PACIFIC SOUTHWEST ASSOCIATION OF CHEMISTRY TEACHERS 0
RECENT METHODS FOR THE PREVENTION OF DENTAL CARIES' HENRY M. LEICESTER College of Physicians and Surgeons, San Francisco, California
DENTAL caries is essentially a chemical problem, for it involves the destruction of an inorganic salt and of a protein hinder of the inorganic crystals, brought about by reaction with chemicals formed by the hacteria which are found in the mouth. For a long time it was assumed that the mechanism of this destruction was quite simple. Carbohydrates from the food were supposed to he retained on the teeth either by impaction in cracks and crevices or under a mucinous film, called a plaque, which often forms on the tooth surface. These carbohydrates mere then fermented to lactic acid by month bacteria. The acid dissolved the inorganic salt, a form of calcium phosphate, and the residual protein was then destroyed by simple abrasion or by the attack of proteolytic hacteria. It is now known that this picture is far too simple. Carbohydrate fermentation is important in the whole picture, hut there are many other factors involved. Actually, dental caries is a complex process which depends on the interrelation of the tooth itself, with a11 the strengths and weaknesses built into it during its formation, and the various physical, chemical, and biological factors of the oral environment. Prevention or reduction of caries can therefore he attempted either by strengthening the tooth or by changing the oral conditions. Perhaps in the end a combination of both methods will prove most effective in preventing decay, hut a t present the majority of attempts at caries reduction have been confined to one or the other of these two methods. Procedures designed to alter the oral environment in order to discourage the growth of hacteria which may cause caries have not so far been completely successful. In the light of the fermentation theory, the most obvious method is reduction in intake of carbohydrate foods, especially sugar. This has not proved practical on a large scale because most people prefer to eat many sweet foods and do not concern themselves with possible tooth decay a year later. Reliance on good tooth hrushing to remove carbohydrates before they can ferment encounters similar psychological difficulties. Both these methods are known to he effec-
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Presented in part at the Second PawAmerican Dental Congress, Ciudad Trujillo, Dominican Republic, October 9, 1956.
tive, hut it is doubtful if either will lead to a very marked reduction in the general incidence of caries. Addition of therapeutic substances to toothpastes is a method which suffers from some of the disadvantages of attempts to improve tooth brushing habits. However, if spectacular results are promised because of some impressive mediral claim, patients may be more willing to use a dentifrice than if they are merely told they should do so. A number of therapeutic dentifrices have appeared on the market in recent years, and farreaching claims have. been made for some of them. A few have even had clinical tests, hut the results up to now have not been outstanding. Chlorophyll dentifrices received much publicity, but have shown no value. Ammoniated dentifrices showed some ability to reduce caries, hut the over-all effect was so slight that their use seems hardly justified. Penicillin dentifrices actually do reduce decay to a marked degree, but antibiotics are perhaps too potent to use in a toothpaste. There is too much danger of sensitizing the patient or of producing resistant strains of microorganisms which might prevent proper use of these drugs in a later medical emergency. The socalled "anti-enzyme" dentifrices rely on the presence of an agent which is supposed to remain in the dental plaque and prevent the growth of acid-producing bacteria. They have showu some promise, hut they have not yet been sufficiently tested on a clinical scale to speak positively of their success. Their advertising, which stresses that their users need not brush their teeth very often, seems to he a definite set-back to attempts to improve tooth brushing habits (I). The second major method of attack on caries is the alteration of tooth structure in some way to make it more resistant. Here greater success has been attained through the use of fluorides which can be applied in various ways and with varying degrees of success. FLUORIDES MOST EFFECTIVE
Essentially, fluorides are effective because they can be huilt into the mineral structures of enamel and dentin. The inorganic salt of the teeth is a basic tricalcium phosphate called hydroxyapatite. It can be JOURNAL OF CHEMICAL EDUCATION
