Mesial migration and lower third molar tilt

The angulation of lower third molars is compared in two groups of radiographs of children aged 10.1--13.0 years, one group showing lower second deciduous molars present, the other showing mesial migration of lower first permanent molars and impaction of second premolars following early extraction of lower second deciduous molars. The lower third molar crypts are significantly less tilted in the group where deciduous molars had been removed, the difference between the means of the two groups being 12.5 degrees (p less than 0.001). The findings are discussed in relation to the hypothesis that early mesial migration improves the orientation of third molar crypts by enabling them to develop further forward. There is some evidence that the mean angle of tilt may be reduced by about 20 degrees when mesial migration occurs before third molar crypts begin to form.