Sameer Puri D.D.S. 
One of the keys to a successful CEREC restoration is the prep. There is no doubt in my mind that if your prep is not adequate, you will be disappointed with your final margins.
The difference between a good restoration and a great restoration is the quality of your preparation. Having said that, the most critical part of the prep is finishing your margins. You can leave the internal rough and you will get a bit of overmilling. However if you leave your margins rough, you will have open margins or a restoration that doesn’t seat. My recommendation is to smooth out every single preparation at the end with a finishing diamond. If you want clean, pristine margins, you need to provide the machine with a clean pristine preparation.
There are a lot of great burs out there. Here is what I have been using lately – the 847-016 Fine Grit Modified End Taper from Coltene Whaledent. This is from the Alpen series of diamond instruments. Frankly, you can use any of the available burs out there, just make sure you use something similar in shape so that you don’t create sharp points in the internal of your preparation.
Every margin should be glass smooth – a finishing diamond as the last step in your preparation will help you achieve excellent marginal fit.
