THE LABORATORY EVALUATION OF PROPHYLACTIC DENTIFRICES 475 against caries, gingivitis, calvulus, etc., but only products intended to inhibit caries are considered here. To test the caries inhibiting properties of a product efficiently two requirements are necessary: (a) Knowledge of the aetiology of the carious process. (b) Knowledge of the mechanism by which the product is intended to interfere with this process. If these were known, specific tests could be designed to check that the product does, in fact, perform its allotted function. Although Miller's acidogenic theory of caries is largely accepted as being true, the detail is by no means established. However, the wide acceptance of the acidogenic theory and the un- doubted effect of fluoride in reducing dental decay has led to the popularity of the study of enamel solubility in acid in the investigation of potential cariostatic agents. ENAMEL SOLUBILITY In essence the method is simple in that the solubility of enamel in a suitable acid buffer - usually lactate or acetate - before and after treatment with the test material is determined. Practical difficulties arise, however, in that no two teeth seem to behave in the same way even when efforts are made to standardise conditions. The use of ground enamel, prepared by the method of Manly and Hodge (3) was introduced to overcome this problem. By this technique a large batch of ground enamel, free from fines and graded by sieving to give a standard material, can be prepared and its solubility characteristics in acid buffers determined accurately. A large number of tests can then be run on the rest of the batch of enamel. Further, if a suitable mesh size is selected (e.g. 25-72 mesh) the enamel can easily be separated from toothpaste solids by sedimentation and this avoids tedious filtration procedures. In determining solubility changes, some of the early work was performed on a weight-loss basis, e.g. Manly and Bibby (4), but most studies now depend on a determination of calcium, usually by a microtitration with ethylenediamine tetraacetic acid (EDTA), or of phos- phorus, by a colorimetric procedure. Radiotracer techniques have also been used (5). Numerous criticisms and drawbacks to powdered enamel are apparent, e.g. a large surface area is exposed compared with intact enamel and

476 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS agitation during treatment may cause attrition of the enamel pieces with the result that minute particles appear in suspension: these are difficult to remove and affect results. The use of intact enamel would certainly be more realistic. Of the many studies made to investigate the solubility characteristics of tooth enamel, those of Gray (6, 7) in which he showed that the rate of solution is predominantly diffusion controlled, are probably the most detailed. Using pieces of enamel of constant area and by careful control of the experimental conditions, particularly agitation, he proved that sound enamel from different teeth dissolved in acid buffer at a similar rate, provided the surface enamel was first removed. This is necessary because of adsorption or incorporation into surface enamel of salivary calcium phosphate, salivary organic material and numerous trace elements which tend to reduce its solubility. ,-. 200 [ • •5o. o • 50, 1. Control 2. SnF a toothpaste (1:3water slurry) • 3. SnF• soln. 0'1% • x 2 x ß TIME ( hours ) Figure 2. Solubility of untreated and treated tooth enamel (continuous immersion in acid). The equipment shown in Fig. 1 •vas constructed to carry out this type of work and it consists of a constant speed motor (1300 rpm) geared to drive six Perspex rods. The tubes which hold the acid have a baffle at the bottom of each to limit vortex formation. The apparatus spans a constant temperature water bath so that the lower two thirds of the tubes are