B.J. Moxham

THE EFFECTS OF ROOT TRANSECTION ON THE UNIMPEDED ERUPTION RATE OF THE RABBIT MANDIBULAR INCISOR

Abstract A study of the effects of root transection on the unimpeded eruption rate of the right mandibular incisor of male Californian rabbits of average weight 3 kg was undertaken. Following a period of one week during which control unimpeded eruption rates were determined, the root was divided transversely into proximal and distal segments in 18 rabbits with the proximal segment pinned in five animals. Unimpeded eruption rates for the operated tooth were then determined thrice weekly for 5 weeks. The distal segment in six rabbits erupted at a mean daily rate of approximately 230 μm, the proximal segment showing little or no movement. In other rabbits, however, though similar rates were observed for the distal segment, the proximal segment also erupted forwards and may have contributed to this movement. Four rabbits were maintained for 6 months following transection and the consequent effects of preventing eruption on the proliferative basal tissues were similar to those previously reported for the rat. These results would indicate no significant differences in the eruptive mechanisms of the rabbit and rat mandibular incisor.

The effects of some vaso-active drugs on the eruption of the rabbit mandibular incisor

Abstract Because the forces of tooth eruption may be generated by vascular/tissue hydrostatic pressures, the effects on the rabbit incisor of hypotensive agents (hexomethonium, guanethidine and hydrallazine) and of noradrenaline and acetylcholine were investigated using a technique for continuously monitoring tooth movements. Following the intravenous administration of hexomethonium (10 mg/kg), guanethidine (3 mg/kg) or hydrallazine (1 mg/kg), the rates of eruption-like movements increased for approximately 2 h and then the hexomethonium ones returned to control values, the other two showed a similar trend to return. In some rabbits treated with guanethidine or hydrallazine, however, the initial response was an intrusion of the incisor. Injection of noradrenaline (0.003 mg/kg) was associated with intrusive movements initially. Acetylcholine (0.01 mg/kg) was associated with a rapid extrusive movement initially where there was little or no prior eruption. Where eruption was present, reactions were difficult to show. These initial effects of noradrenaline and acetylcholine were occasionally followed by eruption-like movements, the rates of which were much higher than expected. Changes in periodontal tissue hydrostatic pressure could be responsible for all the drug effects.

A quantitative comparison of the ultrastructure of the periodontal ligaments of impeded and unimpeded rat incisors

The periodontal ligaments of impeded and unimpeded rat mandibular incisors were examined to find structural correlates for the known functional differences between the tissues. The structures quantified were fibroblasts (area and membrane length, endoplasmic reticulum, mitochondria, microtubules, lysosomes, intracellular collagen profiles, intercellular contacts), oxytalan fibres, collagen fibrils and ground substance. The only changes seen on rendering a tooth unimpeded were an increase in the number of microtubules within the fibroblasts, an increase in the number of simplified desmosomes between the fibroblasts and a decreased amount of ground substance within the extracellular matrix. The results show that it is possible for a connective tissue to undergo marked changes in function, turnover and biomechanical properties without major structural changes.

A quantitative assessment of the effects of axially directed extrusive loads on displacement of the impeded and unimpeded rabbit mandibular incisor.

Abstract Extrusive loads of 0.01–1.0 N were applied to the impeded mandibular incisors of 12 rabbits. Tooth position was monitored continuously with a variable capacitance displacement transducer. On suddenly applying an extrusive load, a biphasic displacement of the tooth was recorded. A rapid extrusive phase was followed by a more gradual extrusion. A similar, but intrusive, biphasic recovery response was seen on suddenly removing the load. The group-mean displacements showed that for all phases the responses were force-dependent, though not linearly graded. For a given load, the group-mean displacements during the first phases of the extrusive and recovery cycles did not show significant differences. However, the displacement during the second phase of the extrusive cycle was significantly greater than that of the recovery cycle for a load between 0.2 and 1.0 N. Extrusive loads were also applied to unimpeded teeth. Though no differences between impeded and unimpeded teeth were observed in the biphasic displacement patterns, significantly greater mobility was seen for the unimpeded incisors in all phases. The findings show that biomechanical changes of the periodontal tissues, of importance to studies on tooth eruption, occur in the chronically unimpeded tooth.

Interactions between thyroxine, hydrocortisone and cyclophosphamide in their effects on the eruption of the rat mandibular incisor.

The effects on the unimpeded eruption rates of the rat mandibular incisor of daily doses of thyroxine (0.5 mg/kg) and hydrocortisone (12.5 mg/kg) and of a single injection of cyclophosphamide (40 mg/kg), either alone or in combination, were studied over approximately 15 days. Both the thyroxine and hydrocortisone alone produced increases in eruption, whereas a decrease was recorded with cyclophosphamide alone. When thyroxine was combined with hydrocortisone, the increase in eruption rate was greater than that with thyroxine or hydrocortisone alone. When cyclophosphamide was combined with hydrocortisone, there was no difference compared with cyclophosphamide alone. These results obtained with drug combinations are consistent with eruption having a multifactorial basis.

