GLASS/CERAMICS FOR INLAYS AND SHELL CROWNS

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Tooth-colored ceramics are attractive alternatives to amalgam because of their proven biocompatibility, chemical inertness, esthetic appearance, wear resistance, low thermal conductivity and low thermal diffusivity. To ensure fracture resistance, it has been assumed that posterior ceramic restorations must be at least 1.5 mm thick. Recent studies of ceramic crowns luted with resin cement suggest that less tooth reduction may be possible because of improved stress transfer between the crown and resin cement. Finite element stress analyses of all-ceramic crowns indicate that tensile or shear stress levels do not increase appreciably when the crown thickness is reduced from 1.5 mm to 0.5 mm. A method for optimizing the thermal processing protocol has been demonstrated for lithia-based glass-ceramics during the previous period of the Center grant that improves the production capability for desired microstructures. Tougher ceramics, coatings that seal or bridge flaws along the internal surface, and resin cements with high elastic moduli, and improved dentin bonding materials and techniques can greatly reduce the risk of clinical fracture. These improvements may allow much more conservative inlays and shell crown (thin, partial-coverage crown forms) to be designed and tested in vitro and in vivo environments. Furthermore, the use of thinner, smaller inlays and shell crowns will lead to shorter tooth-preparation time, less tooth trauma during preparation, shorter time for removal if required, and conservation of remaining tooth structure. The objective of this Project is to test the hypothesis that optimum nucleation and crystallization combined with flaw control by surface coating and the use of resin cement with high elastic modulus can produce glass-ceramics inlays, onlays, onlays, and crowns of reduced thickness, sufficient toughness, fracture resistance, and opacity to ensure the safety and efficacy of conservative ceramic restorations. Two-dimensional and three-dimensional finite element analyses will be performed to determine compatible elastic moduli for combinations of glass-ceramic and resin cement for shell crown designs to minimize high- risk stress concentration areas. Four novel technological developments will be employed for the analyses proposed: 1) the use of DTA exotherm data to optimize thermal processing schedules of glass-ceramics for microstructure refinement, 2) the use of protective surface coatings and adhesive resin cement with high elastic moduli to minimize the risk of ceramic prosthesis fracture, 3) the use of fractal analysis and fractographic observations to characterize fracture patterns, and 4) the integrated design of conservative, glass-ceramic inlays, onlays, and shell crowns of toughened glass-ceramic for use with specially formulated, high modulus, anticariogenic (controlled release) resin cements. Two classes of glass-ceramic, a chain-silica system and a high- strength barium-mica system, will be refined by compositional and thermal processing control to increase fracture toughness, bi-axial flexure strength, and chemical durability compared with Dicor glass-ceramic, the control material. An alumina-core system for all-ceramic restorations will also be investigated to test generalizability of the coating methods, fractal analyses, and conservative design options. P50DE093070006 Dental composites have not been successful replacement materials for dental amalgams due to poor wear characteristics, marginal leakage, technique sensitivity, and post-operative sensitivity. The main deficiency of these materials is the large volumetric shrinkage as a direct result of polymerization of substituted acrylate-type monomers and the small number of active polymeric chains into the network structure, which is the primary determinant of stress distribution within a composite. Defects, i.e., dangling chain ends, which act as stress intensity factors are created by a number of mechanisms such as chain termination, increasing viscosity, and contamination of the polymerization. The majority of dental composites achieve less than 80% conversion of reactive groups and thus their mechanical and physical properties are largely dominated by defect structures. Another deficiency of dental composites occurs via hydrolysis of the filler-resin interface resulting in degradation of physical properties. The objective of this project is to design and develop an improved polymer based composite which polymerizes with near zero shrinkage and through a synergistic combination of a silane copolymer with a sol-gel derived filler particle exhibits better hydrolytic stability than conventional dental composites. The resins of interest are vinyl cycloaliphatic anhydrides which, either during or following free radical polymerization, can undergo a ring-opening reaction that yields a net specific volume increase of 17%. A novel class of reactive polymers will be the interfacial bonding agents for the coupling of the polymer with the reinforcing phase of the composite. The reinforcing phase will be based upon a sol-gel derived gel glass which has a controlled concentration of surface silanol groups. The specific aims of this project are: Aim 1: To test the hypothesis that the polymerization shrinkage of conventional dental composite resin systems can be partially offset by the hydrolysis of cyclic anhydrides. Aim 2: To test the hypothesis that the mechanical properties of a dental resin system can be enhanced by compositional modifications designed to achieve a glass transition temperature close to but above normal temperatures of the oral environment. Aim 3: To test the hypothesis that the modulus and resistance to wear of copolymers based upon cyclic anhydrides will increase by treatment with a metal ion. Aim 4: To test the hypothesis that the concentration of surface silanol groups of a high silica containing bioactive gel glass, to be used as the reinforcing phase, can be controlled to optimize the hydrolytic stability of the composite system. Aim 5: To test the hypothesis that application of a silane coupling agent to the reinforcing phase via a copolymer or methacrylate type monomers and methacryloxy propyl triethoxysilane will improve the retention of mechanical properties of the composite system after storage in water. Aim 6: Demonstrate the biocompatibility of the optimum composite system.