In the mixed dentition, three other anomalies of eruption are fairly common:
1. Infraoccluded primary teeth usually exfoliate provided that the permanent successors are present, but they should be kept under review. If they are not shed and eruption of the permanent tooth is seriously delayed, or if the infraocclusion becomes very marked, then they should be extracted and a space maintainer fitted if appropriate.
2. Impaction of the upper first permanent molar into the distal of the upper second primary molar causing resorption. It is possible to disimpact the tooth with an appliance, but the problem usually resolves spontaneously when the primary molar is shed. The resorption may cause pain if it involves the pulp, in which case the primary molar should be removed. This allows the permanent molar to move rapidly mesially, and a space maintainer or an active appliance to move it distally should be considered.
3. Second premolars in unfavourable positions are sometimes seen as incidental findings on panoramic radiographs, but fortunately they usually correct spontaneously and eventually erupt satisfactorily. Very occasionally this does not happen, and a few cases have been reported of a lower second premolar migrating towards the mandibular ramus. Upper or lower second premolars that are blocked out of the arch because of crowding usually erupt, but are displaced lingually.