1993 年 40 巻 1 号 p. 17-28
The mandibular first permanent molar has been reported to be the first permanent tooth to erupt. However, according to the recent reports, a considerable increase in the number of children has been observed with the eruption of the mandibular first permanent molar after the eruption of the mandibular central incisor. The purpose of this study is to investigate whether there is any morphological difference in the mandible including the mandibular corpus and the ramus between the subjects with the mandibular first permanent molar erupting prior to the mandibular central incisor (Group A) and the subjects with the mandibular central incisor erupting prior to the mandibular first permanent molar (Group B). Lateral, posteroanterior, 45-degree oblique cephalometric radiographs and dental plaster models were used as the materials. Based on the data statistically analyzed, the results are summarized as follows: 1. No significant differences were found in the conventional angular lateral cephalometric measurements between the two groups. 2. No significant difference was found in the mandibular length between the two groups. 3. Significant differences were found in the mandibular corpus and the ramus between the two groups. In other words, the children in Group A have a narrower ramus, longer corpus and a larger eruption space for the mandibular first permanent molar than the children in Group B. These results indicate that even though there was no significant difference in the anteroposterior dimension of the mandible between the two groups, the internal proportion of the mandible represented as the ratio between the corpus and ramus was significantly different. This may suggest that the different proportional constituents of the mandible have a close relationship with the eruption timing of the mandibular first permanent molar.