Many of the parents in my practice are surprised to see that primary or baby teeth have roots.
The only baby teeth they had ever seen had no roots at all and they never really thought about what keeps them in.
Actually, the baby teeth have very strong roots that hold them firm when chewing and biting. As the adult teeth begin to grow, they push against the baby tooth roots and dissolve them away.
The timing of tooth eruption can be found at this link.
The mechanism of tooth eruption is not completely understood, but we do know that they only begin to move when the roots start to develop.
Apparently, the ligaments holding the tooth to the jaw bone pull and push the teeth toward the surface and out of the bone. Pressure on bone or tooth makes it dissolve away.
The pressure from the moving tooth makes the bone move out of the way and eventually the roots of teeth will as well.
Although most of the time the permanent teeth erupt in the correct direction, sometimes the tooth starts off going in the wrong directing and can end up impacted into other permanent teeth.
Many wisdom teeth are removed because they are impacted. Interestingly, the wisdom teeth tend not to dissolve the other molars they are pressing against.
Pressure from the moving permanent teeth causes the primary roots to dissolve and when they are all dissolved away, the baby tooth falls out.
The Shark Teeth article discusses what to do if they don’t fall out normally.
Impacted canine teeth are fairly commonly found against the roots of permanent incisors and have caused serious permanent tooth root damage, and even tooth loss.