Are you aware that if you place all-porcelain restorations chances are that at least some of them are milled?
Do you know which of your all-porcelain restorations are mills or presses? After posing that question you may be asking "who cares?" It comes down to the different limitations of each method, and this is something to be aware of if you desire the best fitting, longest lasting restorations.
While both methods are limited by properties of the restorative material being used, a critical thing to know about milling is the diameter and shape of the instruments being used for our chosen restorative material. Where this is key, particularly in regards to the internal of our restorations, is in understanding that the burs being used to mill the restoration must reach every portion of the restoration.
This means if there are areas that are smaller than the tip of the milling instrument(s) one of two things can happen. The first is the instrument(s) will have to go deeper to completely mill out these areas which while it will allow the restoration to fully seat, will result in a less precise fit. The other possibility is that the milling instrument(s) stop short of completely milling out these small areas resulting in a restoration that binds and does not fully seat. In my experience the best way to manage this is by using burs of a slightly wider tip diameter when finishing my preparations and assuring that I'm able to "flow" these instruments into all areas of the preparation.
If we think about pressing, our labs can press to more irregularities than they can mill to; however, it does not mean they should press to these irregularities, because it doesn't mean that they will necessarily provide the best long-term survival. What both milling and pressing have in common is that the smoother and more flowing our preparations, the better our restorations will fit and last. Additionally, given the proper preparations, both methods can result in excellent restorations. Now if you are getting milled restorations from your lab, have a conversation about the instruments that are being used to mill them.
John R. Carson, DDS, PC, Spear Visiting Faculty. [ www.johnrcarsondds.com ]