Deciduous teeth sometimes become ankylosed or joined/fused to the jaw bone, and do not shed when they are due to be exfoliated. If the period of ankylosis is long, and the child grows rapidly, the ankylosed deciduous teeth may remain at their low level while surrounding adult teeth erupt higher and higher, resulting in submergence of the ankylosed teeth. In severe cases, they end up below gum level.
Ankylosed baby teeth have to be extracted so that they do not obstruct the eruption of permanent teeth below them. If there are no permanent teeth underneath the ankylosed teeth, then the orthodontist will have to assess whether they can be left in the mouth till their roots have resorbed away, or they should be extracted and the surrounding teeth aligned with orthodontic treatment.