OPTIMIZED STRESS STATES IN CERAMIC-BASED RESTORATIONS

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Approximately 90% of the fixed prostheses made today are meta-ceramic restoration. Recent metal-ceramic prostheses that are based on porcelain margins, thinner metal copings, resin bonding, supporting implants, and cantilever designs are associated with an increased risk of structural failure. In addition, no method has yet been established to reliably assess the thermal contraction compatibility status of simple biomaterial or trimaterial metal-ceramic strips or more complex, single-unit or multiple-unit protheses. In spite of numerous studies on the thermal contraction compatibility of metal-ceramic systems, no model has yet been developed to predict residual stresses in clinical prostheses based on constituent properties such as thermal contraction coefficients, elastic moduli, and viscosity values. Furthermore, no study has yet analyzed the relative magnitudes or locations of potentially damaging tensile stresses caused by metal-ceramic contraction differences in clinically relevant prosthesis designs so that critical zones of potential failure can be identified. The initial focus of this five-year research program will be to establish practical screening tests that differentiate thermally compatible from incompatible systems and to identify critical zones of residual tensile stress caused by thermal incompatibility in selected prosthesis designs. In addition, potential failure mechanisms will be investigated as well as methods of strengthening ceramic-based prostheses based on analysis of cooling rate variations, thermal tempering treatment, and ion exchange treatment to induce protective compressive stresses that alert the residual stress profiles in the ceramic structures prior to their placement in the oral cavity. Four novel technological developments will be employed for the analyses proposed: 1) a three-dimensional visco-elastic element for finite element analysis of incompatibility stresses, 2) thermal tempering in liquid media to alter residual stress profiles and to reduce tensile stresses in critical-risk areas of metal-ceramic and all-ceramic prostheses, 3) single and double ion-exchange strengthening treatments based on potassium-enriched and rubidium-enriched slurries that are fired at low temperatures over short time periods, and 4) micro-miniature semiconductor strain gages (gage length approximately 100 mu) in combination with PhotoStress coatings that produce photoelastic stress patterns within the surface of loaded prostheses of actual-size models or enlarged-model designs to validate stress profiles predicted by finite element analysis. The proposed research studies will be designed to assess transient and residual stress states caused by thermal contraction incompatibility, to correlate experimental incompatibility date with those obtained from visco-elastic models that have been developed to predict incompatibility stresses, and to develop thermal processing protocols to optimize the resulting stress distributions in these protheses. The overall objective of the proposed research is to develop a predictive model, to analyze failure risk factors for metal-ceramic and glass-ceramic restorations based on analytical and experimental models, and to alter residual stress distributions that will enhance the margin of safety for all-ceramic and metal-ceramic prostheses by control of thermal processing, tempering, ion exchange, chemical etching, and design factors.