Kuhn proposed in his Structure of Scientific Revolutions (1962) that the theoretical framework of a science (paradigm) determines how each generation of researchers construes a causal sequence. Paradigm change is infrequent and revolutionary; thereafter previous knowledge and ideas become partially redundant. This paper discusses two paradigms central to cariology. The first concerns the most successful caries-preventive agent: fluoride. When it was thought that fluoride had to be present during tooth mineralisation to ‘improve’ the biological apatite and the ‘caries resistance’ of the teeth, systemic fluoride administration was necessary for maximum benefit. Caries reduction therefore had to be balanced against increasing dental fluorosis. The ‘caries resistance’ concept was shown to be erroneous 25 years ago, but the new paradigm is not yet fully adopted in public health dentistry, so we still await real breakthroughs in more effective use of fluorides for caries prevention. The second paradigm is that caries is a transmittable, infectious disease: even one caused by specific microorganisms. This paradigm would require caries prevention by vaccination, but there is evidence that caries is not a classical infectious disease. Rather it results from an ecological shift in the tooth-surface biofilm, leading to a mineral imbalance between plaque fluid and tooth and hence net loss of tooth mineral. Therefore, caries belongs to common ‘complex’ or ‘multifactorial’ diseases, such as cancer, cardiovascular diseases, diabetes, in which many genetic, environmental and behavioural risk factors interact. The paper emphasises how these paradigm changes raise new research questions which need to be addressed to make caries prevention and treatment more cost-effective.

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Changing paradigms in concepts on dental caries: consequences for oral health care.

Tooth enamel is a unique mineralized tissue in that it is acellular, is more highly mineralized, and is comprised of individual crystallites that are larger and more oriented than other mineralized tissues. Dental enamel forms by matrix- mediated biomineralization. Enamel crystallites precipitate from a supersaturated solution within a well-delineated biological compartment. Mature enamel crystallites are comprised of non-stoichiometric carbonated calcium hydroxyapatite. The earliest crystallites appear suddenly at the dentino-enamel junction (DEJ) as rapidly growing thin ribbons. The shape and growth patterns of these crystallites can be interpreted as evidence for a precursor phase of octacalcium phosphate (OCP). An OCP crystal displays on its (100) face a surface that may act as a template for hydroxyapatite (OHAp) precipitation. Octacalcium phosphate is less stable than hydroxyapatite and can hydrolyze to OHAp. During this process, one unit cell of octacalcium phosphate is converted into two unit cell...

Molecular Mechanisms of Dental Enamel Formation

A few significant physico-chemical aspects of hydroxylapatite, a biological material of extensive contemporary inter-disciplinary research, have been reviewed. Updated information on preparation techniques, characterization, isomorphous substitutions, and solubility equilibria is provided. Recent research trends and aspects which require further clarification have been emphasized.

Some physico-chemical aspects of hydroxylapatite

SINCE the early observations1 on reduction of caries with increasing fluoride concentrations in drinking water, the tacit assumption was made that fluoride incorporated in dental enamel acts as a cariostatic agent An appealing explanation for the possible mechanism involved is based on the nature of the enamel mineral of which hydroxyapatite (OHA), Ca5OH(PO4)3, is considered to be the prototype Replacement of fluoride for the hydroxyl would yield fluorapatite (FA) which has a lower solubility Analyses of surface layers of enamel, however, show that the fluoride contents are far below the level expected for FA It is relevant then to ascertain to what extent partial substitution of the fluoride for the hydroxyl group in HA affects its solubility

Fluoridated Hydroxyapatite Solubility and Caries Formation

Orthodontic appliances and enamel demineralization. Part 2. Prevention and treatment of lesions

N A recent review, it was concluded that the mechanism of action of fluoride in reducing dental caries was its solubility-reducing effect on enamel.' Although this theory is supported by considerable laboratory evidence, it is contradicted by the observation of Volker2 that mottled enamel is as soluble as normal enamel. Mottled teeth are caries resistant, and yet it was concluded that the fluoride content in the enamel of these teeth was insufficient to reduce the solubility. A reconciliation of Volker's finding with the above theory is only possible if the fluoride is unevenly distributed, that is, if it occurs in the outer enamel in concentrations capable of reducing the solubility. The purpose of the present study was to observe the distribution of fluoride in the enamel with the hope that a better understanding of its caries-reducing effect might be attained.

The distribution of fluoride in human enamel.

