484 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS PANEL STARTS WITH CONTROL PASTE I 5 WEEKS I I BASE LINE TEST FLUORIDE I / I I GROUP I CONTROL I I i i i i I TEST I TEST 2 ] GROUP Tr i CONTROLl 4-8 EKS I I •,, ! I I GROUP Tr I FLUORIDE I 4- 6 EEKS I Figure 16. Design of panel trial. Most of the tests mentioned depend upon assessing changes in enamel solubility. There are probably numerous reasons why this principle is so popular, e.g. Miller's acidogenic theory of caries emphasises the destructive role of acid naturally occurring fluoride affects the structure of teeth - excessive amounts cause mottling- and these teeth are more resistant to caries. However, some agents have been found which produce a marked reduction in solubility, e.g. lead, indium and zinc, but which have no effect on caries (23). The ability of fluoride to reduce enamel solubility is well documented, but the really critical caries inhibiting effect may not be this at all. Indeed, Jenkins (24) quotes a number of ways in which fluoride could inhibit the carious process, viz:

THE LABORATORY EVALUATION OF PROPHYLACTIC DENTIFRICES 485 3'8' pH 3.7 GROUP I BASE LINE TEST 1 TEST 2 Figure 17. 19 persons on sequence--control, fluoride, control. 1. Cause morphological changes in teeth resulting in shallower fissures. 2. Slow decalcification and accelerate recalcification. 3. Promote apatite formation and improve crystallinity. 4. Inhibit the production of polysaccharide and acid by plaque bacteria. The second and fourth processes could possibly be achieved by a topical application. That bacteria are essential to the carious process is known' from studies on germ-free animals and so this is obviously an important area to consider when techniques are being selected to evaluate cariostatic agents. The need for a method to assess the effect of agents on the carious process as a whole is shown by the various attempts which have been made