N A PREVIOUS report* we stated the various factors which entered into the problem of dental caries and attempted to give a general survey of the whole field of research as applied to this subject. And in continuing our investigations during the past year we have confined our attention almost solely to one factor in the process of caries, as stated in our former report, namely the resistance of the tooth. It is then, our purpose in this report to state the work which we have done as correlated with that of others who are engaged in the same field, and set forth whatever conclusions we may hold at the present time. In order that our former statements may be at hand in this discussion we take the liberty of repeating the same.

*Official Bulletin of the National Dental Association, Vol. 1, No. 4, page 16.

RESISTANCE OF THE TOOTH.

That caries of the teeth is largely dependent upon the character and quality of the teeth nas in all ages found a wide acceptance. Many have claimed that from clinical data there could be no doubt but that the so-called "soft" teeth were more liable to caries that those of a harder and more perfect variety. The susceptibility of such teeth was attributed to a deficiency in amount and organization of their inorganic constituents. Several observers have made investigations in the attempt to gain some definite information in this regard.

Among the results which have been obtained, is the work of G. V. Black*, who in the compilation of a large number of analyses of teeth came to the following conclusions. "No diminution of density or percentage of lime salts occurs in the teeth of women during the process of child-bearing because of that fact. Any basis for accounting for caries in this or that case on account of "frail teeth", or 'teeth below the medium in structure', 'poorly calcified teeth,' etc., must disappear." This later statement he based upon the fact that the specific gravity and lime salt content of carious teeth was equal to that of immune teeth.

*Dental Cosmos, Vol. 37, page 353.

In this regard W. D. Miller stated that his observations lead him to the view that "the enamel of different persons, as well as the different teeth of the same person and different parts of the same teeth, does show difference in resistance to acid; likewise the thickness of the enamel cap as well as the integrity of the enamel surface, freedom from fissures, bruises, cracks, weak lines of imperfect calcification of prisms, etc., as far as these come within the sphere of infection**** ***** the resistance of the enamel is only one of the factors (of caries) and its import will be found to vary greatly in different cases."

H. P. Pickerill, in his work on dental caries** has done much to show the variance in enamel surface of teeth in a graphic manner. He succeeded in rubbing graphite and silver nitrate into the surface of the enamel and demonstrated that the permeation of certain forms of "soft" teeth was much greater than that of the hard varieties. In his conclusion he states, "That these departures from the normal structure are potent factors in lessening the resistance of the teeth to caries cannot be doubted. They must favor considerably the retention of food-stuff and micro-organisms; they expose a larger and more vulnerable surface to the action of the acid produced; they negative both mechanical and physiological prophylactic measures; and the fissures which have been described, since they allow of the penetration of silver nitrate and graphite, must permit the penetration of lactic acid, if not of carbohydrates. fact ceteris paribus, it may be quite logically inferred that the amount of graphite which an enamel surface retains is a measure of the amount of food stuff it would tend to retain, and a demonstration of the situations of its retention. Added to which is the demonstrable fact that teeth so affected are not infrequently likewise the subject of caries."

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In addition to these we have the writings of Boedecker, Caush, Von Abner, Fliechmann, Von Beust, Baumgartner, and many others who believe that the dentin and enamel are capable of change in structure and density due to an internal metabolism. Also those of Erdheim who has found that rats from which the parathyroids have been removed have poorly formed enamel. From all of these writings we see that there are many who hold that the susceptibility of the teeth to caries is largely determined by the perfection of the formation of the teeth, and that changes in the susceptibility may be due to changes in the enamel and dentin as the result of internal and nutritional causes.

In the light of our present knowledge there can be little doubt but that certain teeth are more In spite of the susceptible to caries than others. fact that the percentages of lime salts in such teeth do not vary perceptibly from the normal, it seems very evident that the manner of organization of the various enamels and dentins may determine to a certain extent their permeability and solubility. In addition to this, the degree of glaze upon the enamel surface, and the perfection of union of enamel surfaces in the sulei of teeth is

*Dental Cosmos, Vol. 47, page 30.

**Pickerill, Prevention of Dental Caries, and Oral Sepsis.

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a large factor in the determination of the amount of food stuffs retained in its immediate vicinity. But altho may grant that the resistance of the tooth is a factor in the process of caries, the opinion that it is a predominating factor has not obtained general acceptance. We have so frequently seen teeth which were soft and imperfectly formed which, even in the stress periods of life of the individual, have suffered little from caries, and we have seen the strongest and best formed teeth in other individuals readily succumb to the ravages of caries. So that, recognizing the effect which the resistance of the teeth may have as a factor in dental caries, we are forced in our search for the determining causes, to turn to those factors which lie without the teeth as exprest in the attacking force.

