flatten at their points of contact sufficiently to permit of their standing alone. If, now, several of these stacks or columns be placed in close contact, some idea may be obtained of the structure of enamel.

The enamel rods are stronger than the cement substance which binds them together, so that in breaking or cutting the enamel it is inclined to split along the length of the rods. This, then, is known as the line of cleavage, and is a most important feature in cavity preparation. It is surprising to what extent the general rule applies that The direction of the enamel rods is from the center of

FIG. 1.

nature's effort to so present the ends of the rods as to best withstand the stress of mastication, or possibly more properly, incision or biting.

Our second diagram (Fig. 2) represents a central perpendicular section bucco-lingually through a bicuspid. In all of its parts the enamel rods are cut parallel with their length, but they do not all stand at right angles to the surface of the tooth. They do so most nearly in the central portion of the buccal and lingual surfaces, but as we near the region of the points of the cusps from both the occlusal and axial surfaces we find considerable inclination of the rods toward the points of the cusps, while

FIG. 2.

the crown of the tooth toward the surface, or, to modify this for practical purposes, that The enamel rods are most commonly at right angles to the surfaces on which they are found. In every tooth, however, there are certain portions in which the enamel rods approach the surface at more or less of an inclination from such a perpendicular.

With the aid of the charts I shall endeavor to set these before you.

In Fig. 1 we have represented a central perpendicular section through an incisor. You will note that the enamel rods are cut parallel with their length in all of its parts, but they are not all at right angles to the surface of the tooth. They are nearly so in the central portion of both the labial and lingual surfaces, but as we near the incisal there is sharp inclination in what would seem to be

in the region of the groove we find an opposite inclination. This, again, seems to be nature's effort to present as strong a "front" as possible toward the source of the stress of mastication.

The dotted lines extending from the horns of the pulp to the dento-enamel junction directly under the points of the cusps represent the recessional lines of the pulp, or the path along which the horns of the pulp receded as the tooth developed from its earliest to its present or adult stage. Time does not permit my going into this in proportion to its important bearing on cavity preparation. I can only say that their location in all teeth should be carefully studied.

It is here that unintentional exposure of the pulp very often occurs, for the reason that in the process of the development of the tooth and recession of the pulp this tract is not always completely

EXTENSION OF CAVITY MARGINS FOR THE PREVENTION OF RECURRENCE OF DE

CAY.

In taking up the second great principle mentioned in my opening statement, namely, extension of cavity margins for the prevention of the recurrence of decay, we must again lay a foundation by a short journey into other realms. This time it is pathology.

With reference to the manner in which teeth decay, there are two great groups, namely, pit and fissure cavities and smooth-surface cavities. The former are found wherever pits and fissures occur, i.e. upon the occlusal surfaces of bicuspids and molars, upon the occlusal third of the buccal surfaces of both upper and lower molars, upon the lingual surfaces of the upper anterior teeth -more often the laterals-and upon the occlusal third of the lingual surfaces of the upper molars where the disto-lingual groove extends over the marginal ridge on to the lingual surface and terminates in a pit. In a word, they are decays starting in structural defects.

Smooth-surface cavities, the other great group, and comprising by far the greater number of cavities, occur, as the name indicates, upon portions of the surfaces of teeth which do not present pits or fissures for the beginning of the process of decay. The process of decay, so far as the dentin is concerned, is identical in both of these groups, but the processes in the enamel are quite different and require different treatment in cavity preparation with reference to "extension for prevention."

Pit and fissure cavities occur in surfaces which are kept habitually clean by the excursions of food and saliva over their surfaces, but which, because of structural defects, permit lodgment of the micro-organisms in the recesses of the pits and grooves. Here they cover themselves with a gelatinoid substance which makes their hiding-place secure, so to speak. Their nest or nidus is thus

Their acid products dissolve out the lime salts which hold the enamel rods together, and thus form a channel for the progress of the micro-organisms toward the dentin and a general undermining of the enamel beneath the surface. This is termed backward decay. The only evidence upon the surface of such a process may be a darkened line in the groove or a spot in the pit, pressure upon which with a cutting instrument may cause the surface to cave in.

The process upon reaching the dentoenamel junction finds less resistance here. than in the enamel, and the spread is more rapid. Because of the inclination of the enamel rods toward a groove or pit, we have the cone-shaped decay in

FIG. 3.

a

the enamel here, with its apex toward the surface and its base toward the pulp. The decay penetrates the dentin in the form of a cone with its apex toward the pulp, and its base toward the dentoenamel junction. Someone asks, Why cone-shaped in the dentin? Because the decay had its commencement in the center of the area involved at the dentoenamel junction. The dentinal tubules being entered there first, the micro-organisms penetrate farthest because of their early start. This whole process is shown diagrammatically in the occlusal portion of the tooth in Fig. 3, a. cone is larger in the dentin than in the enamel because the dentin is preferable to the micro-organisms, is softer, etc.

