I see this every day. A child comes in with his permanent lower incisors coming in behind the baby teeth. We sometimes call it "Shark" teeth. The baby teeth have not come out like they were supposed to. It is most common with the lower front teeth when the child is six years old and then the upper back molars when the child is around eleven years old. A common "emergency" is when a parent notices a permanent tooth coming in behind a primary (baby) tooth. It's not really an emergancy, but needs to be addressed.
In the normal course of events, the permanent teeth slowly dissolves the baby tooth root as it comes in up under the baby tooth. Finally, there is not much root left. The baby tooth gets loose and eventually falls out. The permanent tooth then comes in where the baby tooth used to be. Well, it doesn't always happen just like that.
If the child doesn't have enough room for the permanent teeth, then the permanent tooth may not come in right up under the baby tooth. Even when there is lots of room, the new tooth may not be able to resorb the baby tooth root fast enough. It then takes the path of least resistance, which is to come in behind the baby tooth. That means there is nothing "pushing" the baby tooth and there may be quite a lot of baby tooth root left.
The good news is that the permanent tooth will tend to move forward into the correct position on it's own IF there is nothing in the way and there is enough room. That usually means the dentist will need to remove the retained baby tooth and make more room by removing the adjacent baby teeth or making the baby teeth more slender by "disking" them. If the new tooth hasn't come in very much (you haven't waited too long), then the new tooth will slowly move forward. This takes a few weeks or months to move forward into a better position. They may even straighten out a little if they were coming in crooked.
Often teeth come in pairs, so if one tooth is not coming in correctly its partner on the other side won't either. A lot of parents hold out hope the baby tooth will fall out on it's own. Some children are very aggressive in wiggling their teeth. Some just let them hanging there. Remember however, that there may be more root on that baby tooth than you think, otherwise it would have come out by now. Most of the time the dentist has to get in there and get the tooth. Even if you can get that tooth out, is there enough room for it to come forward? Many times teeth come in funny because there is some inherent crowding. If the dentist needs to get it out, it's not a big deal. Most kids do quite well for this. Plus, they get to have their tooth to put under the pillow for the tooth fairy!
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