Since the time of making this statement there have appeared several contributions which have a bearing upon this phase of the subject. The first was that of A. W. Wellings, L. D. S., in a paper entitled "The Staining of Dental and Adjacent Tissues of Some Rodents by Means of Trypan Blue Introduced into the Animals During Life," which was given at the VIth International Dental Congress in London. In this paper he described the general and intense staining powers of this dye when introduced into the living body and carried by the blood stream to the tissues. His investigation was especially directed toward the question of the extent to which the circulation thru the pulp of the tooth might influence the dentin and enamel, taking the depth of the penetration of the trypan blue as an indication depth of penetration of the circulation. In his conclusions he states, "The matrix of dentin and bone is stained very lightly, the color disappearing quite soon. The walls of dentinal tubules and haversian canals do not stain, neither do the dentinal fibrils, but the periosteum stains deeply. Enamel develop

of the

In the Journal of Allied Dental Societies for 1914, page 391, Dr. W. J. Gies reports work done in his laboratory along similar lines to that of Wellings.

As to the results obtained he describes the coloration of the teeth as follows: "It was found that in all cases the pulp had been stained a deep blue to bluish black, and the dentin was similarly, tho less intensely, pigmented. In some cases the pigment colored the entire mass between the dentin and the external enamel surface a pale, tho perceptibly blue, indicating the trypan blue had past internally from the pulp thru the dentin into the enamel, or externally from the alveolar membrane, or both."

In a paper entitled, "Studies of Internal Secretions in Their Relation to the development and Condition of the Teeth"* Dr. Gies reported work done similar to the former researches of Erdheim, Toyofuku and Pappenheimer on the dental effects of Thyroparathyroidectomy and Thymectomy in rats and other animals. Among his opening remarks he makes the following statement. "Speaking with apparent conviction and authority on the present status of knowledge of dental caries, Bunting said: 'It is pretty well considered that altho it may be possible for the structure of the tooth to enter into the question to a certain extent, it is not looked upon as being responsible for the sudden and obscure changes in susceptibility, except by those few believers in the mutability of the tooth in response to general physiological change. So that investigation today is being focussed upon the environment of the tooth, the saliva, ingested food, etc., seeking in them the changeable element which determines caries.' We happen to be among the 'few believers in the mutability of the tooth in response to general physiological change,' i. e., to systemic nutritional influences, especially destructive ones." After reporting a long series of

*Dental Cosmos LVII, page 276.

We

First. In the formation of the tooth the odontoblasts and the enameloblasts are simultaneously forming dentin and enamel, and both require calcium salts from the circulation in the formation of their respective structures. It is true that a portion of the nutrition for the enamel formation is stored in the stellate reticulum, but the bulk of the enamel building material is taken from the circulation. know that either continuous or temporary arrest of the general nutritive functions causes a corresponding effect upon the quality of tooth structures which are formed at that time. Enamel formed under such conditions will be imperfect and incomplete in both form and quality. Dentin so formed will contain places of imperfection and defects known as "Interglobular spaces." There

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is little doubt, as has been stated, that the imperfections in the formation of the enamel induce the inception of caries, other conditions being favorable. Also dentin which is poor in its developmental structure offers little resistance to the progress of caries and favors its rapid spread and penetration. Such developmental defects are seen in individuals who during the period of tooth formation have suffered from rachitis, scurvy, infectious fevers and other malnutritions, especially those which have a direct effect upon the calcium metabolism. All such teeth possess a permanently lowered resistence to the attack of dental caries which fact must be recognized, but as many such teeth never suffer from caries we cannot say that it is an all predominating or determining factor in the process of caries.

Second. As regards the effect of the general metabolism upon the life history and caries susceptibility of mature and fully formed tooth tissues information is less positive and more difficult to obtain.

We do know that mature dentin does undergo constructive changes as the result of certain forms of irritation. In such cases the dentinal tubuli which normally are patent and contain organic fibrils, odontoblastic prolongations from the periphery of the pulp, may and do become obliterated by calcific deposition. Accepted opinion holds that the calcium salts so deposited are introduced into the dentinal tubuli from the circulation of the pulp by the process of osmosis. That constructive calcification of the dentin does occur is unmistakably seen in thousands of sections of teeth which have been made.

