When is pre-op scanning a good idea in digital scanning?

Pre-op scanning allows you to either use the preoperative tooth as the template for creating the new restoration or simply to overlay it as a means of re-establishing preexisting occlusion. Pre-op scanning is a good idea when the occlusal anatomy and occlusion are ideal. An example may be in the replacement of a viable crown with recurrent decay. It is not a good idea when we are creating a restoration for a tooth with a worn or broken existing restoration.