Tooth size, spacing, and crowding in relation to eruption or impaction of third molars.

A study was conducted for the purpose of comparing mesiodistal tooth diameters, sum of tooth widths, and the degree of spacing or crowding in the dentitions of two groups of young adults. The subjects in the first group had complete dentitions including third molars (M3s). In the second group, all M3s had been extracted for failure to erupt. Tooth size and consequently the sum of mesiodistal tooth widths were found to be greater in the extraction group than in the group with complete dentitions. The relationship between tooth size and M3 impaction was most pronounced in female subjects. The sum of mesiodistal tooth widths is apparently one factor of importance in the cause of M3 impaction in women. The results suggest that tooth size in girls could be used at preadolescent age as one predictive variable for third molar eruption. The sexual dimorphism demonstrated regarding the relationship between tooth size and M3 impaction implies that studies in the field of M3 development and eruption should be performed on male and female subjects separately. The dental arches in the extraction group tended to be more crowded than in the group with complete dentitions. However, the differences were small and significant only as they related to the upper arches in males.