A Sintered Dental Zirconia
The ceramic is sintered and the framework contracts to the final dimension. The non-sintered zirconia blanks result following a cold pressing process that compacts the zirconia powders. In this way we obtain a very small pore size and a good distribution of the components within the blank. The next step is machining by immersion in solutions of various metal (cerium, bismuth, iron or a combination thereof) and the coloring of the restorations. As it goes through the last sintering phase the color is developed. The solution concentration has a direct impact on the final shade. A satisfactory coloration can be obtained using concentrations as low as 0.01mol%. For a good result we must follow the production instructions as the final sintering temperature influences the color obtained. The zirconia framework acquires its final mechanical properties at the end of the sintering process when it suffers a contraction at about 25%, which means it returns to its correct dimensions. In order to optimize the fitting of the restoration it’s imperative to know the exact volume shrinkage information for every zirconia blank block. The vast majority of blocks have barcodes that give information regarding the density of the milling block to the computer and so we can adequately mill the framework oversize.
Milled oversized zirconia framework, detached from the blank, and the sintering is mandatory in the final stages to eliminate any stress induced by the surface milling action and to achieve the proper density. Previously sintered zirconia structure is adjusted in order to have a proper shape for ceramic veneering
In order to achieve an aesthetic appearance of the zirconia ceramic restoration a multilayer covering technique is used for veneering zirconia framework with compatible ceramics. Layering ceramics on zirconia framework
Figures presented above are part of a zirconia ceramic restoration manufactured in the dental laboratory. The manufacturing process took a few days due to specificity of materials used and fabrication. Through CAD/CAM implementation this final dental restoration was done in an effective way due to prompt framework digital design, precise milling and adjustment of zirconia framework. After veneering, the obtained zirconia ceramic restoration met all aesthetic requirements mainly thanks to zirconia framework properties.
Compared to the alternative methods, milling full blocks of sintered zirconia takes a lot of time. This is costlier because requires a more frequent change of the diamond burs. That is why non-sintered zirconia can be considered a more convenient solution
In the beginning, the application of high-crystalline zirconia in dental restoration was limited to substructures because of its high opacity. Veneering is usually applied over zirconia cores to instate a more natural appearance. Regarding veneered-zirconia restorations porcelain compatibility is a concern. The latest studies regarding ceramic restorations report a large amount of porcelain chipping, cracking, delamination and fractures.
The most important downsides reported on zirconia by most studies refer to the wear and even clinical failure of the coating material and not so much to the resistance of the supporting structure. The fractures of the coating material also known as chipping are reported as the most frequent clinical problem, regardless of the applied zirconia veneer system. The fracture rates of the coating (veneer) are at 2-9% for crowns after 2 to 3 years and at 3-36% for dental bridges after 1 to 5 years. It is very important that the whole system zirconia core with ceramic coating to have a good stability in time.