Extraction of a primary second molar allows significant mesial migration of the first
permanent molar in that quadrant, causing potentially severe local crowding with
displacement or impaction of the second premolar, especially in the upper arch where
mesial drift is greatest (904HFig. 14.4). How severe this is depends on the degree of
crowding, and in a spaced arch the extraction has little effect. In principle, however,
the loss of a primary second molar should be avoided if at all possible, especially in
the upper arch. A space maintainer, either removable or fixed, can be considered,
unless the patient's caries rate is high or the oral hygiene is poor. Primary second
molar extractions should never be balanced on the contralateral side as there is very
little effect on the centreline and the potential crowding becomes complicated even
further.
In general, there is no need to compensate primary tooth extractions with extractions
in the opposing arch.
Key Points
Mixed dentition extractions
- Early loss of primary teeth generally worsens crowding.
- Primary second molars should be preserved if at all possible.
- Extraction of primary canines may help incisor crowding.
- Benefit of this must be balanced against the trauma of extractions.
- Extraction of primary canines should be balanced on the contralateral side.
905H
Fig. 14.4 (a) and (b) Localized crowding of upper second premolars due to the early
loss of primary upper second molars. (Photos courtesy of Professor J. H. Nunn.)
14.3.4 Enforced extraction of first permanent molars
First permanent molars are very rarely the teeth of choice for extraction for
orthodontic reasons⎯in practice their removal often makes treatment more difficult.
- The space they provide is remote from the labial segments and is poorly placed
either for the relief of anterior crowding or for overjet reduction. - Depending on the timing of the extractions, much of the space is lost to mesial
migration of the second molars, especially in the upper arch (see 906HSection 14.3.3).