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CHAPTER VIII.

Dental Caries.

THE destruction of the tissues of the teeth, commonly known as dental caries, is a process distinctly allied, both in its chemical and bacteriological aspects, to the general phenomena of putrefaction.

Dental caries, like putrefaction, is rarely caused by one species. of organism, and the reaction of the medium in both caries and putrefaction undergoes fluctuation from acid to alkali; and in both processes an alternation of species takes place according to the phase of phenomenon.

The disintegration of enamel, dentine, and cementum is brought about in the first instance by the action of various organic acids, mainly lactic, produced by the vital activity of bacteria; subsequently the various digestive ferments also produced by the organisms come into operation, dissolving the decalcified matrix of the cementum and dentine. What, however, occurs in the first stages of dental caries, and why certain races of men and the majority of animals appear to be immune, is a much more difficult matter to elucidate.

Artificial caries, so well demonstrated by Miller, is by no means difficult to reproduce by exposing cubes of dentine to the action of micro-organisms of the acid-producing class in solutions containing carbohydrates.

Caries of enamel is much more difficult to reproduce artificially, uniform denudation of the enamel generally taking place in carbohydrate cultivations; and as the process of natural caries generally originates at some point of the enamel surface rather than over the whole tooth, much interest centres around the initiation of the process.

Very many theories have been advanced at various times to explain the commencement of enamel destruction, ill-developed and pathological conditions of the enamel structure, deficiency of

lime salts said to be associated with insufficient calcium salts in the water of a country-diet, civilisation, alteration of physiological relationship in position of the teeth in the jaw from evolutionary causes, &c. Many, if not all, of these factors may have their place in predisposing causes, but the ultimate liberating cause is often overlooked in the multiplication of predisposing ones.

Read1 has lately advanced the proposition that caries may be due to the alteration in the sort of flour used in bread-making. Thus the finer roller-ground flour has to a large extent replaced the old stone-milled article; and Read states that the amount of acidity in terms of lactic acid produced by chewing bread made from either sample is much greater in the case of the finer roller flour.

Samples of bread made from the two flours were chewed for equal lengths of time and the acidity determined; in every case the acidity of the roller flour was in excess of the stone-milled. That such an acidity might predispose to caries is by no means improbable, but the subject is not yet sufficiently investigated to draw definite conclusions.

Sim Wallace considers that "the cause of the prevalence of dental caries is that the natural food-stuffs are to a large extent ridded of their accompanying fibrous parts," and that due to the same cause "the micro-organisms of the mouth lodge and multiply, and augment the rapidity and intensity of the acid fermentation."

That such a factor does occur is certain, but it hardly explains the fact that animals, dogs, cats, monkeys, rabbits, &c., which exhibit no dental caries, are yet found to have food particles frequently lodged between their teeth after a meal, the fibrous matters themselves remaining impacted. Interstitial caries is rare in animals and yet commonly occurs in man.

The researches of Miller, Mummery, and more recently Leon Williams, have shown that the enamel surface in sheltered positions is often covered with a film-like layer of bacteria, and that under such a layer definite disintegration of the enamel is taking place. Moreover many of the bacteria found in this film may be seen in the spaces hollowed out between the enamel prisms; the spaces evidently have been formed by the action of the acids produced by the bacteria; moreover spreading from these points of attack

1 Journ. Brit. Dent. Assoc., 1900.

2 "Cause of Decay in Teeth."

and passing inwards to the dentine may be seen organisms well in advance of the general process.

The acid produced by the bacteria, although in small quantities, would eventually inhibit the growth of the organisms were it not neutralised as soon as it is formed by union with the lime salts of the enamel, each fresh increment of acid attacking a fresh portion of tissue, the resulting products diffusing into the saliva.

Miller figures several examples of this bacterial layer attached to the surface of enamel, and also shows the organisms permeating the

FIG. 46. DENTAL CARIES. SECTION OF ENAMEL WITH LAYER OF ATTACHED ORGANISMS AND FORMATION OF PITS BETWEEN ENAMEL RODS.

