detroit mercy or detroit michigan whatever their name is requires new LORs. but i would ask the adcoms at the dental schools you're applying to to verify if it's okay to use the old ones. i personally recommend using new ones.
Could you use the old letters? Sure, its not like they'll be likely to nail you for reusing one. Would it be smart? If you didn't get in with the current ones, probably not. You don't just want to repeat your application, you want to make it better.