Most restorative dentists, myself included, have over adjusted a contact on a porcelain crown when trying it in. While this doesn't happen often, when it does it's extremely frustrating because we're unable to deliver the crown. It also means we have to send the crown back to the lab and reappoint the patient.

There is another option though. If you have the right equipment and supplies you can actually add the contact back yourself. If you're a CEREC user you likely already have a porcelain furnace with a vacuum. If this is the case, then all you need is some add-on porcelain, modeling liquid and a brush.

If you choose to go down this path the first two things you need to know is the coefficient of thermal expansion (CTE) and the firing temperature for your porcelain. Once you know these two parameters you can then pick your add-on porcelain. When picking your add-on material it must have the same CTE and a lower firing temperature than the material you are adding to. If you're a CEREC user nearly all the materials you use (including eMax) have a CTE of approximately 9 and a firing temperature of 820 degrees Celsius or higher. Based on these numbers, one material you can use is VM-9 Add-On from Vita which has a firing temperature of 780 and a CTE of 9. If you're lucky (like me) your lab will also use a porcelain that is compatible with these parameters and one material will be all you need.

Once you have your material selected you simply mix some of it with modeling liquid (or distilled water) until you have a consistency you can handle, and then apply. It is critical that you add the powder to the liquid and that you not let it dry out. If it does dry out, it's best to discard the mix because you'll end up with a porous low quality porcelain after it's fired.

It's important to be aware that you must add to a clean surface. Also, the porcelain you are adding will shrink when it's fired so you must over-build the area you are adding. While you can re-glaze the area, it's typically not needed as you can typically achieve the desired finish with polishing alone. Finally, if you're going to try adding a contact to a crown made by your lab, I strongly suggest you talk to them first.

John R. Carson, DDS, PC, Spear Visiting Faculty. [ www.johnrcarsondds.com ]

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