Knocked out no, surprisingly enough.
I've had four removed - one each side, top and bottom. I was ten, and had a very 'advanced mouth', so four had to come out to let the brace do its job. Other little girls competed over who was first in getting a bra. I lost that but I won the first in getting a brace contest. The upside was that it was off by the time other classmates were having them put them.