Monolithic and semi-monolithic translucent zirconium-dioxide restorations : aspects on design, material and strength
Abstract: Although many clinical reports have shown high rates of clinical success associated with fixed dental restorations made of traditional zirconium-dioxide (zirconia), clinical failure due to improper design aiming to achieve high strength and optimal aesthetics, still occurs. Previous clinical and laboratory studies indicated that veneering porcelain and the connector represent the weak parts of the fully-veneered zirconia restorations where failure may occur. Although the clinical performance of such restorations has recently been reported to be comparable to metal-ceramic restorations, more improvements in design are still required. Restorations with monolithic design made of modified translucent zirconia materials offer an excellent solution for these clinical problems. One of the advantages of monolithic restoration is that such restorations can be prepared without the weak veneering material. Thus, this restoration design has a much higher load-bearing capacity compared to the veneered restorations since it provides additional space for the high-strength zirconia material. With regard to strength and aesthetics, the translucency of the former generation of monolithic translucent zirconia, which comes with equivalent mechanical properties to traditional zirconia, is insufficient. Recently, monolithic zirconia with high translucent properties was developed for highly aesthetic clinical uses. These new systems of translucent zirconia materials have limited capacity in terms of fracture strength and fracture toughness properties. Further, earlier studies have shown doubtful aging stability for these materials. Maintaining well-known strength properties of zirconia restorations while providing high aesthetic outcome is the ultimate goal for dental restorations such as single crowns (SCs) and fixed dental prostheses (FDPs). The optimum design for restorations made of the former generation of translucent zirconia could help to prevent the risks associated with bilayered restorations and overcome the limitations of high-translucent monolithic restorations. Based on clinical needs and previous clinical observations, the overall aim of this thesis was to evaluate translucent zirconia restorations regarding the effect of design modifications, used to enhance the aesthetics, on fracture resistance. In the first three studies, I, II, and III, all the SCs and the FDPs were artificially aged and loaded to fracture. Fracture mode analysis in the different studies was performed visually and microscopically. In study I, fracture strength and fracture mode of veneered translucent zirconia SCs designed with different porcelain layer thicknesses were evaluated. The outcomes showed that translucent zirconia SCs can be veneered with minimal thickness layer of 0.5 mm porcelain without showing significantly reduced fracture strength compared to traditionally veneered (1.0-2.0 mm) SCs. Fracture strength of micro-veneered SCs with a layer of porcelain (0.3 mm) is lower than that of traditionally veneered SCs but still within range of what may be considered clinically sufficient. Porcelain layers of 2.0 mm or thicker should be used only where the expected loads are low. All the SCs in groups 2.5 and 2.0 and more than 80% of the SCs in groups 1.0, 0.8 and 0.5 showed cohesive fracture mode. Conversely, there were significantly (p≤0.05) more complete fractures in group 0.3 compared to all other groups. Study II described different designs of partially veneered monolithic (semi-monolithic) SCs made of translucent zirconia and evaluated the effect of those designs on fracture resistance and fracture mode of SCs made of two generations of translucent zirconia materials. The results demonstrated that translucent and high-translucent zirconia SCs might be used in combination with a 0.3 mm micro-coating porcelain layer with semi-monolithic design to enhance the aesthetic properties of restorations without significantly decreasing fracture resistance of the SCs. If a 0.5 mm porcelain layer is needed for a semi-monolithic SCs, wave design or cap design might be used to increase fracture resistance. The SCs made of translucent zirconia showed higher fracture loads compared to those made of high-translucent zirconia, regardless of design. All monolithic SCs, semi-monolithic SCs with 0.3 mm buccal veneer (100%), and all but one of semi-monolithic SCs with cap design (95%) showed complete fractures. Semi-monolithic SCs with wave design and semi-monolithic SCs with a 0.5 mm buccal veneer showed (70% and 55%, respectively) cohesive veneer fractures. Study III investigated the load-bearing capacity and failure mode of monolithic zirconia FDPs with different connector designs gained by using different embrasure shaping methods. The results showed that sharp embrasures and interproximal separations made with diamond discs significantly decrease the load-bearing capacity of monolithic zirconia FDPs compared to monolithic FDPs made with blunt embrasures (p<0.001). Blunt embrasures in combination with localized porcelain build-up produce monolithic FDPs with high load-bearing capacity in relation to loads that might be expected under clinical use. Fracture mode of the FDPs fabricated with sharp embrasures, and interproximal disc separations differed significantly compared to the FDPs with no occlusal embrasures, the FDPs with blunt embrasures, and the FDPs with interproximal porcelain separations (p < 0.001). Finally, study IV in this thesis aimed to evaluate the influence of the framework designs on the stress distribution within tooth-supported semi-monolithic FDPs made of translucent zirconia material under simulated loads using a three-dimensional finite element analysis (3D-FEA). Simplified 3D solid models of prepared abutment teeth with different 3-unit FDPs based on the designs were created. The designs of 3-unit FDPs included monolithic zirconia, semi-monolithic zirconia with 0.3 mm veneer thickness, semi-monolithic zirconia with 0.5 mm veneer thickness, semi-monolithic zirconia with 0.5 mm veneer thickness supported with cap design, and semi-monolithic zirconia with 0.5 mm veneer thickness supported with wave design. The elastic properties of the components (bone, dentine, cement, translucent zirconia, and veneering porcelain) were gained from the standard references for FEA. Simulated static loading force (300 N) was applied at 10° oblique direction over six points in the occlusal surfaces of the FDPs. Maximum principal stress, shear stress, and safety factor were calculated and analyzed among the different models. The findings confirmed that framework and veneer designs play a significant role in the stress distribution of the partially veneered zirconia FDPs under loading. The FDPs with zirconia frameworks with cap design minimize the maximum principal tensile stress in the veneering porcelain. The FDPs with 0.3 mm-veneering porcelain show low maximum principal tensile stress in the veneering porcelain, but the highest maximum shear stress at the zirconia-veneer interface. The FDPs with wave design of zirconia frameworks minimize the maximum shear stress considerably.
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