Factors influencing the localization of mandibular third molars in twins

Abstract Purpose The aim of the present study was to evaluate the genetic and environmental factors influencing the localization of mandibular third molars by analyzing the panoramic radiographs of twins. We examined the mandibular third molars of Japanese monozygotic (MZ) and dizygotic (DZ) twins recruited by the Osaka University Center for Twin Research. Study design The study included 49 pairs (98 participants) of MZ twins and 11 pairs (22 participants) of DZ twins. Using panoramic radiography, we evaluated the degree of eruption of mandibular third molars based on the height of the alveolus bone and the third-molar-space/crown-width ratio. Using co-twin control analysis and a generalized linear mixed model, we evaluated the effects of various factors, including sex, age, body height, number of teeth, length of the lower dental arch, existence of a second molar, bruxism, and past orthodontic therapy. Results Body height, the third-molar-space/crown-width ratio, and the length of the mandibular dental arch were related to the degree of mandibular third-molar eruption and strongly influenced by genetic factors rather than common or unique environmental factors. Conclusions The degrees of third-molar eruption were more similar among MZ twins than among DZ twins; therefore, genetic factors are expected to have a more significant influence than environmental factors. These results help identify the trend of third-molar eruption from a young age, which will allow us to advise the early extraction of mandibular third molars for patients with a short stature, narrow retromolar space, or short mandibular dental arch. In addition, if the genes that influence the degree of eruption were identified, we would be better equipped to make a prediction regarding an individual’s risk of impaction, and indications for extraction may change.