You deduct medical expenses in the year that you pay the provider, not the year that you incurred the expense or the year that you received the bill.
If you take out a loan to pay the medical expenses, you deduct the expenses in the year that you use the money from the loan to pay the provider. But if your debt is to the provider of the medical services, not to a bank or other lender, then you have not yet paid the expense and you cannot deduct it.
