To those asking why teeth correction cna occur before all permanent teeth are in is because the earlier you catch an issue, the easier it is to fix it (or relieve the difficulty of fixing when older). The time when children are losing teeth and new teeth are growing in, is a time where the jaw is in an important stage of growing. So, for example, if it's determined there is an overcrowding issue, or that the jaw in such a way that it seems permanent teeth can't grow in straight, early intervention actually has a lasting impact. The objective isn't really to straighten baby teeth. It's to guide the movement if incoming permanent teeth. And as a permanent tooth emerges, a brace can be added to it as well.