The other expression that is close to "ta ta" is "ta-da," which means "voila, there you go!" If he said "ta-da" at the end of whatever procedure he was doing for you, that is an expression that means "we're done." However, that would seem rather overly slang-y for a dentist to say to a client whose native language is not English... You got me beat! It might help if you tell us what you did in response to his request. This might give us a sense that you did what he wanted you to do, and give us a clue as to what he might have said.