[Histological studies on the controlled eruption to the rabbit lower incisor]

In order to investigate the mechanism of eruption, the rabbit lower incisors under the influence of controlled eruption were examined. The first experiment was carried out by repeated resection of the incisal edges of both upper and lower incisors so that the rate of eruption was accelerated. The second experiment was to crown the incisors for the purpose of inhibiting the eruption. In the control group, the periodontal ligament was made up of three layers. Fibers of the inner layer were continuous to the Sharpey's fibers of the cementum, and those of the outer layer were to the alveolar bone. Fibers of the middle layer ran parallel to the tooth axis. In the repeated resection group, the distance between the root end and the bottom of socket was increased, indicating the extrusion of the tooth took place. The odontogenic tissue showed increased frequency of mitosis, and the formative end of the enamel as well as the cementum shifted cervically. The eruption rate measured by the movement of metal bead embedded in the dentin, demonstrated that the rate was increased about 2 times in the repeated resection group, while the rate of growth measured by the Tetracycline method remained 1.6 times of the control group. This discrepancy was supposed to be due to the extrusion of the tooth. In case of the crowing group, the opposite was observed; the rate of eruption was reduced to about a third. Whereas the rate of growth to about a half, probably due to the intrusion of the tooth. In the crowing group, the distance between the root end and the bottom of socket was reduced, resulting the resorption of the bone tissue, and the root became sinuous, particularly at the labial side. In the lingual side, mitotic figures of the odontogenic tissue were rarely observed. The middle layer of the periodontal ligament increased in breadth in the repeated resection group, while it reduced in breadth in the crowing group. Indicating the middle layer plays an important part to the mechanism of eruption.