CHAPTER 10 Principles of Removable Partial Denture Design
Some of the biomechanical considerations of removable partial denture design were presented in Chapter 4. The strategy of selecting component parts for a partial denture to help control movement of the prosthesis under functional load has been highlighted as a method to be considered for logical partial denture design. The requirements for movement control are generally functions of whether the prosthesis will be tooth supported or tooth-tissue supported.
For a tooth-supported prosthesis, the movement potential is less because resistance to functional loading is provided by the teeth. Teeth do not vary widely in their ability to provide this support; consequently, designs for prostheses are less variable. This is the case even though the amount of supporting bone, the crown-to-root ratios, the crown and root morphologies, and the tooth number and position in the arch relative to edentulous spaces are well established and may be variable for tooth- and tooth-tissue–supported removable partial dentures (RPDs). For a tooth-tissue–supported prosthesis, the residual ridge (remaining alveolar bone and overlying connective tissue covered with mucosa) presents with variable potential for support. Not only does the underlying alveolar bone demonstrate a highly variable form following extraction, it continues to change with time. As alveolar bone responds to the loss of teeth, the overlying connective tissue and mucosa undergo change that places the soft tissue at risk for pressure-induced inflammatory changes. This variable tissue support potential adds complexity to design considerations when one is dealing with tooth-tissue–supported prostheses. This occurs because unlike the efficient support provided by teeth, which results in limited prosthesis movement, the reaction of the ridge tissue to functional forces can be highly variable, leading to variable amounts of prosthesis movement. An understanding of the potential sources of functional force from the opposing arch that can have an effect on the movement potential of the prosthesis is helpful.
Factors related to the opposing arch tooth position, the existence and nature of prosthesis support in the opposing arch, and the potential for establishing a harmonious occlusion can greatly influence the partial denture design. Opposing tooth positions that apply forces outside the primary support of the prosthesis can introduce leverage forces that act to dislodge the prosthesis. Such an effect is variable and is based on the nature of the opposing occlusion, because the forces of occlusion differ between natural teeth, removable partial dentures, and complete dentures. In general, removable partial dentures opposing natural teeth will require greater support and stabilization over time because of the greater functional load demands. Therefore, occlusal relationships at maximum intercuspation should be broadly dissipated to the supporting units.
On the basis of the previous discussion, it is clear that two distinctly different types of RPDs exist. Certain points of difference are present between Kennedy Class I and Class II types of partial dentures on the one hand and the Class III type of partial denture on the other. The first consideration is the manner in which each is supported. The Class I type and the distal extension side of the Class II type derive their primary support from tissues underlying the base and secondary support from the abutment teeth (Figure 10-1, A and Figure 10-2). The Class III type derives all of its support from the abutment teeth (Figure 10-1, B and Figure 10-2).
Figure 10-1 A, Kennedy Class I partially edentulous arch. Major support for denture bases must come from residual ridges, tooth support from occlusal rests being effective only at the anterior portion of each base. B, Kennedy Class III, modification 1 partially edentulous arch provides total tooth support for the prosthesis. A removable partial denture made for this arch is totally supported by rests on properly prepared occlusal rest seats on four abutment teeth.
Figure 10-2 Distortion of tissues over the edentulous ridge will be approximately 500 µm under 4 newtons of force, whereas abutment teeth will demonstrate approximately 20 µm of intrusion under the same load.
Third, the need for some kind of indirect retention exists in the distal extension type of partial denture, whereas in the tooth-supported, Class III type, no extension base is present to lift away from the supporting tissues because of the action of sticky foods and the movements of tissues of the mouth against the borders of the denture. This is so because each end of each denture base is secured by a direct retainer on an abutment tooth. Therefore the tooth-supported partial denture does not rotate about a fulcrum, as does the distal extension partial denture.
Fourth, the manner in which the distal extension type of partial denture is supported often necessitates the use of a base material that can be relined to compensate for tissue changes. Acrylic-resin is generally used as a base material for distal extension bases. The Class III partial denture, on the other hand, which is entirely tooth supported, does not require relining except when it is advisable to eliminate an unhygienic, unesthetic, or uncomfortable condition resulting from loss of tissue contact. Metal bases therefore are more frequently used in tooth-supported restorations, because relining is not as likely to be necessary with them.
