Relationship between congenitally missing lower third molars and late formation of tooth germs.

In this investigation, we used longitudinal panoramic radiographs from 96 subjects (47 boys, 49 girls) who did not have any congenital disease. The subjects were selected based on their age at the initial radiograph and the number and frequency of radiographs. Lower left premolars and second molars were used as parameters to identify factors that influence the age of formation of the third molar. We classified teeth into eight formation stages according to the method of Demirjian. The age at the beginning of formation of the lower third molar was determined with a regression equation of longitudinal data. The highest correlation existed between the age at the beginning of formation of the third molar and the formation stage of the second molar. We conclude that late formation of tooth germs is one of the factors that leads to the congenital absence of lower third molars. Furthermore, we assume that when the tooth germ of the lower third molar has not appeared at stage 7 of formation of the second molar, the probability of the third molar being missing is 100%.