assigned the formula 3Ca8(PO&Ca(OH)~, hut actually the hydroxyl group in the crystal lattice can be replaced by varying amounts of other ions such as rhloride, carbonate, or fluoride if these ions are present in the solution from which the apatite precipitates. The number of ions which will thus substitute depends on their concentration in the solution. The hydroxyapatite of teeth comes from the tissue fluids, and so ultimately from the blood, which has a relatively constant content of chloride and carbonate. The fluoride content of the blood can vary widely, however, depending on the amount taken in in the diet, and so variable amounts of this ion can enter the calcifying tooth. Of course, when calcification is complete, no more fluoride can thus be built in. The fluoride may enter the crystal lattice by direct incorporation as the crystal grows, or by subsequent adsorption and ion exchange with hydroxyl groups already in the lattice. I n either case, it forms an integral part of the tooth itself. Once fluoride is present in the lattice it reduces the solubility of the apatite in acids. The local release of fluoride by acids when they do dissolve the apatite may liberate enough fluoride ion a t the exact site of the developing caries to inhibit some of the enzyme systems of the bacteria and thus stop further fermentation. This does not, of course, require a high concentration of fluoride in the mouth as a whole. Both of these mechanisms can act to prevent the progress of decay. Extended clinical observations have shown that fluorides actually are the most effectiva agent now known for reducing caries. I t is therefore of interest to consider some of the ways in which they can be used. FLUORIDATION OF DRINKING WATER
The method which gives the best results is to supply fluorides continuously a t a concentration of about one part per million in the drinking water when the teeth are calcifying. Fluoride is then built into the entire tooth structure and will be present there for the entire life of the tooth. Recently another advantage of this method has been shown. After formation of the tooth is complete, it remains for some time in the gum before it finally erupts. During this period it is still continuously exposed to the tissue fluids, which will contain fluoride as long as this ion is being taken in in the diet. During this time, further exchange of the fluoride from the tissue fluid with ions in the enamel surfare rontinues. As a result, a very high concentration of this element is built up and the enamel surface, mhere caries begins, is even richer in fluoride than the mass of enamel and dentin below. This high concentration has beeu demonstrated experimentally by progressive grinding off of thin layers of enamel and determining their fluoride content ( 2 ) . Such a highly fluoridated surface offers excellent protection against caries, and even if it is later worn away, fluoride is still present beneath to afford more protection. It could be predicted from this that the greatest reduction in caries would come from continued small intake of fluoride in the diet during the early years of life, since calcification of all teeth is complete by the ape of ten, and most have erupted. Clinical experience proves that this is so. If, up to the age of about VOLUME 34, NO. 2, FEBRUARY, 1957
ten, the drinking water continually contains from about 0.7 to 1.0 p.p.m. fluoride, depending on the mean temperature, tooth decay is reduced by about two thirds. This has been noted in many areas where fluoride occurs naturally, and has been specifically checked by carefully controlled ten-year experiments in such cities as Newburgh, New York (S), Brautford, Ont,ario (4), and Grand Rapids, Michigan ( 5 ) . No signs of injury to general health appear in these cities (6) nor in others, such as San Francisco, where fluoridation has been in effect for four years (7). It is natural that the question of the safety of water fluoridation from the standpoint of individual health should have beenraised. In fact., the concentration of 1 p.p.m. was chosen after numerous studies had established that a t this level the dental benefits mere not accompanied by any other physiological effects. The most sensitive organ in the body to the action of fluorides is the developing enamel. Drinking water containing 2 or more p.p.m. fluoride produces a condition known as mottled enamel which is the first sign of any undesirable action. Mottling produces chalky white spots on the tooth surface, and these may later be stained dark by pigments in food. The intensity of such mottling increases with concentration of fluoride taken