CHAPTER 9 Denture Base Considerations
The denture base supports the artificial teeth and consequently receives the functional forces from occlusion and transfers functional forces to supporting oral structures (Figure 9-1). This function is most critical for the distal extension prosthesis, as functional stability and comfort often relate directly to the ability for this transfer of forces to occur without undue movement.
Figure 9-1 A, Class I maxillary distal extension removable partial denture showing tissue side (intaglio) of denture bases. B, Occlusal side of maxillary prosthesis: posterior artificial teeth are attached to the bases. C, Class II modification 1 mandibular distal extension removable partial denture shows intaglio of both extension and modification bases. D, Occlusal side of mandibular prosthesis: posterior artificial teeth are attached to the bases. For both prostheses, the bases are extended within the limits of physiologic activity of surrounding oral structures.
Although its primary purpose is related to masticatory function, the denture base also may add to the cosmetic effect of the replacement, particularly when techniques for tinting and reproducing natural-looking contours are used. Most of the techniques for creating a natural appearance in complete denture bases are applicable equally well to partial denture bases.
Still another function of the denture base is stimulation of the underlying tissues of the residual ridge. Some vertical movement occurs with any denture base, even those supported entirely by abutment teeth, because of the physiologic movement of those teeth under function. It is clearly evident that oral tissues placed under functional stress within their physiologic tolerance maintain their form and tone better than similar tissues suffering from disuse. The term disuse atrophy is applicable to both periodontal tissues and the tissues of a residual ridge.
Denture bases differ in functional purpose and may differ in terms of the material of which they are made. In a tooth-supported prosthesis, the denture base is primarily a span between two abutments supporting artificial teeth. Thus occlusal forces are transferred directly to the abutments through rests. Also, the denture base and the supplied teeth serve to prevent horizontal migration of all abutment teeth in the partially edentulous arch and vertical migration of teeth in the opposing arch.
When only posterior teeth are being replaced, esthetics is usually only a secondary consideration. On the other hand, when anterior teeth are replaced, esthetics may be of primary importance. Theoretically, the tooth-supported partial denture base that replaces anterior teeth must perform the following functions: (1) provide desirable esthetics; (2) support and retain the artificial teeth in such a way that they provide masticatory efficiency and assist in transferring occlusal forces directly to abutment teeth through rests; (3) prevent vertical and horizontal migration of remaining natural teeth; (4) eliminate undesirable food traps (oral cleanliness); and (5) stimulate the underlying tissues.
In a distal extension partial denture, denture bases other than those in tooth-supported modifications must contribute to the support of the denture. Such support is critical to the goal of minimizing functional movement and improving stability of the prosthesis. Although the abutment teeth provide support for the distal extension base, as the distance from the abutment increases, support from the underlying ridge tissues becomes increasingly important. Maximum support from the residual ridge may be obtained by using broad, accurate denture bases, which spread the occlusal load equitably over the entire area available for such support. The space available for a denture base is determined by the structures surrounding the space and by their movement during function. Maximum support for the denture base therefore can be accomplished only by using knowledge of the limiting anatomic structures and of the histologic nature of the basal seat areas, accuracy of the impression, and accuracy of the denture base (Figure 9-2). The first two of these support features relate to the gross size and cellular characteristics of the residual ridge tissues. These are highly variable between patients, and consequently not all residual ridges can provide the same quality of support. Therefore the ability to control functional displacement of the distal extension base is a determination that is unique for individual patients.
Figure 9-2 Maxillary and mandibular distal extension removable partial dentures with resin denture bases. Bases are extended buccally within the physiologic tolerance of border structures. A, Maxillary denture bases cover both the maxillary tuberosities, extend into the pterygomaxillary notches, and provide for adaptation along the posterior border, taking care not to extend beyond the soft palatal flexure. B, Mandibular bilateral distal extension removable partial denture bases cover the retromolar pads and extend into the retromylohyoid fossae. The impression procedure used established buccal shelves as primary stress-bearing areas of basal seats.
The snowshoe principle, which suggests that broad coverage furnishes the best support with the least load per unit area, is the principle of choice for providing maximum support. Therefore support should be the primary consideration in selecting, designing, and fabricating a distal extension partial denture base. Of secondary importance (but to be considered nevertheless) are esthetics, stimulation of the underlying tissues, and oral cleanliness. Methods used to accomplish maximum support of the restoration through its base(s) are presented in Chapters 15 and 16.
In addition to their difference in functional purposes, denture bases vary in material of fabrication. This difference is directly related to their function because of the need for some dentures to be relined.
Because the tooth-supported base has an abutment tooth at each end on which a rest has been placed, future relining or rebasing may not be necessary to reestablish support. Relining is necessary only when tissue changes have occurred beneath the tooth-supported base to the point that poor esthetics or accumulation of debris results. For these reasons alone, tooth-supported bases made soon after extractions should be of a material that permits later relining. Such materials are the denture resins, the most common of which are copolymer and methyl methacrylate resins.
