Porcelain-fused-to-metal Crowns - Posting #6

Ceramometal crowns require careful attention to the desired form of the final crown, especially for the anterior teeth, and the thickness of porcelain required for adequate esthetics, especially for older patients, where the enamel is more transparent.

The key to the preparation is the facial reduction, which is frequently done incorrectly, so that the lab tech must either overcontour the facial surface of the crown, or make the porcelain thin so that it has a poor appearance.

The evaluation of the facial reduction requires that you check the axial reduction at the gingival margin, AND check the taper in relation to the adjacent teeth whose contours the crown will be designed to match.

In order to evaluate the facial axial reduction at the gingival margin, you can do it without caring about the margin form. For example, if you use a horizontal shoulder, an angled shoulder, or a shoulder-bevel (no longer desirable for anterior teeth in the esthetic zone), you can see the axial reduction (horizontal distance from the gingival margin to the axial wall) by looking gingivally down the facial wall and measuring the apparent width of the shoulder from this viewing direction. This viewing direction cancels out the effect of angulation so you get the actual axial reduction automatically. We'd like to get approximately a 1.0 mm axial reduction uniformly from the mesial to distal facial embrasures.

From the gingival shoulder to the incisal edge, we need to make certain that the angulation is correct on the facial surface. You can guarantee that this will be appropriate by comparing the facial taper to that of the middle third of another adjacent tooth, that the crown will match. This should be the angle of the facial reduction for the gingival 2/3 of the surface to the shoulder. The incisal 1/3 will typically have another angle, matching that of the incisal 1/3 of the comparison tooth - referred to as a secondary plane, recognizing that the curved surface of the facial contour will require two planes for uniform axial reduction from gingival to incisal.

The most common error in PFM preparations is to allow the shoulder to remain horizontal into the proximal embrasures, mesially and distally. The problem is that this margin will sometimes be in enamel, and if it is the enamel will be undermined due to the natural incisal divergence of the enamel contours at the margin. Often I have seen preparations done by students in patients that, after removal of the provisional crown, before cementation of the permanent restoration, the gingival enamel was found to be chipped and broken, leaving deficient margins in these areas. It is always best to angle the shoulder gingivally with the end of a shoulder diamond so that the EXTERNAL angles of porcelain AND the enamel are closer to 90 degrees!

The esthetics of shoulder-bevel margins are not acceptable for anterior teeth these days, because of inadequate gingival placement or thin tissue biotype to which dentists don't pay enough attention. For posterior PFMs out of the esthetic zone, the shoulder-bevel can be a nice approach for the facial reduction.

We can discuss other types of porcelain crowns, including full ceramic crowns and CAD/CAM crowns. There are advantages and disadvantages to each, and they can be subtle. How one does a FCC prep on a typodont can be a challenge and I can go into this in detail.

For comments or questions about any of these issues, I will love to have you reply to this posting and I'll answer or discuss anything that concerns you. Be sure to check out our programs at www.dentistCEprogram.com