Role of attrition and occlusal contact in the physiology of the rat incisor. II. Diurnal rhythm in eruption and attrition

Botulinum neurotoxin type A in the masseter muscle: effects on incisor eruption in rabbits

INTRODUCTION

Botulinum neurotoxins are responsible for the paralytic food poisoning, botulism. Commercial formulations such as botulinum neurotoxin type A are increasingly used for various conditions, including cosmetic recontouring of the lower face by injection of the large masseter muscles. The paralysis of a major muscle of mastication lowers occlusal force and thus might affect tooth eruption. The purpose of this study was to investigate the effects of unilateral masseter muscle injection of botulinum neurotoxin type A on the rate of eruption of incisors in a rabbit model. We hypothesized that the teeth would overerupt in an underloaded environment.

METHODS

Forty rabbits were injected with either botulinum neurotoxin type A or saline solution in 1 masseter muscle. Mastication and muscle force production were monitored, and incisor eruption rate was assessed by caliper measurement of grooved teeth.

RESULTS

The injection of saline solution had no effect. The masseter muscle injected with botulinum neurotoxin type A showed a dramatic loss of force 3 weeks after injection despite apparently normal mastication. Incisor eruption rate was significantly decreased for the botulinum neurotoxin type A group, an effect attributed to decreased attrition.

CONCLUSIONS

This study has implications for orthodontics. Although findings from ever-growing rabbit incisors cannot be extrapolated to human teeth, it is clear that botulinum neurotoxin type A caused a decrease in bite force that could influence dental eruption.

Axial movements of rat mandibular incisors measured under artificial respiration with halothane anaesthesia

The axial movements of rat incisors were recorded continuously for over 20 h. The rats were anaesthetized with halothane delivered by intratracheal intubation using an artificial respirator. A haemostatic clamp was used to immobilize the jaw and the displacement detector exhibited high resolution, good linearity and low drift. The previously impeded and unimpeded eruption rates (the average extrusive movements) were 406 and 548 microns/24 h, respectively. The previously unimpeded rate was lower than the normal unimpeded rates obtained by other investigators. It appears that the eruptive movement of the previously unimpeded incisor was inhibited by halothane anaesthesia. Circadian rhythm was not observed in the eruption rates of previously impeded or unimpeded incisors. Following the i.p. injection of a lethal dose of pentobarbital sodium, the incisors retracted. The average intrusive movement was significantly greater in the previously impeded group (29 microns) than in the previously unimpeded group (12 microns) at 30 min after the lethal injection. It is assumed that the elimination of the pushing force at death may cause the stretched periodontal fibres to recoil, thereby pulling the tooth in an intrusive direction. It is also likely that the mechanical strength of the ligament is reduced in the previously unimpeded group by repeated shortening of the incisors; therefore the force needed to retract previously unimpeded incisors may be less than that needed to retract previously impeded incisors.

The daily rhythm of tooth eruption

A video microscope system, which can resolve tooth movements to 1 to 2 microns, was used to monitor the eruption of 17 human maxillary second premolars in the prefunctional phase. Eruptive movement was detected by the change in position of an optical ruling on the erupting tooth relative to a reference ruling on the occluding adjacent teeth. Eruption was observed as the teeth erupted toward the occlusal plane over approximately a 41-hour period, from late Friday afternoon until Sunday noon. Half hourly observations were made during a 6 to 8-hour continuous laboratory session, which was during the day (9 AM to 5 PM) for nine of the subjects, and during the evening (5 to 11 PM) for eight. A similar pattern of eruption was seen for all subjects. The tooth intruded from before to after dinner on the first day, erupted significantly overnight, ceased eruption and approximately maintained its position during the next day, and erupted again during the second night. On the average, a slight intrusion was observed during the day, and statistically significant intrusion was associated with breakfast and dinner. Eruption occurred during the evening observation period, and the rate of evening eruption was significantly greater in children who were supine and relaxed than those who were upright and active. The 24-hour eruption rate was slower for teeth that were within 1.5 mm of the occlusal plane. The circadian eruption rhythm may be related to fluctuations of hormonal levels that affect metabolic activities within the periodontal ligament. It is also possible that a transient reduction in pressures by the cheeks, lips, and tongue during periods of rest allows eruption to occur then.