The distal extension partial denture derives its major support from the residual ridge with its fibrous connective tissue covering. The length and contour of the residual ridge significantly influence the amount of available support and stability (Figure 10-3). Some areas of this residual ridge are firm, with limited displaceability, whereas other areas are displaceable, depending on the thickness and structural character of the tissues overlying the residual alveolar bone. The movement of the base under function determines the occlusal efficiency of the partial denture and also the degree to which the abutment teeth are subjected to torque and tipping stresses.
Figure 10-3 A, The longer the edentulous area covered by the denture base, the greater the potential lever action on the abutment teeth. If an extension base area is 30 mm (ac) and tissue displacement is 2 mm (ab), the amount of movement of the proximal plate on the guiding plane will be approximately 0.25 mm: [α = √ (ab)2 + (ac)2]; arc of the tangent ab/ad = x/cd (2/30 = x/3.75 = 0.25 mm). B, The flat ridge will provide good support, poor stability. C, The sharp spiny ridge will provide poor support, poor to fair stability. D, Displaceable tissue on the ridge will provide poor support and poor stability.
No single impression material can satisfactorily fulfill both of the previously mentioned requirements. Recording the anatomic form of both teeth and supporting tissues will result in inadequate support for the distal extension base. This is so because the cast will not represent the optimum coordinating forms, which require that the ridge must be related to the teeth in a supportive form. This coordination of support maximizes the support capacity for the arch and minimizes movement of the partial denture under function.
The tooth-supported partial denture, which is totally supported by abutment teeth, is retained and stabilized by a clasp at each end of each edentulous space. Because this type of prosthesis does not move under function (other than within the physiologic limitations of tooth support units), the only requirement for such clasps is that they flex sufficiently during placement and removal of the denture to pass over the height of contour of the teeth in approaching or escaping from an undercut area. While in its terminal position on the tooth, a retentive clasp should be passive and should not flex except when one is engaging the undercut area of the tooth for resisting a vertical dislodging force.
Cast retentive arms are generally used for this purpose. These may be of the circumferential type, arising from the body of the clasp and approaching the undercut from an occlusal direction, or of the bar type, arising from the base of the denture and approaching the undercut area from a gingival direction. Each of these two types of cast clasps has its advantages and disadvantages.
In the combination tooth and tissue–supported RPD, because of the anticipated functional movement of the distal extension base, the direct retainer adjacent to the distal extension base must perform still another function, in addition to resisting vertical displacement. Because of the lack of tooth support distally, the denture base will move tissue-ward under function proportionate to the quality (displaceability) of the supporting soft tissues, the accuracy of the denture base, and the total occlusal load applied. Because of this tissue-ward movement, those elements of a clasp that lie in an undercut area mesial to the fulcrum for a distal extension (as is often seen with a distal rest) must be able to flex sufficiently to dissipate stresses that otherwise would be transmitted directly to the abutment tooth as leverage. On the other hand, a clasp used in conjunction with a mesial rest may not transmit as much stress to the abutment tooth because of the reduction in leverage forces that results from a change in the fulcrum position. This serves the purpose of reducing or “breaking” the stress, hence the term stress-breakers, and is a strategy that is often incorporated into partial denture designs through various means. Some dentists strongly believe that a stress-breaker is the best means of preventing leverage from being transmitted to the abutment teeth. Others believe just as strongly that a wrought-wire or bar-type retentive arm more effectively accomplishes this purpose with greater simplicity and ease of application. A retentive clasp arm made of wrought wire can flex more readily in all directions than can the cast half-round clasp arm. Thereby, it may more effectively dissipate those stresses that would otherwise be transmitted to the abutment tooth. A discussion of the limitations of stress-breakers has been presented in Chapter 9.
Only the retentive arm of the circumferential clasp, however, should be made of wrought metal. Reciprocation and stabilization against lateral and torquing movement must be obtained through use of the rigid cast elements that make up the remainder of the clasp. This is called a combination clasp because it is a combination of cast and wrought materials incorporated into one direct retainer. It is frequently used on the terminal abutment for the distal extension partial denture and is indicated where a mesiobuccal but no distobuccal undercut exists, or where a gross tissue undercut, cervical and buccal to the abutment tooth, exists. It must always be remembered that the factors of length and material contribute to the flexibility of clasp arms. From a materials physical property standpoint, a short wrought-wire arm may be a destructive element because of its reduced ability to flex compared with a longer wrought-wire arm. However, in addition to its greater flexibility compared with the cast circumferential clasp, the combination clasp offers the advantages of adjustability, minimum tooth contact, and better esthetics, which justify its occasional use in tooth-supported designs.