in, varying from almost invisible marks a t the lower levels to very promineut stains mhen the level of fluoride in the water is above 6 p.p.m. The function of the teeth is not impaired by such mottling, however. Mottling results from a disturbance in the enzymes involved in the calcification mechanism, which is a complex and progressive process. Even lifelong ingestion of 8 p.p.m. fluoride has no other harmful effects,however (S), and it is only a t very high levels, over 20 p.p.m., that soft tissues in the body may begin to be affected. These facts become understandable mhen it is found that a t the low concentrations used in fluoridation programs there is an equilibrium betveeu absorbed and excreted fluorides which is quickly established in the body. Aside from the small amount which is deposited in the bones and,teeth, there is no build-up or cumulative action of fluoride on the system, and in adults there is almost no fluoride retention (9, 10). Wherever possible, therefore, fluoridation of drinking water to the optimum level should be encouraged. APPLICATION OF FLUORIDES TO THE ENAMEL SURFACE
When fluoridation has not beeu possible, fluorides can be used in another manner. Topical application of solutions of fluoride to the surface of erupted teeth will still result in some ion exchange and the resulting entrance of fluoride into the lattice of the enamel surface. A deposit of calcium fluoride crystals may also be formed on this surface. The uptake of fluoride under these conditions is not continuous, as it is when fluoridated water is drunk, and so the results mould not be expected to be as satisfactory as in the latter case. Nevertheless, clinical experience has shown that topical application of fluoride does reduce caries by about 40y0 in children. The results with adults are conflicting and we are still not sure of its usefulness after about the age of fourteen. However, topical
application is still a method of great value within its limitations ( I I). Until recently, the source of the fluoride ion has been almost entirely sodium fluoride or sodium silicofluoride in aqueous solution. I n the last few years it has been claimed that solutions of stannous fluoride produce better results than an equal amount of sodium fluoride. This may be due to an effect of the tin ion superimposed on the effect of the fluoride, for there is reason to believe that stannous ions may replace calcium in the apatite lattice. There is much preliminary evidence to indicate the increased effectiveness of stannous fluoride ( I Z ) , but final conclusive results have not yet been reported. I t is now possible to obtain a dentifrice containing stannous fluoride and further studies are being conducted with this. We cannot yet affirm, however, that this dentifrice will prove to be more effective than the various therapeutic toothpastes discussed earlier. It can be seen that in fluoride we have a proved, safe agent which will greatly reduce tooth decay when it is properly applied. It is not the final answer, for even in the best case, some caries may occur. Probably a combination of fluoride with some other agent, whether one now being tested or one as yet undiscovered, will provide the final answer to the caries problem. Pros-
pects are bright that such an answer will be found in the not too distant future. LITERATURE CITED (1) LEICESTER, H. M., Ann. Rev. Med., 5, 410 (1954). F., D. E. GARDNER, AND F. A. SMITH,J. Den(2) BRUDEVOLD, tal Research, 35, 420 (1956). (3) AST,D. B., D. J. SMITH,B. WACHS, AND K. T. CANTWELL, J. Am. Dental Assn., 52, 314 (1956).
(4) BROWN, H. K., H. R. MCCLAREN, G. H. JOSIE,AND B. J. STEWART, J. C a d i a n Data1 Assn., 22, 207 (1956).
F. A,, JR., H. T. DEAN,P. JAY, AND J. W. KNUT(5) ARNOLD, SON,Public Health Reports, 71, 652 (1956). (6) SCALESINGER, E. R., D. E. OVERTON, H. C. CHASE,A N D K. T. CANTWELL, J . Am. Dental Assn., 52,296 (1956). (7) Sox, E. D., J. Canadian. Dental Assn., 22, 455 (1956). (8) LEONE,N. C.,M. B. SHIMKIN, F. A. ARNOLD, JR., C. A. STEVENSON, E. R. ZIMMERMANN, P. B. GEISER,AND J . E. LIEBERMAN, "F1uoridation as a Public Health Meawre," A.A.A.S., Washington, D. C., 1954, p. 110. (9) ZIPKIN,I., R. C. LIRINS,F. J . MCCLURE, A N D A. C. STEERE, Public Health Reoorts. 71. 767 (19561. , , (10) CHEN,P. S., JR.,'F. A. SMITH, D. E. GARDXER, J. A . O'BRIEN,AND H. C. HODGE, Proc. Soc. Ezptl. B i d . Med., 92, 879 (1956). (11) BIBBY,B. J., AND F. BRUDEYOLD, "Fluoridation as a. Public Health Measure," A.A.A.S., Washington, D. C., 1954, p. 148. J. C., A.W. RADIRE, W.H. NEBEROALL, AND H.G. (12) MUHLER, DAY,J. Am. Dental Assn., 51,556 (1955).
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