Eruption Status and Congenital Absence of Mandibular Third Molars from the Past to the Present in the Japanese Population

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The eruption status and congenital absence of mandibular third molars were studied over time from the Jomon era to the present. The data were radiographs of buried human skeletons and modern populations. Eruption status was classified as upright, inclined, horizontal and congenital absence. The relationship with the M2-retromolar space was also examined. The eruption style of the mandibular third molars depends on the size of the M2-retromolar space, and as the M2-retromolar gap becomes narrower, the eruption goes from upright to inclined and then to horizontal. In congenital absence, most of the M2-retromolar space is less than 1/2 the mesiodistal diameter of the second molar. Congenital absence of mandibular third molars has been strongly influenced by heredity from prehistoric times to the Edo era. However, from the 1930s to the present, it has been more influenced by the environment, particularly food habits, with congenital absence of mandibular third molars declining as dietary habits becomes more favorable.