GOLD BANDS AND VULCANITE.

87

loop acts to contract the circle by drawing the ends of the wire together. Such an appliance would act only on the teeth, but in the following illustration, Fig. 37, a similar

principle is used in a case of maxillary fissure connected with

a fissure of the palate.

There was a wide gap

between the lateral incisor and the

FIG. 87.

canine tooth, and the opening extended through the alveolar arch and into the nasal passage.

It was desired to bring the maxillae into contact, and a

88

READY INVENTION A NECESSITY.

fixture like the last would only act upon the teeth, tending to give them a wrong inclination. This fixture was made to embrace the gum as well as the teeth with the wire running through the length of the vulcanite to give both stability, and the terminal hooks were drawn toward each other when in situ, with silver wire wound around and twisted with a pair of pliers. In the process of twisting, the gap was seen to perceptibly close and ultimately came together and united. The application of these principles to the movement of teeth is one of the most responsible duties the dentist is called upon to perform.

Each and every one of these mechanical powers can be made to do our bidding; and, equally, each one of them may become a formidable engine of disaster.

When applied in the mouth they should have constant watchfulness and care. Not one of them but in the hands of empirics would cause the destruction of those valuable organs they can be made to conserve.

To any one who has become deeply interested by experience with these cases, there is a fascination about them which will lead him to new experiments in the treatment of almost every case.

In fact, success depends quite as much on ready invention of means to meet any emergency as upon the exercise of any knowledge, or the use of any appliances already proved.

CHAPTER VI.

IMPRESSIONS AND MODELS.

In all complicated cases of irregularity, impressions and models are a preliminary necessity in making suitable appliances for correction; and in a majority of cases correct models of both upper and lower jaws, properly articulated, are a valuable aid in making a diagnosis of the case. The positions of the teeth, within or without the line, their inclination, rotation, and articulation, can be much more closely studied from models than in the mouth. Often it will be seen that an opinion formed upon the observation of the living structures may be an erroneous one when the models are closely studied.

The probable effect of the change of position in the side teeth can not always be prognosticated by looking at them in the mouth; but by observing accurately how they articulate in a model, and carefully noting the inclination of the occluding surfaces and the tendency of the same surfaces if the proposed change were made, a much better opinion can be formed of the propriety of movement. Nothing is more injudicious than the moving of teeth into a position which can not be maintained by the articulation of the opposing jaw; and nothing is more powerful in carrying moved teeth back to their former state than the continued action of inclined surfaces in ordinary mastication, favoring such return.

A proper knowledge of the case requires models and impressions, and such models are better taken in plaster of Paris than in any other known substance. While the merits

90

PLASTER VERSUS PLASTIC SUBSTANCES.

of plaster are generally conceded, there is also a feeling that it is especially or only adapted to mouths denuded of teeth, and that its use where teeth are scattered or straggled is inapplicable. Experience proves the reverse of this to be the fact. Plastic substances, such as wax, gutta-percha, etc., are only adapted to simple surfaces; and in no case where there are intricate, devious, or tortuous details can they be relied upon. Their very nature-namely, their plasticity-is what condemns them; and the opposite characteristics of plasternamely, hard, rigid, unyielding, brittle-are those which make it peculiarly applicable for impressions in all difficult cases. That it will adapt itself accurately to all the details of form and irregularity, and will break with a clean, sharp, welldefined fracture on removal, are the peculiar qualities which stamp its superiority.

Therefore it is that the most difficult cases are those which require plaster, and in the more simple ones there is nothing as a rule to make plastic substances a necessity.

An objection has been made to plaster on account of the difficulties attending its use; but, with an experience far less than is required to become proficient in most skillful manipulations, it will be found the most simple and accommodating of substances. This false notion has led to innumerable inventions of steps preliminary to the use of the plaster. A variety of cups or trays have been constructed especially designed for plaster, but the proper use of the material does not justify any such especial inventions. The common form of tray made of britannia, which can be changed in shape somewhat by bending, equally applicable for wax, is the most simple as well as the best that has been introduced for taking impressions of all cases except those of most extraordinary oral deformities. Neither is the preliminary operation of taking an impression in wax, cutting out a portion of the wax and supplementing with plaster, or any other preliminary device, at all necessary. A simple cup of an approximate form, and only large enough to inclose the parts from which

HOW TO TAKE PLASTER IMPRESSIONS.

91

an impression is required, is all that is necessary. To put the plaster in such a tray, carry it steadily to its place, and remove it at the proper moment, constitutes the whole proceeding.

But while the process is a very direct and a very simple one, success will come only from a careful attention to the details. The plaster must be fine and strong, and set with moderate rapidity. The so-called "dentists' plaster" in the market has not proved in the author's experience to possess any advantages. The quality known among the manufacturers as "superfine" is far preferable. When in proper condition—that is, where it has not become deteriorated by long exposure to a damp atmosphere-it possesses all the desirable qualities for impressions, models, and general use. For ordinary use it sets with all the rapidity that proper manipulation will permit, while this property can be much accelerated by the addition of a little salt.

The details of taking an impression of a case of irregularity can be best met in substantially the following manner :

Select a cup which shall just inclose all the teeth required to be shown, and bend it to the general conformation of the dental arch. If it is an old cup which has been often used, it is preferable, as the roughened surface facilitates adherence of the plaster; if a new one, the inner surface should be made quite rough, so that the cup will not be liable to come away, leaving the plaster in the mouth.

A common stoneware coffee-cup is the most suitable vessel for mixing the plaster in, and a small table-knife the most suitable instrument. Fill the cup about one third full of tepid water (tepid water, only because it is pleasanter to the mouth; not hot water, because that affects the time of setting),. and throw in a pinch of salt, the quantity necessary being determined by a little experience in working that particular quality of plaster. Shake into the water all the plaster that will readily sink under the surface, stir the mass until it is observed that it is becoming stiffer, and place it in the tray