Comparison of rates of enamel synthesis in impeded and unimpeded rat incisors

Periodic intubations of rats with solutions of fluoride (F) lead to the appearance of bands of disrupted pigmentation in continuously erupting incisors. Distances between fluorotic bands reflect time intervals between intubations. In this experiment, the periodicity of fluorotic banding was used for estimation of the rate of enamel synthesis in impeded and unimpeded rat incisors. Rats kept on a low-F diet and distilled water were intubated two or four times per week with 2 mg NaF/150 g body weight. In a group of rats, one of the mandibular incisors was cut at the gingival margin after two weeks, and intubations were continued for an additional two weeks. In another group of F-intubated rats, incisors were cut or notched at the gingival margin twice, six days apart. Control rats either received the same periodic F intubations or were maintained on the low-F diet without intubation. Measurements of spacing between fluorotic bands were identical in impeded and unimpeded teeth, even though the latter erupted at a faster rate. In an unimpeded mandibular incisors, there was a significant elongation of the secretory zone and a shortening of the pigmentation zone, resulting in reduced pigmentation intensity of the erupted portions of the teeth. The results show that the rate of enamel synthesis is independent of the eruption rate.

Circadian rhythms of gingival epithelium in young mice

Five-week-old Brookhaven National Laboratory (BNL), Swiss-albino male mice were maintained on a 12-hour light/dark cycle and were subcutaneously injected with 3H-thymidine (1 micro Ci/gm body weight) one hour prior to sacrifice. Twenty-four mice (three each time-period) were killed at 3 hour intervals for 24 hours. Autoradiographs were prepared from 5-micron thick paraffin embedded decalcified sections of maxillary first molars and surrounding tissues. Labeling index was determined over the cells of the basal layer of the gingival epithelium. Crevicular and crestal gingival epithelium revealed a circadian rhythmic pattern in DNA synthesis in which two peaks were evident. Peak labeling of 5-week-old mouse gingiva exhibited a higher value than previously reported for nontraumatized gingiva. Rodents are nocturnal, as was peak activity, but a second peak is also evident in the light period. Consequently, not all peaks are synonymous with the period of animal activity and feeding. The observed continuous high labeling indices represent a significant potential of gingival epithelium to replenish its cells in response to physiological wear and tear.

Autoradiographic evaluation of circadian periodicity in cell proliferation in the periodontal ligament of the young mouse

Five-week-old BNL male mice were maintained on a 12-h light/dark cycle, subcutaneously injected with [3H]-thymidine 1 h before death and killed every 3 h for 24 h. Autoradiographs were made from demineralized, paraffin-wax embedded sagittal sections of maxillae. The labelling index was calculated for fibroblast-like cells of the periodontal ligament mesial to the first molar. A bimodal distribution of labelled cells was seen. Labelling index was highest at 0900 h (light period); a second, lower, peak was seen at 2400 h (dark period). Cell proliferation was significantly increased during the light period. The increased activity during the dark period may represent a superimposed secondary, ultradian, rhythm, differences in periodontal cell subtypes or may be related to the feeding and activity cycle of the mouse. Thus there is a 24-h cyclic rhythmicity in the proliferation of periodontal-ligament cells in the mouse.

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.

Circadian mitotic rhythms in the cervical tissues of the rat maxillary incisor

A pronounced 24-hour fluctuation occurs in the mitotically active tissues of the rat maxillary incisor. High and low periods of activity were seen during the daytime as well as at night. A consistent increase in the number of mitotic figures occurred approximately four hours subsequent to a change in the light-dark regimen.

Effect of fluoride on eruption rates of rat incisors

The addition of either 25 or 100 ppm fluoride to the drinking water of Wistar rats produced a banded appearance on the mandibular incisors but had no significant effect on the impeded or unimpeded eruption rates of the incisors.

Role of attrition and occlusal contact in the physiology of the rat incisor. IV. Prevention of attrition in the articulating incisor

Basal growth in the rat incisor is controlled by occlusal restraint. The result of these two antagonistic forces is eruption, which is mediated via the periodontal ligament. The amount of eruption depends on the relation between the quality of the periodontal tissues and the amount of occlusal interference. If eruption does not rise above a certain minimum, odontogenesis proceeds abnormally.