Development of the Maxillo-mandibular First Molars and Developmental Changes in the Alveolar Bone Morphology : Three-dimensional Observation by X-ray CT

P(論文)

To investigate the development of the maxillo-mandibular first molars and accompanying developmental changes in the alveolar bone morphology during the period from the intra-jaw bone stage to the completion stage of tooth eruption, dry skulls were three-dimensionally observed using X-ray CT. The specimens were 34 dry skulls composed of 17 skulls in the deciduous dentition period (dental age II A) and 17 skulls in the first half of the mixed dentition period (dental age IIC, IIIA). The following results were obtained : 1. By observing the horizontally sectioned images, the maxillary first molars approached the area on the deciduous dentition line from the palatal side, during the period from the deciduous dentition period to the first half of the mixed dentition period. The mandibular first molars approached the area on the deciduous dentition line from the buccal side. 2. By observing the vertically sectioned images, development of the maxillary alveolar bone was higher than that of the mandibular alveolar bone during the period from the deciduous dentition period to the first half of the mixed dentition period. Although differences in the development of the alveolar bone between the maxilla and mandible were noted in the deciduous dentition period, the size of the maxilla was similar to that of the mandible in the first half of the mixed dentition period. Furthermore, positional changes in the buccal direction were marked in the maxillary first molars, which were located on the buccal side of the mandibular teeth in the first half of the mixed dentition period. 3. Observation of the vertically sectioned images also showed that the inclination angle of the coronal axis of the first molars was large in both the maxilla and mandible in the first half of the mixed dentition period, which was marked in the mandible in particular. These results showed that in the first half of the mixed dentition period, in which the early occlusion of the first molars is established, development of the maxillo-mandibular alveolar bones gradually becomes harmonized, and occlusion occurs by the incli- nation of the maxillo-mandibular tooth axis.