Several important experimental procedures have been devised to study the factors influencing the development of caries during the post-eruptive period. Three general procedures are of special importance. These are the controlled studies of human nutrition, the use of experimental animals, and the in vitro studies of caries development in extracted teeth. Because of the difficulty of the direct control of human groups, studies on human beings are difficult to carry out and interpret. On the other hand, much informa-tion has been obtained by the use of experimental animals and by the in vitro studies.
A. HUMAN NUTRITIONAL STUDIES
Numerous attempts have been made to demonstrate a relationship be-tween the presence of carbohydrates in the diets of human beings and the
97. R. F. Sognnaes, J. Am. Dental Assoc. 37, 676 (1948).
98. R. F. Sognnaes and J. H. Shaw, unpublished; see J. H. Shaw, J. Dental Med.
9, 12 (1954).
99. R. F. Sognnaes, Am. J. Diseases Children 75, 792 (1948); G. Toverud, reference 95, page 34.
TABLE VI
ACCUMULATED DENTAL DECAY AS AVERAGE DMF RATES PER PERSON FOR 12,753 ADULTS ACCORDING TO AGE.GROUP AND SEX (100)
Age
incidence of dental decay. The results are influenced greatly by the criteria used for the initial lesions, by the methods of examination (in particular whether radiographie methods have been used), and by the method of ex-pression of the data. The most common method of expressing the caries experience of the population is as the D M F value per mouth or per 100 teeth. The D M F is simply the number of decayed, missing, or filled teeth found in each individual or in 100 teeth examined.
The D M F rate is strongly dependent on the age of the individuals ex-amined. A typical example of the dental decay experience of the adult American population is given by the data of Hollander and Dunning (100) (Table VI). From an examination of more than 12,000 adults, the D M F value per individual for the 30-34 year age group was found to be about 20 and thereafter increased only slowly, so that in the age group 60-64, the D M F value was about 24. No significant sex difference was observable.
Ancient man, as observed from the skeletons of past civilizations, and primitive tribes, such as Eskimos and African tribes when isolated from civilization, generally show relatively low rates of dental decay. When placed in contact with modern civilization, primitive tribes show a greatly intensified rate of dental decay, presumably as a result of a change in diets.
An example of such observations are those reported by Price (101). Some 800 North American Indians and Eskimos were examined and classified according to their apparent contact with civilization. The Eskimos and Indians who had been completely isolated had very excellent teeth showing only one cavity per 1000 teeth. Those in close contact with civilization showed a much greater percentage of carious teeth, for the Eskimos, 13, and, for the Indians, 21.5 per 100 teeth. The isolated groups lived on meat and fish, primarily, whereas those in contact with trading posts included much flour, sugar, and other condensed foods.
100. F. Hollander and J. M. Dunning, J. Dental Research, 18, 43 (1939).
101. W. A. Price, J. Am. Dent. Assoc. 23, 417 (1936).
The enforced reduction in average consumption of purified carbohydrates during World War II has been used to explain the marked reduction of dental decay which occurred in the period subsequent to the war. Such studies assume that the only significant factor changing was the carbohy-drate consumption. Actually, the delay in the appearance of a reduced caries rate, as indicated above, has been interpreted as signifying both pre-emptive and post-eruptive effects of the diets.
In a broad sense, the above evidence has been deduced by studies of geographically isolated groups, and the normal population in civilized areas is taken as the control group. The same concept has been used directly to establish groups for which the diets could be assigned and controlled. Such groups include institutions such as orphan asylums and homes for the in-sane. Another type is composed of individuals with some medical disturb-ance requiring careful dietary control. Particularly suitable for the latter type are diabetics. Boyd {102) found that groups of diabetic children with carefully controlled consumptions of carbohydrates showed an incidence of dental decay much less than that of the average population. A very extensive comparison of institutionalized children on diets having variable amounts of carbohydrates was made by Mack and Urbach (103).
Three orphan asylums were selected, and three standardized diets, differing in the amount of total carbohydrates and in many other nutritional factors, were used for the institutions. After two years, no correlation between the carbohydrate content of the diets and the caries incidence could be estab-lished.
In most dietary studies, the fluoride factor usually has not been given proper consideration. Since fluoride exerts a major influence, dietary ef-fects ascribed to carbohydrates could in some instances be the results of varying amounts of fluoride. Also, as will be shown below, comparisons based on the crude establishment of total carbohydrate components of the diet is an undue oversimplification, since not only the amount but the type and the physical condition are important influences.