Photomicrograph and specimen by Dr. Leon Williams. x 500.

enamel substratum. Miller also showed, by a series of admirably conducted experiments, that the familiar appearance of caries is due to the various acids evolved from the carbohydrate constituents of a normal diet.

Leber and Rottenstein endeavoured to produce artificial caries by placing normal teeth in a mixture composed of the various constituents of a normal diet. They failed to produce caries experimentally, and came to the conclusion that it was not the result of bacterial activity. They admitted that their experimental

flasks, &c., gave off obnoxious smells, and that ordinary putrefaction occurred, in fact they abandoned their experiments largely on this account. Miller however by so adjusting the condition that a constant acid reaction was present, obtained strikingly confirmatory results of the rôle played by organisms producing acid fermentation in the pathological condition known as dental caries.

Leber and Rottenstein's failure is of easy explanation, but although they failed in their main object their work is of considerable importance in the etiology of caries as demonstrating by a negative result the necessity of an acid fermentation to initiate the process.

In the ordinary phenomenon of putrefaction an acid reaction may be present, but very soon gives place to distinct alkalinity and the evolution of evil-smelling gases-indol, skatol, sulphuretted hydrogen, and, as a rule, ammonia.

The phenomena of putrefaction are by no means all produced by a single species or race of organisms, one succeeding another as the conditions suitable for their development arise (see p. 21).

But more important still is the observation of Maly,1 who took the mucous membrane of the stomach, placed it in a solution of cane sugar and kept the mixture at the body temperature for several days. The lactic acid arising from the decomposition of the sugar was neutralised from time to time, and it was found that the process of acid production continued until all the carbohydrate present had been converted into lactate; and then, and not till then, did putrefactive odours become manifest.

This, then, fully explains the failure of Leber and Rottenstein, and the success of Miller's experiments.

It will be noticed, moreover, that the acid produced by the organisms requires to be neutralised from time to time; otherwise, owing to the increased acidity of the fluid, the organisms cease their activity, the very acid produced acting as a check to the growth of the bacteria producing it.

In examining the statistics of the incidence of caries as occurring in various races of mankind, and in considering the diet of such people, we find that those races whose diet is mainly meat show a far smaller percentage of caries than do the races whose diet is principally carbohydrate.

'Herman's Handbuch, Bd. v. (2), S. 239.

It is moreover observed that the teeth of persons suffering from enteric and other diseases necessitating a prolonged subsistence, during convalescence, on a diet composed of a considerable proportion of carbohydrate, often show an increased amount of caries unless most careful cleansing of the mouth is carried out by the attendants. Millers, bakers, and persons engaged in sweet-stuff factories invariably exhibit a marked amount of caries; in millers and bakers particularly, marginal cavities are common. Caries is moreover more common in children of both sexes than in adults, and more frequent in adult woman than in man. All the various questions relating to fermentation point to food-stuff composition as of great importance in the etiology of dental caries, but in applying this general principle we are met with several obstacles which apparently militate a good deal against the acceptance of the theory of food-stuff origin of caries.

One of the chief and main difficulties in discussing the whole problem is the attempt to explain the process of dental caries without having regard to the importance of several rather than a single predisposing cause; the explanations advanced by various observers are none of them sufficient per se to elucidate the problem in its entirety, but taken together a very good working hypothesis may be obtained. Dental caries is not a specific disease due to a certain specific micro-organism; it is no definite "entity," but a process occurring through the operation of certain biological and physical laws.

It has been suggested by some observers that a deficiency in the amount of lime salts of teeth may contribute to the early development of dental caries.

Black, and later C. Tomes, estimated the percentage composition of a large number of teeth and found that there was no appreciable variation, and that there was no evidence that the incidence of caries could be associated with a decrease in the percentage of the lime salts present.

It is known however from an empirical point of view that some teeth are apparently more liable to the inroads of bacteria than are others, although the researches just cited show that the susceptibility to caries probably does not lie in the lime salt content.

Leon Williams has demonstrated the plaques of micro-organisms on the enamel surface of teeth, and the incipient caries occurring under these plaques, and I have myself constantly observed these