Close-up work from a distance requires long focal lengths. The longest macro focusing lens I'm familiar with is 300mm, and I don't think it would be enough for what you propose. Another possible solution might be something on the order of a 500mm lens with macro extension(s).
I would think the most practical solution for your situation would be a fiber optic camera, such as those used for inspecting the insides of rifle bores and small tubing. Since the electronics are all in the camera, the fiber optic part can be sterilized separately. This would require the end of the cable being very close, such as inside the patient's mouth.
You didn't say if the pictures were going to be still shots or video, but the principle is the same, though for still shots, you could have the dentist move his tools and hands out of the way.