting, a transformed castability value, Cv,t, was used. A series of alloys was selected so that effects of major elements on Cv, t and, hence, on Cv could be determined. Compositions were chosen so that correlated effects would be avoided. Assuming a linear dependence on the concentrations of individual elements, one may use the following equation to describe Cv,t: (Formula: see text) where each term employs an elemental concentration in weight percent, (Ei), and a coefficient, Ki, for the ith term. Because Eis are constant for each alloy, Ki = fi (TA, TM) = gi (TC, TM). The temperature-dependent coefficients, Ki, were determined for seven elements and for the (Ni/(Cr) ratio. It was also found that Si and Be produce a synergistic effect. The results help our understanding of the casting behavior of Ni-Cr dental alloys; this approach may be useful in the design of dental alloys.

Effects of Porcelain/Alloy Interfacial Diffusion Zones on Thermo-mechanical Strain

Chemical bonding between dental porcelains and alloys results from interdiffusion of porcelain and metal ions. An interfacial diffusion zone is created which, most likely, has properties different from those of bulk materials. The changed interface might affect experimental measurements of thermo-mechanical strain. To determine the magnitude and conditions under which this would occur, the interface was modeled as the intermediate layer of a three-layered porcelain-veneered split-metal ring. Layer thicknesses and coefficients of thermal expansion were varied, and the effects on gap change after cooling through 500°C were calculated. Results are presented in a series of 14 Figs., ten curves each, which depict not only interfacial effects, but are extended for use in interpretation of the effects of properties of opaques and glazes as well. Under most conditions, the interface will not affect experimental measurements; some special exceptions are noted.