Primary retention for the removable partial denture is accomplished mechanically by placing retaining elements on the abutment teeth. Secondary retention is provided by the intimate relationship of denture bases and major connectors (maxillary) with the underlying tissues. The latter is similar to the retention of complete dentures and is proportionate to the accuracy of the impression registration, the accuracy of the fit of the denture bases, and the total area of contact involved.
Retention of denture bases has been described as the result of the following forces: (1) adhesion, which is the attraction of saliva to the denture and tissues; (2) cohesion, which is the attraction of the molecules of saliva to each other; (3) atmospheric pressure, which is dependent on a border seal and results in a partial vacuum beneath the denture base when a dislodging force is applied; (4) physiologic molding of the tissues around the polished surfaces of the denture; and (5) the effects of gravity on the mandibular denture.
Adhesion and cohesion are effective when there is perfect apposition of the impressioned surface of the denture to the mucous membrane surfaces. These forces lose their effectiveness if any horizontal displacement of the dentures breaks the continuity of this contact. Atmospheric pressure is effective primarily as a rescue force when extreme dislodging forces are applied to the denture. It depends on a perfect border seal to keep the pressure applied on only one side of the denture. The presence of air on the impression surface would neutralize the pressure of the air against the polished surface. Because each of these forces is directly proportional to the area covered by the dentures, the dentures should be extended to the limits of the supporting structures.
The molding of the soft tissues around the polished surfaces of denture bases helps to perfect the border seal. Also, it forms a mechanical lock at certain locations on the dentures, provided these surfaces are properly prepared. This lock is developed automatically and without effort by the patient if the impression is made with an understanding of the anatomic possibilities.*
Few partial dentures are made without some mechanical retention. Retention from the denture bases may contribute significantly to the overall retention of the partial denture and therefore must not be discounted. Denture bases should be designed and fabricated so that they will contribute as much retention to the partial denture as possible. However, it is questionable whether atmospheric pressure plays as important a role in retention of removable partial dentures because a border seal cannot be obtained as readily as it can be with complete dentures. Therefore adhesion and cohesion gained by excellent apposition of the denture base and soft tissues of the basal seat play an important retentive role.
Acrylic-resin bases are attached to the partial denture framework by means of a minor connector designed so that a space exists between the framework and the underlying tissues of the residual ridge (Figure 9-3). Relief of at least a 20-gauge thickness over the basal seat areas of the master cast is used to create a raised platform on the investment cast on which the pattern for the retentive frame is formed (Figure 9-4). Thus after casting, the portion of the retentive framework to which the acrylic-resin base will be attached will stand away from the tissue surface sufficiently to permit a flow of acrylic-resin base material beneath its surface.
Figure 9-3 Mandibular Class II, modification 1 wax pattern developed on an investment cast. Adequate provision is made for attaching the resin base to the major connector on the edentulous side by way of a ladderlike minor connector and a butt-type joint. A similar minor connector design will be used for modification space. Note: Relief space beneath the minor connectors is established by relief wax placed on the original master cast and duplicated in this refractory cast. This allows processed resin to surround the minor connectors in creating the denture base.
Figure 9-4 Unlike the minor connector designs in Figure 9-3, the designs used for this prosthesis have a plastic mesh pattern. Although such designs can be reinforced to be more rigid, the bulk of the connector itself may contribute to weakening of the resin base. A more open type of minor connector seems preferable.
The retentive framework for the base should be embedded in the base material with sufficient thickness of resin (1.5 mm) to allow for relieving if this becomes necessary during the denture adjustment period or during relining procedures. Thickness is also necessary to avoid weakness and subsequent fracture of the acrylic-resin base material surrounding the metal framework.
The use of plastic mesh patterns in forming the retentive framework is generally less satisfactory than a more open framework (see Figure 9-4). Less weakening of the resin by the embedded framework results from use of the more open form. Pieces of 12- or 14-gauge half-round wax and 18-gauge round wax are used to form a ladderlike framework rather than the finer latticework of the mesh pattern. The precise design of the retentive framework, other than that it should be located both buccally and lingually, is not as important as its effective rigidity and strength when it is embedded in the acrylic resin base. It should also be free of interference with future adjustment, should not interfere with arrangement of artificial teeth, and should be open enough to avoid weakening any portion of the attached acrylic-resin. Designing the retentive framework for denture bases by having elements located buccal and lingual to the residual ridge not only will strengthen the acrylic-resin base but also will minimize distortion of the base created by the release of inherent strains in the acrylic-resin base during use or storage of the restoration (Figure 9-5).
Figure 9-5 Note that minor connectors by which resin denture bases will be attached to the framework are open, ladderlike configurations that extend on both buccal and lingual surfaces. This not only provides excellent attachment of the resin bases, but minimizes warping of bases resulting from the release of inherent strains in compression-molded resin.