Removal of the proliferative odontogenic base (i.e. root resection) of the lower right incisor was carried out in sixteen out of twenty-nine adult male Wistar rats from five litters of approximately the same age and with an average weight of 325 g. The remaining thirteen rats acted as controls. Except for five specimens where the unimpeded eruption rate (u.e.r.) was severely reduced, the u.e.r. of the remaining eleven root-resected teeth attained control levels 4-7 days after operation, and this level was retained for approximately 12 days when, as the base of the tooth approached the level of the alveolar crest, the u.e.r. decreased. In some cases the root-resected tooth was shed soon after attaining the level of the alveolar crest, whereas in other cases it was retained in this situation without further eruption for periods of up to 6 weeks. Assuming that the mechanism of tooth eruption in the root-resected tooth is similar to that in the control tooth, the following mechanisms do not appear essential to the eruptive process: (1) root elongation, (2) pulp cell proliferation and dentine formation, (3) fundic bone deposition, (4) tissue fluid pressure; and the cushioned hammock ligament is not a necessary structure. It is suggested that the force of eruption resides in the periodontal membrane which retains its characteristic histological structure following root resection. In two cases of root transection carried out on the lower right incisor of adult male Lister hooded rats weighing 400 g each, with plaster of Paris gauze placed between the two segments to prevent ankylosis, an u.e.r. similar to control levels was attained. © 1969.
