CHAPTER IV.

PHYSIOLOGY AND PATHOLOGY.

THE movement of teeth in correcting irregularities is based on an anatomical and a physiological fact. The anatomical, that the teeth are placed upon the maxillæ surrounded by vascular, elastic, bony processes, which are easily moved, absorbed, and reproduced; the roots penetrating but little into the true maxillæ, and in their movement affecting the maxillæ but slightly if at all.

The physiological fact being that bone will yield or become absorbed under certain influences, and also be reproduced.

That teeth can be moved and become firm in their new positions, and that they are moved frequently at nearly all periods of life, is a matter of common observation.

Teeth which have been used as supports for a plate with clasps are very often, through the bad adaptation of the plate, or through the strain arising from mastication on the artificial teeth, drawn away from their original places, and assume and become firm in new positions.

In like manner we frequently find, where scattered teeth are extracted from both jaws, the remaining ones change position and assume new relations on occlusion.

So, from one cause or another, there is more or less. change of position going on through life, and not inconsistent with their healthy retention in the jaw nor their firmness.

In moving teeth the power used creates a pressure which produces absorption.

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FUNCTION OF ABSORPTION AND REPRODUCTION.

The function of reproduction is nature's means of coming to the rescue and restoring lost parts.

In correcting irregularities it is not probable that there is any lateral movement at the apex of the root. In that respect it is virtually a fixed point. The tooth may be driven up into its socket under pressure or elongated under strain, but rarely, if ever, do we find evidence of lateral movement at the apex. The greatest motion is at the cutting or grinding ends, and the least at the apices.

The only exceptions I have ever seen or thought I saw were where, in consequence of loss of front teeth, the molars have seemed to travel bodily forward without tipping.

The function of absorption and reproduction may or may not go on coincidently, simultaneously, and with equal rapidity.

That bone will become absorbed under pressure, and that bone must be deposited to fill up the socket of a displaced tooth before it will become fixed in its new position, is used as an argument that such a state exists always as a consequence of change in the position of a tooth, and that the success of the movement is dependent upon both these functions. In a slow movement this is probably the case, in a rapid movement it is doubtful.

To account for certain results which have been accomplished by this theory alone, we must believe that the function of reproduction goes on with greater rapidity than has ever been proved.

For example, the superior dental arch has been frequently enlarged to a considerable degree within a very few days.

If the enlargement depended solely upon absorption and reproduction, it would necessitate an entire change of substance in the external alveolar walls, as the movement has often been sufficient to displace the process entirely; but we see the bony covering remains intact, neither impaired nor perceptibly diminished. By its integrity the vital connec

MOVEMENT NOT DEPENDENT UPON ABSORPTION.

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tion and condition of the teeth were maintained until nature filled up the sockets behind them.

In a case now under observation, it is but forty-eight hours (at the present writing) since I applied a fixture across the roof of the mouth of a miss of thirteen years, for the purpose of widening the dental arch.

It has already, by careful measurement, accomplished that result to the extent of half the diameter of the bicuspids.

The patient says there has been no tenderness sufficient to disturb her sleep nor interfere with her mastication.

No one for a moment will suppose that, coincident with that outward movement, there has been an absorption of the external alveolar wall to the extent that the movement indicates.

If the movement is due alone to absorption, then reproduction must be equally rapid, as the external alveolar walls bear all the evidence of undiminished integrity.

Nor is it altogether explained by the theory of Mr. Tomes that the pressure has lifted the teeth partially from their sockets, and, owing to the conical form of their roots, they have simply moved against the farther wall. This hypothesis may account for a limited lateral movement, but if there were elongation we should discover it in soreness or otherwise on the occlusion of the teeth. The circumstances are analogous to the separation of teeth with wedges preparatory to filling proximal cavities, which is performed every day by dentists and at all ages.

In such cases any elongation of the teeth acted upon would be immediately noticed.

Patients are very susceptible to the most trifling elongation, as is daily evidenced in finishing off fillings which are inserted upon grinding surfaces, the removal of the diameter of a hair often being perceptibly noticed.

If, therefore, this lateral movement of the teeth were to be attributed to their being lifted from their sockets, its effects would be shown in every case where the teeth were

60 MOVEMENT DEPENDENT UPON ELASTICITY OF THE ALVEOLI,

spread for filling, where there were antagonizing teeth; but, so far from this being the case, we often find by the complaints of patients that the soreness is not confined to the teeth in contact with the wedge, but is distributed to several contiguous teeth; very often the patient saying that a tooth two or three removes from the wedge is the most painful.

Furthermore, the slight interference with the natural occlusion of the masticating surfaces while the wedge is in the mouth, is fully accounted for by a purely lateral movement which breaks up temporarily their perfect articulation.

In a reverse movement, as for example the contraction of the superior dental arch, we find no such rapid progress is made as in an outward movement, for the reason that the large body of bone resists the pressure as against both expansion and compression.

It is probable that in the last-named movement we make progress no faster than the bone is absorbed, and here we get another proof that the function of absorption progresses more rapidly than that of reproduction; for, while we may be but a few weeks in carrying a tooth to a certain position, we find it is as many months before deposition of new bone has made it solid in its new location.

The enlargement of the arch can be accomplished with great rapidity and with perfect safety; so rapidly as to preclude the idea that the external walls of process are broken down by absorption to permit it, and the only conclusion is that the vascularity of the alveoli permits an elasticity which allows the teeth to be moved outwardly, carrying the external processes along with them.

This process is not necessarily absorbed at any subsequent period, simply because pressure has been brought against it. It has not been stretched beyond its powers of elasticity, no sequestrum has been formed, and it will remain the same process and continue to perform its functions until deposition of new bone shall have filled up to it and the whole become solid.

AS WELL AS UPON ABSORPTION.

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Nevertheless there must be a limit to the rapid movement of teeth outwardly, lest destruction of the process

ensue.

In the movement of the anterior teeth of the lower jaw it is not probable that absorption plays any more important part, either in expanding or contracting the arch. The processes are so thin upon both external and internal surfaces that they would be likely to yield before they became absorbed, and therefore the principal action would be one of reproduction of bone to make them solid.

The movement of a single incisor or canine of the lower jaw can be made inward or outward with much the same readiness, save only that in an inward movement the tooth is apt to bind on an adjoining tooth, and thus retard it. The resistance of the internal and external alveolar wall is much the same.

It has been maintained by some that teeth can not be moved with safety faster than bone can be absorbed or reproduced, but an extended experience has shown that in the outward movement of the superior arch and in the movement in either direction of the anterior teeth of the lower jaw, the idea is fallacious.

The functions of absorption and reproduction vary, however, very much at different ages and under different systemic conditions, but are most active during the formative period of the tissues, so that operations undertaken in youth with impunity could not be carried out at other periods of life without bringing the vitality of the organs acted upon into jeopardy.

It may be assumed as a rule that as soon after eruption as it becomes certain that an irregular denture is inevitable, there is no longer justification for delay, and that after that period every year increases the difficulties, both mechanical and pathological, and prejudices the stability of the dental apparatus. Consequently, if at the age of eight years one of the superior centrals has locked within the lower teeth, im