But can we say, conversely, that under different conditions the circulation thru the pulp may deplete or extract from the dentin lime salts which they contain and thereby weaken the dentinal matrix? We know that in certain forms of rachitis and osteomalacia that mature and formed bone may lose its lime salts and be

come softened. And a priori it has been inferred by many that the dentin may be depleted under similar circumstances. There are two facts which are in contraindication to this view. One, that altho there are several points of structural similarity between bone and dentin, there is reason to believe that dentin is a more stable structure than bone, and is not so dependent upon metabolic changes or nutritional support. When the pulp of the tooth dies and is removed there is but little subsequent change in the dentin. It becomes somewhat more brittle due to the loss of vitality of its organic substance, but the inorganic material is so well organized that the dentin may perform its function for many years after its pulpal nutrition has been discontinued. The same cannot be said of bone. And, second, in the thousands upon thousands of sections of teeth which have been made but few have ever been reported which gave evidence of any deterioration or depletion of formed dentin under systemic malnutrition. We can recognize the constructive changes. why cannot we see the destructive changes if they so frequently occur?

Inasmuch as it is the enamel which is first attacked by the inception of a large percent of dental caries, the theory of nutritional effect of the caries susceptibility of teeth must establish that a degenerative change may take place in the periphery of the enamel as a result of nutritional changes in the pulp. All that has been said as to our present doubt of nutritional depletion of the dentin applies to the enamel also, as any such depleting influence must needs pass thru the dentin on its way from the pulp to the enamel. In addition to this, we have not as yet had sufficient evidence to assume that the enamel contains any set or system of canals, nor any considerable amount of organic substance. When we consider the method by which the enamel is formed and its histological structure the probability of its nutri

tional mutability thru the pulp of the tooth seems very slight. Just as it has been difficult in times past for those observers who hold to this theory to prove that their contention is true, so also is it equally difficult to prove that such is not the case. Our conclusions and present view of this question is that from a histological standpoint, and a careful consideration of the facts in the case we can see little evidence in support of the statement that teeth may become more susceptible to caries because of nutritional depletion by way of the pulp.

Third. But on the other hand, there is abundant clinical evidence that a connection exists between the general nutrition and the condition and structure of the enamel. Every observant operator has seen cases in which the enamel underwent a distinct change in color and structural density co-incident with a change in the general health or metabolism of the individual. So also have they seen a decided change for the better in the enamel of cases which have recently arrived at a time of life when immunity to caries might properly be expected. So that in spite of all theorizing to the contrary there is a just and reasonable ground for believing that the resistant properties of the tooth are dependent upon the general metabolism and nutrition to a greater or less extent and in this connection we have become imprest with the fact that there is a third method or avenue by which this might be accomplished, namely, thru the saliva. By this we mean that there is a possibility of enamel change because of nutritional disturbances which have influenced the character of the saliva and which in turn has produced an appreciable change in the external surface of the enamel. This view of the proble n seemed to be of sufficient importance to warrent us in spending a major portion of our time during the past year in the study of the effect of saliva upon tooth surfaces and its relation to dental caries.

The results which we have to report do not in any way indicate the amount of time and labor which has been expended to make the study a careful and trustworthy one, and altho some of the efforts have met with disappointment, still, in the opinion of those who have been engaged in the work, much of the data obtained has a special significance to the problem of caries susceptibility.

In his work on "Prevention of Dental Caries and Oral Sepsis," H. P. Pickerill describes a graphic method of determining the porosity of the enamel surfaces and the variations in same. This he accomplished by impregnating the enamel surfaces with graphite and silver nitrate taking the depth to which these materials could be introduced as indicative of the relative density and completeness of their organization. His views as to the importance of this porosity of enamel surface in relation to dental caries are stated above.* One of the most significant statements which he made in this regard was that unerupted teeth were considerably more porous than those which had been in place in the mouth for some time. From which he reasoned that the saliva exerted a hardening effect upon the surface of the enamel after the eruption of the tooth.

As to the exact nature of this hardening process we have little accurate information. We may infer that in the unerupted tooth there are certain external openings which are microscopic in their size but patent enough to allow the entrance of fluids such as a solution of silver nitrate. When in the presence of saliva these openings become filled up to a greater or less extent, and are no longer deeply penetrable by the silver nitrate solution, it is very possible that this obstruction of the enamel openings is by deposition of salts from the saliva by a physico-chemical process which is similar to that of petrification or the for

*See page 248.