One of the most important steps in the design of any monolithic restoration is proper functional adjustment during design using the 3Shape virtual articulator. This is especially true for zirconia as this material ideally should not be ground on after sintering… or at least ground on as little as possible to avoid damaging the structure potentially leading to premature failure.
The first thing to understand is exactly where the Maxilla should be. Figure 1 shows the anatomically average position of the maxilla according to Bonwil. As you can see, the incisal plane is parallel to the interocular line, the maxilla is positioned midway between the mandible and the Frankfort Plane, and the maxilla leans downward at a 10 degree tilt from posterior to anterior.
Unfortunately, the initial placement of scanned models into the articulator is NOT correct (figure 2). While 3Shape DOES offer a system that automatically record the position of the maxilla by using calibration objects, it is articulator specific and is not consistently accurate in terms of occlusion so I no longer use this approach. Suffice to say the reasons for this are beyond the scope of this blog.
Four things need to be done to correct the maxillary position.
1) Center the maxillary occlusal plane between the bows by raising it about 12mm (figure 3).
2) Tilt the maxilla downward to about 10 degrees (figure 4).
3) Move the maxillary central incisal edge to about 110mm from the condyles (figure 5)
4) Ensure hat the occlusal plane is parallel to the articulator bows (figure 6).
Once this is done, you can save and name the placement algorithm by clicking the “Save” icon in the Virtual Articulatin dialog box (figure 7). In the future, you can automatically perform the placement by clicking the placement icon right next to it. Be aware that this doesn’t always work perfectly and you may need to tweak the resulting maxillary placement.