Rich Rosenblatt 
There can be lots of reasons. The first reason that I tell all users that they don't have to do crowns in one visit is when they first get their CEREC unit. It can be very frustrating when you are adapting to a new technology and having to learn how to schedule appointments in a different way. It definitely takes some getting used to, and I recommend prepping and imaging the case and getting a temp on there. This way you can design the case on some down time and not turn your schedule upside-down that day.
Today in my office was a second reason that I would not seat a case on the same visit. This was a case where the patient had some interesting dentistry done. Many years ago she must have had a large carious lesion on tooth #30. The doctor used pins and composite to do the build-up, but it seems as if the pins were not going down the long axis of the tooth. See the attached PA and bitewing. She was having sensitivity to cold and a bit to hot, and I was not sure if those pins were going into the pulp or not. The 2-D films made the situation look worse than the actual case. (This is where cone beam would have been great!) Once I finished prepping, the margin was relatively deep on the lingual. I don't believe that crown lengthening is necessary, but if it does need to happen it will make it easier to access the area if the crown is temporarily cemented. I mainly use e.max in the posterior, and trying to cut them off once they are bonded on is just not fun!
I decided on this specific case to do use e.max and have her wear it temporarily and see if the symptoms disappear and the tissue responds positively. The pins were not going toward the pulp, but toward the CEJ, so I'm hoping we will be ok. If in about 4-6 weeks her symptoms are gone, then I will remove it and bond it permanently.
I don't believe there is a right or wrong way to do this, but it is generally how I would handle a case of this nature. What would you do?
