Richard Rosenblatt I’m going to pass on a tip that I learned from my good friend and colleague Mark Fleming many years ago at a cerecdoctors.com course about prepping an inlay or onlay. Inlays and onlays can be one of the toughest restorations to do with CAD/CAM. The reason this is so is that the passive seating of the restoration is very dependent on an ideal prep. Doctors have the tendency to prep with undercuts when doing inlay or onlay type restorations. If the prep has an undercut in the area of the proximal box, it will lead to an open margin at the interproximal wall. The other problem that occurs during the prep is that doctors try to be so conservative during the prep of an inlay that we start from the internal of the tooth when prepping and then work their way to the interproximal. They are using a cylindrical shaped bur and when prepping from inside the prep and then extending to the exit wall, they find themselves creating a S-Curve on that exit wall due to the shape of the bur. The milling unit is not a fan of prepping S-Curves. It can really struggle with this. These preps need straight exit walls at the interproximal. This is where the fantastic tip from Dr Fleming shines. When prepping an inlay or onlay, start prepping from the outside of the tooth at the exit wall and move into the tooth. This will create a straight line and it will be easier to angle the bur in a way to make that wall divergent so you will not have an undercut.
Below is a photo of a case that I did last week. I prepped the occlusal aspect but did not break contact from the internal. I actually move the bur to the outside of the prep and prep inward to connect things. Once I prepped #30, doing the prep for 31 MO was much easier. Look at the screen shot of the preps. The exit walls are divergent and there are no S-curves. This will help to insure a fantastic fitting restoration and great contours that are hard to create when doing things direct.
