Effects of Sintering Shrinkage on the Marginal and Internal Fit of CAD/CAM-faricated Zirconia Frameworks

Problem:

Even thickness of continuous copings (#11 &12) produced good fitting. Five unit bridge (#6 & 10) with small pontic (P3) produced good fitting too. But entire bridge with large pontic area (P1 & P2) failed to seat positive with mesial sides (A & B) open and distal sides (C & D) tight.

Reason:

Marginal gaps on the pontic side were significantly larger than those on the non-pontic side. When the distance between abutments become larger the discrepancy between the pontic and non-pontic sides also become larger.

Drop the high temperature on the firing cycle by 40 degrees C. Evaluate the outcome. Continue to drop the temperature until you achieve the desired chroma strength and translucency.

Note:

You will not diminish the strength of the zirconia, which is locked in before you reach 1400 degrees C.

The most commonly used strength is the 70% level. The lower the % the darker the chroma

Apply the White Opaque Effect Liquid on the inside of the crown. See opaque instructions for use (IFU) for more details.

At least 9 square mm. Less than may work but it could be courting failure.

We recommend no thinner than .7mm.

Consider the white opaque if you just need mask abutments or dark stumps. The easiest way s to use our high strength LIVE zirconia. Due to its grain structure, it disperses light differently and appears somewhat less translucent than Beyond Plus. If you only need an opaquey gingival 2/3rds, do a micro cutback on the incisal 1/3rd and lay some incisal there. It looks really good with the LIVE.

A sintering program is one of the greatest influences on a restoration's final chroma strength and accuracy.

Not only is the high end temperature often the culprit in 'bleaching' the shade, the length of the cycle may also lower the chroma strength.

It's important to know the actual temperature in the chamber and not just rely on the sintering oven's digital display. This is true across many different manufacturers and oven lifespans. Even new ovens may benefit from validating the sintering program's high temperature. B&D Dental Technologies offers high temperature process control rings also referred to as 'oven calibration rings'. While these are not an absolute, definitive, scientific measurement of the muffle's highest temperature, they do provide a good gauge and reality check for your oven.

The White Opaque effect liquid is designed to preserve translucent incisal esthetics while masking the cervical portion of restorations that may be shade compromised due to underlying metal or extremely dark dentin. The need for masking is often exacerbated by a crown design requiring very thin axial walls.

The white opaque liquid is available in three levels of intensity or concentrations: 1, 2, and 3. 1 is “softer” and 3 is the most intense. Level 2 is the most common.

The timing of the liquid application depends on whether you are milling unshaded or pre-shaded discs:

• If you are using pre-shaded zirconia, apply one coat of the white opaque liquid to the inside of the crown once it has been cleaned and is ready to sinter.

• If you are using unshaded zirconia, clean the disc, color it and then, this is critical, DRY it thoroughly before applying one coat of the white opaque liquid. If you apply the white opaquer before you color, the wetting of the colorant will diffuse the white opaquer through the crown.

In both cases, it is recommended that you ‘warm up’ the prepared crown with a heat source such as a heat lamp, toaster oven or even a hair dryer. The purpose of slightly raising the crown temperature is so that the opaque liquid dries instantaneously on the surface and avoids any capillary action that may pull it to the facial surface. Use a fairly small brush, perhaps a #1 round sable. I put a drop of white opaque on my staining tray and paint it from there. When you apply it to a dry surface it will slightly absorb into the surface but not float around and raise the value excessively. Apply only one coat. Applying more than one coat will likely result in a visible line at the end of the prep.

Sinter normally once the white opaquer has been applied. The intaglio surface post-sintered may look a little different but the observable surface should be opacified enough to block out most anything. Application of one coat will not add any substantial thickness to impact the fit.

Finally, we recommend that you test the white opaque on a scrap piece of zirconia and keep it as a kind of shade guide for future restorations.

To extend its shelf life, we recommend refrigeration to reduce temperature and humidity. Keep out of direct sunlight and away from heat to minimize evaporation. Depending on use, it may congeal before it is fully consumed.

NOTE: The white opaque liquids may also be used to create hypo-calcification spots on the outside of the crown.