The

In the preparation of cavities of the pit and fissure group, it is only necessary to remove the enamel covering the area undermined by caries, and form the cavity in the dentin. Further extension for

prevention of recurrence of decay is not necessary, except where it may be required to follow out deep grooves to places where a smooth margin may be given the filling or restoration.

Smooth-surface cavities have their beginnings on surfaces which are free from structural defects. Micro-organisms must therefore attach themselves to the surfaces in areas which are more or less hidden from the excursions of the food and saliva, as in the proximal surfaces, and in habitually unclean areas, left so by scant use of the toothbrush, as at the gingival of the buccal and labial surfaces. An example is shown on the diagram, Fig. 3, at b. Here again the micro-organisms collect and cover themselves with a gelatinoid substance, their acid products dissolving out the lime salts of the enamel, but because of the difference in the inclination of the enamel rods as compared with those at a, the decay spreads laterally on the surface, each part of the widened area of beginning decay penetrating in the line of the length of the enamel rods, which gives a conical area of decay with the base of the cone at the surface, the apex toward the pulp. This, you will note, is just the reverse of the process at a, while the decay in the dentin is identical. Or, to repeat, immediately the enamel is penetrated, decay spreads laterally along the dento-enamel junction in every direction. As each new dentinal tubule is reached, a spreading of the decay along it toward the pulp of the tooth occurs. In the first tubules entered it goes deepest because begun there first, this giving a conical area, as shown at a.

The extension of such a cavity as is shown at b from the center of the surface far toward the mesial and distal angles of the tooth is so necessary that the rule should be to cut close to the angles in every case, but never past the angles. Herein alone lies safety, for if this is not done, and susceptibility to caries continues, decay is almost sure to recur to the mesial or distal or both, and the filling will be lost. Why to the angles? Because this is an area of natural immunity. I might mention that in a record

of ten thousand cases examined in the dental clinic of the Northwestern University at Chicago but nine were found. to have caries beginning at these angles of the teeth, and all of these had some natural or acquired fault of occlusion -teeth twisted, not occluding, or abscessed, thereby preventing normal use. In laying the gingival wall of your cavity you should be guided by the age of the patient. If such cavities are filled in young persons without pushing the gingiva well back so that the margin of the cavity can be extended well under it, the natural shrinkage of the gingiva as age advances will expose the enamel between the gingiva and the filling.

THE SEVEN STEPS IN CAVITY PREPARATION.

To some of you gentlemen, grown gray in the practice of your profession, it may sound cumbersome when I say that Black's system contemplates seven distinct steps in the preparation of cavities in the teeth. Both from my experience as an accountant and as a dental teacher, I am most willing to subscribe to the principle that "any system is better than none." This in no sense is an apology for the one I am expounding, for if I were not satisfied with its merits I should not teach it. I can go farther, and say, for the comfort of any who will adopt them, that with increased use these steps become almost second nature. We follow their sequence without realizing it, even as we drive our automobiles. I can at this time scarcely do more than recount, and give a brief explanation of each step.

(1) Outline form. This, in a word, is the mental picture we form of the area to be included in the finished filling or restoration. Yes, I grant you that we may have cause to change this as we open up the cavity, but is it not better to work with a definite end in view than to just cut and drill away until we conclude "That will do"? Conditions may not permit the preparation of an ideal cavity in many cases, but is it not better to have an ideal toward which to work,

even if we cannot attain it? Outline form, then, is the form of the area of the tooth surface to be included within the outline or enamel margins of the finished cavity. The laying out and cutting to these lines should be the first thing considered and accomplished. Here is where we first have use for our knowledge of the inclination of the enamel rods, as well as extension of cavity margins for prevention of the recurrence of decay. If the cavity be of the pit and fissure class it means, in addition to breaking down all undermined enamel, the following out of grooves to a point where a smooth finish may be given the margin of the filling. If of the smooth-surface group, it means the laying of cavity margins in areas of immunity or those areas kept clean by the excursions of food and saliva over their surfaces.

(2) Resistance form. This is the form given a cavity to best enable the restoration or filling to withstand the stress of mastication, and is accomplished in its highest form by combining a flat seat or floor for the filling, cut at right angles to the direction of the stress of mastication-which is usually at right angles to the long axis of the tooth-together. with definite angles at the floor junction with the surrounding walls. A round bur has no place here. This, then, is our first encounter with the third important principle mentioned in my opening remarks, namely, the establishment of definite angles at the junction of the surrounding walls with the floors of cavities.

(3) Retention form is the form given a cavity to prevent the filling from being displaced by such lateral or tipping force as may be brought against it. In simple cavities the retention form is obtained by so placing the opposing walls as to make them strictly parallel, or slightly-and I want to accent that word "slightly" undercut in order that the filling material may be firmly held in place. Retention form is required in its most perfect aspect in proximal cavities on bicuspids and molars, and is made in the form of a step cut over on the occlusal surface in more or less the shape of a dovetail. Let me impress upon you that in

the use of this step you do away with unnecessarily deep cavities on the proximal surfaces, and it should always be used, unless, perchance, the tooth which would ordinarily approximate the filling is to be permanently absent. Formerly, pits and grooves were depended upon for retention form, but they have proved "a delusion and a snare." The pot-leg, made usually with a round bur, has no place in modern dentistry.

