On the Deflecting Wrinkle Appearing in the Mandibular Second Deciduous Molars of the Japanese Residing in Kyushu, Japan

The deflecting wrinkle appearing in the mandibular second deciduous molars of the Japanese residing in Kyushu, Japan, was studied by the authors, utilizing the hard plaster models of the mandibular teeth. Five hundred and thirteen mandibular second deciduous molars (280 males and 233 females) were examined in this study, and the appearance frequency of the deflecting wrinkle as well as its relations with the crown measurement values or other primitive traits were investigated. Also, the deflecting wrinkle that appeared in the first deciduous molars and first molars adjacent to the second deciduous molars was studied for comparison. The results of the investigation are summarized as follows:1. The appearance of deflecting wrinkle in the mandibular second deciduous molars is 75% while that of the first molar is 43%. A significantly positive correlation is observed between both teeth.2. Positive correlations are observed between the deflecting wrinkle and the 7th cusp and also with the central ridge of metaconid in the study of the correlations of the deflecting wrinkle and other primitive traits.3. No correlation is observed at all between the distal trigonid crest of the mandibular first deciduous molars and the deflecting wrinkle of the mandibular second deciduous molars' However, the deflecting wrinkle of the second molars are considered to be one of the variations of the distal trigonid crest of the first deciduous molars.4. The correlations between the deflecting wrinkle of the second deciduous molars and the central ridge of metaconid of the first molars and that of the central ridge of metaconid of both teeth are observed to be positive significantly.5. In the relationship between the deflecting wrinkle and crown measurement (mesiodistal, buccolingual diameters), such tendency was seen as the better the development of the deflecting wrinkle, the larger the tooth size is.That the deflecting wrinkle of the mandibular second deciduous molars appears in the 75% of the Japanese residing in Kyushu is considered to strongly indicate the so-called Mongoloid racial tooth characteristic. In view of its correlations with other traits and measured values, the deflecting wrinkle is believed to be one of the primitive traits.