The amount of stress transferred to the supporting edentulous ridge(s) and the abutment teeth will depend on: (1) the direction and magnitude of the force; (2) the length of the denture base lever arm(s); (3) the quality of resistance (support from the edentulous ridges and remaining natural teeth); and (4) the design characteristics of the partial denture. As was stated in Chapter 7, the location of the rest, the design of the minor connector as it relates to its corresponding guiding plane, and the location of the retentive arm are all factors that influence how a clasp system functions. The greater the surface area contact of each minor connector to its corresponding guiding plane, the more horizontal the distribution of force (Figure 10-4).
Figure 10-4 1, Maximum contact of the proximal plate minor connector with the guiding plane produces a more horizontal distribution of stress to the abutment teeth. 2, Minimum contact or disengagement of the minor connector with the guiding plane allows rotation around the fulcrum located on the mesio-occlusal rest, producing a more vertical distribution of stress to the ridge area. 3, Minor connector contact with the guiding plane from the marginal ridge to the junction of the middle and gingival thirds of the abutment tooth distributes load vertically to the ridge and horizontally to the abutment tooth. F is the location of the fulcrum of movement for the distal extension base.
The design of the partial denture framework should be systematically developed and outlined on an accurate diagnostic cast based on the following prosthesis concepts: where the prosthesis is supported, how the support is connected, how the prosthesis is retained, how the retention and support are connected, and how edentulous base support is connected.
In developing the design, it is first necessary to determine how the partial denture is to be supported. In an entirely tooth-supported partial denture, the most ideal location for the support units (rests) is on prepared rest seats on the occlusal, cingulum, or incisal surface of the abutment adjacent to each edentulous space (see Figure 10-1, B). The type of rest and amount of support required must be based on interpretation of the diagnostic data collected from the patient. In evaluating the potential support that an abutment tooth can provide, consideration should be given to (1) periodontal health; (2) crown and root morphologies; (3) crown-to-root ratio; (4) bone index area (how tooth has responded to previous stress); (5) location of the tooth in the arch; (6) relationship of the tooth to other support units (length of edentulous span); and (7) the opposing dentition. (For a more in-depth understanding of these considerations, review Chapters 6 and 12.)
In a tooth and tissue–supported partial denture, attention to these same considerations must be given to the abutment teeth. However, equitable support must come from the edentulous ridge areas. In evaluating the potential support available from edentulous ridge areas, consideration must be given to (1) the quality of the residual ridge, which includes contour and quality of the supporting bone (how the bone has responded to previous stress) and quality of the supporting mucosa; (2) the extent to which the residual ridge will be covered by the denture base; (3) the type and accuracy of the impression registration; (4) the accuracy of the denture base; (5) the design characteristics of the component parts of the partial denture framework; and (6) the anticipated occlusal load. A full explanation of tissue support for extension base partial dentures is found in Chapter 16.
Denture base areas adjacent to abutment teeth are primarily tooth supported. As one proceeds away from the abutment teeth, they become more tissue supported. Therefore it is necessary to incorporate characteristics in the partial denture design that will distribute the functional load equitably between the abutment teeth and the supporting tissues of the edentulous ridge. Locating tooth support units (rests) on the principal abutment teeth and designing the minor connectors that are adjacent to the edentulous areas to contact the guiding planes in such a manner that the functional load is dispersed equitably between the available tooth and tissue supporting units will provide designs with controlled distribution of support (see Figure 10-4).
The second step in systematic development of the design for any removable partial denture is to connect the tooth and tissue support units. This connection is facilitated by designing and locating major and minor connectors in compliance with the basic principles and concepts presented in Chapter 5. Major connectors must be rigid so that forces applied to any portion of the denture can be effectively distributed to the supporting structures. Minor connectors arising from the major connector make it possible to transfer functional stress to each abutment tooth through its connection to the corresponding rest and also to transfer the effects of the retainers, rests, and stabilizing components to the remainder of the denture and throughout the dental arch.
The third step is to determine how the removable partial denture is to be retained. The retention must be sufficient to resist reasonable dislodging forces. As was stated in Chapter 7, retention is accomplished by placement of mechanical retaining elements (clasps) on the abutment teeth and by the intimate relationship of the denture bases and major connectors (maxillary) with the underlying tissues. The key to selecting a successful clasp design for any given situation is to choose one that will (1) avoid direct transmission of tipping or torquing forces to the abutment; (2) ac/>