Human incisor teeth, fixed with 10% formalin (pH 6.5), were exposed to disinfected lactobacillus drink (Calpis) at 37°C. The morphological changes in the natural surfaces of the labial enamel were observed for exposure times of from 3 hours to 3 days by means of the scanning electron microscope. The outer enamel layers which possessed demineralized zones in the subsurface enamel due to exposure for 24 hours or 3 days were observed morphologically by transmitted or polarized microscopy and microradiography as well as scanning electron microscopy, and were also studied by X-ray microanalysis and the Vickers hardness test.
The surfaces of the crystallites on the natural enamel surface were demineralized and were fused together at 3 to 6 hours after exposure. After 24 hours they were either partilly dissolved away or peeled off and the crystallites of 100-150 nm in diameter laid bare. After exposure to lactobacillus drink for 3 days, these large crystallites occupied the total surface area of the enamel, assuming a surface layer approximately 1.5 pm thick.
The outer enamel layer which possessed a demineralized zone in the subsurface enamel could be identified as the following one layer and three zones in order from the surface: (1) the surface layer described above; (2) the remineralized zone which was comparatively sparse and easily disintegrated physically; (3) the demineralized zone where the bodies and peripheries of enamel rods were dissolved remarkably; and (4) the demineralized zone front where mainly only the peripheries of the enamel rods were dissolved.

View full abstract

There has been considerable interest concerning the use of the laser in many medical applications. As with electrical heat energy, the laser is recognized as having considerable potential as a special tool for medical and dental treatment. Reliable performance of the laser, however, is necessary for medical and dental applications. Experimental results of a laser impaction onto tooth enamel in vitro for the purpose of caries prevention have been reported using extracted human or animal teeth, but few in vivo experiments have been reported in the literature because of the lacking of a suitable device for guiding the laser beam. Thus, the developement of an optical manipulator for the laser is necessary for its clinical use. In this report a useful optical manipulator for dental use of an Nd-YAG laser was described. The optical munipulator enables the passage of a normal and a giant pulse beam of the YAG laser. The device consists of 4 arms for beam passage, 8 articulating joints for reflection, and terminal hand-piece, which can also be operated with a foot-switch. Each elbow contains a prism reflector and allows 360°C rotation at each joint, thus providing up to 6 degrees of freedom in beam positioning. Transmission through the optical systems is more than 65%. Directing the laser beam onto a given area of oral tissue is simple and it can be precisely controlled by handling of the terminal hand-piece with the guide beam of a He-Ne laser.

View full abstract

The accumulation of lead, harmful to the living body, in hard tissues was investigated in relation to nutrition. An examination was also made of the quantitative changes in Ca and P, measureing the constituent elements of hard tissues after lead administration. Lead-administered rats were fed on 5%, 18%, and 30% protein diets for 4 weeks. Lead accumulation was largest in the group of rats fed on the 5% protein diet and was more remarkable in young rats than in adults. Quantitative changes in Ca and P were observed mainly in the femurs.

View full abstract

Extracted teeth are not suitable for many experiments, for example, regarding enamel solubility and plaque adhesion, because they have no reproducibility. So it is necessary to make a test piece that closely resembles enamel, is uniform, and applicable to such experimentation.
The enamel disk molding method is as follows.
1) Thoroughly mix 325 mesh enamel powder refined from many extracted teeth with polymer solution.
2) Dry and pulverise it to 250 mesh.
3) Take 200 mg of 250 mesh powder and fill a metal mold with it.
4) Press it with a pressure of 18 ton/cm².
The enamel powder is reduced to an enamel disk which is from 0.7 to 1.0 mm in thickness and 13 mm in diameter.
Polymer solution of polymethyl methacrylate (PMMA), epoxy (EP), or polystyrene (PS) can be used to mold an enamel disk. The concentration of polymer solution can be 10%, 20%, or 30%.
A 10% PMMA-enamel disk is better than any others judging from its handling and attributes. It specific gravity of 2.27±0.07, a Vickers hardness number of 79.80±4.47 and a strength of rupture in bending of 0.928±0.059. The coefficients of variation of the Vickers hardness number and strength of rupture in bending are relatively small, 5.60% and 6.36%, respectively, in five-time repetitive tests.
The purpose of this study, to get a uniform test piece, was achieved.

