– The prevalence of malocclusion was studied in 6,398 Swedish schoolchildren They were divided into two groups. Group 1 consisted of 2,664 boys and 2,795 girls in the town of Umeå. The children were examined at 7, 10 and 13 years of age. Group 2 was made up of 429 boys and 510 girls referred from the district of Västerbotten and examined only once. Dental anomalies of the permanent teeth, and space and occlusal anomalies were recorded. Of the children in Group 1 73.8% had some form of anomaly. Of these, 52.3% were occlusal anomalies, 32.6% space anomalies and 14.9% dental anomalies. Of the referred children (Group 2) 44.8% had occlusal anomalies, while space and dental anomalies were equally common (29.4% and 25.8%, respectively).

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It has been known for a long time that the organic matrix of the enamel could be histologically studied only if the preservation through a sufficiently long fixation could be established.' The penetration of the fixative is likely to be mediated by the more-or-less hydrated organic stroma, which, it is suggested, exists in a non-fibrous form,2 possibly a gel.3 An increasing number of publications report that the water phase in enamel seems to occupy a larger part than had previously been supposed.4 A diffusion of liquid through the enamel was microscopically demonstrated by Bergman.5'6It was thought that there might be a difference in the permeability between freshly obtained untreated and intentionally coagulated dental hard tissues and that sodium ions (Na22) would be a suitable test substance. Materials and MethodsThe material of this study comprises sixteen pairs of intact, homologous premolars, from boys and girls eleven to thirteen years old, which were removed for orthodontic indications. Four of these pairs were used for the first part of the experiment, which was performed as follows.Immediately after the extraction, the teeth were placed in Ringer's solution. One tooth in each pair was boiled for 5 minutes in this solution in order to coagulate the organic substances. The apical part of the root was cut with a diamond disk at a level 2-3 mm. from the enamel-cementum border.The remaining part of the root was covered with Tenax wax, which formed a funnel to the root pulp (Fig. 1). These preparations of the teeth were made immediately after the extractions with great care, so that the crown was never allowed to dry. The crown was submerged in a vessel containing Ringer's solution of known volume. With a fineglass pipette, a certain amount of Na22-labeled saturated NaCl solution (specific activity, 1 mc/ml) was injected into the pulp cavity of the tooth. At various time intervals, samples of the surrounding Ringer's solution were analyzed for activity in welltype and solid-crystal detectors operating with a single-channel analyzer.For the second part of the experiment, four pairs of intact homologous premolars were used. In an attempt to limit the effect to the enamel, the crown of one tooth in each pair was submerged down to the enamel-cementum border in the boiling Ringer's solution for 2 minutes. The rest of the tooth was protected by a shield of aluminum foil. The control tooth was left untreated. As in the first part of the experiment, the roots were cut off and Na22 injected into the pulp chamber. The activities in the surrounding Ringer's solution were measured as before.In order to check some variables, the following test experiments were made:1. The concentration of the Na22Cl solution was lowered to 0.5 per cent.2. The boiling time was varied from 1 to 10 minutes.3. Identical experiments were performed in which the amount of injected Na22 was varied.4. To elucidate the effect of boiling on the size of the hydroxyapatite crystals, two glass capillary tubes inner diameter 2 mm. 1299 permission. at U...

The dental ages of 149 children born and grown up in a district with naturally occurring fluoride in the drinking water (maximum level 2.0 mg/l) were compared with those of 181 control children born and grown up in a neighbouring low-fluoride district (less than or equal to 0.1 mg/l). The children were 9--12 years old. In neither group did the dental ages prove to differ from normality. It is concluded that concentrations below the highest recommended level fluoride occurring naturally in the drinking water do not seem to have any long-term effect on the individual physiologic maturation, as judged from the dental age of the growing child.