Recording the eruption of the rabbit mandibular incisor using a device for continuously monitoring tooth movements.

Abstract Using a variable capacitance displacement transducer, continuous records were made of the eruption of the incisors of 67 rabbits distributed into 4 groups. The incisors in 3 groups were impeded before monitoring their movements under respectively, urethane, urethane and chlorolose, and pentobarbitone anaesthesia. In the 4th group (anaesthetized with urethane), the incisors were maintained unimpeded for 5 days before monitoring their movements. The incisors showed movements resembling eruption throughout the 4 h of recording. However, there was variation in the rates, both in individual rabbits and between different rabbits in the same experimental group. For a few rabbits, no eruption was recorded, possibly because they were deeply anaesthetised. Excluding such animals, the group mean rate of movement of impeded incisors under urethane or urethane and chlorolose, declined from about 3.0μm/5 min to about 1.0 μ/5 min during the last half-hour of recording. Under pentobarbitone anaesthesia, the mean rate remained within the range of 2.0 to 2.6 μm/5 min throughout the 4-h recording period. As incisors unimpeded for several days before monitoring showed increase in their rates of movement compared with the impeded rates, the extrusive movements must be eruptive movements.

The effects of axially-directed extrusive loads on movements of the mandibular incisor of the rabbit

Abstract Using a variable capacitance displacement tranducer to record tooth movements continuously, the effects of applying axially-directed extrusive loads of up to 200 g (2 N) on movements of the mandibular incisors of 6 male Californian rabbits (average weight 3.4 kg) were studied. On applying a load of 1 g (0.01 N) or more, a biphasic movement of the tooth was recorded which could be clearly distinguished from eruptive movements. Initially, a rapid, almost instantaneous, extrusive phase was observed, followed by a second phase during which there was a more gradual extrusion. A similar, but intrusive, biphasic recovery response was observed on removing the load. The responses to the extrusive loads were force-dependent. Immediately after the animals were killed, responses were similar to those in live animals.

Fenestrated capillaries in the connective tissues of the periodontal ligament.

A quantitative ultrastructural study was undertaken to assess the number and distribution of fenestrated capillaries in the periodontal ligaments of the rat dentition. Experiments were conducted on four animals using the right mandibular incisors and first molars. For the incisor, analysis was performed in three sites along the length of the periodontal ligament. Regardless of site, there was 0.1 fenestration per micron2 of endothelium. In the basal region, the incisor periodontal ligament contained 3.5 X 10(6) fenestrations per mm3 of tissue whereas more occlusally it contained 1 X 10(6) fenestrations per mm3 of tissue. For the molar, the capillaries showed more fenestrations, 0.4 fenestration per micron2 of endothelium. There were also more per unit volume of tissue, 12 X 10(6) fenestrations per mm3. Thus, the vasculatures of the rat incisor and molar periodontal ligaments exhibit significant numbers of fenestrations which, in the case of the incisor, are not distributed uniformly along its length.

The mobility of the lathyritic rabbit mandibular incisor in response to axially-directed extrusive loads

Axially-directed extrusive loads of 0.01-0.1 N were applied to the lathyritic mandibular incisors of 12 rabbits. Tooth position was monitored continuously with a variable capacitance displacement transducer. Compared with pair-fed controls, lathyritic teeth showed markedly reduced resistance to extrusive loading, indicating that the periodontal collagen has a role in supporting the tooth. The pattern of tooth mobility, however, was little affected. The results may have implications for the eruptive mechanism. That eruption rates can appear unchanged by lathyrogens might not preclude an effect of the drug upon the eruptive force, assuming that there is a concomitant decrease in the resistance of the periodontal tissues to eruption. The teeth of rabbits in the pair-fed control group showed increased resistance to extrusive loading compared with animals fed ad libitum, indicating that changes in food and water intake may also influence the mechanical properties of the periodontium.

The effects of section and stimulation of the cervical sympathetic trunk on eruption of the rabbit mandibular incisor

Abstract The immediate and short-term effects of section and stimulation of the cervical sympathetic trunk on the eruption of the ipsilateral mandibular incisor in rabbits were investigated using a displacement transducer which enables continuous monitoring of tooth movement. Compared with mean rates of eruption determined for a control group, 30 min following sympathectomy in 12 rabbits, the rates were significantly increased for approx. 1 h. The effects of stimulation for 3 min of the peripheral cut end of the cervical sympathetic trunk were assessed using a further group of 11 rabbits which showed eruptive movements. The pulse duration for every stimulation was 1 ms. In 8 rabbits, intrusive movements resulted where the stimulus strength and frequency exceeded 2 V and 3 Hz. For 3 rabbits, eruption was uninterrupted. The results were consistent with a haemodynamic hypothesis of eruption.