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Distortions in the marginal region during manufacture are detrimental to the long-term success of fixed dental prostheses (FDPs). The aim of the present in vitro study was to evaluate changes in marginal fit because of the veneering process of four-unit FDPs made from different zirconia materials. Two groups of FDPs with 10 specimens each were machined from white body zirconia blanks (VITA In-Ceram YZ Cubes, KaVo Everest ZS-Blanks) by means of computer-aided design/computer-aided manufacturing systems and subsequently sintered to their final density. The marginal and internal gaps of the frameworks were determined using a replica technique. Afterwards, frameworks were veneered with the recommended ceramics (VITA VM9, GC Initial Zr), and fitting accuracy was evaluated again. Statistical analyses were performed by Student's t-test with the level of significance chosen at 0.05. With one framework/veneering system (Everest ZS-Blanks/Initial Zr), both marginal gap (P = 0.019) and internal gap (P = 0.001) at the premolar retainer were significantly affected by the veneering process. The resulting distortions were directed towards the lumina of the retainers and the mean values of the measured gaps decreased by about 23.8 mum (marginal gap) and 27.3 mum (internal gap). For the other system (In-Ceram YZ Cubes/VM9), no distortions because of the veneering were found. The veneering of zirconia restorations may lead to significant changes in the marginal fit, while the combination of core and veneering material used influences the resulting distortions.

From glasses with the composition 51.9 SiO 2 ·21.2 MgO·21.2 Al 2 O 3 ·5.7 ZrO 2 (in mol %), high and low-quartz solid solution glass ceramics were formed upon heat treatment at 950°C for different dwell times. A combination of X-ray diffractometry, transmission electron microscopy, and scanning transmission electron microscopy with annular dark field imaging and energy-dispersive X-ray analysis revealed the temporal evolution of the volume crystallization. It is found that the crystallization of high-quartz solid solution is nucleated either directly at star-shaped ZrO 2 crystallites or within a zone of Zr-depleted glass surrounding the latter and proceeds isotropically into the sample volume, thereby expelling secondary, circular ZrO 2 precipitates. With advancing annealing time, the quartz solid solution is depleted in Mg and Al since indialite (Mg 2 Al 4 Si 5 O 18 ) and spinel (MgAl 2 O 4 ) are formed as well, thus enabling a change from high-quartz solid solution to the low-quartz modification while sample cooling. Upon analyses of samples heat treated for different soaking times, different states of crystallization were observed. Using electron diffraction and nanoanalytical techniques, a growth model with special emphasis on the early stages of volume crystallization was developed. INTRODUCTIONGlass ceramics based on the MgO−Al 2 O 3 −SiO 2 (MAS) system are biocompatible 1,2 and show excellent mechanical properties, 3−17 such as a mechanical strength of up to 450 MPa, high Young's modulus (up to 140 GPa), and high hardness (Vicker's hardness of up to 14 GPa). These properties depend on the composition of the base glass and on the type and concentration of additives, that is, their effect on nucleation and crystallization. In previous papers, the effect of the nucleating agents TiO 2 , 5−9 ZrO 2 , 8,10−12 or a mixture of both 13−15 was reported, and the effect of the partial replacement of MgO against ZnO 9,15−17 was studied as well.In order to achieve good mechanical properties in MAS glass ceramics, it is quite essential to control the precipitation of crystal phases possessing large thermal expansion coefficients, such as the low-temperature quartz modification (α-quartz) and spinel (MgAl 2 O 4 ), by a tailored annealing procedure. In the MgO−Al 2 O 3 −SiO 2 system, both the annealing temperature 5 and the annealing time affect the concentration of MgO and Al 2 O 3 in the quartz (solid solution) crystals. 18 At comparably low annealing temperatures, β-quartz-solid-solution, with a structure similar to the high-temperature modification of quartz (β-quartz), is formed. This modification does not transform to the low-temperature modification of quartz upon cooling the sample to room temperature because it is stabilized by the incorporation of equimolar concentrations of both MgO and Al 2 O 3 in the crystal lattice. Increasing the crystallization temperature or increasing the crystallization time leads to the depletion of MgO and Al 2 O 3 . The quartz crystals then possess

The aim of the present in vitro study was to evaluate the influence of different processing routes on the fitting accuracy of four-unit zirconia fixed dental prostheses (FDPs) fabricated by computer-aided design/computer-aided manufacturing (CAD/CAM). Three groups of zirconia frameworks with ten specimens each were fabricated. Frameworks of one group (CerconCAM) were produced by means of a laboratory CAM-only system. The other frameworks were made with different CAD/CAM systems; on the one hand by in-laboratory production (CerconCAD/CAM) and on the other hand by centralized production in a milling center (Compartis) after forwarding geometrical data. Frameworks were then veneered with the recommended ceramics, and marginal accuracy was determined using a replica technique. Horizontal marginal discrepancy, vertical marginal discrepancy, absolute marginal discrepancy, and marginal gap were evaluated. Statistical analyses were performed by one-way analysis of variance (ANOVA), with the level of significance chosen at 0.05. Mean horizontal discrepancies ranged between 22 μm (CerconCAM) and 58 μm (Compartis), vertical discrepancies ranged between 63 μm (CerconCAD/CAM) and 162 μm (CerconCAM), and absolute marginal discrepancies ranged between 94 μm (CerconCAD/CAM) and 181 μm (CerconCAM). The marginal gap varied between 72 μm (CerconCAD/CAM) and 112 μm (CerconCAM, Compartis). Statistical analysis revealed that, with all measurements, the marginal accuracy of the zirconia FDPs was significantly influenced by the processing route used (p < 0.05). Within the limitations of this study, all restorations showed a clinically acceptable marginal accuracy; however, the results suggest that the CAD/CAM systems are more precise than the CAM-only system for the manufacture of four-unit FDPs.