Woah, that's the first time I've heard of such a thing. Is there a particular reason as to why your dentist wants to submit a handwritten LOR? Anyway I think it's still possible, since you can get your evaluator to upload a pdf (doesn't have to be a word document).
Definitely call the schools you're applying to. Chances are most won't care although it'll raise some eyebrows.
Maybe because OP's dentist is a man/woman of the older generation who does not feel comfortable using a computer? My dentist handwrote his letter, then asked his assistant to type it up and upload it onto AADSAS. Of course he has to read it over and sign. I imagine my father - a dentist himself would do the exact same thing if someone asked him for a letter.