HOW DENTAL STATISTICS ARE SECURED IN THE METROPOLITAN LIFE INSURANCE COMPANY

¹ Metropolitan Life Insurance Company, Home Office, New York City

In order to present, in generalized form, the relations of hereditary and environmental effects, as they appear concretely in the dentures of the fifteen pairs of twins examined, two sets of graphs were constructed, one representing composites of the X and Y measurements made for fifteen of the twins (fig. 17); another representing composites of the X and Y measurements for the fifteen co-twins (fig. 18). This undertaking involved the necessity of dividing the twins into two groups; one, designated A, containing one of each pair of twins; the other, designated B, containing the second of each pair of twins. It was aimed in this way to distribute differences between co-twins as evenly as possible in the two groups.

The assignment of the twins to these two groups was accomplished in the following manner (fig. 17): The total of the X measurements for each twin was found, and the pairs of twins were put in array. The first, fourth, fifth, eighth, ninth, twelfth, thirteenth, sixteenth, seventeenth, twentieth, twenty-first, twenty-fourth, twenty-fifth, twenty-eighth and twenty-ninth, were assigned to the A group. The second, third, sixth, seventh, tenth, eleventh, fourteenth, fifteenth, eighteenth, nineteenth, twenty-second, twenty-third, twenty-sixth, twenty-seventh, and thirtieth were assigned to the B group. The average X and average Y measurements for each tooth, for each group as a whole, were calculated (Table 35), and these measurements were plotted for the lower arch of the A group to form a graph in the shape of an arch (fig. 18), with a similarly constructed arch for the B group superimposed upon it. A similar procedure was carried out for the upper arch. The degree of correspondence in the superimposed arch forms, and the coincidence of cusps, are measures in a general way of the extent to which the thesis here maintained is supported, when the entire number of twins examined is considered.

Hence, the positions into which the teeth erupt are not determined by the allocation of a kind of " no-man's land" between opposing physical forces, where the inward pressure of the lips and cheeks neutralises the outward pressure of the tongue. Except in individual [See figure in the PDF file] cases where abnormal habits have interfered with intrinsic forces, the arch form, previsioned in the germ cells, is carried to fruition in the adult, and there is very little justification to assume that environment, either intra-oral or extra-oral, under average conditions in life, plays a very great role. Therefore, every interference with the general form of the dental arch may be violating and opposing inherent forces, and our degree of success may depend merely upon the elasticity of Wolff's Law, which says in effect that the growth of bone depends upon the need for it.

This may serve to explain some not infrequent and baffling failures, in the nature of reversions, which confront us subsequent to those [See table in the PDF file] corrective endeavors that aim at an " ideal" or " harmonious" arch form. The stimulation towards development, artificially created by an appliance, may disappear with the removal of that appliance. It does disappear, at least in some cases; and, until the proportion of these to the whole number of successfully treated cases in that group is determined, it is incumbent on us to be more conservative in every way in treatment. For it can no longer be said that with mechanical appliances we are aiding nature to attain an ideal condition which, unaided, she could not produce on account of some extrinsic developmental handicap of epigenetic origin. In the mass of the population the arch form is inherited, and orthodontic interferences purporting to change this developmental limitation set by heredity fall into the category of surgical intervention, and find their justification or refutation on the same grounds that such procedures do, namely:

(1) The possible value of the intervention, and its possible dangers.

(2) The chances of success, computed on the basis of the whole number of treated cases.

(3) The demonstration of a distinct extrinsic or epigenetic cause, which can be counteracted.

The elucidation of these questions devolves upon every individual who practises orthodontia, and it is his bounden duty to justify his procedures. It is not an obligation (as stressed in a recent editorial challenge entitled " We approach an era of extraction") of the geneticist to show which cases are inherited or what the heredity limits for a given case are: the onus is rather on those who practise orthodontia, to justify their procedures in a manner acceptable to statisticians, or to differentiate genetic and epigenetic factors in individual cases.

This brings up anew the old requisite of scientific orthodontic procedure; namely, a classification based upon etiology of types of cases and criteria for distinguishing individual cases. The possibility and method of achieving such a classification has been revealed in this essay. If this modest contribution serves to stimulate increasing contributions to our literature along those lines the costly and time-consuming efforts of the author will have been amply requited. It is manifestly a greater achievement to practise orthodontia in a manner calculated to eliminate reversions, with the goal of an optimum state for the individual in mind, than to strive after a preconceived ideal in every case. The condition of individual optimum does not coincide necessarily with a collective ideal.

The accumulation of the necessary data upon which to found a classification based on etiology, differentiating primarily between extrinsic and intrinsic forces, is one in which the whole profession could participate. If every practitioner merely reported carefully to the literature the identical twins in his practice or acquaintance, sufficient data would be available to found this classification, not only for types of cases but also for individual teeth in three dimensions of space, and to secure criteria for distinguishing in individual cases extrinsic from intrinsic causes of mal-occlusion. In this direction lies our greatest hope of divorcing empiricism from the practice of orthodontia-through a method which constitutes, so to speak, a flank attack upon problems that have defied solution by frontal attack through the orthodox channels of dental research.