Degeneracy: Its Causes, Signs and Results

by Eugene S. Talbot

FIG. 90.

There are three characteristics of the normal arch. Independent of temperamental peculiarities, the line extending from one cuspid to the other should be an arc of a circle, not an angle or straight line; the lines from the cuspids to the third molar should be straight, curving neither in nor out, the sides not approximating parallel lines. Absolute bilateral uniformity is not implied in this, as the two sides of the human jaw are rarely, if ever, wholly alike. A uniform arch necessitates uniformity of development between the arch of the maxilla and the arch of the teeth and a correct position of the individual teeth in their relation to each other. When there is inharmonious development between the jaws and the teeth, as may happen when one parent has a small maxilla with correspondingly small teeth, and the other a large one, with correspondingly large teeth, if the child inherit the jaw of one and the teeth of the other, irregularities must follow. Such difference in diameter between the arch of the maxilla and that of the crowns of the teeth is a constitutional cause of irregularity. When there is a difference between these diameters the line formed by the teeth must either fall outside or within the arch of the maxilla and irregularities of arrangements result. The primary division of irregularities is the V-shaped and saddle-shaped arches. We have the V-shaped variety (Fig. [90], one of the typical forms), where the apex of a triangle is formed by the incisors, the base of the triangle being a line connecting the first two molars. If, because of premature or tardy extraction, the first molars move forward, or by coincidence of the arch of the maxilla and the arch of the crown of the teeth in trying to accommodate itself to the lesser arch of the maxilla, the arch becomes a broken line, forming an angle at the incisors. This angle results from two causes: the thinness of the process at this point and the diminution of resistance which must follow.

When the permanent bicuspids erupt under a favourable condition, so that their greatest diameter is in a line with the greater diameter of both cuspids and first molar, they will be held firmly in place, since the greatest pressure is on this very line. On the other hand, when the bicuspids are erupted after their proper time, while the cuspids progress duly, and meeting no resistance fall into their proper places, but the bicuspids adapt themselves as best they can to the space left for them, and if the arch of the maxilla does not coincide with that of the crowns, they must fall within or without the arch. Now, if the first molar have moved forward, diminishing the space, the bicuspid must erupt either within or without the arch.

FIG. 91.

To understand why they are generally found within the arch, the shape of the molar and cuspids must be kept in mind. A transverse section of their crowns shows their proximal walls not to be parallel, but wedge-shaped, their diameter being greater on the buccal than on the palatal side. When the crowned bicuspid falls within the greatest diameter of these teeth, finding more room within the arch, they naturally slip in the direction of least resistance, i.e., toward the palate. A local cause for the same condition is found in the fact that the crown of the bicuspids, before their eruption, was held between the roots of the temporary molars, and, as these form an arch of a smaller circle than that of the permanent teeth, the bicuspids will be found generally inside the arch. From both causes occurs an inward curvature, which is termed the saddle-shaped arch (Fig. [91]). It should be noted here that, since the V-shaped irregularity is found anterior to the cuspid, the upper incisors are always projecting beyond the lower; the saddle-shaped irregularity is invariably posterior to the cuspid from an inward curve. The incisors never project. Both forms contract the arch; the V-shaped anteriorly, the saddle-shaped posteriorly. In both forms the forward movement of the first molar is the local cause.

Deformities of the dental arch are due, first, to arrest of development of the jaws, and, second, in the nature of the deformity, to the order of eruption of teeth, which rarely erupt twice alike. From an evolution standpoint these deformities are atavistic. The V-shaped reverts to the reptilian type; the saddle-shaped to the lower mammals. In the gorilla, the nearest to man in dentition, there is a very distinct approach to the saddle shape. In the chimpanzee it remains. The orang-outang exhibits less of this tendency. The arch of some of the cebidæ very nearly approaches man. It all depends upon the extent of prognathism. When that is reduced the arch appears and rectangular arrangements of the teeth are lost. Most carnivors exhibit a distinct approach to the saddle shape. Some felines have a shortening of the jaw, partly obliterating the tendency, but in most canidæ it is quite marked.

These are facts which cannot be overlooked, since, from the very nature of development and eruption of the teeth, they cannot take any other form. The arrangement of the crowns of the cuspid (canine) in the jaw before eruption is such that, no matter what the local condition of the jaws or teeth may be the V-shaped or saddle-shaped dental arch must be produced.