Praise be to Allah.
Shaykh Saalih al-Fawzaan was asked about straightening the teeth and he said, If that is necessary, such as if there is some deformity in the teeth that needs to be corrected, then there is nothing wrong with it. But if it is not necessary then it is not permitted., rather it is forbidden to file the teeth or make gaps between them for the purpose of beautification, and there is a strong warning against doing so because it is a kind of messing about and altering the creation of Allaah.
But if this is done for the purpose of medical treatment, or to remove a deformity or for some other need, such as when a person is unable to eat unless his teeth are corrected, then there is nothing wrong with that.
With regard to removing extra teeth, Shaykh Ibn Jibreen said: there is nothing wrong with removing an extra tooth, because it deforms the appearance and makes things difficult for a person… but it is not permissible to file them or make gaps between them because of the prohibition on doing so.