You can only include the medical and dental expenses you paid this year, regardless of when the services were provided.
If you pay medical expenses by check, the day you mail or deliver the check generally is the date of payment. If you use a "pay-by-phone" or "online" account to pay your medical expenses, the date reported on the statement of the financial institution showing when payment was made is the date of payment. If you use a credit card, include medical expenses you charge to your credit card in the year the charge is made, not when you actually pay the amount charged.
See IRS Publication 17 Your Federal Income Tax (For Individuals) for more information.