A histological study of root-resected and root-transected rat incisors when eruption ceases, shortly before they are...

Resection of the odontogenic region or root transection of normal (impeded) rat lower incisors showed that eruption ceased from 1 to 13 weeks when the base of the resected teeth (87.5%) or of the distal segment of the transected ones (86%) reached the alveolar-crest region. When the operated teeth reached the crestal region, the enamel-related periodontal tissues were absent and the periodontal ligament (PDL) was the only periodontal tissue that remained. The PDL of the crestal region may be considered as mature PDL, showing a length of approx. 5-6 6 mm at the mesial face of the tooth, 4-5 mm at lingual face and 1 mm at distal face; from these limits towards the apical end of the socket the PDL becomes gradually immature. The mature PDL seems not to have a role in the process of tooth eruption. Several factors can be suggested to explain the more frequent retention, at the crestal region of the socket, of impeded rather than unimpeded incisors submitted to the same procedures. The connective tissue that develops between the base of the tooth and the bone that fills the alveolus may have more time to organize itself in impeded than in unimpeded teeth, which erupt at a faster rate; this tissue could support and retain the impeded operated teeth longer than the unimpeded ones. The decrease in the mechanical properties of the PDL in the unimpeded condition may ease the traumatic effects and lead to exfoliation. Eruption might be stopped by the increase in occlusal forces, per unit area of root surface, as the root becomes shorter; this effect is likely to be greater in impeded than unimpeded teeth.