Classification of Abutment Teeth

The subject of abutment preparations may be grouped as follows: (1) those abutment teeth that require only minor modifications to their coronal portions, (2) those that are to have restorations other than complete coverage crowns, and (3) those that are to have crowns (complete coverage).

Abutment teeth that require only minor modifications include teeth with sound enamel, those with small restorations not involved in the removable partial denture design, those with acceptable restorations that will be involved in the removable partial denture design, and those that have existing crown restorations requiring minor modification that will not jeopardize the integrity of the crown. The latter may exist as an individual crown or as the abutment of a fixed partial denture.

The use of unprotected abutments has been discussed previously. Although complete coverage of all abutments may be desirable, it is not always possible or practical. The decision to use unprotected abutments involves certain risks of which the patient must be advised and includes responsibility for maintaining oral hygiene and caries control. Making crown restorations fit existing denture clasps is a difficult task; however, the fact that it is possible to do may influence the decision to use uncrowned but otherwise sound teeth as abutments.

Complete coverage restorations provide the best possible support for occlusal rests. If the patient’s economic status or other factors beyond the control of the dentist prevent the use of complete coverage restorations, then an amalgam alloy restoration, if properly condensed, is capable of supporting an occlusal rest without appreciable flow for a long period. Any existing silver amalgam alloy restoration about which there is any doubt should be replaced with new amalgam restorations. This should be done before guiding planes and occlusal rest seats are prepared, to allow the restoration to reach maximum strength and be polished.

Continued improvement in dimensional stability, strength, and wear resistance of composite resin restorations will add another dimension to the preparation and modification of abutment teeth for removable partial dentures that should be less invasive than placement of complete coverage restorations and more economical.

Sequence of Abutment Preparations on Sound Enamel or Existing Restorations

Abutment preparations on sound enamel or on existing restorations that have been judged as acceptable should be done in the following order:

1. Proximal surfaces parallel to the path of placement should be prepared to provide guiding planes (Figure 14-1A).

2. Tooth contours should be modified (Figure 14-1B and C), lowering the height of contour so that (a) the origin of circumferential clasp arms may be placed well below the occlusal surface, preferably at the junction of the middle and gingival thirds; (b) retentive clasp terminals may be placed in the gingival third of the crown for better esthetics and better mechanical advantage; and (c) reciprocal clasp arms may be placed on and above a height of contour that is no higher than the cervical portion of the middle third of the crown of the abutment tooth.

3. After alterations of axial contours are accomplished and before rest seat preparations are instituted, an impression of the arch should be made in irreversible hydrocolloid and a cast formed in a fast-setting stone. This cast can be returned to the surveyor to determine the adequacy of axial alterations before proceeding with rest seat preparations. If axial surfaces require additional axial recontouring, this can be performed during the same appointment and without compromise.

4. Occlusal rest areas should be prepared that will direct occlusal forces along the long axis of the abutment tooth (Figure 14-1D). Mouth preparation should follow the removable partial denture design that was outlined on the diagnostic cast at the time the cast was surveyed and the treatment plan confirmed. Proposed changes to abutment teeth should be made on the diagnostic cast and outlined in colored pencil to indicate the area, amount, and angulation of the modification to be done (see Chapter 12). Although occlusal rest seats may also be prepared on the diagnostic cast, indication of their location in colored pencil is usually sufficient for the experienced dentist because rest preparations follow a definite pattern (see Chapter 6).

Figure 14-1 Abutment contours should be altered during mouth preparations in the following sequence. A, The proximal surface is prepared parallel to the path of placement to create a guiding plane. B, Height of contour on the buccal and lingual surfaces is lowered when necessary to permit the retentive clasp terminus to be located within the gingival third of the crown, bracing part of the retentive arm at the junction of the middle and gingival thirds of the crown, and the reciprocal clasp arm on the opposite side of tooth to be placed no higher than the cervical portion of the middle third of the crown. C, The area of the tooth at which the retentive clasp arm originates should be altered if necessary to permit a more direct approach to the gingival third of the tooth: (1) incorrect position of retentive clasp arm; (2) area of tooth modified to accommodate better position of retentive clasp arm; (3) more ideal position of retentive clasp arm. D, Occlusal rest preparation that will direct occlusal forces along the long axis of the tooth should be the final step in mouth preparations.

Abutment Preparations Using Conservative Restorations

Conventional inlay preparations are permissible on the proximal surface of a tooth not to be contacted by a minor connector of the removable partial denture. On the other hand, proximal and occlusal surfaces that support minor connectors and occlusal rests require somewhat different treatment. The extent of occlusal coverage (i.e., whether cusps are covered) will be governed by the usual factors, such as the extent of caries, the presence of unsupported enamel walls, and the extent of occlusal abrasion and attrition.

When an inlay is the restoration of choice for an abutment tooth, certain modifications of the outline form are necessary. To prevent the buccal and lingual proximal margins from lying at or near the minor connector or the occlusal rest, these margins must be extended well beyond the line angles of the tooth. This additional extension may be accomplished by widening the conventional box preparation. However, the margin of a cast restoration produced for such a preparation may be quite thin and may be damaged by the clasp during placement or removal of the removable partial denture. This hazard may be avoided by extending the outline of the box beyond the line angle, thus producing a strong restoration-to-tooth junction.

