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B2 ◊Page B1 LAB TRIBUNE Dental Tribune Middle East & Africa Edition | 2/2017 Fig. 8: After sintering: smooth colour transition and ideal basic shade for completing the bridge Figs 3a and b: First, the restoration was designed in full contour and then cut back in the visible esthetic region Fig. 9: Comparison between white opaque zirconium oxide (su- perimposed simulation at the top margin) and the Zenostar Zr framework Fig. 4: Nesting of the bridge framework in the CAM software Fig. 10: After the liner and foundation firing… Fig. 5: After milling: high precision result with excellent marginal accuracy (incisal, occlusal) Fig. 11: ... the vestibular anterior surfaces were veneered individu- ally. Fig. 6: Shading the interior crown surfaces and basal surfaces Fig. 12: After final firing: the monolithic crowns did not appear brighter than the veneered crowns Fig. 7: Customized framework prior to sintering Fig. 13: Finished bridge: harmonious shade effects and homoge- neous surface texture restoration, we used it as the basis for the design of the final restoration. The natural wear facets that formed during the temporization period should be reflected in the final res- toration. A conventional impression of the oral situation was taken in the practice. In the lab, a model and a gingival mask were prepared and scanned. First, the working model together with the temporary bridge was digitized. Next, we scanned the model togeth- er with the abutments, the opposing jaw model and the bite registration. Finally, the abutments were scanned individually one after the other be- cause the abutment shoulders were located subgingivally and could therefore not be captured accurately enough with the model scan alone (Figs 2a and b). CAD construction First, the position of the digitized model was defined in the design software according to the common insert direction of the abutments. In the second step, the shoulder lines of the abutments were marked and the thickness of the cement gap was defined. The shoulder line rep- resents the “preparation margin” of the restoration. In this case, we set the cement gap to 0.2 mm and the cement space to 0.4 mm. The thickness of the cement gap at the marginal border was set to 0.1 mm. In our experience, these settings result in an excellent accuracy of fit of the restoration on the model and in the patient’s mouth, eliminating the need for later adjustments. At the end, the design of the restora- tion was checked once more against the individual design parameters. If the wall thickness is lower than the minimum acceptable, the software will issue a warning and enable an automated remediation step. The final restoration was designed using the full-contour long-term temporary as a basis. The full con- tours of teeth 13 to 23 were reduced by 0.9 mm on the vestibular aspect to make space for the partial veneers (Figs 3a and b). The incisal border was left fully contoured as a large num- ber of functional movements occur in this area. The fully contoured shapes of the posterior teeth and the palatal sur- faces of the anterior teeth were left unaltered to ensure a maximum lev- el of strength in the final restoration. There was a risk that the abutments might shimmer through as grey ar- eas. For this reason, we decided to use translucent zirconium oxide. The layer thickness appeared to be adequate to mask the abutments. Milling The completed CAD design divides a basic crown framework into 18,000 to 20,000 coordinates and generates a harmonious surface texture and perfect marginal seal. The completed design was trans- ferred to the CAM unit. We use the V3 CAM version, which gives us the option to choose between various output formats. The Zenocam® 3.2 format is our preferred output op- tion because, in contrast to the open STL format, it delivers information on the specified cement gap, im- plant axes and restoration margins. The CAM software uses this informa- tion to calculate milling parameters that distinguish between the differ- ent areas of the restoration. For in- stance, when milling the restoration margins, the unit reduces the speed, infeed and feed rate to prevent thin crown margins from breaking or fracturing. As a result, even wafer- thin cervical margins having a thick- ness of as little as 0.1 mm can be re- liably milled and require only very little reworking after the sintering process. In less sensitive areas, the unit uses a higher milling speed. After the output format has been entered, the milling strategy is chosen. In this case, a milling strategy using 2.5 mm, 1.0 mm and 0.7 mm burs was select- ed for the manufacture of the bridge. The option of using a 0.3 mm bur was not taken as it was not needed for the restoration in