well, it depends. once the xray is done and they know where the root is, if the surgery is possible, they can remove the root (and possibly a bit of jaw bone). this means that the root is no longer there but of course also that the bottom incisor will no longer be growing which in effect would mean that the top incisor would keep overgrowing but as long as she has regular filing, her quality of life would be very good - she should not have any pain although she may not enjoy regular trips to the vets. if we got to that stage, it would be fab.
if the root is positioned where they cannot safely get it out, then it's a whole different story.. but i really hope this will not be the case. she only has three legs as well, she's had it rough. she is a sweet little girl as well...