INLAY CAVITY PREPARATION FOR POSTERIOR TEETH
by Richard V. Tucker, D.D.S.

The steps required in the placement of a cast gold restoration are as follows: diagnosis, tooth preparation, impressions, models, wax-up, investing, casting, cementation, and finishing. A flaw in any of these steps or lack of attention to the finest detail could result in a failure or less than satisfactory result.

The single step which contributes to a large number of failures, and that which requires a special effort from the operating dentist, is the cavity preparation.

A well prepared cavity not only meets the standard criteria of a satisfactory outline form, resistance form, retention form, and the other requirements established years ago by G.V. Black, it also must be smooth and precise in both its internal and external form. It is this precise, smooth properly tapered cavity with sharp margins and internal angles that makes it possible to carry out each of the other steps with precision.

Consider these steps mentioned above in relation to the cavity preparation. The proper diagnosis and treatment plan is of little value if the treatment is not adequately carried out while preparing the tooth. The preparation should encompass that which was considered when it was determined that the cast gold restoration was indicated. It involves occlusal coverage, esthetic considerations and preventive concepts.

A good impression is more easily obtained if the cavity is smooth and sharp; without this sharpness it is difficult to evaluate the impression. The models are enhanced also by the fine definition of a good cavity and results in fewer areas that are fragile. A satisfactory wax pattern is more easily obtained from the model of a smooth cavity preparation with a proper taper and well-defined margins. The resultant casting, which can be no better than the wax pattern, is likely to have fewer flaws and discrepancies than if the preparation of the tooth is rough and irregular. The last consideration would be that a properly extended cavity with sharp smooth margins makes it possible to finish the restoration on the tooth and to produce a more perfectly adapted gold inlay.

Dr. George Ellsperman, an outstanding teacher and critic of operative dentistry, invariably spoke of his concern with cavity detail, design and fine margins. Considering the importance ascribed to the cavity preparation, which is the theme of this paper, a short discussion of cavity instrumentation will be presented. The following steps in the procedure will involve the posterior Class II cavity.

1. After anesthetic has been administered, a rubber dam should be placed to give better visibility, tissue retraction, and ease of operation. When it is apparent that a procedure can be done better with the dam removed, such as discing of a margin on the distal surface of the most posterior tooth, then the preparation should be completed as nearly as possible before removing the dam for final completion.

2. The tooth is opened with either a #56 carbide bur for a bicuspid tooth or a #57 carbide bur for a molar. Care is used to avoid overcutting. First the occlusal outline is established. At the same time as the occlusal outline is being established, a flat pulpal floor of proper depth and the occlusal walls of a uniform taper are being prepared with the same bur. Then the flat gingival floor of proper depth is formed at the same time as the proximal and axial walls are being formed. All of the walls should have a uniform taper and draw from the gingival and pulpal floor to the cavosurface margins.

3. The double ended off-angle chisel #42S (15-10-16) is first used to form the proximal half of the distal axial line angles and to plane the distal buccal and the distal lingual walls. The same instrument is then placed on the edge of the pulpal floor and with one slice toward the gingival, one of the distal axial line angles is formed. The other distal axial line angle is formed in the same manner and the axial wall planed at that time. The #42S chisel also can be used to smooth any flaws or discrepancies on the pulpal and gingival floors at this time. The same procedure is used on the mesial proximal area of the tooth with the off-angle chisel #43S (15-10-16). Either the mesial or distal proximal walls of the tooth should be planed first, before the corresponding axial wall is planed, so that sharp crisp axial proximal line angles are formed.

4. The external gingival bevel is placed with the double-ended gingival marginal trimmers. The #232 Tucker (19-98-10-16) is used to form the distal gingival bevel and the #233 Tucker (10-78-10-16) is used to form the mesial gingival bevel. The double-ended instruments should be used for ease of operation. The gingival bevel should be made smooth and of uniform width, with the trimmers planing toward the proximal walls.

5. There is some difference of opinion regarding the use of a bevel on the occlusal margin. When the tooth being prepared has sound occlusal enamel that has not been bruised or crazed, it may be justified to place little or no bevel. The purpose of the bevel is to remove any irregularities in the preparation or unsupported enamel rods at the cavosurface margin. This will give the cavosurface margin a nice smooth, flowing outline. When the cusps are steep, little or no bevel is placed. When the cusps are shallow, a more distinct bevel of approximately five degrees can be placed. The occlusal bevel is placed with the #56 or #57 carbide bur. A disc can be used to straighten and smooth the proximal walls, if there is space enough. When the space between the proximal walls and the adjacent tooth is minimal, a chisel may be used to plane and smooth the proximal walls and cavosurface margins. The proximal walls should be a single plane.

6. When it is necessary to cover a cusp or cusps with gold, it can be accomplished effectively with a #7404 twelve-fluted, round ended bur. This bur produces a hollow ground relief and allows for a bulk of gold at the cavosurface margin. This finishing bur is used in a painting motion to establish a smooth, well defined, flowing outline to the extended cavity. This Technique is not be used for coverage of maxillary buccal cusps where esthetics is important. In such cases occlusal reduction is done with a #56 or #57 carbide in a single plane. The cavosurface margin on the buccal is disced with a fine cuttle disc to remove any unsupported enamel rods.

It has been demonstrated that there is a relationship between surface roughness of the cavity or the casting and the consequent retention. It would seem unreasonable to apply this fact to posterior inlays that present little or no retentive problems. However, the axial walls of three quarters, seven eights, or full crowns need not be polished with discs. Buccal and or lingual margins of full crowns, that are in enamel and will be finished, should be smoothed with a #7404 twelve-bladed bur. It has been shown that properly placed pins serve well to increase the retentive quality of a casting. If a tooth is destroyed to the extent that retention is a problem, the placement of pins is a simple solution.

In view of the fundamental necessity to prepare a fine cavity to accomplish a satisfactory gold inlay, it is hoped that this paper and the Technique that has been described will be of some assistance.