The diamond burs of two dental CAD/CAM systems (GN and CD) were examined if they could be used to fabricate up to 21 ceramic full crowns without fracture.After one, 11, and 21 machining times, the surfaces of the diamond burs were observed and the number of particles captured on SEM pictures was counted.The average surface roughness of the crowns was also measured.All diamond burs could be used to fabricate 21 ceramic crowns without fracture. A significant decrease in the number of diamond particles was found on the surfaces of GN burs after 11 and 21 machining times, but not on those of CD burs. The average surface roughness of GN crowns significantly increased with increase in the number of machining times. A significant positive correlation was found between the average surface roughness and the number of diamond particles.
The purpose of this study was to investigate the effect of repeated machining up to 51 times using the same diamond bu: on machining accuracy of inner and outer surfaces of CAD/CAM (computer-aided designing and computer -aided manufactur ing) machined ceramic crowns. The surface topography of machined crowns was examined using photographs . It was fount that machining accuracy was not affected by the number of machining times . In all measuring points, the inner surface wa; machined to a dimension larger than the die model (i.e., increased gap) , whereas the outer surface was machined to a dimen sion smaller than the crown model (i.e., smaller crown) . Photo observation showed that cervical contour was machined it a clear, rounded form from 1st to 11th crowns.
Full crowns were fabricated from three different materials (titanium, ceramic, and resin composite) at different rest diameters using a dental CAD/CAM system, and then their marginal discrepancy measured. The deflection of cylindrical specimens at 500gf was measured using the same materials and diameters as for the accuracy measurement. Marginal discrepancy decreased as rest diameter increased, and at the same rest diameter the titanium crown had lower marginal discrepancy than ceramic and composite crowns. A significant correlation was found between marginal discrepancy of the crowns and deflection of the materials. This correlation was aptly expressed in the regression equation, MG=4.54DF+53.9, where MG represented marginal discrepancy and DF represented deflection. This equation can be used as a good measure for new materials which have different modulus of elasticity.
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