(4) Convenience form, though not of such prime importance as other steps, should not be neglected. It represents certain modifications in a cavity which will render the form more convenient for placing the filling material; often, by cutting a wall to a certain inclination the plugger point will reach some portion, or at a more available angle, enabling the operator to pack gold more accurately and securely in certain otherwise inaccessible parts of the cavity. This is exemplified in the proximal cavity on a cuspid by cutting past the mesiolabial angle somewhat on to the labial surface.

A second order of convenience form consists of slight undercuts (not potlegs) placed usually in point angles to serve as starting-points for packing gold. A point angle in dentistry is where three walls meet. In proximal cavities they are placed in the axio-linguo-gingival and axio-bucco-gingival angles, and should rarely be placed below the level of the gingival wall. They should be cut at the expense of the buccal and axial, or lingual and axial walls. They should be in the shape of a wedge or cone with its base at, not in, the gingival wall, and tapering off to nothing half-way up the axial wall, so to speak.

(5) Removal of remaining carious dentin. Before proceeding to this step the rubber dam should be applied, for should the pulp be exposed in the accomplishment of this operation we find ourselves in better position to master the situation, and proceed in more nearly aseptic surroundings.

Generally, when the cavity has been given the treatment thus far mentioned, there will be no carious dentin remain

ing, but in the larger cavities it will often be a question whether or not the pulp will be exposed in removing the carious dentin overlying it. Do not, therefore, cut toward the pulp until the cavity is otherwise prepared, as it will then be easier to give an exposed pulp the treatment which may be indicated. When this stage in cavity preparation is reached, any remaining softened dentin may be removed with large spoon excavators. Very often, by working around the edges, a large cake of leathery decay may be lifted off a pathological pulp almost painlessly, whereas the patient would not allow you to "get near it" by going in at the center.

(6) Finishing the enamel walls and beveling the cavo-surface angles. If your treatment of the enamel walls thus far has been done with a proper observance of the inclination of the enamel rods, there will remain little to be done now so far as smoothing is concerned, other than running over them with a planing motion with a sharp chisel. When the walls have been satisfactorily smoothed the cavo-surface angle of the enamel should be cut to a distinct bevel outward, also by a planing motion with a chisel, enamel hatchet, or gingival margin trimmer, used lightly. This bevel should include about one-fourth the thickness of the enamel wall, and should be at an angle of from six to ten centigrades from the plane of the enamel wall. The object is to cut away any loose ends of or any unsupported enamel rods which might afterward fall away and render the margin imperfect. We have all seen the dark outline surrounding the other fellow's fillings. A second object is to safeguard the cavo-surface angle against possible checking in packing filling material. I am asked so often if this may not be done with small stones and even fissure burs in the engine, that I am going to answer right here, No, not in my hands, for with such an agent I lose that tactile feature which is so valuable in this delicate work. If I were to use a stone at all, it would be an Arkansas stone, and that only succeeding my cutting instruments, for I would value only

its polishing characteristics. Sometimes, when the walls are just right, a very fine polishing strip may be used on cavities in the anterior teeth, but this merely supplements, not supplants, the sharp cutting instruments.

(7) Cavity toilet consists in freeing the cavity from chips and dust then remaining. This should be done by wiping the cavity in all its parts very thoroughly with dry cotton or spunk held in the pliers. Do not use alcohol, for after drying with the air-syringe something will be left coating the walls.

Let me give you this final rule with all the force at my command: If, after all the cutting is done, moisture of any kind should enter the cavity, that portion of the cavity should be thoroughly dried and freshened with the cutting instruments.

PREPARATION OF CAVITIES BY CLASSES.

You will remember I said that with reference to the manner in which they decay, cavities are divided into two groups, pit and fissure cavities and smooth-surface cavities. Now, with reference to the similarity of treatment and instrumentation, they are divided into five classes, as follows:

Class 1-Pit and fissure cavities, found wherever structural defects occur. Class II-Proximal cavities on bicuspids and molars.

Class III-Proximal cavities on incisors and cuspids which do not require the removal and restoration of the incisal angle.

Class IV-Proximal cavities on incisors and cuspids which do require the removal and restoration of the incisal angle.

Class v-Gingival third cavities-not pit cavities-occurring on the labial, buccal, or lingual surfaces of the teeth.

An analysis of this classification will reveal the fact that class I and group 1 are identical, i.e. pit and fissure cavities, while classes II, III, IV, and v, comprise group 2, or smooth-surface cavities.

Class 1 cavities. To take up the preparation of a simple cavity of class I (see Fig. 4) in the central fossa of