CHAPTER 16 Support for the Distal Extension Denture Base
In a tooth-supported removable partial denture, a metal base or the framework that supports an acrylic-resin base is connected to and is part of a rigid framework that permits the direct transfer of occlusal forces to the abutment teeth through the occlusal rests. Even though the denture base of the modification space(s) in a Kennedy Class III removable partial denture provides support for the supplied teeth, the residual ridge beneath the base is not called on to aid in the support of the removable partial denture. Therefore the resiliency of the ridge tissues, the ridge configuration, and the type of bone that supports these tissues are not factors in denture support. Regardless of the length of the edentulous spans, if the framework is rigid, the abutment teeth are sound enough to carry the additional load, and the occlusal rests are properly formed, support comes entirely from the abutment teeth at either end of that span. Support may be augmented by splinting and by the use of additional abutments, but in any event the abutments are the sole support of the removable restoration.
An impression (and resultant stone cast) records the anatomic form of the teeth and their surrounding structures and is needed to make a tooth-supported removable partial denture. The impression should also record the moving tissues that will border the denture in an unstrained position, so the relationship of the denture base to those tissues may be as accurate as possible. Although underextension of the denture base in a tooth-supported prosthesis is the lesser of two evils, an underextended base may lead to food entrapment and inadequate facial contours, particularly on the buccal and labial sides. To accurately record the moving tissues of the floor of the mouth, an individual impression tray should be used, rather than an ill-fitting or overextended stock tray. This has been discussed at length in Chapters 5 and 15.
The distal extension removable partial denture does not have the advantage of total tooth support because one or more bases are extensions covering the residual ridge distal to the last abutment. It therefore is dependent on the residual ridge for a portion of its support.
The distal extension removable partial denture must depend on the residual ridge for some support, stability, and retention. Indirect retention, to prevent the denture from lifting away from the residual ridge, should also be incorporated in the design. The tooth-supported base is secured at either end by the action of a direct retainer and is supported at either end by a rest, whereas this degree of support and direct retention is lacking in the distal extension prosthesis. For this reason, a distal abutment should be preserved whenever possible. In the event of the loss or absence of a distal abutment tooth, the patient must be made aware of the movements to be expected with a distal extension removable partial denture and the limitations imposed on the dentist when the residual ridge must be used for support, stability, and retention for that part of the prosthesis.
Because one of the stated objectives of prosthodontic treatment is the restoration of function and comfort in an esthetically pleasing manner, maintenance of occlusal contact in distal extension removable partial dentures demands an understanding of the factors that influence residual ridge support. Support from the residual ridge becomes more important as the distance from the last abutment increases and will depend on the following several factors:
The ideal residual ridge to support a denture base would consist of cortical bone that covers relatively dense cancellous bone, with a broad rounded crest with high vertical slopes, and is covered by firm, dense, fibrous connective tissue. Such a residual ridge would optimally support vertical and horizontal stresses placed on it by denture bases. Unfortunately this ideal is seldom encountered.
Easily displaceable tissue will not adequately support a denture base, and tissues that are interposed between a sharp, bony residual ridge and a denture base will not remain in a healthy state. Not only must the nature of the bone of the residual ridge be considered in developing optimum support for the denture base, but also its positional relationship to the direction of forces that will be placed on it.
The crest of the bony mandibular residual ridge is most often cancellous. Because lining mucosa restricts both the buccal and lingual mucosae adjacent to teeth in the mandible, loss of firm mucosa overlying the residual ridge is common following tooth extraction in the posterior mandible. Pressures placed on tissues overlying the crest of the mandibular residual ridge usually result in irritation of these tissues, accompanied by the sequelae of chronic inflammation. Therefore the crest of the mandibular residual ridge cannot be a primary stress-bearing region. The buccal shelf region (bounded by the external oblique line and the crest of the alveolar ridge) seems to be better suited for a primary stress-bearing role because it is covered by relatively firm, dense, fibrous connective tissue supported by cortical bone. In most instances this region bears more of a horizontal relationship to vertical forces than do other regions of the residual ridge (Figure 16-1). The slopes of the residual ridge then would become the primary stress-bearing region for resistance of horizontal and off-vertical forces.
