Why does O'Brian rip out winston's tooth in part 3? What is he trying to prove?

O'Brien wants to show Winston what is left of the humanity that Winston purports is still alive. Winston looks in the mirror and does not see his former self. He has become emaciated creature that has rotted beyond recognition. O'Brien pulls Winston's remaining tooth out as if it were a baby tooth on it's last strand. Winston becomes a metaphor of the human spirit that O'Brien says has long since lost its meaning.