Changes in the eruption order of the first permanent tooth and their relation to season of birth in Japan |
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Authors: | K Nonaka A Ichiki T Miura |
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Institution: | Department of Hygiene, Teikyo University School of Medicine, Tokyo, Japan. |
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Abstract: | In order to examine the eruption order of the first two permanent teeth, kindergarten children 5 to 6 years old were examined in Hakone, Japan. Among a total of 817 children examined from 1976 to 1984, 349 were determined as I-type children, whose mandibular first incisor erupted earlier than the mandibular first molar, and 183 were as M-type children, whose first molar erupted earlier in the mandible. The mandibular I-type rate, i.e., the proportion of the I-type among a total of I- and M-type children, was 66% (349/532). In 1983-1984, the I-type rate was 70% among boys and 62% among girls, but the overall I-type rate did not differ significantly by the sex of the subject or by the year of examination. The I-type rate varied significantly with the season of subject's birth. Those born in October or November showed a significantly lower I-type rate (33%) than the other subjects (P less than 0.001), in spite of a shift of birth season with low I-type rate toward winter in 1983-1984. The difference of the I-type rate according to birth season suggests that the causes responsible for this change are primarily environmental and act at the prenatal or perinatal stage of life. |
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Keywords: | Tooth eruption Eruption order Birth season Environmental factors Evolution |
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