When you have preops that you are trying to stitch to preps in medit, and this could also include relating scanbodies to arches as well, you need enough data that is redundant in both catalog boxes
if you watch this video carefully, you can see how i over trimmed the mesial part of the equation and even though the camera is active, it is not acquiring images. you can see the red box.
once i move the camera to the distal molar, the acquisition starts because the molar has more data points than the premolar and the software / camera recognized the area and started to acquire images.
understanding this will unlock a lot of complicated cases and make the easier for you
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