Night Guard cpt code

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There has been a discussion on whether or not we are using the correct cpt code for a night guard. We take the molds for the guard but send it out to another company who actually makes it.

Should we be using 21085? If so since we aren't making the guard should be using a 52 modifier. Or should we bill using D0393? Or should we be using D7880?

Any help would be great!