Figure 16-1 The dotted portion outlines the crest of the residual ridge, which should be recorded in its anatomic form in impression procedures. Similarly, retromolar pads should not be displaced by impression. Buccal shelf regions are outlined by a herringbone pattern, and selected additional pressures may be placed on these regions for vertical support of the denture base. Lingual slopes of the residual ridge (cross-hatched) may furnish some vertical support to the restoration; however, these regions principally resist the horizontal rotational tendencies of the denture base and should be recorded by the impression in undisplaced form.
The immediate crest of the bone of the maxillary residual ridge may consist primarily of cancellous bone. Unlike in the mandible, oral tissues that overlie the maxillary residual alveolar bone are usually of a firm, dense nature (similar to the mucosa of the hard palate) or can be surgically prepared to support a denture base. The topography of a partially edentulous maxillary arch imposes a restriction on selection of a primary stress-bearing area. In spite of impression procedures, the crestal area of the residual ridge will become the primary stress-bearing area for vertically directed forces. Some resistance to these forces may be obtained by the immediate buccal and lingual slopes of the ridge. Palatal tissues between the medial palatal raphe and the lingual slope of the posterior edentulous ridge are readily displaceable and cannot be considered as primary stress-bearing sites (Figure 16-2). The tissues covering the crest of the maxillary residual ridge must be less displaceable than the tissues that cover palatal areas, or relief of palatal tissues must be provided in the denture bases or for palatal major connectors.
Figure 16-2 The crest of the maxillary residual ridge (herringbone pattern) is the primary supporting region for the maxillary distal extension denture base. Buccal and lingual slopes may furnish limited vertical support to the denture base. It seems logical that their primary role is to counteract the horizontal rotational tendencies of the denture base. The dotted portion outlines the incisive papilla and the median palatal raphe. Relief must be provided for these regions, especially if tissues covering the palatal raphe are less displaceable than those covering the crest of the residual ridge.
The broader the residual ridge coverage, the greater is the distribution of the load, which results in less load per unit area (Figure 16-3). A denture base should cover as much of the residual ridge as possible and should be extended the maximum amount within the physiologic tolerance of the limiting border structures or tissues. Knowledge of these border tissues and the structures that influence their movement is paramount to the development of broad coverage denture bases. In a series of experiments, Kaires has shown that “maximum coverage of denture-bearing areas with large, wide denture bases is of the utmost importance in withstanding both vertical and horizontal stresses.”
Figure 16-3 Comparison of two removable partial dentures for the same patient. The denture on the right has severely underextended bases. Its replacement, with properly extended bases, is on the left. Occlusal forces are more readily distributed to denture-bearing areas by the replacement denture.
It is not within the scope of this text to review the anatomic considerations related to denture bases. The student is referred to several articles listed in the “Selected Reading Resources” regarding this subject.
The residual ridge may be said to have two forms: the anatomic form and the functional form (Figure 16-4). The anatomic form is the surface contour of the ridge when it is not supporting an occlusal load. The functional form of the residual ridge is the surface contour of the ridge when it is supporting a functional load.
Figure 16-4 Comparison of anatomic and functional ridge forms. A, Original master cast with the edentulous area recorded in its anatomic form, using elastic impression material. B, Same cast after the edentulous area has been repoured to its functional form as recorded by the secondary impression.
The anatomic form is recorded by a soft impression material, such as a metallic oxide impression paste, if the entire impression tray is uniformly relieved. Depending on the viscosity of the particular impression material used and the rigidity of the impression tray, it is also the form that can be recorded by mercaptan rubber, silicone, and hydrocolloid impression materials. Distortion and tissue displacement by pressure may result from confinement of the impression material within the tra/>