Wednesday, January 18, 2012
I have blogged before about the difficulty of restoring a single anterior tooth. Perhaps an even more difficult task to complete is a crown underneath a partial denture. It is especially hard if there is a rest or a clasp on that same tooth. Traditionally, we were required to take the partial denture away from the patient and give it to the lab technician for a few days. There, it would be modified by hand on the model until the perfect fit was achieved. Meanwhile, the patient is walking around without their full complement of teeth. Plus, there is a risk that the patient will continually displace the provisional crown with the clasping on the denture during the time they actually get to keep it.
Model and Partial Denture
I have done a few of these cases using CEREC, and the process is much more simple. Correlation mode allows you to replicate the existing crown, from its contours to the occlusion. The idea is that you are basically making a copy of the crown. There is still the need to adjust a bit with the clasps to ensure the proper tightness, as well as make sure there is proper contact with any guide plane.
My business partner Dr. J. David Dart took a different approach that proved even more simple. In this case, the previous crown had been lost, so there was nothing to scan for correlation. There are some CEREC cases we do in which it is nice to have a model for final adjustments. In this case, he took a traditional quadrant impression of the prepared tooth which would allow him to make sure everything fit together correctly. He then seated the partial denture on the model, and scanned the entire preparation, clasps, and guide planes. The Blue cam allows you to roll around a bit to capture those undercuts from the partial. During the trimming step, he then just cut away everything but the preparation. He was then able to visualize every point of contact on that crown, and dial it in perfectly. Following milling and sintering of the e.Max block from Ivoclar Vivadent, he was able to reseat the partial with absolutely no other adjustments to the crown itself. The crown and the partial denture were delivered back to the patient later that day.
Final With Model
Perhaps the only thing I would do differently next time would be to scan the partial denture in the mouth. This would allow us to more accurately capture the occlusion with the new CEREC 4.0 software. It would also allow us to skip the need to have a traditional model made, even if it was just being used as a backup.