(A) X-ray showing the premolar (*) blocked due to migration of adjacent teeth (arrows). (B) At the upper arch, the upper teeth have migrated forward and "stolen" space to the canines which are blocked outside the arch (arrows). (C and D) View of the tilted teeth blocking the premolar (*) corresponding to the radiograph (A).
In contrast, as the following example illustrates, the right lower drift space was lost when the second temporary molar was extracted and no space maintainer was used to prevent tooth migration. The premolar (*) is blocked by the teeth that have tilted on either side of the extraction space.