I've heard this too many times to ignore from former Redcam users who have now upgraded to the Omnicam. That's why I feel compelled to address this issue with this blog. They have issues with the fit of the restorations most likely because someone in the past told them that they should prep their inlays with undercuts.  I've never liked this technique in the past and I like it even less with the Omnicam.

If you look at the picture below, this is what kind of prep you should see when you image. I can see the box forms, a clear margin on the cervical, and all of the internal aspects of the preparation.

When you prep like this and you can see everything and capture those images with the Omnicam, your restorations will fit like a glove. If you have voids or porosities in your prep, then obviously you haven’t captured all the information.

In essence, this is what the software will do when you have an undercut on the internal - it will try not to fill in the undercut. But depending on the placement of the undercut, this could lead to trouble with the fit. You see, anywhere on the box form axial walls, the software will actually try to mill into that space. See the photo below. I essentially took a hand piece and made a gouge in the wall.  Because its on a box form near the external part of the preparation, guess what - the software will try to mill into that undercut. The photo couldn’t be any more clear. Now if that undercut was on the internal aspect, probably not a big deal as the software will try to ignore those. But if you have been using CEREC for any period of time, you know that the software doesn't always behave the way its supposed to.

This is my point - make your life easy and predictable, avoid all undercuts in a preparation as you will have better, more consistent results. If you have undercuts for any reason, try to limit those to the internal part of the preparation. If undercut on the axial wall, the machine will try to mill into those undercuts. And problems will occur.....like an inlay not fitting!!!!!