The effect of partial damage to the enamel-related periodontium combined with root resection on eruption of the rat incisor eruption

Previous work has indicated that the enamel-related periodontium (ERP) has a role in the eruptive process of the rat lower incisor. By combining partial damage of this tissue with resection of the odontogenic organ, we examined the effect of the damage on subsequent incisor eruption. The connective tissue of the enamel-related periodontium was regenerated in less than 2 weeks, showing morphology close to normal. The injured part of the enamel organ was neither regenerated nor repaired, and a cement-like tissue, continuous with the true acellular cement, was formed on the denuded enamel. Before tooth exfoliation, the operated teeth erupted at a slower rate compared with root-resected and sham-operated incisors, probably because of the absence of a substantial part of the enamel organ due to surgical damage. As with the coronal dental follicle and the enamel organ in rat molars, the enamel-related periodontium and the enamel organ of rat incisors may have some control on their eruptive process.     

The effect of corticosteroids and cyclophosphamide on the eruption of resected incisor teeth in the rat

It has been observed that there may be two mechanisms making resected rat incisors erupt during their phase of normal eruption rates, only one of which works during their slow phase of eruption. The effects of cyclophosphamide and corticosteroids on the eruption of non-resected teeth may also be consistent with the presence of two eruptive mechanisms, one responsive to cyclophosphamide and corticosteroids, the other not, or less, responsive to them. To see whether this previous work has, in different experiments, revealed the same pair of mechanisms, the effects of cyclophosphamide and corticosteroids on resected incisors were investigated. Cortisone accelerated their eruption and cyclophosphamide slowed it when they were erupting at normal rates, but neither compound had any effect during the initial slow phase.     

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.     

Measurement of the rate of eruption of the rat incisor

A radiographic method of measuring the rate of eruption of the rat mandibular incisor was developed using the crest of the interincisal bone as reference point. The stability of this point was assessed histologically. The rate of unimpeded eruption of the mandibular incisor of twelve adult rats was measured over 22 days and was 1.85 mm per 48 hr, s.d. 0.22 mm. The errors of the method were measured.     

The effect of intermittent forces on eruption of the rabbit incisor

Rabbits were prepared so that a variable capacitance displacement transducer (VCDT) could be used to monitor the eruptive movements of a previously-amputated lower incisor, and a strain gauge transducer mounted on a relay-controlled plate was used to place precisely controlled intermittent forces opposing the erupting tooth. Forces of one or three g were applied under time conditions of one second on, nine seconds off (1:9); one second on, three seconds off (1:3); and one second on, one second off (1:1). The data indicate that with force present for 10% of the time (1:9), there was little or no effect on eruption rate. In contrast, 50% time (1:1) produced essentially the same intrusive effect as continuous force, and 25% time (1:3) gave an intermediate response. The data are consistent with the hypothesis that force duration is more important than force magnitude for the vertical equilibrium, as is the case for facio-lingual equilibrium.     

Effect of occlusal contact on the rate of eruption of the rat incisor

In three groups of rats, cutting back the incisors out of occlusion intermittently or continuously increased the eruption rate. The mean rate depended on the length of the period during which the tooth lacked occlusal contact. If the period of eliminated occlusion was sufficiently long, the average rate of eruption rose while repeated elimination and renewal of occlusal contact after a while had a declining trend.     

Partial pulp resection increases unimpeded eruption rates of the rat incisor

Thirty rats were divided into 3 equal groups. Following 9 days of induced unimpeded eruption, a single partial pulp resection was performed on the repeatedly-shortened lower left incisor of one group. A total of 3 resections, each repeated after 48 h, was performed on the animals in a second group. The lower left incisor was shortened but the pulp was not resected in the 3rd sham-operated control group. Eruption rates of all of the incisors were recorded daily. Partial resection of the pulp increased the unimpeded rate of the experimental incisors in the first group by 50 per cent at 24 h after operation and an increase occurred 3 times in the second group of rats (by 45, 36 and 25 per cent respectively). The rate returned to control levels 24 h later. No significant differences were observed between the eruption rates of the right maxillary and mandibular incisors of the animals in the 3 groups. The increased rate of eruption may be induced by local regulators from the traumatized pulp that are transmitted to components of the periodontal ligament which provide the force for eruption.     

The effect of lead on the eruption rates of incisor teeth in rats

The effects of lead on the continuously erupting rat incisors under normo-, hyper- and hypofunctional conditions were investigated. Left lower incisors of 20 rats were rendered unimpeded (hypofunctional) by cutting them out of occlusion every 2 days; the right lower incisors of these rats were considered hyperfunctional. Measurements on normally growing teeth (normofunctional) were carried out in a group of ten rats whose teeth were not cut but only marked every 2 days. On day 7 of the experiment, half of the rats from these two groups were given a single intraperitoneal injection of lead acetate (40 mg/kg), and the other half received sodium acetate (22 mg/kg). Another group of 15 rats was used to obtain blood samples for lead determination 1 h, and 10, 20, and 30 days after lead administration. Animals were killed on day 32. Hypofunctional incisors from lead-treated rats erupted more slowly than control ones (P<0.05). These results show a previously unreported toxic effect of heavy metals.     

Acceleration and circadian rhythm of eruption rates in the rat incisor.

The eruption of the rat mandibular incisor was measured at 4 or 6 hr intervals by a photographic method. Acceleration of eruption occurred immediately after the experimental shortening of the incisors with some further acceleration from 12 hr afterwards. A circadian rhythm of eruption rate was found in incisors with functional occlusion and also in those without biting contacts; that is, the eruption was slower during the night when the animals were physically most active.     

The effects of cyclophosphamide on eruption of the continuously growing mandibular incisor of the rat

Single and daily injections of the cytotoxic agent cyclophosphamide (40mg/kg body wt) to rats with unimpeded mandibular incisors both reduced the unimpeded eruption rate, multiple injections bringing eruption to a virtual standstill after 6 days. Histological changes occurred within the incisor following drug injection.     

Dose-response effects of colchicine and vinblastine on unimpeded eruption rates of the rat mandibular incisor

Dose-dependent inhibition of the eruption rate followed single injections of colchicine or vinblastine. 1.5 mg of colchicine/kg of body wt decelerated the eruption rate to 6 per cent of the control value in the initial 24 h after subcutaneous injection and 2.5 mg of vinblastine/kg of body wt to 60 per cent. Doses of colchicine and vinblastine which would reduce the eruption to zero were estimated to be 1.62mg/kg and 14.07mg/kg respectively. Toxicity developed slowly whereas inhibition of eruption appeared quickly. Colchicine toxicity was the less. The inhibitory action of these drugs may be related to disruption of microtubules in the cells which may produce the motive force of eruption. It has yet to be shown how microtubules could produce such a force.     

Vincristine's effect on dentinogenesis in rat incisor

Abstract – Vincristine was administered to 60 rats in four groups in doses of 0,1, 0,3, 0,5 and 0.7mg/kg. Histomorphologic investigation of the dentin and odontoblasts in the maxillary incisors after 1, 2 and 3 weeks revealed a moderate to severe disturbance of dentin production in some parts of the incisors. In some of the affected areas the effect seemed to be reversible in that normal-appearing dentin was found pulpal to the dentinal derangements after 2 and 3 weeks. The distribution and severity of the lesions were similar to those previously observed 3 days after injection of the same doses of vincristine.