UBC Theses and Dissertations
Evaluation of distortion of monolithic zirconia crowns under the influence of different preparation designs and sintering techniques Ahmed, Walaa Magdy
Zirconia has seen a marked increase in its use in dentistry. The sintering of soft milled zirconia is accompanied by high shrinkage, approximately 20–30%. Sintering shrinkage is usually estimated as a single value for each blank, and manufacturers do not provide information on how shrinkage percentage is calculated and whether the estimated shrinkage percentage is based on linear or volumetric changes. In order to compensate for sintering shrinkage, the dimensions of the milled frameworks are enlarged by an appropriate factor, which supposedly corresponds to the estimated shrinkage upon sintering. Since dimensional changes are unavoidable during the processing of zirconia, the purpose of this series of studies was to understand how different preparation designs may affect dimensional changes during sintering, especially when using different sintering protocols, and how that would affect the fitting of the crown (Chapter 1). A systematic overview of how altering sintering protocol could affect the microstructure, mechanical and optical properties of zirconia material was conducted (Chapter 2). Then a systematic review of literature on the factors affecting the marginal fit of zirconia crowns was assessed qualitatively (Chapter 3). Subsequently, the effects of different preparation designs and sintering protocols on the marginal fit of zirconia crowns were investigated (Chapter 4). Afterward, the linear and volumetric dimensional differences between the virtual, milled and sintered copings as a result of the two different sintering protocols were measured (Chapter 5). Our search demonstrated that fast sintering improved the optical properties of zirconia but decreased its flexural strength. There was a lack of studies investigating the effects of different sintering protocols on marginal fit and dimensional changes of zirconia prostheses. There was a significant interaction between the crown thickness, finish line width and sintering protocol on the marginal fit of zirconia crowns. There was also a significant interaction between the coping design, processing stage and sintering protocol on linear and volumetric dimensions of zirconia copings. The combined outcome of this series of experiments allowed the proposition of the ideal combination of design and sintering protocol that results in minimal distortion and improves fitting of zirconia crowns.
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