In this type of preparation, the pulp is particularly vulnerable unless the axial wall is curved to conform to the external proximal curvature of the tooth. When caries is of minimal depth, the gingival seat should have an axial depth at all points about the width of a No. 559 fissure bur. It is of utmost importance that the gingival seat be placed where it can be easily accessed to maintain good oral hygiene. The proximal contour necessary to produce the proper guiding plane surface and the close proximity of the minor connector render this area particularly vulnerable to future caries. Every effort should be made to provide the restoration with maximum resistance and retention, as well as with clinically imperceptible margins. The first requisite can be satisfied by preparing opposing cavity walls 5 degrees or less from parallel and producing flat floors and sharp, clean line angles.

It is sometimes necessary to use an inlay on a mandibular first premolar for the support of an indirect retainer. The narrow occlusal width bucco-lingually and the lingual inclination of the occlusal surface of such a tooth often complicate the two-surface inlay preparation. Even the most exacting occlusal cavity preparation often leaves a thin and weak lingual cusp remaining.

Abutment Preparations Using Crowns

When multiple crowns are to be restored as removable partial denture abutments, it is best that all wax patterns be made at the same time. A cast of the arch with removable dies may be used if they are stable and sufficiently keyed for accuracy. If preferred, contouring wax patterns and making them parallel may be done on a solid cast of the arch (Figure 14-2), with individual dies used to refine margins. Modern impression materials and indirect techniques make either method equally satisfactory.

Figure 14-2 Solid cast of multiple abutment crowns for a removable partial denture. Wax patterns for crown #21, #28, #30, and #31 can be completed at the same time using the identical cast orientation. This allows control of the path of insertion features on all fitting surfaces of the removable prostheses.

The same sequence for preparing teeth in the mouth applies to the contouring of wax patterns. After the cast has been placed on the surveyor to conform to the selected path of placement and after the wax patterns have been preliminarily carved for occlusion and contact, proximal surfaces that are to act as guiding planes are carved parallel to the path of placement with a surveyor blade. Guiding planes are extended from the marginal ridge to the junction of the middle and gingival thirds of the tooth surface involved. One must be careful not to extend the guiding plane to the gingival margin because the minor connector must be relieved when it crosses the gingivae. A guiding plane that includes the occlusal two thirds or even one third of the proximal area is usually adequate without endangering gingival tissues.

After the guiding planes are parallel and any other contouring to accommodate the removable partial denture design is accomplished, occlusal rest seats are carved in the wax pattern. This method has been outlined in Chapter 6.

It should be emphasized that critical areas prepared in wax should not be destroyed by careless spruing or polishing. The wax pattern should be sprued to preserve paralleled surfaces and rest areas. Polishing should consist of little more than burnishing. Rest seat areas should need only refining with round finishing burs. If some interference by spruing is unavoidable, the casting must be returned to the surveyor for proximal surface refinement. This can be done accurately with the aid of a handpiece holder attached to the vertical spindle of the surveyor or some similar machining device.

One of the advantages of making cast restorations for abutment teeth is that mouth preparations that would otherwise have to be done in the mouth may be done on the surveyor with far greater accuracy. It is generally impossible to make several proximal surfaces parallel to one another when preparing them intraorally. The opportunity for contouring wax patterns and making them parallel on the surveyor in relation to a path of placement should be used to its full advantage whenever cast restorations are being made.

The ideal crown restoration for a removable partial denture abutment is the complete coverage crown, which can be carved, cast, and finished to ideally satisfy all requirements for support, stabilization, and retention without compromise for cosmetic reasons (Figure 14-3). Porcelain veneer crowns can be made equally satisfactory but only by the added step of contouring the veneered surface on the surveyor before the final glaze. If this is not done, retentive contours may be excessive or inadequate.

Figure 14-3 Metal ceramic crowns for teeth #4 and #5 demonstrating occlusal rests in metal and evidence of palatal finishing procedures. The distal surface of #4 provides a guide-plane surface that is continued onto a portion of the lingual surface for maximum stabilization.

The three-quarter crown does not permit creation of retentive areas as does the complete coverage crown. However, if buccal or labial surfaces are sound and retentive areas are acceptable or can be made so by slight modification of tooth surfaces, the three-quarter crown is a conservative restoration of merit. The same criteria apply in the decision to leave a portion of an abutment unprotected, as in the decision to leave any tooth unprotected that is to serve as a removable partial denture abutment.

Regardless of the type of crown used, preparation should be made to provide the appropriate depth for the occlusal rest seat. This is best accomplished by altering the axial contours of the tooth to the ideal before preparing the tooth and creating a depression in the prepared tooth at the occlusal rest area (Figure 14-4). Because the location of occlusal rests is established during treatment planning, this information will be known in advance of any tooth preparations. If, for example, double occlusal rests are to be used, this will be known so that the tooth can be prepared to accommodate the depth of both rests. It is inexcusable when waxing a pattern to find that a rest seat has to be made shallower than is desirable because of post-treatment planning. It can also create serious problems when a rest seat has to be made shallow in an existing crown or inlay because its thickness is not known. The opportunity for creating an ideal rest seat (if it has been properly treatment planned) depends only on the few seconds it takes to create a space for it.