THE surface enamel is more resistant to acid than is subsurface enamel, the rate of dissolution of phosphate from the surface being less than that from the subsurface enamel.1-3 With ordinary and polarized light microscopy, as well as with soft x-rays, it has also been shown that it is possible for demineralization of the subsurface enamel to occur without any apparent change in the enamel surface,4 5and when ground sections of teeth were exposed to EDTA the outer portion of enamel was the last to be demineralized.6 The reason for this difference between surface and subsurface enamel has not been established. That fluoride may play a role, at least at the level of 2 ppm in the water supply, is suggested by the work of Jenkins, Armstrong, and Speirs,2 who found that the solubility rate of surface enamel of teeth from areas with 2 ppm fluoride in the water supply was less than that from areas with 0.25 ppm, although there was no difference in the solubility rate of the bulk of the enamel. Since fluoride accumulates in the surface enamel, even in teeth from areas low in fluoride,7 it was considered that fluoride acquisition might generally be responsible for the decreased solubility rate of surface compared to subsurface enamel. Therefore, in the present study an attempt was made to correlate the solubility rate and fluoride concentration of successive layers of enamel from areas with different levels of fluoride in the drinking water.

Solubility Rate and Natural Fluoride Content of Surface and Subsurface Enamel

We describe a simple method for measuring fluoride concentration in biological fluids, in which the fluoride ion-specific electrode is used, and which is based on the principle of single known addition. Difficulties associated with variations in pH, temperature, ionic strength, complexation, and nonideal electrode response are thus minimized, and accuracy and precision are maximized. The mean recovery of fluoride added to plasma or to urine was 101.5 or 100.5%, respectively. Analytical variation (coefficient of variation) was 8-10% for plasma and 1.5-4% for urine. The method was applied to 26 sera obtained during hospital pre-admission examinations, and gave a mean ionic fluoride concentration of 1.95 µmol/liter (range, 0.84-2.90 µmol/liter), in reasonable agreement with values reported for other procedures that are based on totally different principles.

Direct potentiometric determination of total ionic fluoride in biological fluids.

N UMEROUS studies on the caries-reducing effect of fluorides point to drinking water as being the most reliable vehicle for fluoride supplementation.' It therefore becomes of interest to examine the manner by which the concentration of fluoride in the drinking water affects the fluoride content of the enamel. It is evident from the studies done in this field up to 1951 that an increase in the concentration of fluoride in the water supply results in an increase in the fluoride content of enamel.2 In these studies which showed a comparatively small difference between the fluoride content from high compared to low fluoride areas, powdered whole enamel was used for analyses. Using another approach, Jenkins and Speirs3 compared the fluoride content of the surface to that of the subsurface enamel. These workers etched off the outermost enamel with acetic acid and determined the fluoride content in the dissolved enamel and in the residual bulk enamel. By this method more than twice as much fluoride was found in the surface enamel of fluorosed teeth than in the surface enamel of teeth from low fluoride areas. Jenkins and Speirs also concluded that all the fluoride of the enamel was acquired pre-eruptively and that the fluoride content does not vary with age. However, the use of a sampling procedure of grinding rather than etching has shown that posteruptive uptake of fluoride does take place.4 Therefore, it is desirable to take age into consideration when comparing the fluoride content of teeth from different areas. The purpose of the present study was to obtain more information concerning the pre-eruptive and posteruptive acquisition of fluoride by enamel. By using the sampling method of Brudevold and co-workers4 and by working with a number of carefully selected enamel samples, it was hoped that the effect of varying levels of fluoride in the water supply and of age on the distribution of fluoride in enamel would be clarified.

The relation of fluoride in the drinking water to the distribution of fluoride in enamel.

SummaryFluoride in dog plasma was completely ultrafilterable through visking membranes when tested using the centrifuge type apparatus In dogs with a normal water load, the average normal renal fluoride clearance was 27 ml/min; the fluoride : chloride clearance ratio, 19; and the fluoride : creatin-ine clearance ratio, 0077 Fluoride clearance was more rapid than that of simultaneously measured sodium or chloride clearances or urine flow During mannitol diuresis, fluoride was cleared at a rate higher than that of sodium, chloride, or phosphate Fluoride clearance, although greater than urinary flow, varied directly with the flow under mannitol osmotic diuresis Intravenous infusion of hy-pertonic NaNO3 or Na2SO4 solutions, creating a salt diuresis, did not affect fluoride excretion Hypertonic NaCl infusion did increase the fluoride clearance Although fluoride : creatinine clearance ratios were raised greatly by intravenous fluoride, the highest ratios were attained during fluoride infusion plus osm

Dwight E. Gardner

Papers

The distribution of fluoride in human enamel.

Solubility Rate and Natural Fluoride Content of Surface and Subsurface Enamel

Direct potentiometric determination of total ionic fluoride in biological fluids.

The relation of fluoride in the drinking water to the distribution of fluoride in enamel.

Renal clearance of fluoride.