View full abstract

The plaque index (PlI) and gingival index (GI) were measured twice per week for 4 months in ODU plaque-susceptible rats (SUS) and plaque-resistant rats (RES). The two strains of rats showed a morked difference in plaque formation followed by moderate chronic gingivitis in part of the gingiva of their lower incisors when fed a commercially available powder diet.
The mean PlI increased rapidly after one week and continued to increase slowly thereafter, and the mean GI increased week by week in SUS, but there was neither plaque formation nor gingivitis in RES.
We also examined the correlation between PlI and GI monthly in SUS individual rats. There was a negative correlation in the 1st, 2nd, and 3rd months and a positive correlation in the 4th month.

View full abstract

The Effect of premature loss of deciduous molars on the development of malocclusion in permanent dentition was examined. The material was 180 pairs of dental casts of permanent dentiton, 122 pairs of which were obtained through the dental health program for school children in an area without dentists, Koromogawa, Iwate, where cavity filling and extraction were the only therapeutic measures used at the annual visit by the Tohoku University group. The other 58 pairs of casts were obtained from children whose deciduous molars were sufficiently taken care of at the Lion Family Dental Center, Nagoya.
From these materials, the casts of 20 children who had lost more than 7 deciduous molars prematurely were selected as the lost group. The casts of 49 children who had preserved all the deciduous molars normally were selected as the preserved group. No significant difference was found in the frequency of malocclusion of permanent dentition between the two groups. No difference was found in the frequency of functional factors for malocclusion in the two groups.
As for the tooth to denture base discrepancy, the material was divided into two groups: the discrepancy and the non-discrepancy groups. Significant difference was found in the frequency of malocclusion in permanent dentition between the two groups with a risk of less than 0.1%.
Conclusion was that the premature loss of deciduous molars would have no significant effect upon the development of malocclusion of permanent dentition, but congenital etiological factors such as tooth to denture base discrepancy would have significant effect upon it. Accordingly, the extraction of advanced carious deciduous molars might be an affirmative therapeutic measure, especially from the standpoint of efficiency of treatment and sufficiency of dental care for school children.
The information from such a public health program should also be applied to clinical dentistry, especially for the developmental stage of dentition.

View full abstract

The effects of fluoride on thyroid metabolism were studied in rats.
Wistar rats were maintained for 54-58 days on a high (34.5 ppm F) or low (0.31 ppm F) fluoride diet, and were provided distilled water as drinking water in which sodium fluoride was contained at concentrations of 0, 1, 5, 10, 50, 100, and 200ppm F.
The results were as follows:
1. No dose-response relationships were found between fluoride concentration of drinking water and thyroid hormone (T₃ and T₄) values in serum. In the groups exposed to high concentrations of fluoride, moreover, few distinct effects were found. Therefore, it is probable that the fluoride has no serious effect on thyroid metabolism. In groups maintained with 1 ppm F water and low fluoride diet, however, a somewhat high value of T₃ and T₄ was observed. From this, it is possible that the thyroid metabolism is activated by trace amounts of fluoride.
In the groups maintained with low fluoride water (1, 5 and 10ppm F) and high fluoride diet, a little low value of T₃ was observed, but no change of T₄ value was seen. It is possible that some effects appear with ordinary diet (high fluoride diet) itself.
2. No histopathologic changes were observed in any of the experimental groups.
3. In the groups exposed to 100 or 200 ppm F water, TSH and GH values decreased. The weight of the pituitary gland also decreased in the 200 ppm F group. It is possible that the pituitary gland is affected directly by high fluoride.
4. Fluoride concentration in serum was a little higher in the low fluoride diet group than in the high fluoride group, and a similar finding was observed in teeth and bones, although the total intake of fluoride from drinking water was lower in the low fluoride diet group than in the high fluoride diet group. It is possible that the absorption of fluoride from water through the digestive tract is inhibited by the high fluoride diet.
5. A relatively firm homeostasis in serum fluoride level was found in the zero to 10ppm F water groups, and fluoride concentrations in soft tissues were reflected in the serum fluoride level. Therefore, fluoride levels in soft tissues have no worth as an index of fluorosis except at high fluoride doses. Fluoride concentration in hard tissues reflected clearly the fluoride content in drinking water, especially in the low fluoride diet group. This may be a best index of fluoride intake.

View full abstract