question. Next, the job was placed in a virtual Zenostar® blank (Fig. 4). We decided to use a translucent, pre-shaded Ze- nostar T zirconium oxide disc in the shade T sun, because the posterior teeth from 14 to 16 and 24 to 26 were planned to be restored with mono- lithic zirconium oxide. The warm, reddish shade of this disc closely matches the selected tooth shade and allows the A – D shades to be rec- reated efficiently and reproducibly. Next, a sinter support structure was designed to allow the restoration to be sintered in an upright position in the Programat® S1 sintering furnace. The sinter frame minimizes distor- tion during sintering and is instru- mental in achieving a high accuracy of fit in long-span objects. Finally, the program calculated the milling data in a process that took less than three minutes to finish. Then, the milling operation was started. This process was achieved in a Zenotec select S2 milling unit that features 5-axis operation and an 8-disc material changer (Wieland Dental). The absolute precision with which this unit works is evident in the excellent milling results ob- tained on the occlusal and palatal surfaces and at the incisal edge (Fig. 5). Customizing the framework Once the milling was completed, the framework and the sinter support structure were separated from the disc. At the next step, the unsintered bridge was customized with colour- ing liquids using the infiltration technique. The range of Zenostar Color Zr liquids is perfectly suited for this purpose. These liquids are supplied in the standard shades of the A – D shade guide. Additionally, five Effect shades are available for further customizations. We used Ze- nostar Color Zr in shades A2 and A3 as well as the grey-violet Effect shade. To render the infiltration of the indi- vidual liquids visible, the virtually colourless liquids were mixed with a visualizer (Zenostar VisualiZr). First, the interior surfaces of the crowns and the basal surface were infiltrat- ed; followed by approx. 1 mm of the cervical margin, the fissures and the central areas of the palatal surfaces. Infiltration of all these aspects was achieved with Zenostar Color Zr A3 mixed with yellow Zenostar Visual- iZr (Fig. 6). After that, the dentin area up to the incisal third was infiltrated with shade A2 mixed with red Visu- aliZr liquid. The incisal area of the anterior teeth and the cusps of the posteriors were customized with a diluted version of grey-violet Effect shade and Zenotec Color Optimizer mixed with blue VisualiZr liquid (Fig. 7). It is essential to use a separate brush for each shade. After having been allowed to dry for two hours, the framework was sintered in a Pro- gramat S1 sintering furnace. After the sintering process, the res- toration exhibited an excellent accu- racy of fit, without necessitating any adjustments by grinding, e.g. on the insides of the crowns. The advantag- es of the translucent zirconium ox- ide used were obvious at this stage. Due to the colouring liquids, the cervical and dentin areas were beau- tifully accentuated. The incisal areas exhibited a slight greyish-translu- cent sheen, which should facilitate the subsequent layering procedure. Figure 8 shows the smooth transi- tion of the shades. The simulation in figure 9 demonstrates how difficult it would have been for us to achieve the desired tooth shade if we had used opaque white zirconium oxide for the framework. Despite the high translucency of the zirconium ox- ide, the titanium abutments do not show through the framework. Individual framework refine- ments An optimum esthetic outcome is only achieved if the restoration ex- hibits ideal optical properties. A con- trolled brightness value, adequate saturation and translucency and minimized light reflection are es- sential to achieve a pleasing esthetic outcome. If these parameters are not met, the result will never be satisfactory, even if the restoration is veneered with ceramics. The result would simply be a resto- ration that looks good on the model but appears too bright in the mouth. Anterior area Staining the zirconium oxide prior to sintering is the first measure to control the light reflection effects. Application of a liner is the sec- ond measure. The bridge was ve- neered with IPS® e.max Ceram. As the framework already exhibited a pleasing basic shade, we applied a mixture of IPS e.max Ceram ZirLiner Clear and Incisal (70:30). ZirLiner In- cisal reduces the light reflection of zirconium oxide; alternatively Liner 4 may be used. To mix the liners, IPS e.max ZirLiner Build-Up Liquid was added. The result was a mixture with a pleasing consistency, ensuring an even coating. After the firing process, the restoration exhibited a homoge- neous surface and an adequate level of fluorescence. ÿPage B3