Chapter 16: Support for the Distal Extension Denture Base

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.

Factors Influencing the Support of A Distal Extension Base

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:

Contour and Quality of the Residual Ridge

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.

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.

Extent of Residual Ridge Coverage by the Denture Base

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.”

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.

Jan 17, 2015 | Posted by in Prosthodontics | Comments Off on Chapter 16: Support for the Distal Extension Denture Base
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