B. STUDIES USING EXPERIMENTAL ANIMALS (104)
Although rats had been used earlier, the introduction of the Syrian ham-ster as an animal for caries research allowed animal experimentation to become important. The earlier lesions obtained with rats were not typical of human lesions, and factors such as the granule size of foods were of critical importance. More recently, suitable methods of using rats have
102. J. D. Boyd, Am. J. Diseases Children 66, 349 (1943).
103. P. B. Mack and C. Urbach, Soc. for Research in Child Development Monograph No. 46, 13, No. 1 (1948).
104- G. J. Cox, reference 95, p. 55.
been developed. On the other hand, Syrian hamsters present the grave dif-ficulty that hereditary and nutritional factors have not been established to the same extent as for experimental rats. By the use of hamsters, rats, and monkeys, it has been clearly shown by numerous investigators that diets rich in some types of carbohydrates will induce decay in the teeth.
The experiments of Kite, Shaw, and Sognnaes demonstrated that the ef-fect occurs directly in the mouth (105). The general metabolism is not involved because when the same diets were fed to rats by stomach tubes, the animals' teeth were unaffected. The necessity for bacteria in the decay process has been shown at the experimental "germ-free" laboratory at the University of Notre Dame (106). The existence of a period, following erup-tion, of enhanced susceptibility to attack has been established by a num-ber of investigators who placed rats or hamsters on cariogenic carbohydrate diets, at various intervals after the teeth had erupted (107).
It must be noted, however, that hereditary influences exist, and caries-resistant and caries-susceptible strains of rats have been developed (1C8).
Most dietary compositions used for experimentation have contained sucrose or starch as the principal carbohydrate material. But carbohydrates are not identical in their effects, and both chemical and physical differences may influence their cariogenicity (109).
C. I N VITRO CARIES STUDIES
Carious-like lesions can be produced in extracted human teeth by ex-posure to a nutrient medium suitable for bacterial growth and by innocula-tion of the soluinnocula-tion with oral microorganisms. Lesions were produced in the early work of E. Magitot (around 1870) and later by W. D. Miller and many subsequent research workers (110). In vitro studies of this type have been greatly improved by the introduction of the ' 'Artificial Mouth" in which many of the important oral conditions are reproduced. Under appro-priate conditions, the entire tooth structure will be destroyed. In order to produce localized lesions, regular cleansing of the exposed tooth surfaces is required (111).
105. O. W. Kite, J. H. Shaw, and R. F. Sognnaes, J. Nutrition 42, 89 (1950).
106. F. J. Orland, J. R. Blayney, R. W. Harrison, J. A. Reyniers, P. C. Trexler, M. Wagner, H. A. Gordon, and T. D. Luckey, / . Dental Research 33, 147 (1954).
107. D. F. Mitchell and W. G. Shafer, J. Dental Research 28, 424 (1949); F. J. Mc-Clure, / . Nutrition 43, 303 (1951); J. C. Muhler, / . Dental Research 33, 245 (1954).
108. See R. F. Keller, H. R. Hunt, and C. A. Hoppert, J. Dental Research 33, 558 (1954).
109. See H. C. Elliott, Jr., and W. W. Pigman, J. Dental Research 33, 27 (1954).
110. See W. Pigman, J. Am. Dental Assoc. 51, 685 (1955).
111. W. Pigman, W. Hawkins, H. West, and C. Gaston, Oral Surg. Oral Med. and Oral Pathol. 7, 427 (1954); W. Pigman, H. C. Elliott, and R. O. Laffre, J. Dental Re-search 31, 627 (1952).
By the use of the Artificial Mouth, it has been demonstrated that two general types of attack on teeth can be distinguished. These, in turn, are dependent upon the amount of available carbohydrate in the nutrient medium. When glucose is present in the medium to an extent of about 100 mg. % or less, sound teeth are not attacked, but any previously decal-cified matrix protein is rapidly destroyed. When glucose is present to an extent greater than about 300 mg. %, the inorganic portion of the tooth is rapidly removed, but the exposed matrix is attacked only slowly. At in-termediate concentrations, probably 200 to 300%, both types of attack proceed simultaneously, and the entire tooth substance is destroyed {112).