Fifty-four cases of ectopically erupted maxillary permanent first molars were studied with a method using the measurements obtained from photographs of the study casts. The amount of relative loss of anteroposterior arch length of the right and left sides was shown to correlate with the increased mesial angulation of the maxillary permanent first molar. The bilateral ectopic group has a significantly larger mesial angulation of the permanent first molar on a maxilla with shorter anteroposterior arch length. The relative loss of arch length with ectopic eruption of the permanent first molar limits the use of unilateral appliances for treatment. The use of an appliance with bilateral arch support seems preferable.
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